Humoral immune responses during acute rejection in rat lung transplantation

The detailed responses of humoral immunity during acute rejection remain obscure in lung transplantation (LTx). In order to clarify the reactions of alloantibodies (allo-Abs) during acute rejection, we demonstrated the time-course of changes in anti-donor Ab reaction in the peripheral blood and deposition in the grafts using a rat LTx model. Lewis (LEW) rats served as recipients for Brown Norway (BN) lung allografts (MHC fully incompatible combination). The left lung was transplanted orthotopically using a cuff technique. Syngeneic transplants (LEW to LEW) served as control. No immunosuppression therapy was administered in this model. We evaluated the alloreactivity against donor in rat recipients by detecting allo-Abs with a flow cytometric cross-match (FCXM) technique. Recipient serum samples were incubated with donor lymphocytes and stained with anti-rat immunoglobulin (Ig), to determine the titers of circulating allo-Abs in the peripheral blood with a three-color FCXM technique. We also examined the deposition of anti-donor Abs (IgG and IgM) in the grafts with an immunofluorescent method. All allografts were completely rejected and lost their aeration within 6 days after LTx. Strong allo-Abs responses of both IgG and IgM were observed in the peripheral blood during acute rejection. The level of IgM allo-Abs had already significantly increased on day 2 at the time of mild rejection; however, IgG Abs did not elevate until day 6, when the grade of rejection was severe. Circulating IgM levels started decreasing on day 8, whereas IgG Abs continued elevating. On the other hand, no evident deposition of allo-Abs in the grafts was observed until day 6. We have shown in this study that circulating IgM allo-Abs was detected at the time of mild allograft rejection, interestingly, before evident deposition in the graft. It might be suggested that allograft rejection progressed without antibody deposition until severe rejection.