Aggressive skin allograft rejection in CD28-/- mice independent of the CD40/CD40L costimulatory pathway

CD28(-/-) mice have been utilized to study the role of B7/CD28 and B7-CTLA4 interactions. There is evidence that CTLA4 ligation may be critical for tolerance induction. The aim of the current study is to further investigate rejection responses of CD28(-/-) mice and to define the role of B7-CTLA4 interactions in the absence of the CD40 and CD28 pathways. Balb/c skin allografts were transplanted onto C57BL/6 (B6) wild type or CD28(-/-) mice treated with anti-CD40L, CTLA4-Ig, or combination blockade. To investigate the cellular mechanism of rejection in CD28(-/-) recipients, mice were treated with anti-CD4 or anti-CD8 antibodies prior to treatment with costimulation blockade. The fluoroscein dye USE was utilized to study T cell expansion in vivo. Surprisingly, treatment of B6 CD28(-/-) mice with CTLA4-Ig alone (MST 12d), anti-CD40L alone (MST 13d), or combined blockade (MST 13d) had no effect on allograft survival compared to untreated B6 CD28(-/-) mice (MST lid). CD28(-/-) recipients depleted of CD4(+) cells and treated with CTLA4-Ig, anti-CD40L, or combination blockade also did not have prolonged survival compared with untreated mice (MST 10d). In contrast, CD28(-/-) recipients depleted of CD8(+) cells had markedly prolonged allograft survival when treated with either anti-CD40L alone (MST 49d) or with combination blockade (MST 57d). Studies utilizing USE demonstrated that CD28(-/-) CD8(+) T cells are not defective in in vivo proliferation responses compared with wild type CD8 cells. Thus, CD28(-/-) CD8(+) T cells are responsible for aggressive rejection responses of CD28(-/-) mice independent of the CD40 pathway. In addition, CD40L blockade does not result in CD4(+) T cell tolerance in CD28(-/-) recipients, despite an intact B7-CTLA4 pathway. (C) 2001 Elsevier Science B.V. All rights reserved.