AML blasts and monocytic myeloid-derived suppressor cells (M-MDSC) inhibit human mucosal-associated invariant t (MAIT) cells activation.

Abstract Introduction: In acute myeloid leukemia, tumor cells can evade and suppress the host immune system, contributing to tumor progression or relapse. One of these escape mechanisms is the development and/or recruitment of immunoregulatory cells, including myeloid-derived suppressor cells (MDSCs). These cells potently inhibit T-cell activation and proliferation; thus, they are associated with tumor progression as well as relapse after allogenic hematopoietic stem cell transplantation (allo-HSCT). In contrast, the role of mucosal-associated invariant T (MAIT) cells in promoting or controlling tumor growth remains unclear. MAITs are the most abundant innate T lymphocyte subset and are well known for their antimicrobial action via their potent cytotoxic functions. They can be activated not only via their semi-invariant TCR, recognizing the 5-OP-RU microbial ligand presented by MR1 (MHC related protein 1), but also via cytokines (IL-12 and IL-18), thereby highlighting their role beyond bacterial infections. This study investigated the potential anti-tumor role of MAIT cells and, reciprocally, assessed the impact of AML blasts or monocytic MDSCs (M-MDSCs) on MAIT cells function. Methods: Experiments were conducted using total PBMCs, sorted T lymphocytes, monocytes and MAIT cells from healthy donors. These cells were co-cultured with AML cell lines (THP1, OCI-AML3, KG1) or M-MDSC induced either by co-culture with AML blasts or by exposure to GM-CSF and IL-6. MAIT cell activation was assessed through expression of activation and degranulation markers (CD69, CD25, 41BB, CD107a, granzyme B) following TCR-dependent (5-OP-RU) or TCR-independent (IL-12/IL-18) stimulation. Transcriptomic analyses were performed on bulk sorted MAIT cells. Results: AML cell lines were found to spontaneously express surface MR1 but were not able to activate MAIT cells. As expected, in the presence of 5-OP-RU, purified MAIT cells showed direct cytotoxicity against AML cell lines.Interestingly IL-12/18 induced MAIT-mediated AML tumor cell killing, even in the absence of their canonical ligand. These data suggest that MAIT cells may exhibit a direct anti-leukemic activity in an inflammatory context. Unexpectedly, when co-cultured with total PBMCs, AML blasts inhibited both TCR-dependent and TCR-independent MAIT cell activation. This implies the involvement of a third cellular component in this inhibitory effect, which we found to be independent of NK cells or monocytes. MDSCs – known to be increased both in AML and post-HSCT – similarly inhibited TCR-dependent and TCR-independent MAIT cell activation. Transwell experiments showed that AML blast- and MDSC-mediated immunosuppression required cell–cell contact. However, blocking antibodies targeting known inhibitory receptors on MAIT cells (PD-1, CD161, BTLA) failed to restore MAIT cell activation, suggesting alternative suppressive mechanisms. We next assessed the transcriptomic profile of activated MAIT cells in the presence versus absence of blasts. The 5-OP-RU-activated MAITs revealed a profile of T-cell exhaustion and senescence in the presence of AML blasts (increased expression of TOX and of inhibitory receptors LAG-3 and TIM-3, and down-regulation of CD28). Cytokine-activated MAIT cells showed a marked downregulation of NK-cell cytotoxicity signature in the presence of AML blasts, with decrease in several NK-associated activating receptors potentiallyinvolved for TCR-independent cytotoxic responses. Conclusion: This study demonstrates that MAIT cells, when activated via IL-12/IL-18 exert a strong cytotoxic activity against AML blasts, supporting their potential role in anti-leukemic immunity, particularly in the inflammatory context of allo-HSCT. However, AML blasts as well as MDSCs can suppress both TCR-dependent and TCR-independent MAIT cell activation through distinct mechanisms impairing their cytotoxic functions. Besides, this MAIT dysfunction may also have strong clinical implications for the control of microbial infections following allo-HSCT.