C5AR1 (complement C5a receptor 1) is a classical G protein-coupled receptor that is mainly expressed on granulocytes, monocytes, dendritic cells and MDSCs (Myeloid-derived suppressor cells) in several cancer types. Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of immature cells that play a significant role in inhibiting T cell-related responses. C5a can directly bind and activate MDSCs to upregulate C5AR1 expression, and that in turn leads to an increased stimulation of MDSCs by C5a, which further increases immunosuppression. The enhancement of cancer cell invasion and growth by the C5a-C5AR1 system suggests that this system is a possible target for cancer therapy. Currently, there is a lot of focus on research and development of antibody drugs targeting C5AR1. In order to generate a suitable animal model in which to evaluate the in vivo activity of humanized anti-C5AR1 antibodies, we have developed C5AR1 humanized mice (BhC5AR1 mice) in which the murine C5ar1 gene is replaced with the human C5AR1 gene by homologous recombination. The viability of this model is supported by previous studies showing that mouse and human C5a bind human C5AR1 with comparable affinities. Our analysis shows that human C5AR1 was detectable in dendritic cells, granulocytes and macrophages isolated from spleen and dendritic cells isolated from bone marrow in homozygous B-hC5AR1 mice but not in wild type mice. Percentage of T cells, B cells, NK cells, DCs, granulocytes, monocytes, macrophages, CD8+ T cells, CD4+ T cells and Tregs in homozygous B-hC5AR1 mice were similar to those in the wild type mice, demonstrating that introduction of human C5AR1 in place of its mouse counterpart does not change the overall development, differentiation or distribution of these cell types. Following these results, we are currently performing an in vivo efficacy evaluation of an anti-human C5AR1 antibody using the humanized mice. Mouse pancreatic ductal adenocarcinoma Panc02 cells were subcutaneously implanted into homozygous B-hC5AR1 mice (female, 6-7 week-old, n=5). Mice were grouped when tumor volume reached approximately 100 mm3 , at which time they were treated with anti-human C5AR1 antibodies. The final results are still pending. In conclusion, B-hC5AR1 mice are a promising model for preclinical in vivo pharmacodynamic assessment of anti-C5AR1 antibodies.