The roles of Cabin1 and Left1 in T cell development

Abstract

Cabinl binds calcineurin and myocyte enhancer factor 2 (MEF2) through the C-terminal region. In cell lines, these interactions inhibit calcineurin activity following TCR signaling and transcriptional activation of Nur77 by MEF2. The role of these interactions in vivo was investigated using a mutant mouse strain that expresses a truncated Cabin 1 lacking the C-terminal calcineurin and MEF2 binding domains. Although mutant mice exhibited normal lymphocyte development and thymocyte apoptosis, they had elevated levels of serum IgG 1, IgG2b and IgE and produced more IgG1 and IgG2b in response to a T-dependent antigen. The increased antibody production is apparently not due to changes in immunoglobulin class switching as B cells from mutant mice switched to IgG 1 at the same frequency as wildtype B cells in vitro in the presence of IL-4. However, in response to anti-CD3 stimulation, mutant T cells expressed significantly higher levels of IL-2, IL-4, IL-9, IL-13, and IFN-y. Thus, the calcineurin and MEF2 binding domain of Cabin 1 is dispensable for thymocyte development and apoptosis, but is required for proper regulation of T cell cytokine expression and Th2 antibody responses. In our analysis of gene expression in memory CD8+ T cells, we identified a member of the membrane spanning 4A gene family, MS4a4b, which we named Leftl. It was also found to be expressed in Thl, but not Th2 cells. To examine the role of Leftl in T cell development, we created transgenic mice which express Leftl in T cells. These mice revealed a role for Leftl at multiple stages of T cell development.

(cont.) Ectopic expression of Leftl in CD4+CD8+ thymocytes promoted CD8+ lineage commitment, although CD4+ thymocyte development was unaffected. Although mature transgenic T cells were impaired in their response to certain stimuli in vitro, there was a dramatic increase in the proportion of memory-phenotype T cells in vivo. Finally, Th2 cytokine and GATA-3 expression was impaired in Leftl transgenic Th2 cells. Transgenic T cells activated under Th2 polarizing conditions maintained some characteristics of Thl cells, such as the expression of T-bet and IL-12 receptor. Downregulation of Leftl expression is an essential step in commitment to the Th2 lineage.