The Function of the TCR zeta zeta Module in T Cells
Pitcher, Lisa Anne
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The alpha beta T cell receptor complex (TCR) has the unique ability to discriminate and differentially respond to peptide/MHC ligands encountered on the surface of antigen presenting cells. The ligation of the TCR with peptide/MHC complexes is translated into intracellular signals through a conserved sequence motif, termed ITAM, or immunoreceptor tyrosine-based activation motif, which are present in one or more copies in the cytoplasmic portions of the TCR zeta and CD3 gamma, delta and epsilon chains. A distinctive feature of the TCR complex is that it contains ten ITAMs, in contrast to other antigen receptor complexes which contain two or four ITAMs. The ten TCR ITAMs are distributed as one in each CD3 chain and three in the TCR zeta subunit (TCR alpha beta epsilon delta epsilon gamma zeta zeta). It has been proposed that the TCR is comprised of two autonomous signaling modules, TCR zeta zeta and CD3 gamma epsilon/delta epsilon. Following receptor ligation, TCR zeta is the most heavily tyrosine-phosphorylated subunit of the TCR, developing into two stable intermediates of 21- and 23-kDa (p21 and p23). Based on the number of ITAMs it contributes, TCR zeta zeta was initially presumed to be the predominant signaling module in the TCR complex, with p21 and p23 being linked to virtually all aspects of T cell biology. To clearly define the functions of the TCR zeta zeta module, we generated a series of TCR zeta transgenic mice, with modified zeta molecules, that selectively express p21 alone, no p21 or p23, or no phospho-zeta intermediates. In a wild type or high affinity TCR system (P14), T cell development was completely normal in the TCR zeta transgenic lines. Surprisingly, when peripheral T cells were analyzed for their functionality in response to various stimuli, including peptide/MHC-, T cell mitogen- and superantigenic- stimulation, equivalent dose response curves were observed, regardless of phospho-zeta. Notably, these data also eliminated a possible inhibitory role for the partially phosphorylated p21 intermediate of TCR zeta. To more carefully examine the roles of p21 and p23, the TCR zeta transgenics were mated to a TCR transgenic line (HY) bearing T cells with a low affinity TCR. In this system, important roles for phospho-zeta during T cell selection were revealed. Specifically, TCR zeta ITAMs functioned additively during positive selection events. These selection events appear to be independent of traditional signaling pathways, as the signaling capacity of unselected T cells in the absence of all phospho-zeta was equivalent to T cells with wild-type TCR zeta subunits. These results imply that the CD3 gamma epsilon/delta epsilon module is the predominant signaling module in the TCR complex. Our studies also identified a unique role for p21 during negative selection events. The select expression of p21 in T cells attenuated negative selection of thymocytes, resulting in the generation of a population of potentially autoreactive cells. Based on these data, a revised model of TCR signal transmission is proposed. Within this model, the TCR zeta zeta and CD3 gamma epsilon/delta epsilon modules contribute both redundant and unique functions to T cells. The CD3 gamma epsilon/delta epsilon module is primarily responsible for classical TCR-mediated signaling pathways leading to T cell activation. The TCR zeta zeta and CD3 gamma epsilon/delta epsilon modules contribute redundant functions to thymocyte positive selection. These redundant functions are mediated by the ten TCR ITAMs. Phosphorylated intermediates of the TCR zeta zeta module also contribute to thymocyte positive selection, likely through alternative signaling pathways. In addition, the TCR zeta zeta module functions in a unique manner during thymocyte negative selection, with p21 attenuating negative selection of thymocytes. Furthermore, preliminary evidence suggests novel roles for the TCR zeta zeta module in the maintenance of peripheral T cells and in the adaptive immune response to bacterial pathogens.