The role of "indirect" recognition in initiating rejection of skin grafts from major histocompatibility complex class II-deficient mice.

Abstract

In vitro studies have revealed several pathways by which T cells can respond to alloantigens, including CD4+ direct responses to allogeneic class 11 antigens, CD8+ direct responses to allogeneic class I antigens, and CD4+ "indirect" responses to peptides of alloantigens presented in association with responder class 11 molecules. In vivo studies of skin graft rejection, however, have so far provided dear evidence for the contribution of only the two direct pathways and not for indirect recognition. We have used major histocompatibility complex class 11-deficient mice as donors to test the role of indirect recognition in rejection of skin grafts. Class 11-deficient skin was always rejected without delay by normal recipients. Removal of recipient CD8+ cells (to leave the animals dependent on CD4+ function) or depletion of recipient CD4+ cells revealed that CD4+ cells were usually involved and sometimes absolutely required in this rapid rejection. Since the donor grafts lacked class II antigens, the CD4+ cells must have recognized donor antigens presented in association with recipient class 11 molecules. These results therefore indicate that indirect recognition can initiate rapid skin graft rejection.Theories to explain the special importance of major histocompatibility complex (MHC) antigens in graft rejection and the extraordinary strength of the immune response to these antigens are all based on the capacity of T cells to recognize allogeneic MHC antigens directly without the usual requirement that their peptides be processed and presented by recipient antigen presenting cells (APCs) (1-3). As a result, studies of the mechanisms of graft rejection have been dominated by concern with the direct pathways of alloreactivity. A few investigators, however, have considered the possibility that graft rejection might also occur through more classical immunologic mechanisms in which peptides of donor antigens are presented in association with recipient MHC molecules. La Rosa and Talmage (4) described this possibility as "indirect" recognition, but the actual role of this pathway in graft rejection has been difficult to determine.Recently, "knockout" mice lacking MHC class II antigens have been produced by the technique of targeted gene disruption by homologous recombination in embryonic stem cells (5, 6). Since these mice lack the MHC class II antigens responsible for stimulating CD4+ T cells, grafts from these mice can be used to examine rejection in the absence of direct stimulation of these helper T cells. The experiments described in this report suggest that in the absence of donor class II antigens, indirect recognition remains an effective pathway of alloreactivity leading to rapid graft rejection. MATERIALS AND METHODSClass II-Deficient Mice. The development and initial characterization of the class II-deficient mice used in these experiments have been described (5). Briefly, the Apb gene was disrupted in the D3 embryonic stem cell line of 129/Sv origin by the technique of homologous recombination. Cloned e...

Inability to Induce Tolerance Through Direct Antigen Presentation

“…These studies, along with others using MHC class I (CI)-and class II (CII)-deficient donors (11), suggested that indirect antigen presentation is an important pathway mediating MHC CI-and MHC CII-disparate allograft rejection. Additionally, experiments using donor-derived peptides to stimulate alloreactive T cells and donor tissues derived from MHC-deficient mice confirmed indirect antigen presentation as an important mechanism in the rejection of allogeneic organs and tissues (4,5,9,(12)(13)(14)(15)(16)(17)(18).…”

“…The direct antigen presentation pathway is sufficient for the rejection of allogeneic skin Previous studies using MHC CII-/-allogeneic donor skin showed that the indirect antigen presentation pathway to CD4π T cells was sufficient to mediate skin allograft rejection (11). To address whether the direct pathway of CD4π T-cell allorecognition can mediate graft rejection, B6 aBM12 TCR Tg mice were grafted with allogeneic BM12 and syngeneic Direct Class II Antigen Presentation and Tolerance B6 skin.…”

“…In order to show that our particular treatment regimen was effective as compared to those previously reported, we used a modified version of the system developed by Auchincloss and colleagues (11) to determine the effect of combined anti-B7-1, anti-B7-2, and anti-CD154 mAbs regimen on the survival of MHC CI-disparate allogeneic skin. This protocol resulted in prolonged graft survival in all mice and long-term survival in almost half of the recipients tested ( Figure 4B).…”

Intrabody-mediated phenotypic knockout of major histocompatibility complex class I expression in human and monkey cell lines and in primary human keratinocytes

“…These studies have reaffirmed that MHC class I plays a dominant role in skin graft rejection. 62,63 Soluble MHC class I shed by the donor keratinocytes are taken up by recipient's APC and presented to CD4 T cells. The CD4 T cells primed via the indirect pathway of allorecognition mediate inflammatory effects (DTH) as well as activate CD8 CTL that recognize donor MHC class I via the direct pathway.…”

“…25,62,[64][65][66][67] However, ␤2-microglobulin and TAP single/double knockout mice still express residual amounts of the MHCI heavy chain that reach the cell surface and are sufficient to trigger a CTL response. [68][69][70] These data suggest that an almost complete MHC class I knockout phenotype is necessary to prevent CTL triggering and action.…”