CD4-Mediated Islet Xenograft Rejection Requires 'Indirect' Antigen Recognition

Abstract

34 CD4 T cells are both necessary and sufficient for acute cellular immunity to pancreatic islet xenografts (rat-to-mouse). This study determined whether CD4 T cells required antigen presentation by host MHC class II molecules as predicted for indirect (host APC-dependent) antigen recognition. We established a model in which normal CD4 T cells were transferred into immune-deficient animals that were either MHC class II positive or negative by selective breeding of immune-deficient Rag2-/- mice (H-2b) with MHC class II-/- mice (H-2b). F1 mice were intercrossed and F2 mice were selected that were immune deficient (Rag2-/-) and either MHC class II positive or negative (determined by PCR screening). Such animals were rendered diabetic with streptozotocin and grafted with WF (RT1u) rat islet xenografts. Following restoration of blood glucose, animals were reconstituted with 5×106 purified CD4 T cells derived from F1 parental mice. Flow cytometry demonstrated the purity of CD4 T cells both prior to lymphocyte reconstitution and at the conclusive of the experiment(<1% contaminating CD8 T cells or B220+ B cells). All WF rat xenografts survived >60 days in unreconstituted Rag2-/- recipients that were either MHC class II + (n=7) or class II (n=5). Transfer of purified CD4 T cells triggered acute rejection of established WF islet xenografts in 6/6 Rag2-, MHC class II+ recipients in 10.0± 1.1 days. However, CD4 T cells failed to reject WF grafts in MHC class II- Rag2- recipients with 5/5 grafts functioning >60 days following T cell transfer. Nephrectomy of long-term surviving grafts resulting in a return to hyperglycemia in all animals, indicating that normoglycemia was graft-dependent. Prior to nephrectomy, flow cytometry of peripheral blood in Rag2- Class II- recipients receiving CD4 T cells demonstrated that CD4 T cells persisted in these animals despite the inability to trigger xenograft rejection. In conclusion, CD4 T cells are sufficient for acute cellular immunity to islet xenografts and this response requires MHC class II expression from the host. This result suggests that indirect presentation of xenogeneic antigens in association with host MHC class II molecules is the predominant pathway of islet xenograft immunity in vivo.