Allorecognition is initiated by CD4+ and CD8+ T cells recognizing either intact allo-MHC molecules (MHC class II and I, respectively) on donor antigen-presenting cells (APCs)(direct pathway) or allopeptides bound to self-MHC molecules on recipient APCs (indirect pathway)(1). The high precursor frequency of T cells capable of interacting directly with an allo-MHC molecule presumably accounts for the strength and polyclonality of the direct alloresponse, as observed during in vitro mixed leukocyte reactions (MLR). In contrast, T cells recognizing allopeptides on recipient MHC molecules (indirect pathway) are directed to a few dominant determinants and they display a limited T-cell receptor (TCR) Vβ gene usage (2–4). Likewise, it is traditionally accepted that direct alloreactivity represents the driving force behind early acute graft rejection. Alternatively, it is believed that the direct alloresponse rapidly subsides as donor passenger leukocytes vanish, while the indirect alloresponse persists and causes chronic rejection, presumably by promoting delayed-type hypersensitivity and the production of alloantibodies (5, 6). However, these scenarios are based largely on circumstantial evidence and the actual contribution of direct versus indirect allorecognition to the alloresponse and transplant rejection remains open to question.

In this issue of the American Journal of Transplantation, Brennan and colleagues have revisited these questions using a mouse model in which TCR transgenic (tg) CD4+ T cells recognizing allo-MHC antigens either directly (4C) or indirectly (TCR 75) were adoptively transferred to recipients of vascularized heart transplants. It was observed that T cells activated indirectly displayed much earlier and stronger proliferation rates and IFNγ production than T cells activated directly. Next, they assessed polyclonal CD4+ T-cell responses using an ELISPOT assay in B6 mice transplanted with BALB/c skin. They observed that the CD4+ T-cell indirect alloresponse represented 10% of the overall alloresponse 10 days posttransplantation but rose to 20% after 60 days (Table 1). Most strikingly, in recipients of allogeneic hearts, the indirect response represented up to 33% of the CD4+ T-cell-mediated alloresponse when measured using T cells recovered from the transplant itself. In another set of experiments, B6 Rag 1 KO mice, that had accepted BALB/c grafts for 100 days, were adoptively transferred with TCR tg CD4+ T cells. Strikingly, only those T cells interacting with alloantigens in an indirect fashion rejected the allografts. Co-transfer of T cells with allogeneic (donor) dendritic cells could restore allograft rejection via the direct pathway. This result suggests that the direct alloresponse becomes inoperative once the donor passenger leukocytes are no longer present, although this could also reflect tolerogenic effects of alloantigen presentation by graft parenchymal cells.