The P-glycoprotein (P-gp) transporter is normally expressed by subsets of both CD4 and CD8 T cells in mice, and the proportion in each subset that expresses P-gp goes up considerably with age. In addition, CD4 memory T cells that express P-gp are hyporesponsive to T cell receptor (TCR) stimulation in many <italic>in vitro</italic> assays. The physiological significance of P-gp expression in T cells, however, has not been well established. To elucidate whether certain T cell functions may require P-gp, we compared a variety of responses using T cells from normal and P-gp-deficient mice. These comparisons showed that P-gp is not required for proliferation, cytokine secretion, or allospecific cytotoxic responses. We also monitored the development of T cells in different lymphoid compartments to see whether P-gp expression affects T cell development or survival. Although circulating T cells were found to develop normally in P-gp-deficient mice, the relative proportions of T cell subsets in the intestinal lymphoid compartment became dramatically altered. The molecular basis for the hyporesponsive nature of P-gp-expressing CD4 memory T cells was also studied by analyzing TCR activation-dependent events in P-gp<super>high</super> and P-g<super>plow</super> subsets of CD4 memory cells. Recruitment of key signaling proteins, including LAT and PKC-theta, to the stimulator cell interface (immunological synapse) was greatly impaired in P-gp<super>high</super> CD4 memory T cells. In contrast, c-Cbl, a negative regulator of signaling, was clustered at the synapse in a high proportion of CD4 memory T cells regardless of P-gp subset. Moreover, while both c-Cbl and LAT were frequently recruited together to the synapse in individual P-gp<super>low</super> cells, P-gp<super>high</super> cells often showed clustering of c-Cbl without LAT. Alterations in the ratios of positive and negative signaling regulators that assemble at the synapse during activation thus appear to contribute to the hyporesponsiveness of P-gp<super> high</super> CD4 memory T cells. Based on these data, P-gp has a negligible role in peripheral T cell effector functions, but its expression is necessary for normal intestinal lymphocyte development and is apparently linked to other cellular changes that lead to a state of anergy in an age-dependent CD4 memory T cell population.Ph.D.Biological SciencesCellular biologyHealth and Environmental SciencesImmunologyMolecular biologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132340/2/9963774.pdf