Abstract 1128: The Leukocyte Integrin Mac-1 Promotes Acute Cardiac Allograft Rejection

Background: In allograft rejection, donor endothelial cells (EC) are the principal early targets of host leukocytes and alloantibodies. Although the leukocyte integrin Mac-1(αmβ2, CD11b/ CD18) facilitates leukocyte-leukocyte as well as leukocyte-EC interactions, its roles in allograft survival and vasculopathy remains unclear. This study used murine cardiac transplantation to test the hypothesis that Mac-1 deficiency in the recipient reduces accumulation of graft infiltrating cells and attenuates cardiac allograft rejection. Results: In total-allomismatched murine cardiac allografts (BALB/c donor hearts and C57BL/6 recipients), survival averaged 13.8 ± 2.3 days (n=6) for allografts in Mac-1-deficient (Mac-1−/−) recipients compared to 8.3 ± 1.3 days (n = 18, p<0.0001) in wild-type (WT) recipients. At 7 d post-transplantation, allografts in Mac-1−/− recipients had significantly lower parenchymal rejection (PR) scores (2.3 ± 0.4) than WT recipients (3.0 ± 0.4, p<0.01). Allograft accumulation of neutrophils, T cells, and macrophages (Mϕ) significantly diminished in Mac-1−/− compared to WT recipients. To determine the cell type involved in acute graft dysfunction, we performed adoptive transfer experiments. Adoptive transfer of WT, but not Mac-1−/−, Mϕ exacerbated parenchymal rejection of 7-day allografts placed into Mac-1−/− recipients (PR score: 2.3 ± 0.4 for WT vs. 1.5 ± 0.5 for Mac1−/− Mϕ; p < 0.05), and significantly reduced graft survival (8.0 ± 1.4 vs. 13.5 ± 2.6 days, p<0.01). Neither WT nor Mac-1−/− neutrophil adoptive transfer affected graft survival in Mac-1−/− recipients. Mac-1−/− Mϕ co-cultured with allo-EC (BALB/c) expressed significantly lower levels of the co-stimulatory molecules CD40 and CD80, and mixed leukocyte reaction using allo-EC-primed Mac-1−/− Mϕ showed significantly lower antigen-presenting function than WT Mϕ. Despite attenuating acute rejection, recipient Mac-1-deficiency did not prevent graft arterial disease. Conclusion: These studies demonstrate that Mac-1 contributes to acute allograft rejection by modulating macrophage recruitment, antigen presentation, and T cell priming. These observations establish a molecular target for modulating recipient responses to prolong graft survival.