BACKGROUND: Immunosuppressive CD4+Foxp3+ T regulatory cells (Treg) play a vital role in immune regulation. Thus, Treg contribute to the prevention of autoimmune disease and graft-versus-host reactions following allogeneic stem cell transplantation (alloSCT) but also to the inhibition of effective anti-tumor T cell responses. It has previously been suggested that the frequency of Treg is increased in the peripheral blood of patients with multiple myeloma (MM). However, little is known about the presence of Treg in the bone marrow and it is unclear whether allogeneic stem cell transplantation might deplete Treg from this immune compartment.

METHODS: In the present study, we analyzed percentages of CD4+Foxp3+ Treg as well as Treg expression of CD45RA and CCR7 in the bone marrow (BM) and in the peripheral blood of MM patients who had received alloSCT (N=42), in newly diagnosed MM patients (N=18), and in healthy controls (N=15) using flow cytometry. In addition, we performed inhibition assays in order to test the functional relevance of peripheral and BM-residing Treg.

RESULTS: While newly diagnosed MM patients and healthy controls showed no significant difference in the proportions of CD4+Foxp3+ Treg in the bone marrow, percentages of BM-residing CD4+Foxp3+ T regulatory cells were markedly higher (p<0.001 and p<0.01) in patients post alloSCT (3.3±0.3%) than in normal BM (1.0±0.3%) or in BM of untreated MM patients (1.8±0.4%). In both groups of patients (p<0.05) as well as in the healthy controls (p<0.001) percentages of Treg were higher in the peripheral blood than in the bone marrow. While there were no differences regarding the percentages of peripheral Treg between the remaining groups, patients post alloSCT had higher percentages of peripheral Treg than newly diagnosed patients (5.6±0.8 vs. 3.2±0.7%, p<0.05). More than 90% of these donor-derived peripheral and BM-residing Treg expressed a memory T cell phenotype, being negative for CD45RA and CCR7. Importantly, peripheral as well as BM-residing Treg of patients post alloSCT were capable of inhibiting the proliferation of autologous non-Treg CD4+ T cells.

CONCLUSION: Our study demonstrates for the first time an increased frequency of immunosuppressive Treg in the bone marrow of MM patients. Remarkably, in our patients these memory-type Treg were all donor-derived and led to an efficient replenishment of Treg in the periphery. These Treg might be necessary for the prevention of graft-versus-host disease in the transplanted MM patients, however, they might also contribute to the failure of an effective graft-versus-myeloma effect in the majority of the patients.

Disclosure: No relevant conflicts of interest to declare.

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