Abstract

Abstract Myeloproliferative neoplasms represent a group of clonal hematopoietic disorders of which myelofibrosis (MF) is the most aggressive. In the context of myeloid neoplasms, there is a growing recognition of the dysregulation of immune response and T‐cell function as significant contributors to disease progression and immune evasion. We investigated cytotoxic T‐cell exhaustion in MF to restore immune response against malignant cells. Increased expression of inhibitory receptors like CTLA‐4 was observed on cytotoxic T cells from MF patients together with a reduced secretion of IFNɣ and TNFɑ. CTLA‐4 ligands CD80 and CD86 were increased on MF granulocytes and monocytes highlighting a possible role for myeloid cells in suppressing T‐cell activation in MF patients. Unlike healthy donors, the activation of cytotoxic T cells from MF patients was attenuated in the presence of myeloid cells and restored when T cells were cultured alone or treated with anti‐CTLA‐4. Moreover, anti‐CTLA‐4 treatment promoted elimination of neoplastic monocytes and granulocytes in a co‐culture system with cytotoxic T cells. To test CTLA‐4 inhibition in vivo , patient‐derived xenografts were generated by transplanting MF CD34+ cells and by infusing homologous T cells in NSGS mice. CTLA‐4 blockade reduced human myeloid chimerism and led to T‐cell expansion in spleen and bone marrow. Overall, these findings shed light on T‐cell dysfunction in MF and suggest that CTLA‐4 blockade can boost the cytotoxic T cell‐mediated immune response against tumor cells.