C5aR-antagonism alone or in combination with MEK inhibition durably alters the plexiform neurofibroma tumor micro-environment

Abstract ID 98867 Poster Board 249 Mutations in the NF1 tumor suppressor gene, which encodes a negative regulator of RAS signaling cascades, predispose affected individuals to manifestations including development of benign nerve tumors (neurofibromas). The formation and growth of plexiform neurofibroma (PNF) tumors depend on the interplay between tumor cells and the surrounding tumor microenvironment (TME), which is marked by chronic inflammation, myeloid cell expansion, and remodeling of local and systemic immune compartments. Although inhibiting mitogen-activated protein kinase (MEK) signaling downstream of RAS shrinks most established plexiform neurofibromas (PNF), not all patients respond to MEK inhibition and even in those that do the response is not durable. Our aim in this study was to identify and test a druggable target to synergize with MEK inhibitor treatment to promote efficacy and durability of treatment. Using flow cytometry and single cell anlaysis of two murine models and histological examination of human tumors, we discovered an increase in the C5a-C5a receptor (C5aR) system in PNF as characterized by an increase in C5aR1 expressing macrophages, the predominant cell in PNF tumors, which is not normalized by MEK inhibition. Thus, to test the hypothesis that C5aR can synergize with MEK inhibition to modulate inflammatory responses in PNF we treated immunocompetent PNF-bearing Nf1f/f; DhhCre mice, a model which has historically demonstrated robust translational importance, for 60 days with either a MEK inhibitor, a C5aR1 inhibitor, or a combination of the two. We found that therapeutic reduction of C5aR1 activity induced cell death in tumor macrophages and enhanced the engulfment of dying Schwann cells by macrophages but did not affect neurofibroma number or size. Complete or partial genetic depletion of C5aR1 in Nf1f/f; DhhCre mice confirmed these results, suggesting stronger therapy would not increase efficacy of response.We next tested if the combination of MEK and C5aR1 inhibition would improve durability of response. To test this idea, we treated mice for 1 month, and then maintained mice off therapy for one month. Tumors regrew in all groups, but only mice treated with a combination of MEK and C5aR1/2 inhibitors showed altered tissue cellular architecture, expansion of dendritic cells, and increased macrophage MHCII expression. We conclude that C5aRA in combination with MEK inhibition is tolerable and causes durable immunosuppressive effects on the neurofibroma microenvironment. Funding: NIH R33 NS112407 to JQ and NR, and DOD W81XWH-19-1-0816 to NR