Myeloma escape after stem cell transplantation is a consequence of T-cell exhaustion and is prevented by TIGIT blockade

Autologous stem cell transplantation (SCT) remains a standard of care for multiple myeloma (MM) patients and prolongs progression-free survival. A small cohort of patients achieve longterm control of disease, but the majority of patients ultimately relapse, and the mechanisms permitting disease progression remain unclear. In this study, we used a preclinical model of autologous SCT for myeloma where the disease either progressed (MM relapsed) or was controlled. In the bonemarrow (BM), inhibitory receptor expression on CD81 T cells correlated strongly withmyeloma progression after transplant. In conjunction, the costimulatory/adhesion receptor CD226 (DNAM-1) was markedly downregulated. Interestingly, DNAM-12 CD81Tcells in MM-relapsed mice had an exhausted phenotype, characterized by upregulation of multiple inhibitory receptors, including T-cell immunoglobulin and ITIM domains (TIGIT) and programmed cell death protein 1 (PD-1) with decreased T-bet and increased eomesodermin expression. Immune checkpoint blockade using monoclonal antibodies against PD-1 or TIGIT significantly prolonged myeloma control after SCT. Furthermore, CD81 T cells from MM-relapsed mice exhibited high interleukin-10 (IL-10) secretion that was associated with increased TIGIT and PD-1 expression. However, while donor-derived IL-10 inhibited myeloma control post-SCT, this was independent of IL-10 secretion by or signaling to T cells. Instead, the donor myeloid compartment, including colony-stimulating factor 1 receptor-dependent macrophages and an IL-10-secreting dendritic cell population in the BM, promoted myeloma progression. Our findings highlight PD-1 or TIGIT blockade in conjunction with SCT as a potent combination therapy in the treatment of myeloma.