CSF-1R inhibition and lenalidomide synergize to promote myeloma control after autologous stem cell transplantation

Autologous stem cell transplantation (ASCT) with maintenance lenalidomide remains the mainstay of consolidation therapy for eligible multiple myeloma (MM) patients but preventing disease relapse remains a critical unmet need. Here we investigated whether immunosuppressive myeloid populations in bone marrow (BM) correlated with ASCT outcomes. Using a preclinical ASCT model with suboptimal endogenous anti-myeloma activity, we demonstrated that while neither CSF-1R inhibition nor lenalidomide monotherapy significantly improved outcomes, their combination synergistically attenuated disease progression and prolonged survival. Single-cell RNA sequencing revealed that lenalidomide expanded NK-like CD8+ T-cells but paradoxically also increased the frequency of Csf1r+ macrophages. Cell-cell communication analyses identified Csf1r+ macrophages as suppressors of these NK-like and effector-like exhausted (Tphex) CD8 T-cell populations through CD94/NKG2A and PD-L1/PD-1, respectively. CSF-1R blockade depleted these immunosuppressive macrophages, which correlated with decreased expression of inhibitory receptors and enhanced expression of activation markers in Tphex.