Lung allograft rejection in the rat. II. Specific immunological properties of lung grafts.

The immunological mechanism of lung allograft rejection was studied in inbred rats, in order to explain the rapid progress of the rejection response against RT1-incompatible lung grafts. Histological appearances of the graft and of the recipient's spleen were studied, migration patterns of graft and recipient lymphocytes were assessed, and titers of circulating alloantibodies were determined. Histologically, we discriminated four phases of the rejection response in lung grafts: sequentially the latent, vascular, alveolar, and destruction phases. Early in the vascular phase, recipient lymphocytes primarily infiltrated the bronchus-associated lymphoid tissue (BALT) of the graft, causing a local immune response. Concurrent with these local rejection phenomena in the graft, a strong systemic immune response developed in the recipient's spleen, presumably induced by the great number of lymphocytes that migrated from the graft's BALT into the recipient's lymphoid tissues. We conclude that BALT facilitates a fast and intensive interaction between lung graft and recipient that is likely to accelerate the induction of the rejection response both locally in the graft and systemically in the recipient's lymphoid organs.