The alloantibody network following intrathymic immunomodulation of sensitized rat recipients of cardiac allografts

An intrathymic injection of allogeneic spleen cells (2xl0⁷) prevents accelerated (1 cardiac allografts to about 11 days. This effect is do-nor-specific, x-irradiation-sensitive and thymus-de-pendent, and it does not require adjunctive immunosuppressive therapy. We have recently shown that following intrathymic allo-Ag injection, host cell proliferative responses in lymphoid organs are markedly depressed as compared with untreated sensitized recipients. Little is known about how intrathymic immu-nomodulation may affect host humoral alloreactivity. In this work, we studied the dynamic interplay between the humoral responses, both in the circulation and at the graft site of sensitized hosts. Intrathymic allo-Ag exposure triggered a profound change in the utilization pattern of alloreactive IgM, IgG, and IgG subclasses compared with recipients receiving syngeneic cells. Administration of allo-Ag into the thymus at the time of sensitization resulted in an earlier and significantly increased systemic production of IgM, as shown by flow cytometry. Subsequently, isotype switching to IgG occurred prematurely and resulted in elevated levels of IgGl and IgG2a. Indeed, the addition of such allo-Ab enriched serum suppressed the MLR assay in a dose-dependent manner. The binding of Ig to cardiac allografts was analyzed in eluates by flow cytometry, and by immunohistochemical staining at day 1 after transplantation. Intragraft IgM and IgG levels were consistently higher in well-functioning grafts following administration of allo-Ag, as compared with controls. IgG deposits at the graft site consisted predominantly of IgGl and IgG2a, while significant amounts of IgG2b could only be detected in control hosts undergoing accelerated graft rejection. These data document that intrathymic injection of do-nor-specific allo-Ag in sensitized recipients leads to profound alterations of the host humoral allore-sponses, and that such elevated allo-Ab levels interfere with the Ag reactivity or alloresponsive effector cells in vitro. These results support the notion that the pattern of allo-Ab utilization is indicative of the functional status of the alloimmune response in the transplant recipient.