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Intact B7‐H3 signaling promotes allograft prolongation through preferential suppression of Th1 effector responses

Ligands of the B7 family provide both positive and negative costimulatory signals to the CD28 family of receptors on T lymphocytes, the balance of which determines the immune response. B7‐H3 is a member of the B7 family whose function in T‐cell activation has been the subject of some controversy: in autoimmunity and tumor immunity, it has been described as both costimulatory and coinhibitory, while in transplantation, B7‐H3 signaling is thought to contribute to graft rejection. However, we now demonstrate results to the contrary. Signaling through a putative B7‐H3 receptor prolonged allograft survival in a fully MHC‐mismatched cardiac model and promoted a shift toward a Th2 milieu; conversely, B7‐H3 blockade, achieved by use of a blocking antibody, resulted in accelerated rejection, an effect associated with enhanced IFN‐γ production. Finally, graft prolongation achieved by CTLA4 Ig was shortened both by B7‐H3 blockade and the absence of recipient B7‐H3. These findings suggest a coinhibitory role for B7‐H3. However, experience with other CD28/B7 family members suggests that immune redundancy plays a crucial role in determining the functions of various pathways. Given the abundance of conflicting data, it is plausible that, under differing conditions, B7‐H3 may have both positive and negative costimulatory functions.