Relevance of the direct pathway of sensitization in corneal transplantation is dictated by the graft bed microenvironment

Corneal grafts were until recently considered entirely devoid of resident APCs, giving rise to the tenet that alloantigen recognition is mediated exclusively by the indirect (host APC-dependent) pathway. The recent discovery of a resident myeloid corneal dendritic cell population that is normally MHC class II- but can readily up-regulate class II expression during inflammation led us to hypothesize that under certain conditions the direct pathway of allosensitization becomes operative. To test this, corneal allotransplants were performed in either inflamed (high-risk (HR)) or uninflamed (low-risk (LR)) host beds in mice, and the frequencies of host T cells activated via the direct pathway were determined. We found that directly primed CD4(+) T cells were detected in the HR but not LR setting, and these cells displayed a clear Th1 phenotype by 2 wk after grafting. Moreover, the use of MHC class II knockout donor tissue led to significantly enhanced survival of HR but not LR allografts. Finally, we show that donor corneal APC demonstrate high expression of CD40, CD80, and CD86 costimulatory molecules when derived from HR but not LR grafts. These data are the first to report that a functional donor APC-dependent direct response is elicited in corneal transplant hosts when the graft bed is inflamed and underscore the relevance of the graft microenvironment in dictating the pathway of allosensitization.