Single cell RNA sequence reveals C5aR1 inhibition selectively targets pro-tumorigenic M2 macrophages reversing PARP inhibitor resistance

Although Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) have been approved in multiple diseases, including BRCA1/2 mutant breast cancer, responses are usually transient thus requiring the development of combination therapies that can capitalize on PARPi activity. We thus explored mechanisms underlying sensitivity and resistance to PARPi using two intrinsically sensitive and resistant syngeneic murine breast cancer models. Our data indicate that the PARPi sensitive tumor model has a high ratio of M1 anti-tumor/M2 pro-tumor macrophages with the M1/M2 ratio being increased by PARPi. In contrast the PARPi resistant tumor model had very low levels of M1 macrophages and thus a low M1/M2 ratio that was not altered by PARPi. Our data indicate that co-transplantation of the PARPi sensitive and the PARPi resistant tumor results in accumulation of M2 macrophages in the sensitive tumor, rendering the sensitive tumor PARPi resistant. C5aR1 and RPS19/C5aR1 signaling is selectively elevated in the M2 macrophages that are associated with PARPi resistance. Strikingly C5aR1 targeting decreased M2 macrophages, while sparing M1 macrophages rendering PARPi resistant tumors sensitive to PARPi. Consistent with the murine data, human breast cancers with high C5aR1 levels have a poor response to immune checkpoint blockade. Thus targeting C5aR1 may represent a therapeutic approach to selectively deplete M2 macropahges and engender sensitivity to PARPi and potentially other therapeutic approaches. Overall design: scRNA-seq by 10x genomics of two mouse-derived syngeneic transplant (LPA1-T22, and LPA1-T127) single strain transplanted or co-transplanted models treated with olaparib for 4 days, 8 days and 20 days