BACKGROUND: Malignant diseases are frequently accompanied by suppression of cytotoxic lymphocyte function, and a large body of circumstantial evidence suggests that lymphocyte suppression may have implications for disease-free survival in several hematopoietic cancers. In AML, a plethora of immunosuppressive mechanisms have been described including the production of T- and NK-cell-inhibitory factors by AML blasts, a deficient expression of activating receptors on natural killer (NK) cells, and an impaired stimulatory interaction between the CD28 antigen expressed by T cells and contact antigens on AML blasts. We and others have proposed that the production of oxygen radicals (including hydrogen peroxide and its derivatives) by normal and leukemic phagocytic cells may significantly contribute to the state of immunosuppression in AML. The phagocyte-derived oxygen radicals strongly suppress the anti-leukemic cytotoxicity of natural killer (NK) cells with ensuing NK cell death by apoptosis, and inhibitors of oxygen radical formation efficiently rescue NK cells from inhibition and apoptosis. These mechanisms may have implications for AML treatment, as exemplified by the use of the T and NK cell activator interleukin-2 (IL-2) in conjunction with the oxygen radical inhibitor histamine dihydrochloride (HDC) as relapse-protective therapy in AML (
METHODS and RESULTS: The question whether oxygen radical-induced activation apply to CD8+ T cells with spontaneous reactivity against human primary AML blasts has remained unresolved. Autoreactive T cells in AML were recently phenotypically defined as CD3+/8+/45RA+/CCR7 cells (CD8+/45RA+ cells; Blood 100:2132–7). We recovered these cells from human peripheral blood by FACS sorting (FACSAria) with high purity (>98%) and exposed them to exogenous hydrogen peroxide or oxygen radical-producing mononuclear phagocytes. CD8+/45RA+ T cells acquired features of apoptosis at low concentrations of hydrogen peroxide. Also, more than 60% of the CD8+/45RA+ cells acquired features of apoptosis when admixed with autologous mononuclear phagocytes (at a 1:1 phagocyte to T cell ratio, n=11, p<0.0001). HDC, an inhibitor of oxygen radical formation in phagocytes, completely prevented apoptosis in the CD8+/45RA+ T cell subset (n=8, p<0.0001). In the presence of mononuclear phagocytes, HDC synergized with IL-2 in promoting cell cycle proliferation (n=4, p=0.004) and activation (as measured by the acquisition of membrane CD69; n=9, p=0.017) in CD8+/45RA+ cells.
CONCLUSIONS: The phenotype of T cells with spontaneous reactivity against AML blasts is highly sensitive to oxidants. By inhibiting oxygen radical formation, HDC efficiently protects these cytotoxic lymphocytes from apoptosis in an environment of oxidative stress. In addition, HDC synergizes with IL-2 to activate and expand CD8+/45RA+ T cells. These mechanisms of T cell activation may be relevant to the prevention of leukemic relapse reported for post-consolidation treatment with HDC/IL-2 in AML.
Disclosure: Consultancy: Author KH is a consultant to EpiCept Corporation. Ownership Interests:; Author KH holds shares in EpiCept Corporation. Honoraria Information: Author KH has received an honorarium from EpiCept Corporation. Financial Information: Author KH owns rights to royalties for the therapeutic use of HDC.