Mutation of the phospholipase C-gamma1-binding site of LAT affects both positive and negative thymocyte selection.

Linker for activation of T cells (LAT) is a scaffolding adaptor protein that is critical for T cell development and function. A mutation of LAT (Y136F) that disrupts phospholipase C-gamma1 activation and subsequent calcium influx causes a partial block in T cell development and leads to a severe lymphoproliferative disease in homozygous knock-in mice. One possible contribution to the fatal disease of LAT Y136F knock-in mice could be from autoreactive T cells generated in these mice because of altered thymocyte selection. To examine the impact of the LAT Y136F mutation on thymocyte positive and negative selection, we bred this mutation onto the HY T cell receptor (TCR) transgenic, recombination activating gene-2 knockout background. Female mice with this genotype showed a severe defect in positive selection, whereas male mice exhibited a phenotype resembling positive selection (i.e., development and survival of CD8(hi) HY TCR-specific T cells) instead of negative selection. These results support the hypothesis that in non-TCR transgenic, LAT Y136F knock-in mice, altered thymocyte selection leads to the survival and proliferation of autoreactive T cells that would otherwise be negatively selected in the thymus.

An architectural perspective on signaling by the pre-, alphabeta and gammadelta T cell receptors.

The T cell antigen receptor (TCR) is a multimeric complex composed of an antigen-binding clonotypic heterodimer and a signal transducing complex consisting of the CD3 dimers (CD3gammaepsilon and CD3deltaepsilon) and a TCR-zeta homodimer. In all jawed vertebrates there are two T cell lineages, alphabeta and gammadelta, distinguished by the clonotypic subunits contained within their TCRs (TCR-alpha and -beta or TCR-gamma and -delta, respectively). A third receptor complex, the preTCR, is only expressed on immature T cells. The preTCR, which contains the invariant pre-Talpha (pTalpha) chain in lieu of TCR-alpha, plays a critical role in the early development of alphabeta lineage cells. The subunit composition of the signal transducing complexes of the pre-, alphabeta- and gammadeltaTCRs was previously thought to be identical. However, recent data demonstrate that there are significant differences in the signal transducing complexes of these three TCRs. For example, alphabetaTCRs contain both CD3gammaepsilon and CD3deltaepsilon dimers, whereas gammadeltaTCRs contain only CD3gammaepsilon dimers. Moreover, preTCR function appears to be unaffected in the absence of CD3delta, suggesting that CD3deltaepsilon dimers are dispensable for pre-TCR assembly. In this review, we summarize current data relating to the subunit composition of the pre-, alphabeta- and gammadeltaTCRs and discuss how these structural differences may impact receptor signaling and alphabeta/gammadelta lineage determination.

A potential role for CD69 in thymocyte emigration.

The early activation marker, CD69, is transiently expressed on activated mature T cells and on thymocytes that are undergoing positive or negative selection in the thymus. CD69 is a member of the NK gene complex family of C-type lectin-like signaling receptors; however, its function is unknown. In this report, we describe the characterization of mice that constitutively express high levels of surface CD69 on immature and mature T cells throughout development. Constitutive surface expression of CD69 did not affect T cell maturation, signaling through the TCR or thymocyte selection. However, phenotypically and functionally mature thymocytes accumulated in the medulla of CD69 transgenic mice and failed to be exported from the thymus. The retention of mature thymocytes correlated with transgene dose and CD69 surface levels. These results identify a potential role for CD69 in controlling thymocyte export, and suggest that the transient expression of CD69 on thymocytes and T cells may function to regulate thymocyte and T cell trafficking.

A LAT mutation that inhibits T cell development yet induces lymphoproliferation.

Mice homozygous for a single tyrosine mutation in LAT (linker for activation of T cells) exhibited an early block in T cell maturation but later developed a polyclonal lymphoproliferative disorder and signs of autoimmune disease. T cell antigen receptor (TCR)-induced activation of phospholipase C-gamma1 (PLC-gamma1) and of nuclear factor of activated T cells, calcium influx, interleukin-2 production, and cell death were reduced or abrogated in T cells from LAT mutant mice. In contrast, TCR-induced Erk activation was intact. These results identify a critical role for integrated PLC-gamma1 and Ras-Erk signaling through LAT in T cell development and homeostasis.