Recent studies in several labs provide compelling evidence that the innate immune cells can recognize and reject allogeneic non-self in an antigen-specific manner, but the molecular identity of such innate receptors remains poorly defined. By using MHC class I and class II congenic mice, MHC class I pentamers (including donor, host, and third-party), and high throughput screening approaches, as well as immunoprecipitation and mass-spectrometry, we identified the paired-Ig-like receptor (PIR) family of proteins as the innate allorecognition receptors for allospecific macrophages. Specifically, allospecific macrophages express high levels of PIR-A3 that directly binds to H-2Dd (donor) with much higher affinity than H-2Db binding (host). We also cloned PIR-A3 from B6 mice (H-2b) and constructed PIR-A3/Fc fusion protein, and showed that PIR-A3/Fc strongly labels allogeneic BALB/c cells. Moreover, in HEK293T cells that overexpress the PIR-A3 receptor, they bind the H-2Dd pentamer with much higher affinity, thus definitively demonstrating the allospecificity of PIR-A3 to allogeneic MHC class I molecules. We also showed in a reporter system consisting of PIR-A3 extracellular domain fused to CD3e intracellular domain that binding of allogeneic MHC class I molecules to PIR-A3 triggers signaling activities in reporter cells. In our model PIR-A3 expression on macrophages requires CD40 stimulation, primarily due to chromatin remodeling at Pira and Pirb loci. Further in vivo studies showed that treatment of Rag1^-/-γc^-/- mice with PIR-A3/Fc fusion protein inhibited the induction of allospecific macrophages and rejection of allogeneic BALB/c cells, and in the BALB/c-to-B6 heart transplantation model, the PIR-A3/Fc fusion protein inhibited macrophage activation, and significantly prolonged allograft survival.

In conclusion, we identified PIR-A3 as the innate allorecognition receptor that confers macrophages the ability to directly recognize allogeneic non-self in transplant settings, and this findings are significant in developing new anti-rejection therapies.

CITATION INFORMATION: Lan P., Dai H., Liu W., Chen S., Minze L., Dou Y., Zhao Y., Chen W., Xiao X., Pan P., Chen H-.H., Lakkis F., Li X. Identification of Macrophage Innate Receptors in Recognition and Rejection of Allogeneic Non-Self in Transplantation Am J Transplant. 2017;17 (suppl 3).

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