Suppression of human macrophage-mediated cytotoxicity by transgenic swine endothelial cell expression of HLA-G

Background: Xenotransplantation is an appealing alternative to human allotransplantation because of a worldwide shortage of organs. One of the obstacles for xenografts is cellular rejection by the innate immune system, comprised of NK cells, monocytes, and macrophages. In this study the inhibitory function of HLA-G1, a MHC Ib molecule, on macrophage-mediated cytotoxicity was examined. Furthermore, this study also evaluates the suppressive effect of cytokine production by macrophages.

Methods: The expression of inhibitory receptors that interact with HLA-G1, immunoglobulin-like transcript 2 (ILT2), ILT4 and KIR2DL4 (CD158d) on in vitro generated macrophages were examined by flow cytometry. Complementary DNA (cDNA) of HLA-G1, HLA-E and human β2-microglobulin (hβ2m) were prepared and transfected into swine endothelial cells (SECs). The expression of the transgenic genes was evaluated by flow cytometry, and macrophage-mediated SEC cytolysis was assessed using the macrophages.

Results: In vitro generated macrophages expressed not only ILT2 and ILT4 but CD158d as well. The transgenic HLA-G1 on SECs indicated significant suppression in macrophage-mediated cytotoxicity, which was equivalent to that of transgenic HLA-E. Furthermore, the results on real time PCR and ELISA revealed that transgenic HLA-G1 induces the anti-inflammatory cytokines, such as IL-10 and TGF-β, and suppresses iNOS mRNA expression, indicating that transgenic HLA-G1 has suppressive effects in a broad range of transplant rejection.

Conclusion: These results indicate that generating HLA-G1 transgenic pigs can protect porcine grafts from macrophage-mediated cytotoxicity.