Because CD28-mediated T-cell costimulation has a pivotal role in the initiation and maintenance of T-cell responses, we tested the hypothesis that CD28 is critical for the development of graft-versus-host disease (GVHD). We compared the in vivo effects of CD28^−/− T cells transplanted from B6 donor with the CD28 gene deleted by homologous recombination with those of CD28^+/+ T cells transplanted from wild-type C57BL/6 (B6) donor. Fifty million CD28^−/− or CD28^+/+ splenocytes from B6 mice were transplanted into unirradiated (B6 × DBA/2)F1 (BDF1) recipients. Unlike CD28^+/+, CD28^−/− T cells from B6 mice had lower levels of proliferation and interleukin-2 production, had a limited ability to generate cytotoxic T lymphocytes against the recipient, and did not induce immune deficiency, despite survival in the recipient for at least 28 days. The ability to prevent rejection was reduced by the absence of CD28, because as many as 1.0 × 10⁷ CD28^−/− CD8⁺ cells were needed to prevent rejection of major histocompatibility complex (MHC) class-I incompatible marrow in sublethally irradiated (550 cGy) bm1 recipients, whereas 8.0 × 10⁵ CD28^+/+CD8⁺ T cells were sufficient to produce a similar effect, indicating that CD28 on donor CD8⁺ cells helps to eliminate host immunity. Two million CD4⁺CD28^−/− or CD28^+/+ T cells were transplanted into sublethally irradiated (750 cGy), MHC class-II incompatible (B6 × bm12)F1 recipients. With CD28^−/−cells, 44% of the recipients died at a median of 20 days compared with 94% at a median of 15 days with CD28^+/+ cells (P < .001). Two million CD8⁺CD28^−/− or CD28^+/+ T cells were transplanted into sublethally irradiated (750 cGy), MHC class-I incompatible (B6 × bm1) F1 recipients. With CD28^−/−cells, 25% of the recipients died at a median of 41 days compared with 100% at a median of 15 days with CD28^+/+ cells (P < .001). (B6 × bm12)F1 and (B6 × bm1)F1 mice surviving after transplantation of CD28^−/− cells recovered thymocytes, T cells, and B cells in numbers and function comparable with that of irradiation-control F1 mice. We conclude that CD28 contributes to the pathogenesis and the severity of GVHD. Our results suggest that the severity of GVHD could be decreased by the administration of agents that block CD28 function in T lymphocytes.

© 1998 by The American Society of Hematology.