CD300a as a Potential Immune Checkpoint in Breast Cancer: Insights from in vivo and in vitro Models.

Introduction Breast cancer (BC) is a common cancer type in women and a major cause of death. CD300a is an inhibitory receptor expressed on immune cells, particularly mast cells (MCs). CD300a ligands expression was found on several cancer cells. We hypothesized that CD300a has a role in suppression of anti-tumor immunity in BC. Methods To test our hypothesis, we induced the 4T1 BC model in WT and CD300a-KO mice and evaluated, by toluidine blue staining, the activation level of tumor associated MCs. 4T1 cells were co-cultured with WT or CD300a-KO mouse bone marrow derived MCs (BMMCs) and 4T1 cell viability was analyzed using trypan blue. Human MDA-231 BC cells were co-cultured with cord blood derived MCs (hCBMCs) and soluble hCD300a-Fc was added. FACS analysis for MDA-231 cell viability and WB for CD300a signal transduction in CBMCs were performed. Results In vivo, smaller 4T1-cell tumors with more activated MCs were found in CD300a-KO mice. In vitro, 4T1 cell growth was reduced in co-culture with WT BMMCs and further reduced with CD300a-KO BMMCs. Similarly, growth of MDA-231 BC cells in co-culture with CBMCs was reduced following treatment with CD300a-Fc which feasibly interfered with CD300a pro-tumorigenic interactions. CD300a signal transduction pathway was activated in CBMCs co-cultured with MDA-231 cells and this activation was inhibited following treatment with CD300a-Fc. Conclusion Our results would imply that in BC, tumor cells activate CD300a on MCs and probably on other immune cells to downregulate their anti-tumorigenic activity and therefore suggest CD300a as a potential immune-suppressor in BC.