The inducible costimulator plays the major costimulatory role in humoral immune responses in the absence of CD28.

CD28 plays crucial costimulatory roles in T cell proliferation, cytokine production, and germinal center response. Mice that are deficient in the inducible costimulator (ICOS) also have defects in cytokine production and germinal center response. Because the full induction of ICOS in activated T cells depends on CD28 signal, the T cell costimulatory capacity of ICOS in the absence of CD28 has remained unclear.

The B7 family member B7-H3 preferentially down-regulates T helper type 1-mediated immune responses.

We investigated the in vivo function of the B7 family member B7-H3 (also known as B7RP-2) by gene targeting. B7-H3 inhibited T cell proliferation mediated by antibody to T cell receptor or allogeneic antigen-presenting cells. B7-H3-deficient mice developed more severe airway inflammation than did wild-type mice in conditions in which T helper cells differentiated toward type 1 (T(H)1) rather than type 2 (T(H)2).