Study of the immune-microenvironment in Multiple Myeloma: understanding the effects of immunomodulatory drugs and the contribution of immune checkpoints in disease progression

Despite major improvements in the treatment landscape, multiple myeloma (MM) remains an incurable malignancy. Alterations of the bone marrow (BM) immune-microenvironment, due to the presence of malignant plasma cells, characterize the progression of monoclonal gammopathies and the development of osteolytic bone disease. “Immunoparesis” is a common feature of MM patients displaying impaired dendritic, natural killer and T cell functions, whereas the onset of MM osteolytic lesions is associated to an increased prevalence of Th17 cells. Most recently, preclinical studies have also suggested the role of PD-1/PD-L1 pathway in the induction of tumor tolerance and immune evasion. However, contradictory results are currently available on the expression profile of PD-1/PD-L1 axis in MM patients and the possible correlation with the presence of osteolytic bone disease has not yet been explored. Among the therapeutic strategies, the development of the Immunomodulatory drugs represented a paradigm shift in the treatment of MM. Lenalidomide-based regimen is one of the standard of care for MM patients either in frontline or in relapsed setting. However, the use of lenalidomide, alone or in combination with immune checkpoint blockade, to reverse tumor-mediated immune suppression and amplify MM-specific immunity is currently under investigation. Particularly lenalidomide effects on dendritic cells are still unclear. In this study, the potential effect of lenalidomide on dendritic cell differentiation and activity has been investigated. Dendritic cells were differentiated from either primary MM CD14+ cells or from a human monocytic cell line. Lenalidomide, at the concentration range reached in vivo, significantly increased the median intensity expression of HLA-DR, CD86 and CD209 by dendritic cells derived from both BM and peripheral myeloma monocytes and enhanced the production of Interleukin-8, C-C motif chemokine ligand 2 and tumor necrosis factor-α. Consistently, lenalidomide pre-treated dendritic cells showed an increased ability to stimulate autologous CD3+ cell proliferation. Lenalidomide effect on dendritic differentiation was associated with the degradation of the Cereblon-related factors Ikaros and Aiolos in monocytes. Moreover, lenalidomide in vitro treatment blunted mesenchymal stromal cell inhibitory effect on dendritic differentiation inhibiting casein kinase-1α levels. Finally, in vitro data were confirmed in ex-vivo cultures obtained from relapsed myeloma patients treated with lenalidomide at 25 mg/day showing a significant increase of dendritic cell differentiation from peripheral blood monocytes. In conclusion, lenalidomide increased the expression of mature dendritic markers both directly and indirectly and enhanced dendritic cell ability to stimulate T cell proliferation and to release chemokines. This suggests a new possible mechanism by which lenalidomide could exert its anti-myeloma activity. In the second part of the study, the expression profile of PD-1/PD-L1 was evaluated in plasma cells, monocytes and T cells from BM aspirates of patients with monoclonal gammopathies and the results were correlated with clinical data, especially the presence of bone disease. Interestingly, an increased frequency of PD-1+ T cells was observed across the progression of the disease. Moreover, for the first time, it was found a significant relationship between the presence of extensive osteolytic bone disease and a reduced expression of PD-1 on BM CD8+ T cells and PD-L1 on malignant PCs and monocytes in MM patients. It was thus hypothesized that a less immune-suppressive profile could be related to the development of osteolysis and probably mediated by the effect of PD-1/PD-L1 axis on Treg/Th17 cell ratio. These preliminary data thus provide a new mechanism by which PD-1/PD-L1 axis could exert its effect within MM BM microenvironment.