Evidence for direct and indirect pathways in the generation of the alloimmune response against pancreatic islets.

The role of the direct and indirect pathways of alloantigen presentation in the generation of the alloimmune response was dissected using the murine mixed lymphocyte-islet coculture system (MLIC). Stimulator DBA/2J (H-2d) pancreatic islet populations consisted of whole islets (MHC class I+, II+) or FACS-purified beta cells (MHC class I+, II-). Responding C57Bl/6 (H-2b) splenocyte populations were either: (1) untreated; (2) depleted of helper T cells with anti-L3T4 monoclonal antibody plus complement; (3) depleted of cytotoxic T lymphocytes with anti-Lyt2 mAb plus complement; or (4) depleted of antigen-presenting cells by passage through a Sephadex G-10 column. Whole islets were capable of stimulating a significant C57Bl/6 anti-DBA cytotoxic T cell response if the responding population was untreated or treated with complement alone. Depletion of responding splenocytes with either anti-Lyt2 or anti-L3T4 mAb plus complement abrogated the generation of allospecific CTL. If the responding splenocyte population was depleted of APCs, the allo-CTL response against whole islets was decreased, but still significant. If, however, the stimulator population consisted of FACS-purified DBA 2J beta cells, APC-depleted C57Bl 6 splenocytes were incapable of generating any CTL response. Adding responder type (C57Bl/6) APCs back to the microwells restored the capacity for both whole islets and purified beta cells to stimulate a strong allo-CTL response. These data demonstrate that both indirect and direct pathways of alloantigen presentation function in the MLIC.