The right place at the right time: novel B7 family members regulate effector T cell responses.

Recent identification of novel members of the B7-family of costimulatory ligands has illustrated their importance for costimulation, not only for initiation of adaptive immune responses, but also for regulation of activated effector lymphocytes. Two key features that distinguish these novel molecules from classical B7.1 and B7.2 costimulatory ligands are their broader expression in non-lymphoid tissues and their binding to receptors induced on activated T cells. Whereas B7.1/B7.2-CD28 interactions are important for priming naïve T cells, novel costimulatory interactions appear critical in regulating effector lymphocytes at sites of infection in the periphery.