Pattern of Graft- and Host-Specific MHC Class II Expression in Long-Term Murine Cardiac Allografts: Origin of Inflammatory and Vascular Wall Cells

In solid-tissue allografts, donor vascular cells as well as recipient inflammatory cells can express MHC class II molecules. However, it is uncertain how much residual donor endothelium persists and to what extent donor versus recipient MHC class II expression can contribute to the ongoing immune response, especially in long-term grafts. To establish the origin of class-II-expressing cells in the allograft, we evaluated the expression of donor- or recipient-specific MHC class II molecules in murine cardiac allografts.Donor hearts from BALB/c (H-2(d)) mice were transplanted into C57BL/6 (B6, H-2(b)) recipients; BG isografts served as controls. Untreated allografts ceased functioning at approximately 7 days with severe parenchymal rejection. Allografts from recipients treated with anti-CD4 and anti-CD8 MAbs after transplantation were explanted at 8 to 12 weeks and demonstrated intimal fibroproliferative lesions with a mild parenchymal mononuclear cell infiltrate. Class II expression in isografts was limited to epicardial macrophages. Both acutely rejecting and long-term allografts contained abundant macrophages expressing recipient class II molecules. Occasional cells (passenger leukocytes) in untreated, acutely rejecting allografts bore donor class II molecules; long-term allografts contained few such cells. In contrast, vascular endothelial and medial smooth muscle cells consistently expressed donor class II molecules. These results suggest that ongoing MHC class II expression in donor vascular cells, as well as in recipient macrophages, may contribute to sustained activation of host T cells with consequent release of cytokines that ultimately promote the development of graft arteriosclerosis.