Allogeneic hematopoietic cell transplantation led to the discovery of the allogeneic GVL effect, which remains the most convincing evidence that immune cells can cure cancer in humans. However, despite its great paradigmatic and clinical relevance, induction of GVL by conventional allogeneic hematopoietic cell transplantation remains a quite rudimentary form of leukemia immunotherapy. It is toxic and its efficacy is far from optimal. It is therefore sobering that since the discovery of the GVL effect 3 decades ago, the way GVL is induced and manipulated has practically not changed. Preclinical and clinical studies suggest that injection of T cells primed against a single Ag present on neoplastic cells could enhance the GVL effect without causing any GVHD. We therefore contend that Ag-targeted adoptive T-cell immunotherapy represents the future of leukemia immunotherapy, and we discuss the specific strategies that ought to be evaluated to reach this goal. Differences between these strategies hinge on 2 key elements: the nature of the target Ag and the type of Ag receptor expressed on T cells.

The GVL reaction refers to the ability of donor immune cells to eliminate host leukemic cells after allogeneic hematopoietic cell transplantation (AHCT). In 1956, Barnes et al1  were the first to report cure of leukemia in mice after total body irradiation and AHCT. Further studies of GVL in animal models were pioneered by Bortin et al2  in the 1970s.3  The relevance of the GVL reaction in humans was established by the Seattle group in 1979, and key insights into its mechanisms were reported in a landmark study from the International Bone Marrow Transplant Registry in 1990.4,5  Strikingly, the latter study that was based on data from 2254 subjects treated by 142 teams showed that GVL was abrogated if T cells were depleted from the graft or if the AHCT donor was an identical twin.5  On the basis of these data, it was therefore inferred that GVL depended on donor T cells and on the existence of histocompatibility differences between the donor and its recipient (absent among identical twins). Because the International Bone Marrow Transplant Registry study involved HLA-identical siblings, the sole histocompatibility differences between donors and recipients were minor histocompatibility Ags (MiHAs).6  The molecular nature of MiHAs was elucidated in 1990, and the first human MiHA was sequenced in 1995.7,8  Increasing recognition that cure after AHCT for leukemia is largely because of the GVL effect led to the introduction of nonmyeloablative conditioning regimens and donor lymphocyte infusions (DLIs).9,10  Remarkably, DLI can eradicate ≤ 1012 neoplastic cells in patients with chronic myelogenous leukemia,11,12  and the allogeneic GVL effect represents the most convincing evidence that immunotherapy can cure human neoplasias.13,14  Nowadays, AHCT can be viewed primarily as a quest for the GVL effect.12  In AHC transplant recipients, the effect of GVL on leukemia-free survival is so important that it supersedes the deleterious effect of GVHD and protracted immunodeficiency.

After conventional AHCT (unmanipulated graft), GVL is T-cell dependent: it is abrogated after T-cell depletion of the graft, or when the leukemic cells are MHC-deficient and therefore cannot interact with donor T cells.5,15  Studies of numerous experimental models have firmly established that GVL specifically depends on recognition of host histocompatibility Ags on leukemic cells. Indeed, (1) GVL is not observed after syngeneic AHCT5 ; (2) when parental strain rats (A and B) were used as donors for F1 hybrid recipients (AB) bearing leukemic cells derived from parental strain A, both A and B donors induced GVHD, but only B-strain donors induced GVL16 ; (3) in H2b and H2bm1 recipients bearing H2b leukemic cells, GVHD follows H2b→H2bm1 and H2bm1→H2b AHCT, but GVL is elicited only in H2bm1→H2b AHCT17 ; (4) GVL can be enhanced by pre-AHCT donor immunization against host MHC or MiHAs2,3 ; and (5) GVHD, which results from alloreactivity, correlates with low leukemic relapse rates after AHCT.4,18  Two conclusions can be inferred from these studies. First, in MHC-identical donor-recipient pairs, the most common situation in human AHCT, GVL depends on donor T cells that recognize host MiHAs. Second, that GVL does not occur in the presence of GVHD targeted to Ags absent on leukemic cells16,17  means that GVL requires direct interactions between donor T cells and leukemic cells and cannot be ascribed solely to paracrine (eg, cytokine-mediated) effects of GVHD.

Natural killer (NK) cells, which rapidly recover to normal blood levels after AHCT, may also contribute to the GVL effect.19,20  However, for reasons that remain unclear, NK cells display antileukemic activity predominantly if not exclusively against myeloid leukemias. Depending on the context, NK cell–mediated GVL may involve MHC I–dependent and –independent mechanisms. NK cells express inhibitory killer immunoglobulin-like receptors (KIRs) that prevent NK-cell killing on binding to their cognate MHC I molecule on target cells. Benjamin et al20  and Ruggeri et al21  have shown in patients with acute myeloid leukemia (AML) treated by T cell–depleted haploidentical AHCT, that NK cells can kill AML cells that do not express the ligands for donor inhibitory KIRs. After HLA-matched AHCT (where inhibitory KIRs are irrelevant), several clinical studies suggest that expression of specific activating NK-cell receptors on donor cells is associated with a decreased risk of AML relapse.19,20,22  The nature of the (non-MHC) ligands recognized by activating NK receptors on myeloid cells remains unknown and is a subject of great interest. A more in-depth evaluation of the role of NK cells in GVL can be found in recent review articles.

Nature of MiHAs

MHC molecules present peptides at the cell surface. Under normal circumstances (in the absence of infection), all of these peptides originate from proteolytic degradation of cell proteins.23  Some MHC-associated peptides (MAPs) are polymorphic; they are present in some persons, but in other MHC-matched subjects they are absent or present a slightly different amino acid sequence.7,24  For historical reasons, polymorphic MAPs are referred to as MiHAs. They are a consequence of any form of accumulated genetic variation that hinders MAP generation (eg, gene deletion) or the structure of a MAP (eg, single nucleotide polymorphisms).6,24,25  MiHAs are essentially genetic polymorphisms viewed from a T-cell perspective.

Do leukemia-associated Ags contribute to the GVL effect?

Hundreds of tumor Ags have been found on various types of neoplastic cells, and leukemia cells are no exception to this rule.26  A comprehensive and up-to-date database on human T cell–defined tumor Ags can be found at http://www.cancerimmunity.org/peptidedatabase/Tcellepitopes.htm. Although some leukemia-associated Ags (LAAs) are only overexpressed on leukemic cells (relative to normal cells), other LAAs are truly specific to leukemic cells. Such Ags should therefore be seen as neo-Ags by donor T cells. Therefore, donor T cells would be expected to respond at least to some host LAAs. However, the lack of GVL in the absence of alloreactivity (eg, identical twins) means that LAAs are not sufficient to elicit curative antileukemic response after conventional AHCT. This conclusion must be tempered however, by the fact that the amount of data on GVL after syngeneic AHCT in humans remains limited. Further studies of anti-LAA immune responses in the syngeneic setting would be of great interest. Furthermore, despite the lack of direct evidence that LAAs play a role in GVL, evidence suggests that LAAs are immunogenic and might contribute to GVL initiated by MiHA-responsive T cells. Expansion of donor T cells specific for the PR1 LAA (an HLA-A2–associated peptide derived from proteinase 3) was found after AHCT in subjects treated for chronic myelogenous leukemia.27  Likewise, expansion of CD8 T cells specific for the WT1 LAA correlated with the occurrence of GVL in 10 subjects treated for acute lymphoblastic leukemia.28  Moreover, recent studies provide compelling evidence that MiHA-triggered GVL initiates Ag spreading that leads to T-cell responses against LAAs and B-cell responses against differentiation Ags. Thus, after AHCT in patients with chronic lymphocytic leukemia, achievement of remission correlated with the development LAA-specific T cells and antibodies against B-cell lineage Ags.29,30  Furthermore, in a human subject with melanoma, vaccination with MAGE-A Ags led to expansion of CD8 T cells specific for a tumor-specific peptide that was not present in the vaccine, and the latter T cells contributed to regression of melanoma metastases.31  Finally, in an MiHA-mismatched mouse model of nonmyeloablative AHCT for treatment of prostate cancer, post-AHCT vaccination of recipients against tumor Ags enhanced the graft-versus-tumor effect, thereby suggesting that MiHA-specific T cells may collaborate with tumor-specific T cells.32  Together, these studies suggest that, although alloreactivity is necessary, it may not be sufficient for effective GVL which may also require immune responses against LAAs.

Why are T cells specific for host MiHAs more effective than LAA-specific T cells for leukemia immunotherapy? The potential role of immunoediting

The immune system acts as an extrinsic tumor suppressor that neoplastic cells must evade to survive. One key component of immunoevasion is immunoediting whereby neoplastic cells that express highly immunogenic tumor Ags are eliminated.33  Down-regulation of immunogenic tumor Ags is used by tumor cells to evade immune surveillance.34-36  According to this paradigm, leukemic cells have undergone negative selection pressure by recipient's LAA-specific T cells, and the sole leukemic cells that were “successful” are those that did not express highly immunogenic LAAs. However, before AHCT, leukemic cells have never encountered MiHA-specific T cells (which are present only in allogeneic subjects) and have not “evolved” an evasive strategy toward MiHA-specific T cells; even leukemia cells with highly immunogenic MiHAs were under no negative selection pressure. We therefore surmise that because of immunoediting, leukemic cells are prepared to evade LAA-specific but not MiHA-specific T cells: leukemic cells do not express highly immunogenic LAAs but can express highly immunogenic MiHAs. Furthermore, with rare exceptions, donor T cells have never encountered host MiHAs and have not been tolerized against them. We therefore propose that because of immunoediting, LAAs are less immunogenic than MiHAs, explaining the overwhelming importance of MiHAs to elicit a strong GVL. This hypothesis is testable and would require comparing the immunogenicity of human MiHAs and LAAs. Nonetheless, the putative advantage of MiHAs over LAAs is not permanent: after AHCT, immunoediting may affect immune responses against leukemic cells in many ways and may lead to reduced expression of all Ags expressed on leukemic cells.37 

Relation between GVHD and GVL

The most commonly asked question in the field of AHCT is whether GVHD and GVL can be separated. It is generally accepted that GVHD and GVL are linked but can be separated in several clinical and experimental models. We do not want to provide a comprehensive discussion of all relevant experimental models because this fundamental issue has been addressed in excellent review articles.38-41  Nonetheless, we would like to address the GVHD-GVL conundrum from a different perspective by asking the following question: if both GVHD and GVL result from alloreactivity (directed specifically against MiHAs in the case of MHC-matched persons), how can alloreactive T cells spare normal cells (no GVHD) and kill leukemic cells (GVL)? We propose a simple answer that integrates data from numerous reports. We posit that the susceptibility to alloreactive T cells is different for various cell types and can be ranked as follows: hematolymphoid cells > thymic epithelial cells > epithelial cells from the gut, liver, and skin > other organs that are spared by GVHD (eg, heart, kidney) (Figure 1). Implicit to this model, the clinical correlates of hematolymphoid GVHD are complete donor chimerism and GVL, whereas destruction of thymic epithelial cells causes immune deficiency, and damage to other epithelial cells leads to typical clinical signs of GVHD.

Figure 1

Schematic working model for the relation between susceptibility to alloreactive T cells and GVHD and GVL. Of note, organs such as the heart and kidney are spared by GVHD and are therefore not shown in this figure.

Figure 1

Schematic working model for the relation between susceptibility to alloreactive T cells and GVHD and GVL. Of note, organs such as the heart and kidney are spared by GVHD and are therefore not shown in this figure.

The idea that hematolymphoid cells are more sensitive than other cell types to alloreactive T cells is supported by several observations: (1) in patients who do not receive any myeloablative regimen and have GVHD induced by blood transfusion, pancytopenia (because of BM aplasia) is the main cause of death42 ; (2) likewise, in mouse models of AHCT where nonmyeloablated recipients receive allogeneic lymphoid cells (but not hematopoietic progenitors), pancytopenia is the first sign of GVHD43,44 ; (3) after conventional AHCT, the rate of clearance of recipients' hematolymphoid cells in the early posttransplantation period predicts the occurrence of GVHD in humans45 ; and (4) after DLI, conversion from mixed chimerism to full donor chimerism (elimination of host hematolymphoid cells) is an absolute prerequisite for the occurrence GVL and is often but not always followed by “epithelial GVHD.”12 

Proliferation of alloreactive T cells is initiated in secondary lymphoid organs.46  Activated T cells must then extravasate from secondary lymphoid organs and infiltrate epithelia to cause GVHD.47  The thymic epithelium is excessively sensitive to damage even by very small numbers of alloreactive cells that can be detected in the thymus as early as days 2 or 3 after AHCT.48,49  Thymic GVHD of course hampers immune reconstitution.50,51  In addition, thymic failure is a watershed event that enhances GVHD in 2 ways: by preventing generation of natural CD4 regulatory T cells that mitigate GVHD52-54  and by allowing thymic export of host-reactive T cells that have not undergone proper negative selection.55  Alike thymic epithelial cells, stromal cells in other hematolymphoid organs are damaged by alloreactive T cells. Thus, after MHC-mismatched AHCT, alloreactive CD4 T cells were found to rapidly eliminate BM osteoblasts.56  Destruction of osteoblasts and of other constituents of the hematopoietic stem cell niche (eg, Nestin+ mesenchymal stem cells57 ) could contribute to GVHD-associated BM hypoplasia. In addition, evidence suggests that GVHD-induced damage to the stroma of secondary lymphoid organs hampers reconstitution of peripheral T-cell pools: T cells from lymphopenic GVHD+ mice are able to expand normally when transferred into GVHD mice, but injection of T cells from healthy donors does not correct the T-cell deficit of GVHD+ mice.50  Finally, host-reactive T cells infiltrate many target organs such as the skin, liver, and gastrointestinal tract; recruit other cell types; and orchestrate complex immunopathologic events.58-60 

In the proposed model, GVL induced by conventional AHCT simply results from hematolymphoid GVHD. Accordingly, the question of whether GVL can be separated from GVHD might be formulated differently: can GVHD be limited to the hematolymphoid compartment? We believe that models reporting on preservation of GVL without GVHD essentially represent systems in which GVHD occurs but remains limited to hematolymphoid cells. An attractive way to dampen alloreactivity and thereby limit GVHD to the hematolymphoid compartment is to manipulate the numbers of regulatory T cells.61,62  However, as long as we are unable to separate alloreactive T cells that mediate GVHD and GVL, transforming a graded phenomenon (the strength of alloreactivity) into a binary output (hematolymphoid GVHD but no systemic GVHD) may remain difficult.

The term adoptive T-cell immunotherapy (ATCI) refers to transfusion of T lymphocytes that may come from different types of donors: the patient (autologous), a genetically identical donor (syngeneic), or a nonidentical donor (allogeneic). Conventional AHCT represents a simple form of ATCI because it involves transfer of unselected T cells. Despite its great paradigmatic and clinical relevance, the GVL effect induced under these conditions is still a quite rudimentary form of leukemia immunotherapy. First, it lacks specificity and is therefore highly toxic; unselected allogeneic T cells react against a multitude of host MiHAs and thereby induce GVHD in 60% of recipients. Second, it induces only an attenuated form of GVL reaction because donor T cells are not being primed (preactivated) against specific Ags expressed on leukemic cells before injection into the patient. Although primed T cells are resistant to tolerance induction, naive T cells can be tolerized by tumor cells.63-66 

More recently, ATCI with Ag-specific “activated” T cells has been used, mainly in 3 distinct settings. First, autologous T cells harvested from tumors (and presumably specific for tumor Ags) and expanded in vitro in the presence of IL-2 are being used for treatment of metastatic human solid tumors.67,68  Second, ATCI with allogeneic CD8 T cells directed against viral Ags (expanded in vitro in the presence of their cognate Ag) is used with great success for treatment of human CMV, EBV, and adenovirus infection.69,70  Finally, the use of allogeneic MiHA-specific T cells for cancer treatment has been evaluated in animal models and a landmark phase 1 clinical study.71-73  It must be stressed that the goal in ATCI is not to discover a “one size fits all” off the shelf blockbuster. It would be impossible to treat all hematologic malignancies by targeting the same Ag in all patients. ATCI should rather be considered as a form of personalized medicine in which the target epitope is determined by the HLA genotype of the subject and the proteome of the leukemic cells. For example, the PR-1 LAA is present uniquely on myeloid leukemic cells from HLA-A2+ subjects, and the HA-3 MiHA is expressed only on cells from HLA-A1 subjects bearing the R allele of the KIAA0020 gene.24,27  Because of these constraints, ATCI has been an academic endeavor led by a few centers with no input from large companies. We hope that things will change in the future, but, for the time being, cellular immunotherapy in general and ATCI in particular remain mostly academic enterprises. We therefore deem it important to present a tentative frame for translational research in ATCI of leukemia. To this end, we summarize herein the key insights gained from preclinical and clinical studies and present the different Ag-specific ATCI approaches that need to be explored for next-generation leukemia immunotherapy. The key features of these various approaches are summarized in Table 1. For the sake of brevity, we limit our discussion to ATCI with Ag-specific T cells and refer the reader to seminal articles for discussion of 2 other promising strategies for leukemia immunotherapy: injection of allogeneic NK cells and manipulation of regulatory T cells.21,61 

Table 1

Key features of different strategies for ATCI of leukemia

Key featuresAg receptor
Endogenous TCR
Transfected TCR
CARs
MiHAs*LAAs*MiHAs*LAAs*Cell-surface molecules*
MHC restricted − 
Dependent on the recipient's MiHA genotype − − − 
Dependent on LAA expression by leukemic cells − − − 
Requires massive in vitro expansion of Ag-specific T cells − − − 
Ag receptor is monoclonal − − 
Limited to cell-surface molecules (generally not leukemia specific) − − − − 
Target Ag can originate from all cell compartments − 
T cells can be allogeneic or autologous  
Key featuresAg receptor
Endogenous TCR
Transfected TCR
CARs
MiHAs*LAAs*MiHAs*LAAs*Cell-surface molecules*
MHC restricted − 
Dependent on the recipient's MiHA genotype − − − 
Dependent on LAA expression by leukemic cells − − − 
Requires massive in vitro expansion of Ag-specific T cells − − − 
Ag receptor is monoclonal − − 
Limited to cell-surface molecules (generally not leukemia specific) − − − − 
Target Ag can originate from all cell compartments − 
T cells can be allogeneic or autologous  
*

Target Ag.

Allogeneic T cells are required.

Ex vivo generation of fit MiHA- and LAA-specific T cells

In healthy preimmune subjects, T cells that can recognize MiHAs and LAAs are mostly if not exclusively in the naive T-cell compartment. The frequency of Ag-specific T cells in naive persons is ∼ 10−5, and ∼ 109 T cells are needed for ATCI of leukemia.72,74  This means that Ag-specific ATCI requires massive expansion of Ag-specific T cells, which has to be performed ex vivo in humans. Unfortunately, most methods for ex vivo expansion lead to an exhaustion of Ag-primed T cells.72,75  Exhausted T cells have shortened telomeres and lose functional attributes in a hierarchical manner, beginning with the ability to produce IL-2 and to proliferate in vivo.76  That is a big drawback because a major study of ATCI in patients with metastatic melanoma has shown that objective antitumor response correlated with longer telomeres of the infused cells and their long-term persistence in vivo.77  After Ag stimulation, the fittest T cells in terms of self-renewal potential are memory stem cells (TSCM, CD44loCD62LhiSca-1hiCD122hiBcl-2hi) and, to a lesser extent, central memory T cells (TCM, CD44hiCD62Lhi).78,79  The challenge here is therefore to develop ex vivo culture conditions that promote expansion of TSCM and TCM as opposed to effector memory T cells (TEM, CD44hiCD62Llo). Recent reports suggest that 2 strategies may help achieve this goal. First, Gattinoni et al80  have found that induction of Wnt-β-catenin signaling blocked differentiation of CD8 T cells into TEM and promoted the generation of TSCM with substantial proliferative and antitumor properties. Second, 2 teams have reported that provision of specific metabolic conditions during Ag stimulation (eg, inhibition of the mammalian target of rapamycin pathway or modulation of fatty acid metabolism) may promote the generation of memory as opposed to effector T cells.81,82  Further studies are needed to evaluate the merits of these various approaches for ex vivo generation of clinically relevant numbers of fit Ag-specific TCM and TSCM.

Another attractive strategy for ex vivo production of MiHA- or LAA-specific T cells is to transfect Ag-specific TCRs in polyclonal T-cell populations. This approach has 2 main advantages: it minimizes the need for T-cell expansion and it is flexible because TCR transgenes can be inserted in a variety of autologous or allogeneic T-cell subsets.79,83  Transfecting MiHA-specific TCRs into autologous T cells would obviate the need for an allogeneic donor. Care must be taken, however, to avoid host reactivity that might result from mispairing of introduced TCR chains with endogenous TCR chains.84  In addition, it is noteworthy that the T-cell population reactive to any given Ag is usually diversified and contains polyclonal TCRs. It is therefore theoretically possible that ATCI with monoclonal TCRs might be less effective than with polyclonal TCRs. Furthermore, the specific T-cell subset in which TCR should be inserted remains controversial. A report from Hinrichs et al85  suggests that when autologous cells are used for ATCI, naive T cells offer the best potential. Relative to TCM and TEM, naive T cells retained longer telomeres and displayed minimal signs of exhaustion. However, injection of (TCR-transfected) allogeneic naive T cells might be dangerous because GVHD is induced primarily by naive (MiHA-responsive) T cells.86  For allogeneic cells, TCR transfection into TCM might therefore represent a safer alternative.79,87 

GVL mediated by T cells specific for a single MiHA

Estimates based on mouse data suggest that the number of MiHA differences between 2 persons may be ∼ 30-50.6,88,89  This raises the question, how many MiHAs must be recognized or targeted to induce GVHD and GVL? When T cells reactive to a single host MiHA were injected into irradiated recipient mice, no GVHD has ever been observed. This test has been done with 25 different MiHAs, using naive donors or donors specifically primed against the target MiHA.71,90,91  Hence, at least in mice, it is impossible to induce GVHD by injecting T cells targeted to a single MiHA, even when the MiHA is ubiquitously expressed. The number of MiHAs that need to be recognized to elicit GVHD is unknown. By contrast, T cells primed against a single MiHA can eradicate leukemic cells as long as the target MiHA is immunodominant (highly immunogenic).71,92  Of note, the innocuity of T cells targeted to a single minor was recently challenged when Warren et al72  documented lung injury in 3 recipients of anti-MiHA CD8 T-cell clones. The T-cell clones had been expanded in vitro in the presence of anti-CD3 Ab and IL-2. Whether lung toxicity in these subjects resulted from cognate interactions of the T-cell clones with the lung parenchyma as opposed to some non–Ag-specific toxicity of the activated T cells remains a matter of conjecture. Indeed, T cells expanded in vitro in the presence of IL-2 can cause lung toxicity by TCR-independent mechanisms, by launching an inflammatory reaction in the first organ where they are trapped.93,94  Nevertheless, on the basis of the report by Warren et al72  it would appear hazardous to treat patients with T cells whose target Ag is expressed in the lungs.

H7a (formerly B6dom1), the first immunodominant MiHA discovered in mice, has been studied extensively.95,96  H7 allelic products originate from a single nucleotide polymorphism in the Stt3b gene. The sequence of the H7 peptide is KAPDNRETL in H7a+ mice (eg, C57BL/10) and KAPDNRDTL in H7b+ mice (eg, C3H.SW).96  Injection of anti-H7a T cells cured not only leukemia but also established melanoma.71,73  Eradication of melanoma cells depended on 2 T-cell effector mechanisms: direct killing of neoplastic cells by granule exocytosis and inhibition of angiogenesis by IFN-γ. Importantly, even though H7a is ubiquitously expressed, injection of anti-H7a T cells caused neither GVHD nor any untoward toxicity (eg, vitiligo) to recipients. The efficacy and innocuity of anti-H7a T cells are explained, at least in part, by the fact that anti-H7a effector T cells express high levels of Vla-4,73  and that the ligand of Vla-4 (Vcam-1) is expressed almost exclusively on BM microvessels and tumor neovessels. Therefore, anti-H7a effector T cells extravasate primarily in the BM and the tumor bed, where, in the presence of their target MiHA, they proliferate rapidly and extensively and kill neoplastic cells.73,97,98  It should be of interest to determine whether up-regulation of Vla-4 is a general feature of MiHA-specific effector cells and whether Vcam-1–dependent homing of MiHA-reactive T cells to the BM and solid tumors might explain that hematopoietic cells (Figure 1) and solid tumors are particularly susceptible to MiHA-specific T cells.73  The notion that solid tumors can be more susceptible to T-cell attack than nonhematopoietic cells has been supported in a study in which mice with intracranial melanoma were immunized against a melanoma-associated Ag, dopachrome tautomerase, which normal melanocytes and glial cells also express. Quite remarkably, immunotherapy led to rejection of intracranial tumor cells without damaging the brain despite sharing the target Ag.99  Although the mechanisms regulating the susceptibility of nonhematopoietic tissues and tumors to T-cell attack have yet to be worked out, studies on H7a and dopachrome tautomerase suggest that, at least in certain contexts, solid tumors may be unduly sensitive to T-cell attack.

In a phase 1 study reported in 2010, 7 patients with recurrent leukemia after AHCT were treated with donor-derived ex vivo–expanded MiHA-specific CD8 T-cell clones. Two salient findings emerged from that study.72  First, injected T cells migrated to the BM, and 5 of 7 patients achieved complete remission. However, adoptively transferred T cells failed to persist in vivo and remissions were transient. This important study shows that T cells targeted to a single MiHA can induce a potent GVL reaction in humans. However, it also shows that current methods for ex vivo expansion of T cells specific for noninfectious epitopes are inadequate.

Discovery of human MiHAs

Studies of leukemia immunotherapy with anti-H7a CD8 T cells suggest that it may be possible to obtain a strong GVL without GVHD by targeting MiHAs that have a wide tissue distribution. However, we believe that MiHAs with a wide tissue distribution are not ideal targets; “Ag excess” promotes functional exhaustion and physical demise of MiHA-reactive T cells.98,100  Thus, ideal targets would be MiHAs expressed on hematopoietic cells.101  At the present time, only 30% of patients with leukemia would be eligible for hematopoietic MiHA-based ATCI because of the limited number of human MiHAs that have been molecularly characterized.102  Hence, it is imperative to develop and exploit high-throughput methods for MiHA discovery.103,104 

Is a single Ag target sufficient for successful ATCI of leukemia?

Irrespective of the nature of the Ag (MiHA, LAA, cell-surface molecule), it may seem surprising that targeting a single Ag would be sufficient to cure leukemia or other types of cancer. A priori, injection of monospecific T cells should increase the risk of immune escape by selection of Ag loss variants. However, we contend that infiltration of the BM or solid tumors by activated T cells specific for a single Ag launches 2 series of events that facilitate eradication of all neoplastic cells, including Ag loss variants: angiostasis and epitope spreading. First and foremost, although all neoplasias (including leukemias) depend on generation of neovessels, IFN-γ and TNF-α secreted by activated T cells are perhaps the most potent antiangiogenic molecules available. As a consequence, secretion of these 2 cytokines in the tumor bed leads to ischemic death of neoplastic cells and bystander eradication of Ag loss variants.73,105,106  Notably, the antiangiogenic effect of IFN-γ was essential for eradication of melanoma cells by anti-H7a T cells.73  Second, in several models killing of tumor cells by cytotoxic T lymphocytes initiates Ag spreading, that is, priming and recruitment and of T-cell clones specific for Ags (MiHAs and LAAs) different from the initial Ag target.29,31,32  Ag spreading can even extend to B-cell responses against cell-surface molecules.30 

Nevertheless, an alternative approach to leukemia immunotherapy pioneered by Bordignon et al107,108  has also yielded promising results: injection of polyclonal allogeneic lymphocytes transduced to express a suicide gene. Expression of a suicide gene confers to cells sensitivity to a molecule (prodrug) that is nontoxic to nonexpressing cells. Thus, when recipients of genetically modified lymphocytes present signs of GVHD, the prodrug is administered to terminate the antihost alloreaction. In essence, this strategy exploits the fact that hematopoietic cells are more susceptible than epithelial cells to attack by alloreactive T cells and that brisk termination of GVHD may not totally abrogate the GVL reaction (Figure 1). In a landmark study, 23 patients received donor lymphocyte infusions with lymphocytes transduced to express the HSV-TK suicide gene for relapse of hematologic malignancies occurring after AHCT.107  Suicide gene transfer is safe and allows at least for some level of GVL activity, but it is limited by the development of transgene-specific CD8 T-cell immune responses that limit the in vivo persistence of genetically modified cells.107,109  Therefore, several new cell and gene transfer approaches are being evaluated in phase 1-2 clinical studies.108 

The lack of evidence that LAAs contribute to the GVL after conventional AHCT suggest that naive LAA-specific T cells are not sufficient to elicit curative antileukemic responses. Therefore, ATCI of leukemia with LAA-specific T cells remains largely unexplored. However, it remains possible that activated LAA-specific T cells may have more potent antileukemic activity than naive LAA-reactive T cells. Consistent with this, impressive results in patients with solid tumors treated by tumor Ag-targeted ATCI83,110,111  suggest that leukemia immunotherapy with LAA-primed T cells might have significant therapeutic potential. TCRs recognize MHC-associated peptides. Whether the peptide is an MiHA or an LAA, recognition by TCRs is MHC restricted. Accordingly, ATCI that is based on TCR recognition of LAA peptides is fraught with caveats similar to those encountered with MiHA-based ATCI (Table 1): (1) the need for individualized Ag targeting; LAA targeting must be tailored according to the HLA genotype and the proteome of leukemic cells; and (2) irrespective of whether endogenous TCRs or transfected TCRs are used, generation of large numbers of fit LAA-specific T cells will be demanding. However, if high-frequency immunogenic LAAs are found, donor selection for LAA-targeted ATCI would be easier than for MiHA-targeted ATCI. In the case of LAA, any HLA-matched subject is a potential donor, whereas when targeting MiHAs, the donor must be HLA-matched and negative for the target MiHA. Furthermore, in the case of ATCI with non–TCR-transfected T cells, LAAs present another advantage over MiHAs: LAA-specific T cells can be generated from autologous T cells and do not require an allogeneic donor. To this end, it might be advantageous to expand LAA-specific T cells from ex vivo–generated naive autologous T cells that have not been subjected to immunoediting in vivo.112  However, T cells from a healthy allogeneic donor may have an advantage over autologous T cells: healthy subjects that have not been subjected to chemotherapy may have a superior thymic function and a more diversified T-cell repertoire.113 

To circumvent the problem of MHC restriction inherent to targeting TCR epitopes, an alternative strategy is now being explored by several groups: targeting cell-surface molecules present on leukemic cells with T cells transfected with chimeric Ag receptors (CARs). CARs are typically composed of fusion proteins between single-chain variable fragments from monoclonal antibodies and intracellular signaling domains such as the CD3ζ-chain.114,115  Thus, CAR-expressing T cells see the world like B cells but react like T cells. Adoptive transfer of autologous anti–CD19-CAR-expressing T cells recently led to regression of a B-cell lymphoma (and of normal CD19+ cells) in a patient, providing a proof of principle that CARs represent a promising approach.116  Accordingly, anti–CD19-CARs are generating substantial enthusiasm, and their value will probably be evaluated in a multiple institutional trial.115  MHC-independent Ag recognition makes CARs broadly applicable. However, there is a significant caveat in seeing the world with a CAR (ie, with an Ab): you can only see cell-surface Ags. This is problematic because cell-surface Ags that are truly leukemia specific and can be recognized by Abs are exceedingly rare. In contrast, because TCRs recognize peptides derived from all cell compartments,117,118  all leukemic cells presumably express MHC-restricted targets (MiHAs and LAAs).

The division of labor between B cells and T cells was recently found to be conserved in jawless and jawed vertebrates, in a remarkable example of convergent evolution.119  Over the past 500 million years, survival of all vertebrates on this planet has depended on a combination TCR- and Ab-mediated recognition. We anticipate that both recognition systems will prove valuable in ATCI of leukemia. Nonetheless, development of next-generation leukemia immunotherapy is fraught with 2 main obstacles. The first is “philosophical”: the vestigial (and soon obsolete) reluctance to move from blockbuster medicine to personalized medicine. The second is methodologic: we need to develop reliable methods for ex vivo generation of large numbers of fit Ag-specific T cells. We can envision many strategies that differ with regard to the nature of the target Ag and the type of Ag receptor expressed on T cells (Table 1). However, it would be impossible to evaluate in vivo the fitness of T cells generated with all Ag/receptor combinations. We therefore need to establish reliable criteria for in vitro prediction of in vivo T-cell fitness. We propose as a working hypothesis that after in vitro priming and expansion, it might be possible to predict the fitness of Ag-primed T cells by estimating their telomere length and polyfunctionality index (eg, expression of CD107, CCL4, and TNFα; production of IFN-γ and IL-2).77,120 

K.V. is supported by a studentship from the Cole Foundation. Work on leukemia immunotherapy in the authors' laboratory has been supported by grants from the Canadian Cancer Society (C.P.) and the Fonds de la Recherche en Santé du Québec (D.-C.R.). C.P. holds a Canada Research Chair in Immunobiology.

Contribution: K.V. and C.P. wrote the first draft of the paper. All authors wrote the final version of the article.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Correspondence: Claude Perreault, Institute for Research in Immunology and Cancer, PO Box 6128, Station Centre-Ville, Montreal, QC, Canada H3C 3J7; e-mail: claude.perreault@umontreal.ca.

1
Barnes
 
DW
Corp
 
MJ
Loutit
 
JF
Neal
 
FE
Treatment of murine leukaemia with X rays and homologous bone marrow; preliminary communication.
Br Med J
1957
, vol. 
2
 
4993
(pg. 
626
-
627
)
2
Bortin
 
MM
Truitt
 
RL
Rimm
 
AA
Bach
 
FH
Graft-versus-leukaemia reactivity induced by alloimmunisation without augmentation of graft-versus-host reactivity.
Nature
1979
, vol. 
281
 
5731
(pg. 
490
-
491
)
3
Truitt
 
RL
The Mortimer M. Bortin Lecture: to destroy by the reaction of immunity: the search for separation of graft-versus-leukemia and graft-versus-host.
Biol Blood Marrow Transplant
2004
, vol. 
10
 
8
(pg. 
505
-
523
)
4
Weiden
 
PL
Flournoy
 
N
Thomas
 
ED
, et al. 
Antileukemic effect of graft-versus-host disease in human recipients of allogeneic-marrow grafts.
N Engl J Med
1979
, vol. 
300
 
19
(pg. 
1068
-
1073
)
5
Horowitz
 
MM
Gale
 
RP
Sondel
 
PM
, et al. 
Graft-versus-leukemia reactions after bone marrow transplantation.
Blood
1990
, vol. 
75
 
3
(pg. 
555
-
562
)
6
Perreault
 
C
Decary
 
F
Brochu
 
S
Gyger
 
M
Belanger
 
R
Roy
 
D
Minor histocompatibility antigens.
Blood
1990
, vol. 
76
 
7
(pg. 
1269
-
1280
)
7
Rotzschke
 
O
Falk
 
K
Wallny
 
HJ
Faath
 
S
Rammensee
 
HG
Characterization of naturally occurring minor histocompatibility peptides including H-4 and H-Y.
Science
1990
, vol. 
249
 
4966
(pg. 
283
-
287
)
8
den Haan
 
JM
Sherman
 
NE
Blokland
 
E
, et al. 
Identification of a graft versus host disease-associated human minor histocompatibility antigen.
Science
1995
, vol. 
268
 
5216
(pg. 
1476
-
1480
)
9
Kolb
 
HJ
Schattenberg
 
A
Goldman
 
JM
, et al. 
Graft-versus-leukemia effect of donor lymphocyte transfusions in marrow grafted patients. European Group for Blood and Marrow Transplantation Working Party Chronic Leukemia.
Blood
1995
, vol. 
86
 
5
(pg. 
2041
-
2050
)
10
Bethge
 
WA
Hegenbart
 
U
Stuart
 
MJ
, et al. 
Adoptive immunotherapy with donor lymphocyte infusions after allogeneic hematopoietic cell transplantation following nonmyeloablative conditioning.
Blood
2004
, vol. 
103
 
3
(pg. 
790
-
795
)
11
Collins
 
RH
Shpilberg
 
O
Drobyski
 
WR
, et al. 
Donor leukocyte infusions in 140 patients with relapsed malignancy after allogeneic bone marrow transplantation.
J Clin Oncol
1997
, vol. 
15
 
2
(pg. 
433
-
444
)
12
Kolb
 
H-J
Schmid
 
C
Barrett
 
AJ
Schendel
 
DJ
Graft-versus-leukemia reactions in allogeneic chimeras.
Blood
2004
, vol. 
103
 
3
(pg. 
767
-
776
)
13
Bleakley
 
M
Riddell
 
SR
Molecules and mechanisms of the graft-versus-leukaemia effect.
Nat Rev Cancer
2004
, vol. 
4
 
5
(pg. 
371
-
380
)
14
Gyurkocza
 
B
Storb
 
R
Storer
 
BE
, et al. 
Nonmyeloablative allogeneic hematopoietic cell transplantation in patients with acute myeloid leukemia.
J Clin Oncol
2010
, vol. 
28
 
17
(pg. 
2859
-
2867
)
15
Matte-Martone
 
C
Liu
 
J
Jain
 
D
McNiff
 
J
Shlomchik
 
WD
CD8+ but not CD4+ T cells require cognate interactions with target tissues to mediate GVHD across only minor H antigens whereas both CD4+ and CD8+ T cells require direct leukemic contact to mediate GVL.
Blood
2008
, vol. 
111
 
7
(pg. 
3884
-
3892
)
16
Kloosterman
 
TC
Martens
 
AC
van Bekkum
 
DW
Hagenbeek
 
A
Graft-versus-leukemia in rat MHC-mismatched bone marrow transplantation is merely an allogeneic effect.
Bone Marrow Transplant
1995
, vol. 
15
 
4
(pg. 
583
-
590
)
17
Reddy
 
P
Maeda
 
Y
Liu
 
C
Krijanovski
 
OI
Korngold
 
R
Ferrara
 
JL
A crucial role for antigen-presenting cells and alloantigen expression in graft-versus-leukemia responses.
Nat Med
2005
, vol. 
11
 
11
(pg. 
1244
-
1249
)
18
Thepot
 
S
Zhou
 
J
Perrot
 
A
, et al. 
The graft-versus-leukemia effect is mainly restricted to NIH-defined chronic graft-versus-host disease after reduced intensity conditioning before allogeneic stem cell transplantation.
Leukemia
2010
, vol. 
24
 
11
(pg. 
1852
-
1858
)
19
Barrett
 
AJ
Understanding and harnessing the graft-versus-leukaemia effect.
Br J Haematol
2008
, vol. 
142
 
6
(pg. 
877
-
888
)
20
Benjamin
 
JE
Gill
 
S
Negrin
 
RS
Biology and clinical effects of natural killer cells in allogeneic transplantation.
Curr Opin Oncol
2010
, vol. 
22
 
2
(pg. 
130
-
137
)
21
Ruggeri
 
L
Mancusi
 
A
Capanni
 
M
, et al. 
Donor natural killer cell allorecognition of missing self in haploidentical hematopoietic transplantation for acute myeloid leukemia: challenging its predictive value.
Blood
2007
, vol. 
110
 
1
(pg. 
433
-
17
)
22
Cooley
 
S
Weisdorf
 
DJ
Guethlein
 
LA
, et al. 
Donor selection for natural killer cell receptor genes leads to superior survival after unrelated transplantation for acute myelogenous leukemia.
Blood
2010
, vol. 
116
 
14
(pg. 
2411
-
2419
)
23
Perreault
 
C
The origin and role of MHC class I-associated self-peptides.
Prog Mol Biol Transl Sci
2010
, vol. 
92
 (pg. 
41
-
60
)
24
Spierings
 
E
Hendriks
 
M
Absi
 
L
, et al. 
Phenotype frequencies of autosomal minor histocompatibility antigens display significant differences among populations.
PLoS Genet
2007
, vol. 
3
 
6
pg. 
e103
 
25
Roopenian
 
D
Choi
 
EY
Brown
 
A
The immunogenomics of minor histocompatibility antigens.
Immunol Rev
2002
, vol. 
190
 (pg. 
86
-
94
)
26
Rezvani
 
K
Grube
 
M
Brenchley
 
JM
, et al. 
Functional leukemia-associated antigen-specific memory CD8+ T cells exist in healthy individuals and in patients with chronic myelogenous leukemia before and after stem cell transplantation.
Blood
2003
, vol. 
102
 
8
(pg. 
2892
-
2900
)
27
Molldrem
 
JJ
Lee
 
PP
Wang
 
C
, et al. 
Evidence that specific T lymphocytes may participate in the elimination of chronic myelogenous leukemia.
Nat Med
2000
, vol. 
6
 
9
(pg. 
1018
-
1023
)
28
Rezvani
 
K
Yong
 
AS
Savani
 
BN
, et al. 
Graft-versus-leukemia effects associated with detectable Wilms tumor-1 specific T lymphocytes after allogeneic stem-cell transplantation for acute lymphoblastic leukemia.
Blood
2007
, vol. 
110
 
6
(pg. 
1924
-
1932
)
29
Nishida
 
T
Hudecek
 
M
Kostic
 
A
, et al. 
Development of tumor-reactive T cells after nonmyeloablative allogeneic hematopoietic stem cell transplant for chronic lymphocytic leukemia.
Clin Cancer Res
2009
, vol. 
15
 
14
(pg. 
4759
-
4768
)
30
Marina
 
O
Hainz
 
U
Biernacki
 
MA
, et al. 
Serologic markers of effective tumor immunity against chronic lymphocytic leukemia include nonmutated B-cell antigens.
Cancer Res
2010
, vol. 
70
 
4
(pg. 
1344
-
1355
)
31
Corbiere
 
V
Chapiro
 
J
Stroobant
 
V
, et al. 
Antigen spreading contributes to MAGE vaccination-induced regression of melanoma metastases.
Cancer Res
2011
, vol. 
71
 
4
(pg. 
1253
-
1262
)
32
Hess Michelini
 
R
Freschi
 
M
Manzo
 
T
, et al. 
Concomitant tumor and minor histocompatibility antigen-specific immunity initiate rejection and maintain remission from established spontaneous solid tumors.
Cancer Res
2010
, vol. 
70
 
9
(pg. 
3505
-
3514
)
33
Vesely
 
MD
Kershaw
 
MH
Schreiber
 
RD
Smyth
 
MJ
Natural innate and adaptive immunity to cancer.
Annu Rev Immunol
2011
, vol. 
29
 (pg. 
235
-
271
)
34
Beatty
 
GL
Paterson
 
Y
IFN-g can promote tumor evasion of the immune system in vivo by down-regulating cellular levels of an endogenous tumor antigen.
J Immunol
2000
, vol. 
165
 
10
(pg. 
5502
-
5508
)
35
Sanchez-Perez
 
L
Kottke
 
T
Diaz
 
RM
, et al. 
Potent selection of antigen loss variants of B16 melanoma following inflammatory killing of melanocytes in vivo.
Cancer Res
2005
, vol. 
65
 
5
(pg. 
2009
-
2017
)
36
Kmieciak
 
M
Knutson
 
KL
Dumur
 
CI
Manjili
 
MH
HER-2/neu antigen loss and relapse of mammary carcinoma are actively induced by T cell-mediated anti-tumor immune responses.
Eur J Immunol
2007
, vol. 
37
 
3
(pg. 
675
-
685
)
37
Vago
 
L
Perna
 
SK
Zanussi
 
M
, et al. 
Loss of mismatched HLA in leukemia after stem-cell transplantation.
N Engl J Med
2009
, vol. 
361
 
5
(pg. 
478
-
488
)
38
Welniak
 
LA
Blazar
 
BR
Murphy
 
WJ
Immunobiology of allogeneic hematopoietic stem cell transplantation.
Annu Rev Immunol
2007
, vol. 
25
 (pg. 
139
-
170
)
39
Kolb
 
H-J
Graft-versus-leukemia effects of transplantation and donor lymphocytes.
Blood
2008
, vol. 
112
 
12
(pg. 
4371
-
4383
)
40
Jenq
 
RR
van den Brink
 
MR
Allogeneic haematopoietic stem cell transplantation: individualized stem cell and immune therapy of cancer.
Nat Rev Cancer
2010
, vol. 
10
 
3
(pg. 
213
-
221
)
41
Pavletic
 
SZ
Kumar
 
S
Mohty
 
M
, et al. 
NCI first international workshop on the biology, prevention, and treatment of relapse after allogeneic hematopoietic stem cell transplantation: report from the committee on the epidemiology and natural history of relapse following allogeneic cell transplantation.
Biol Blood Marrow Transplant
2010
, vol. 
16
 
7
(pg. 
871
-
890
)
42
Dwyre
 
DM
Holland
 
PV
Transfusion-associated graft-versus-host disease.
Vox Sang
2008
, vol. 
95
 
2
(pg. 
85
-
93
)
43
Mori
 
T
Nishimura
 
T
Ikeda
 
Y
Hotta
 
T
Yagita
 
H
Ando
 
K
Involvement of Fas-mediated apoptosis in the hematopoietic progenitor cells of graft-versus-host reaction-associated myelosuppression.
Blood
1998
, vol. 
92
 
1
(pg. 
101
-
107
)
44
Sprent
 
J
Surh
 
CD
Agus
 
D
Hurd
 
M
Sutton
 
S
Heath
 
WR
Profound atrophy of the bone marrow reflecting major histocompatibility complex class II-restricted destruction of stem cells by CD4+ cells.
J Exp Med
1994
, vol. 
180
 
1
(pg. 
307
-
317
)
45
Gyger
 
M
Baron
 
C
Forest
 
L
, et al. 
Quantitative assessment of hematopoietic chimerism after allogeneic bone marrow transplantation has predictive value for the occurrence of irreversible graft failure and graft-vs.-host disease.
Exp Hematol
1998
, vol. 
26
 
5
(pg. 
426
-
434
)
46
Beilhack
 
A
Schulz
 
S
Baker
 
J
, et al. 
Prevention of acute graft-versus-host disease by blocking T-cell entry to secondary lymphoid organs.
Blood
2008
, vol. 
111
 
5
(pg. 
2919
-
2928
)
47
Brochu
 
S
Baron
 
C
Hetu
 
F
Roy
 
DC
Perreault
 
C
Oligoclonal expansion of CTLs directed against a restricted number of dominant minor histocompatibility antigens in hemopoietic chimeras.
J Immunol
1995
, vol. 
155
 
11
(pg. 
5104
-
5114
)
48
Krenger
 
W
Hollander
 
GA
The immunopathology of thymic GVHD.
Semin Immunopathol
2008
, vol. 
30
 
4
(pg. 
439
-
456
)
49
Na
 
IK
Lu
 
SX
Yim
 
NL
, et al. 
The cytolytic molecules Fas ligand and TRAIL are required for murine thymic graft-versus-host disease.
J Clin Invest
2010
, vol. 
120
 
1
(pg. 
343
-
356
)
50
Dulude
 
G
Roy
 
DC
Perreault
 
C
The effect of graft-versus-host disease on T cell production and homeostasis.
J Exp Med
1999
, vol. 
189
 
8
(pg. 
1329
-
1342
)
51
Mackall
 
CL
Hakim
 
FT
Velardi
 
A
Ferrara
 
JL
Cooke
 
KR
Deeg
 
HJ
The immune system in graft-vs.host disease: target and effector organ.
Graft-vs-Host Disease
2005
3rd ed
New York, NY
Marcel Dekker Inc
(pg. 
195
-
228
)
52
Johnson
 
BD
Becker
 
EE
LaBelle
 
JL
Truitt
 
RL
Role of immunoregulatory donor T cells in suppression of graft-versus-host disease following donor leukocyte infusion therapy.
J Immunol
1999
, vol. 
163
 
12
(pg. 
6479
-
6487
)
53
Cohen
 
JL
Trenado
 
A
Vasey
 
D
Klatzmann
 
D
Salomon
 
BL
CD4+CD25+ immunoregulatory T cells: new therapeutics for graft-versus-host disease.
J Exp Med
2002
, vol. 
196
 
3
(pg. 
401
-
406
)
54
Trenado
 
A
Sudres
 
M
Tang
 
Q
, et al. 
Ex vivo-expanded CD4+ CD25+ immunoregulatory T cells prevent graft-versus-host-disease by inhibiting activation/differentiation of pathogenic T cells.
J Immunol
2006
, vol. 
176
 
2
(pg. 
1266
-
1273
)
55
Krenger
 
W
Blazar
 
BR
Hollander
 
GA
Thymic T-cell development in allogeneic stem cell transplantation.
Blood
2011
, vol. 
117
 
25
(pg. 
6768
-
6776
)
56
Shono
 
Y
Ueha
 
S
Wang
 
Y
, et al. 
Bone marrow graft-versus-host disease: early destruction of hematopoietic niche after MHC-mismatched hematopoietic stem cell transplantation.
Blood
2010
, vol. 
115
 
26
(pg. 
5401
-
5411
)
57
Mendez-Ferrer
 
S
Michurina
 
TV
Ferraro
 
F
, et al. 
Mesenchymal and haematopoietic stem cells form a unique bone marrow niche.
Nature
2010
, vol. 
466
 
7308
(pg. 
829
-
834
)
58
Miklos
 
DB
Kim
 
HT
Miller
 
KH
, et al. 
Antibody responses to H-Y minor histocompatibility antigens correlate with chronic graft-versus-host disease and disease remission.
Blood
2005
, vol. 
105
 
7
(pg. 
2973
-
2978
)
59
Socie
 
G
Blazar
 
BR
Acute graft-versus-host disease; from the bench to the bedside.
Blood
2009
, vol. 
114
 
20
(pg. 
4327
-
4336
)
60
Giroux
 
M
Delisle
 
J-S
Gauthier
 
S-D
, et al. 
SMAD3 prevents graft-versus-host disease by restraining Th1 differentiation and granulocyte-mediated tissue damage.
Blood
2011
, vol. 
117
 
5
(pg. 
1734
-
1744
)
61
Maury
 
S
Lemoine
 
FM
Hicheri
 
Y
, et al. 
CD4+CD25+ regulatory T cell depletion improves the graft-versus-tumor effect of donor lymphocytes after allogeneic hematopoietic stem cell transplantation.
Sci Transl Med
2010
, vol. 
2
 
41
pg. 
41ra52
 
62
Bastien
 
JP
Krosl
 
G
Therien
 
C
, et al. 
Photodepletion differentially affects CD4+ Tregs versus CD4+ effector T cells from patients with chronic graft-versus-host disease.
Blood
2010
, vol. 
116
 
23
(pg. 
4859
-
4869
)
63
Inaba
 
M
Kurasawa
 
K
Mamura
 
M
Kumano
 
K
Saito
 
Y
Iwamoto
 
I
Primed T cells are more resistant to Fas-mediated activation-induced cell death than naive T cells.
J Immunol
1999
, vol. 
163
 
3
(pg. 
1315
-
1320
)
64
Yang
 
J
Brook
 
MO
Carvalho-Gaspar
 
M
, et al. 
Allograft rejection mediated by memory T cells is resistant to regulation.
Proc Natl Acad Sci U S A
2007
, vol. 
104
 
50
(pg. 
19954
-
19959
)
65
Massague
 
J
TGFbeta in cancer.
Cell
2008
, vol. 
134
 
2
(pg. 
215
-
230
)
66
Hanahan
 
D
Weinberg
 
RA
Hallmarks of cancer: the next generation.
Cell
2011
, vol. 
144
 
5
(pg. 
646
-
674
)
67
Rosenberg
 
SA
Restifo
 
NP
Yang
 
JC
Morgan
 
RA
Dudley
 
ME
Adoptive cell transfer: a clinical path to effective cancer immunotherapy.
Nat Rev Cancer
2008
, vol. 
8
 
4
(pg. 
299
-
308
)
68
Rosenberg
 
SA
Dudley
 
ME
Adoptive cell therapy for the treatment of patients with metastatic melanoma.
Curr Opin Immunol
2009
, vol. 
21
 
2
(pg. 
233
-
240
)
69
Leen
 
AM
Myers
 
GD
Sili
 
U
, et al. 
Monoculture-derived T lymphocytes specific for multiple viruses expand and produce clinically relevant effects in immunocompromised individuals.
Nat Med
2006
, vol. 
12
 
10
(pg. 
1160
-
1166
)
70
Heslop
 
HE
Slobod
 
KS
Pule
 
MA
, et al. 
Long-term outcome of EBV-specific T-cell infusions to prevent or treat EBV-related lymphoproliferative disease in transplant recipients.
Blood
2010
, vol. 
115
 
5
(pg. 
925
-
935
)
71
Fontaine
 
P
Roy-Proulx
 
G
Knafo
 
L
Baron
 
C
Roy
 
DC
Perreault
 
C
Adoptive transfer of T lymphocytes targeted to a single immunodominant minor histocompatibility antigen eradicates leukemia cells without causing graft-versus-host disease.
Nat Med
2001
, vol. 
7
 
7
(pg. 
789
-
794
)
72
Warren
 
EH
Fujii
 
N
Akatsuka
 
Y
, et al. 
Therapy of relapsed leukemia after allogeneic hematopoietic cell transplant with T cells specific for minor histocompatibility antigens.
Blood
2010
, vol. 
115
 
19
(pg. 
3869
-
3878
)
73
Meunier
 
MC
Delisle
 
JS
Bergeron
 
J
Rineau
 
V
Baron
 
C
Perreault
 
C
T cells targeted against a single minor histocompatibility antigen can cure solid tumors.
Nat Med
2005
, vol. 
11
 
11
(pg. 
1222
-
1229
)
74
Jenkins
 
MK
Chu
 
HH
McLachlan
 
JB
Moon
 
JJ
On the composition of the preimmune repertoire of T cells specific for peptide-major histocompatibility complex ligands.
Annu Rev Immunol
2010
, vol. 
28
 (pg. 
275
-
294
)
75
Ahmadzadeh
 
M
Johnson
 
LA
Heemskerk
 
B
, et al. 
Tumor antigen-specific CD8 T cells infiltrating the tumor express high levels of PD-1 and are functionally impaired.
Blood
2009
, vol. 
114
 
8
(pg. 
1537
-
1544
)
76
Wherry
 
EJ
T cell exhaustion.
Nat Immunol
2011
, vol. 
131
 
6
(pg. 
492
-
499
)
77
Rosenberg
 
SA
Yang
 
JC
Sherry
 
RM
, et al. 
Durable complete responses in heavily pretreated patients with metastatic melanoma using T cell transfer immunotherapy.
Clin Cancer Res
2011
 
PMID 21498393
78
Zhang
 
Y
Joe
 
G
Hexner
 
E
Zhu
 
J
Emerson
 
SG
Host-reactive CD8+ memory stem cells in graft-versus-host disease.
Nat Med
2005
, vol. 
11
 
12
(pg. 
1299
-
1305
)
79
Turtle
 
CJ
Riddell
 
SR
Genetically retargeting CD8+ lymphocyte subsets for cancer immunotherapy.
Curr Opin Immunol
2011
, vol. 
23
 
2
(pg. 
299
-
305
)
80
Gattinoni
 
L
Zhong
 
XS
Palmer
 
DC
, et al. 
Wnt signaling arrests effector T cell differentiation and generates CD8+ memory stem cells.
Nat Med
2009
, vol. 
15
 
7
(pg. 
808
-
813
)
81
Araki
 
K
Turner
 
AP
Shaffer
 
VO
, et al. 
mTOR regulates memory CD8 T-cell differentiation.
Nature
2009
, vol. 
460
 
7251
(pg. 
108
-
112
)
82
Pearce
 
EL
Walsh
 
MC
Cejas
 
PJ
, et al. 
Enhancing CD8 T-cell memory by modulating fatty acid metabolism.
Nature
2009
, vol. 
460
 
7251
(pg. 
103
-
107
)
83
Robbins
 
PF
Morgan
 
RA
Feldman
 
SA
, et al. 
Tumor regression in patients with metastatic synovial cell sarcoma and melanoma using genetically engineered lymphocytes reactive with NY-ESO-1.
J Clin Oncol
2011
, vol. 
29
 
7
(pg. 
917
-
924
)
84
Bendle
 
GM
Linnemann
 
C
Hooijkaas
 
AI
, et al. 
Lethal graft-versus-host disease in mouse models of T cell receptor gene therapy.
Nat Med
2010
, vol. 
16
 
5
(pg. 
565
-
570
)
85
Hinrichs
 
CS
Borman
 
ZA
Gattinoni
 
L
, et al. 
Human effector CD8+ T cells derived from naive rather than memory subsets possess superior traits for adoptive immunotherapy.
Blood
2011
, vol. 
117
 
3
(pg. 
808
-
814
)
86
Shlomchik
 
WD
Graft-versus-host disease.
Nat Rev Immunol
2007
, vol. 
7
 
5
(pg. 
340
-
352
)
87
Wang
 
X
Berger
 
C
Wong
 
CW
Forman
 
SJ
Riddell
 
SR
Jensen
 
MC
Engraftment of human central memory-derived effector CD8+ T cells in immunodeficient mice.
Blood
2011
, vol. 
117
 
6
(pg. 
1888
-
1898
)
88
Loveland
 
B
Simpson
 
E
The non-MHC transplantation antigens–neither weak nor minor.
Immunol Today
1986
, vol. 
7
 
7–8
(pg. 
223
-
229
)
89
Sykes
 
M
Woods
 
K
Sachs
 
DH
Paul
 
WE
Transplantation immunology.
Fundamental Immunology
2008
6th ed
Philadelphia, PA
Lippincott Williams & Wilkins
(pg. 
1426
-
1488
)
90
Korngold
 
R
Leighton
 
C
Mobraaten
 
LE
Berger
 
MA
Inter-strain graft-vs.-host disease T-cell responses to immunodominant minor histocompatibility antigens.
Biol Blood Marrow Transplant
1997
, vol. 
3
 
2
(pg. 
57
-
64
)
91
Blazar
 
BR
Roopenian
 
DC
Taylor
 
PA
Christianson
 
GJ
Panoskaltsis-Mortari
 
A
Vallera
 
DA
Lack of GVHD across classical, single minor histocompatibility (miH) locus barriers in mice.
Transplantation
1996
, vol. 
61
 
4
(pg. 
619
-
624
)
92
Pion
 
S
Fontaine
 
P
Baron
 
C
Gyger
 
M
Perreault
 
C
Immunodominant minor histocompatibility antigens expressed by mouse leukemic cells can serve as effective targets for T cell immunotherapy.
J Clin Invest
1995
, vol. 
95
 
4
(pg. 
1561
-
1568
)
93
Law
 
TM
Motzer
 
RJ
Mazumdar
 
M
, et al. 
Phase III randomized trial of interleukin-2 with or without lymphokine-activated killer cells in the treatment of patients with advanced renal cell carcinoma.
Cancer
1995
, vol. 
76
 
5
(pg. 
824
-
832
)
94
Clark
 
JG
Madtes
 
DK
Hackman
 
RC
Chen
 
W
Cheever
 
MA
Martin
 
PJ
Lung injury induced by alloreactive Th1 cells is characterized by host-derived mononuclear cell inflammation and activation of alveolar macrophages.
J Immunol
1998
, vol. 
161
 
4
(pg. 
1913
-
1920
)
95
Perreault
 
C
Jutras
 
J
Roy
 
DC
Filep
 
JG
Brochu
 
S
Identification of an immunodominant mouse minor histocompatibility antigen (MiHA). T cell response to a single dominant MiHA causes graft-versus-host disease.
J Clin Invest
1996
, vol. 
98
 
3
(pg. 
622
-
628
)
96
McBride
 
K
Baron
 
C
Picard
 
S
, et al. 
The model B6dom1 minor histocompatibility antigen is encoded by a mouse homolog of the yeast STT3 gene.
Immunogenetics
2002
, vol. 
54
 
8
(pg. 
562
-
569
)
97
Meunier
 
MC
Roy-Proulx
 
G
Labrecque
 
N
Perreault
 
C
Tissue distribution of target antigen has a decisive influence on the outcome of adoptive cancer immunotherapy.
Blood
2003
, vol. 
101
 
2
(pg. 
766
-
770
)
98
Meunier
 
MC
Baron
 
C
Perreault
 
C
Two host factors regulate persistence of H7a-specific T cells injected in tumor bearing mice.
PLoS One
2009
, vol. 
4
 
1
pg. 
e4116
 
99
Bridle
 
BW
Li
 
J
Jiang
 
S
, et al. 
Immunotherapy can reject intracranial tumor cells without damaging the brain despite sharing the target antigen.
J Immunol
2010
, vol. 
184
 
8
(pg. 
4269
-
4275
)
100
Asakura
 
S
Hashimoto
 
D
Takashima
 
S
, et al. 
Alloantigen expression on non-hematopoietic cells reduces graft-versus-leukemia effects in mice.
J Clin Invest
2010
, vol. 
120
 
7
(pg. 
2370
-
2378
)
101
Mutis
 
T
Goulmy
 
E
Hematopoietic system-specific antigens as targets for cellular immunotherapy of hematological malignancies.
Semin Hematol
2002
, vol. 
39
 
1
(pg. 
23
-
31
)
102
Bleakley
 
M
Riddell
 
SR
Exploiting T cells specific for human minor histocompatibility antigens for therapy of leukemia.
Immunol Cell Biol
2011
, vol. 
89
 
3
(pg. 
396
-
407
)
103
Bleakley
 
M
Otterud
 
BE
Richardt
 
JL
, et al. 
Leukemia-associated minor histocompatibility antigen discovery using T-cell clones isolated by in vitro stimulation of naive CD8+ T cells.
Blood
2010
, vol. 
115
 
23
(pg. 
4923
-
4933
)
104
Van Bergen
 
CA
Rutten
 
CE
Van Der Meijden
 
ED
, et al. 
High-throughput characterization of 10 new minor histocompatibility antigens by whole genome association scanning.
Cancer Res
2010
, vol. 
70
 
22
(pg. 
9073
-
9083
)
105
Qin
 
Z
Schwartzkopff
 
J
Pradera
 
F
, et al. 
A critical requirement of interferongamma-mediated angiostasis for tumor rejection by CD8+ T cells.
Cancer Res
2003
, vol. 
63
 
14
(pg. 
4095
-
4100
)
106
Zhang
 
B
Karrison
 
T
Rowley
 
DA
Schreiber
 
H
IFN-g- and TNF-dependent bystander eradication of antigen-loss variants in established mouse cancers.
J Clin Invest
2008
, vol. 
118
 
4
(pg. 
1398
-
1404
)
107
Traversari
 
C
Marktel
 
S
Magnani
 
Z
, et al. 
The potential immunogenicity of the TK suicide gene does not prevent full clinical benefit associated with the use of TK-transduced donor lymphocytes in HSCT for hematologic malignancies.
Blood
2007
, vol. 
109
 
11
(pg. 
4708
-
4715
)
108
Lupo-Stanghellini
 
MT
Provasi
 
E
Bondanza
 
A
Ciceri
 
F
Bordignon
 
C
Bonini
 
C
Clinical impact of suicide gene therapy in allogeneic hematopoietic stem cell transplantation.
Hum Gene Ther
2010
, vol. 
21
 
3
(pg. 
241
-
250
)
109
Berger
 
C
Flowers
 
ME
Warren
 
EH
Riddell
 
SR
Analysis of transgene-specific immune responses that limit the in vivo persistence of adoptively transferred HSV-TK-modified donor T cells after allogeneic hematopoietic cell transplantation.
Blood
2006
, vol. 
107
 
6
(pg. 
2294
-
2302
)
110
Dudley
 
ME
Gross
 
CA
Langhan
 
MM
, et al. 
CD8+ enriched ”young“ tumor infiltrating lymphocytes can mediate regression of metastatic melanoma.
Clin Cancer Res
2010
, vol. 
16
 
24
(pg. 
6122
-
6131
)
111
Parkhurst
 
MR
Yang
 
JC
Langan
 
RC
, et al. 
T cells targeting carcinoembryonic antigen can mediate regression of metastatic colorectal cancer but induce severe transient colitis.
Mol Ther
2011
, vol. 
19
 
3
(pg. 
620
-
626
)
112
Awong
 
G
Herer
 
E
La Motte-Mohs
 
RN
Zuniga-Pflucker
 
JC
Human CD8 T cells generated in vitro from hematopoietic stem cells are functionally mature.
BMC Immunol
2011
, vol. 
12
 
1
pg. 
22
 
113
Hakim
 
FT
Memon
 
SA
Cepeda
 
R
, et al. 
Age-dependent incidence, time course, and consequences of thymic renewal in adults.
J Clin Invest
2005
, vol. 
115
 
4
(pg. 
930
-
939
)
114
Sadelain
 
M
Brentjens
 
R
Riviere
 
I
The promise and potential pitfalls of chimeric antigen receptors.
Curr Opin Immunol
2009
, vol. 
21
 
2
(pg. 
215
-
223
)
115
Kohn
 
DB
Dotti
 
G
Brentjens
 
R
, et al. 
CARs on track in the clinic.
Mol Ther
2011
, vol. 
19
 
3
(pg. 
432
-
438
)
116
Kochenderfer
 
JN
Wilson
 
WH
Janik
 
JE
, et al. 
Eradication of B-lineage cells and regression of lymphoma in a patient treated with autologous T cells genetically engineered to recognize CD19.
Blood
2010
, vol. 
116
 
20
(pg. 
4099
-
4102
)
117
Fortier
 
MH
Caron
 
E
Hardy
 
MP
, et al. 
The MHC class I peptide repertoire is molded by the transcriptome.
J Exp Med
2008
, vol. 
205
 
3
(pg. 
595
-
610
)
118
de Verteuil
 
D
Muratore-Schroeder
 
TL
Granados
 
DP
, et al. 
Deletion of immunoproteasome subunits imprints on the transcriptome and has a broad impact on peptides presented by major histocompatibility complex I molecules.
Mol Cell Proteomics
2010
, vol. 
9
 
9
(pg. 
2034
-
2047
)
119
Guo
 
P
Hirano
 
M
Herrin
 
BR
, et al. 
Dual nature of the adaptive immune system in lampreys.
Nature
2009
, vol. 
459
 
7248
(pg. 
796
-
801
)
120
Seder
 
RA
Darrah
 
PA
Roederer
 
M
T-cell quality in memory and protection: implications for vaccine design.
Nat Rev Immunol
2008
, vol. 
8
 
4
(pg. 
247
-
258
)