Figure 5.

OX40-OX40L signaling causes effector T cell proliferation, which generates a primary immune response. Effector T cells undergo apoptosis following a primary immune response; however, OX40-OX40L signaling mediates the transition of some into resting memory T cells. Re-exposure to the same antigen converts resting memory T cells into effector memory T cells expressing OX40. OX40-OX40L ligation on effector memory T cells causes effector memory T cell proliferation, leading to a secondary immune response.