Primary vascularization of allografts governs their immunogenicity and susceptibility to tolerogenesis

We investigated the influence of allograft primary vascularization on alloimmunity, rejection, and tolerance in mice. First, we showed that fully allogeneic primarily vascularized and conventional skin transplants were rejected at the same pace. Remarkably, however, short-term treatment of mice with anti-CD40L Abs achieved long-term survival of vascularized skin and cardiac transplants but not conventional skin grafts. Nonvascularized skin transplants triggered vigorous direct and indirect proinflammatory type 1 T cell responses (IL-2 and IFN-γ), whereas primarily vascularized skin allografts failed to trigger a significant indirect alloresponse. A similar lack of indirect alloreactivity was also observed after placement of different vascularized organ transplants, including hearts and kidneys, whereas hearts placed under the skin (nonvascularized) triggered potent indirect alloresponses. Altogether, these results suggest that primary vascularization of allografts is associated with a lack of indirect T cell alloreactivity. Finally, we show that long-term survival of vascularized skin allografts induced by anti-CD40L Abs was associated with a combined lack of indirect alloresponse and a shift of the direct alloresponse toward a type 2 cytokine (IL-4, IL-10)-secretion pattern but no activation/expansion of Foxp3(+) regulatory T cells. Therefore, primary vascularization of allografts governs their immunogenicity and tolerogenicity.

Both rejection and tolerance of allografts can occur in the absence of secondary lymphoid tissues

In this study, we showed that aly/aly mice, which are devoid of lymph nodes and Peyer's patches, acutely rejected fully allogeneic skin and heart grafts. They mounted potent inflammatory direct alloresponses but failed to develop indirect alloreactivity after transplantation. Remarkably, skin allografts also were rejected acutely by splenectomized aly/aly (aly/aly-spl(-)) mice devoid of all secondary lymphoid organs. In these recipients, the rejection was mediated by alloreactive CD8(+) T cells presumably primed in the bone marrow. In contrast, cardiac transplants were not rejected by aly/aly-spl(-) mice. Actually, aly/aly-spl(-) mice that spontaneously accepted a heart allotransplant and displayed donor-specific tolerance also accepted skin grafts from the same, but not a third-party, donor via a mechanism involving CD4(+) regulatory T cells producing IL-10 cytokine. Therefore, direct priming of alloreactive T cells, as well as rejection and regulatory tolerance of allogeneic transplants, can occur in recipient mice lacking secondary lymphoid organs.

Modulation of alloreactivity to MHC-derived peptides and transplantation tolerance

The recognition by T cells of intact foreign MHC molecules at the surface of transplanted cells (direct pathway) was originally thought to represent the driving force behind acute rejection of allogeneic transplants. Over the past decade, however, a body of evidence has been provided demonstrating that T cell recognition of donor peptides presented by recipient APCs (indirect pathway) is sufficient on its own to ensure both acute and chronic rejection of allografts. While the direct allorecognition leads to an exceptionally vigorous inflammatory T cell response, it is thought to be short lived due to the rapid depletion of donor professional APCs and it can be controlled with a short course of immunosuppressive drugs including calcineurin inhibitors. In contrast, while the indirect alloresponse is oligoclonal and much weaker, it is long-lived and tends to spread to formerly cryptic determinants on donor and self-tissue specific antigens. This feature of indirect alloreactivity is presumably associated with the sustained presence of recipient professional APCs that can maintain a chronic inflammatory response similar to that observed in autoimmune diseases. Consequently, the indirect alloresponse may be more difficult to suppress than its direct counterpart. On the other hand, there is accumulating evidence showing that administration of alloantigen in a "tolerogenic fashion" mediates allograft acceptance via the activation of regulatory T cells recognizing donor antigens via the indirect allorecognition pathway. Apparently, these regulatory T cells can suppress both direct and indirect inflammatory T cell responses to donor antigens. This suggests that modulation of indirect alloreactivity may represent the best strategy to achieve long-term donor-specific tolerance to allotransplants.