Introduction: The immune tumor microenvironment (TME) in multiple myeloma (MM) is a complex and dynamic ecosystem characterized by interactions between malignant plasma cells and various immune cell populations, which collectively contribute to disease progression, immune evasion, and therapeutic resistance. Modulation of immune responses by immune checkpoint molecules expressed on malignant cells and immune cells within the TME helps the cancer evade immune surveillance and promotes tumor survival.
Methods: In this study, we mapped inhibitory and costimulatory immune checkpoints in the TME of patients with MGUS and active MM using multiparameter flow cytometry. Our pipeline was designed to profile the expression of inhibitory and costimulatory immune checkpoint molecules on various subsets of immune cell types within the adaptive and innate TME in bone marrow samples from patients with MGUS and active MM, as well as from healthy donors (HD).
Results: By profiling the adaptive TME, we observed a significantly higher expression of CD27 on cytotoxic T cells, helper T cell subtypes, and γδ T cells. Conversely, the expression levels of OX40 and PD-1 were significantly downregulated on T cell subsets in both MGUS and active MM compared to HD. However, we noted significantly higher expressions of TIGIT, 2B4, and DNAM-1, in contrast to the low levels of receptors 4-1BB, CTLA-4, TIM-3, LAG-3, and BTLA. Moreover, statistically significant differences between the MGUS stage and active MM were observed in the expression of TIGIT, PD-1, CD27, and 2B4 on cytotoxic T cells and helper T cell subtypes. On NK cells of innate immunity, we observed a decrease in OX40 and PD-1 in both MGUS and MM, as well as a decrease in TIM-3 only in MM, and an increase in CD27 in MM compared to HD. The expression level of CTLA-4 was significantly lower in MGUS patients compared to active MM. Upregulation of TIGIT was detected on NKT cells in active MM compared to HD. On B lymphocytes, we observed upregulation of CD27 and downregulation of PD-1, LAG-3, and 4-1BB, along with a decrease in TIM-3 expression in MGUS and OX40 in MM compared to HD. The expression profile of malignant plasma cells showed high levels of the immune checkpoints PD-1, CD27, BTLA, ICOS, CTLA-4, DNAM-1, TIM-3, and TIGIT, compared to low levels of LAG-3, 2B4, 4-1BB, and OX40.
Conclusions: In conclusion, we observed a significantly higher expression of CD27 on T cell subsets in the premalignant MGUS stage. In contrast, in active MM, we found increased expression of inhibitory immune checkpoints 2B4, PD-1, and TIGIT on T cells, suggesting their involvement in the transformation from MGUS to MM. A better understanding of the TME, including both inhibitory and costimulatory immune checkpoints in precursor conditions and active MM, may provide the basis for new therapeutic approaches.
This study was supported by SRDA grants APVV-16-0484 (JJ), APVV-20-0183 (JJ), APVV-19-0212 (DC), and NextGenerationEU project No. 09I03-03-V02-00031 (DC & KS).
