PD-1 (Programmed death-1) is mainly expressed on the surface of T cells and primary B cells. The two PD-1 ligands, PD-L1 and PD-L2, are widely expressed on antigen-presenting cells (APCs). PD-1 interacts with its ligands and plays an important role in the negative regulation of the immune response. PD-L1 protein expression is detected in many human tumor tissues. PD-L1 expression in tumor cells could be induced by the microenvironment of tumor cells. PD-L1 expression is favorable for tumorigenesis and growth, for induction of anti-tumor T Cell Apoptosis, and for escaping responses by the immune system. Inhibition of PD-1 binding to its ligand can result in tumor cells that are exposed to the killing version of the immune system, and thus is a target for cancer treatments. PD-L1 (Programmed cell death ligand-1), also known as B7-H1 and CD274, is mainly expressed in antigen-presenting cells (APCs) and activated T cells, and is one of the two ligands of PD-1. The interaction between PD1 and PD-L1 plays an important role in the negative regulation of the immune response. PD-L1 is highly expressed in a variety of solid tumors. PD-1 and PD-L1 interactions can reduce T Cell Activation and promote tumor immune escape. The PD-1/PD-L1 signaling pathway can be blocked and antitumor immune response can be restored by using by anti-PD-1 or anti-PD-L1 antibodies to block the binding of PD1 to PD-L1.
C5AR1（complement C5a receptor 1） is a classical G protein-coupled receptor that is mainly expressed in granulocytes and monocytes and is also expressed in a variety of tumor cells in association with poor prognosis in tumor types. In cancer research, the application direction of C5AR1 target is to treat lung cancer in combination with PD-1 antibody. Both C5AR1 and C5L2 (C5AR2) are receptors for C5a, which can directly or indirectly activate bone marrow-derived suppressor cells (MDSCs). C5AR1 is present on the cell surface of MDSCs, and the level of expression is directly proportional to the degree of tumor infiltration by MDSCs. C5a can directly bind and activate MDSCs to express C5AR1, while more C5AR1 leads to more stimulation of MDSCs by C5a, which increases immunosuppression. It has been reported that combined inhibition of C5a/C5AR1 and PD-1 signaling has a synergistic anti-tumor effect and becomes a potential target for cancer immunotherapy.
The exon 2 of mouse PD-1 gene that encode the extracellular domain was replaced by human PD-1 exon 2 in B-hPD-1/hPD-L1/hC5AR1 mice. The exon 3 of mouse Pdl1 gene that encode the extracellular domain was replaced by human PD-L1 exon 3 in B-hPD-1/hPD-L1/hC5AR1 mice. The exon 2 of mouse C5ar1 gene that encodes the full-length protein was replaced by human C5AR1 exon 2 in B-hPD-1/hPD-L1/hC5AR1 mice. This treble knock-in model was developed by breeding the B-hPD-1/hPD-L1 mice and the B-hC5AR1 mice together.
Protein expression analysis
Strain specific PD-1, PD-L1, C5AR1 expression analysis in homozygous B-hPD-1/hPD-L1/hC5AR1 mice by flow cytometry. Splenocytes were collected from wild-type C57BL/6 mice (+/+) and homozygous B-hPD-1/hPD-L1/hC5AR1 mice (H/H) for PD-1 and PD-L1 expression analysis; Bone marrow was collected from wild-type C57BL/6 mice (+/+) and homozygous B-hPD-1/hPD-L1/hC5AR1 mice (H/H) for C5AR1 expression analysis. Analyses were performed by flow cytometry with species-specific anti-PD-1, anti-PD-L1 and anti-C5AR1 antibody. Mouse PD-1, PD-L1, C5AR1 were detectable in wild-type C57BL/6 mice. Human PD-1, PD-L1, C5AR1 were exclusively detectable in homozygous B-hPD-1/hPD-L1/hC5AR1 mice but not in wild-type mice.