NKG2D-mediated cytotoxicity of CD4 cytotoxic T cells in multiple myeloma

• CD4 CTLs are significantly increased and clonally expanded in the bone marrow of patients with multiple myeloma.

• CD4 CTLs directly kill myeloma cells in an NKG2D-dependent manner, and the abundance of NKG2D+CD4+ CTLs correlates with improved survival.

Emerging evidence indicates that CD4+ T cells contribute to antitumor immunity beyond their traditional roles as helpers or regulators. However, the specific subset of CD4+ T cells mediating beneficial outcomes in patients with multiple myeloma remains unclear. Here, we performed single-cell RNA sequencing and T cell receptor sequencing on CD4+ T cells sorted from the bone marrow of patients across the stages of myeloma progression. We identified several distinct states of CD4+ cytotoxic T lymphocytes (CTLs) that were significantly increased and clonally expanded in myeloma patients. CD4+ CTLs displayed transcriptional and phenotypic characteristics indicative of cytotoxicity, demonstrating their ability to directly kill myeloma cells. This cytotoxicity, however, was abrogated by NKG2D blockade. Notably, the abundance of NKG2D+CD4+ CTLs correlated with improved survival in myeloma patients. Our findings suggest that harnessing CD4+ CTLs could lead to novel strategies for enhancing immunotherapy outcomes in multiple myeloma.