| T lymphocyte activation requires the recognition of antigen by the T cell receptor (TCR) as well as a second, or costimulatory signal. Many proteins with potential costimulatory activity exist on T cells but it is unclear how or if their roles overlap. CD28 is widely regarded as the most important costimulatory protein, and yet it is unclear exactly how and wherein the process of T cell activation CD28 functions. Using DO11.10 TCR transgenic T cells deficient in either CD28 or another costimulatory molecule, CD40L, we demonstrate both in vitro and in vivo, using an adoptive transfer system, that CD28 plays a crucial role in initiating TH responses while CD40L is necessary to maintain responses.; CD28 may contribute to the strength of signals generated by the TCR or it could initiate distinct signaling pathways, with integration occurring in the nucleus. CD28 increases proliferation, survival, and production of IL-2 and the effector cytokines, IFN-γ and IL-4. It is unclear if these multiple effects of CD28 costimulation are regulated by a single signaling pathway or if CD28 initiates multiple pathways, each leading to a distinct downstream effect.; To study the contribution of individual signaling domains within the cytoplasmic tail we have used a retroviral-mediated gene transfer system to reconstitute CD28−/− T cells with various cytoplasmic domain mutants. Using live cell imaging, we have studied the role of CD28 in early activation events. Signaling motifs that act in a redundant manner regulate CD28-mediated IL-2 production and proliferation. However, IL-4 production is driven by a specific combination of signaling domains suggesting that different functional mechanisms underlie the induction of distinct T cell responses by CD28. CD28 localization to the synapse occurs directly after T cell:APC contact and co-localization with the TCR improves CD28 function. CD28 increases the likelihood that a T cell will flux calcium after antigen-APC contact and serves to regulate the magnitude and duration of the calcium response. Thus, CD28 functions in the very proximal events of T cell activation, contributing to the strength of TCR signals, but also providing regulation of these signals that results in production of TH2 cytokines. |
