Langerhans cells, orthotopic corneal allografts, and direct and indirect pathways of T-cell allorecognition.

PURPOSE To determine after orthotopic corneal allografting the role of Langerhans cells in activation of T cells via the direct and indirect pathways of allorecognition and the relationship between these pathways and the rapidity of graft rejection. METHODS Corneas from eyes of normal mice and from eyes after superficial cauterization were grafted to eyes of major histocompatibility complex (MHC) and/or minor histocompatibility (H)-disparate recipient mice. The grafts were analyzed through time for content of class 1 MHC- bearing Langerhans cells and for rejection or acceptance. Graft recipients were evaluated for acquisition of delayed hypersensitivity (DH) and cytotoxic T cells (Tc) directed at donor MHC and minor H alloantigens. RESULTS Langerhans cells migrated more rapidly into epithelium of cauterized grafts than normal grafts. Unlike normal grafts, the vast majority of cauterized allografts were rejected within 2 weeks. Normal grafts induced neither DH nor Tc directed at donor MHC antigens, whereas cauterized grafts induced both DH and Tc specific for donor MHC. All grafts induced DH directed at donor minor H antigens, but only rejected grafts correlated with acquisition of Tc directed at donor minor H antigens. CONCLUSIONS. The rapidity of orthotopic corneal allograft rejection correlated with density of Langerhans cells within epithelium and with acquisition of donor-specific DH and Tc. Although recipient-derived Langerhans cells promoted minor H-specific, self-MHC-restricted T cells (indirect pathway) and subacute graft rejection, donor-derived Langerhans cells promoted early, acute rejection in conjunction with allogeneic MHC-specific Tc (direct pathway).