CD43 is arguably one of the most abundant proteins on the T cell surface. It has been estimated to cover up to 28% of the surface area on rat T cells. CD43 has been proposed to function as both a co-stimulatory molecule and a negative regulating molecule. These contradictory hypotheses are based on the observation that cross-linking CD43 and knocking out CD43 have the same effect; in both situations the T cells have augmented responses to stimulus. We have reconciled these two seemingly contradictory findings. First, we find that CD43 crosslinking leads to decreased CD43 expression. Second, we find that when T cells interact with APC, CD43 is naturally excluded from this contact site. Together, these data support the hypothesis that, due to its abundance and highly glycosylated nature, CD43 acts as a barrier to T cell interactions. We propose that this is a unique function of CD43, since CD45, another large, abundant, highly glycosylation protein does not modulate from the contact area. In addition, we hypothesize that CD43 does not passively modulate from the T cell-APC interaction site, but is an active barrier that is removed by specific signal transduction events. The goal of this grant is to identify the molecular interactions and signal transduction events that regulate CD43 barrier function, and to characterize the effect of this barrier function on T cell immune responses. To achieve these goals, we propose the following specific aims: (1) Determine whether CD43 movement occurs through cytoplasmic interactions with the cytoskeleton. We hypothesize that an inside/out signal from the TCR induces CD43 movement via a direct or indirect interaction with the actin cytoskeleton. Mutant CD43 molecules will be transfected into a CD43-/- T cell clone to determine the role of the highly charged extra-cellular region of CD43 and the intercellular region in the modulation of CD43 from the T cell-APC interaction site. Cytoskeleton involvement in CD43 movement out of the T cell-APC interaction site will be studied by using specific inhibitors of cytoskeletal elements and by examining co-localization of these elements with CD43. (2) To elucidate the signal transduction pathways involved in CD43 modulation from the T cell-APC interaction site. We hypothesize leads to the phosphorylation and modulation of CD43 via the association of intracellular protein intermediates. To test this hypothesis, we will determine the pathways involved in the signal transduction from the T cell receptor to the cytoplasmic tail of CD43. (3) To elucidate the nature of the immune response in CD3-deficient mice. We hypothesize that CD43 provides a barrier to all cellular interactions including those that lead to cellular activation as well as cellular down-regulation and death. We will test this hypothesis by investigating the antigen- specific response in vitro and in vivo of TCR transgenic mice bred to the CD43-/- strain. The secondary or chronic immune response will be tested by either breeding the CD43-/- mice to autoimmune prone strains and assessing whether there is increased severity of early onset of disease (Th 1 responses), or by testing where there is increased airway inflammation in an allergic airway inflammation model (Th2 response).