Augmentation of Recipient Adaptive Alloimmunity by Donor Passenger Lymphocytes within the Transplant.

Chronic rejection of solid organ allografts remains the major cause of transplant failure. Donor-derived tissue-resident lymphocytes are transferred to the recipient during transplantation, but their impact on alloimmunity is unknown. Using mouse cardiac transplant models, we show that graft-versus-host recognition by passenger donor CD4 T cells markedly augments recipient cellular and humoral alloimmunity, resulting in more severe allograft vasculopathy and early graft failure.

Diversity of the CD4 T Cell Alloresponse: The Short and the Long of It.

MHC alloantigen is recognized by two pathways: "directly," intact on donor cells, or "indirectly," as self-restricted allopeptide. The duration of each pathway, and its relative contribution to allograft vasculopathy, remain unclear. Using a murine model of chronic allograft rejection, we report that direct-pathway CD4 T cell alloresponses, as well as indirect-pathway responses against MHC class II alloantigen, are curtailed by rapid elimination of donor hematopoietic antigen-presenting cells.

Copresentation of intact and processed MHC alloantigen by recipient dendritic cells enables delivery of linked help to alloreactive CD8 T cells by indirect-pathway CD4 T cells.

In transplantation, direct-pathway CD8 T cells that recognize alloantigen on donor cells require CD4 help for activation and cytolytic function. The ability of indirect-pathway CD4 T cells to provide this help remains unexplained, because a fundamental requirement for epitope linkage is seemingly broken. The simultaneous presentation, by host dendritic cells (DCs), of both intact MHC class I alloantigen and processed alloantigen would deliver linked help, but has not been demonstrated definitively.

Germinal center alloantibody responses are mediated exclusively by indirect-pathway CD4 T follicular helper cells.

The durable alloantibody responses that develop in organ transplant patients indicate long-lived plasma cell output from T-dependent germinal centers (GCs), but which of the two pathways of CD4 T cell allorecognition is responsible for generating allospecific T follicular helper cells remains unclear. This was addressed by reconstituting T cell-deficient mice with monoclonal populations of TCR-transgenic CD4 T cells that recognized alloantigen only as conformationally intact protein (direct pathway) or only as self-restricted allopeptide (indirect pathway) and then assessing the alloantibody response to a heart graft. Recipients reconstituted with indirect-pathway CD4 T cells developed long-lasting IgG alloantibody responses, with splenic GCs and allospecific bone marrow plasma cells readily detectable 50 d after heart transplantation.

Donor CD4 T cells contribute to cardiac allograft vasculopathy by providing help for autoantibody production.

Background: The development of autoantibody after heart transplantation is increasingly associated with poor graft outcome, but what triggers its development and whether it has a direct causative role in graft rejection is not clear. Here, we study the development of antinuclear autoantibody in an established mouse model of heart allograft vasculopathy.

Methods And Results: Humoral vascular changes, including endothelial complement staining, were present in bm12 heart grafts, explanted 50 days after transplantation.

Abrogation of antibody-mediated allograft rejection by regulatory CD4 T cells with indirect allospecificity.

Alloantibody is an important effector mechanism for allograft rejection. In this study, we tested the hypothesis that regulatory T cells with indirect allospecificity can prevent humoral rejection by using a rat transplant model in which acute rejection of MHC class I-disparate PVG.R8 heart grafts by PVG.

Dendritic cells internalise and re-present conformationally intact soluble MHC class I alloantigen for generation of alloantibody.

Following organ transplantation soluble MHC class I is released from the graft and may contribute to alloimmunity. We determined in a well-established rat model whether DC are able to internalise soluble MHC class I alloantigen and then re-present intact alloantigen to B cells and T cells for generation of an alloantibody or CD8 T cell response. PVG.

Evidence of macrophage receptors capable of direct recognition of xenogeneic epitopes without opsonization.

Background: We have previously demonstrated that porcine livers perfused with human blood remove most of the erythrocytes from three units of human blood over the course of a 72-h extracorporeal perfusion. Red blood cell loss did not appear to involve classical complement pathway-mediated hemolysis, but instead resulted from porcine Kupffer cell phagocytosis.

Methods: We developed a method incorporating collagenase digestion and metrizamide separation to isolate and maintain porcine Kupffer cells in primary culture.

Classical pathway complement destruction is not responsible for the loss of human erythrocytes during porcine liver perfusion.

Background: Porcine livers perfused with human blood destroy 85% of human erythrocytes (red blood cells [RBC]) during prolonged extracorporeal perfusion, raising the possibility of a complement-mediated graft-versus-host effect.

Methods: Isolated porcine livers were perfused with fresh human blood. Plasma samples were analyzed for complement production by reverse CH50 analysis and porcine immunoglobulin class and specificity by enzyme-linked immunosorbent assay (ELISA) and flow cytometry.