T-cell receptor signaling in CD4+CD8+ double positive thymocytes determines cell survival and lineage commitment, but the genetic and molecular basis of this process is poorly defined. To address this issue, we used ethylnitrosourea mutagenesis to identify a novel T-lineage-specific gene, Themis, which is critical for the completion of positive selection. Themis contains a tandem repeat of a unique globular domain (CABIT), which includes a cysteine motif that defines a family of 5 uncharacterized vertebrate proteins with orthologs in most animal species. Themis-deficient thymocytes showed no major impairment in early T-cell receptor signaling, but exhibited altered expression of cell cycle and survival genes before and during positive selection. These data suggest a unique role for Themis in sustaining positive selection.

THEMIS, a T cell specific protein that is highly expressed in CD4+CD8+ thymocytes, has a crucial role in positive selection and T cell development. THEMIS lacks defined catalytic domains but contains two tandem repeats of a distinctive (CABIT) module of unknown function. Here, we show that THEMIS directly regulated the catalytic activity of the protein tyrosine phosphatase SHP-1. This action was mediated by the CABIT modules, which bound to the SHP-1 phosphatase domain and promoted or stabilized oxidation of the SHP-1 catalytic cysteine, inhibiting SHP-1 tyrosine phosphatase activity. Reduction of SHP-1 alleviated the developmental block in Themis−/− thymocytes. Thus, THEMIS facilitates thymocyte positive selection by enhancing the T cell antigen receptor signaling response to low affinity ligands.

Adaptive tolerance is a process by which T cells become desensitized when Ag stimulation persists following an initial immune response in vivo. To examine the biochemical changes in TCR signaling present in this state, we used a mouse model in which Rag2−/− TCR-transgenic CD4+ T cells were transferred into CD3ε−/− recipients expressing their cognate Ag. Compared with naive T cells, adaptively tolerant T cells had normal levels of TCR and slightly increased levels of CD4. Following activation with anti-TCR and anti-CD4 mAbs, the predominant signaling block in the tolerant cells was at the level of Zap70 kinase activity, which was decreased 75% in vitro. Phosphorylations of the Zap70 substrates (linker of activated T cells and phospholipase Cγ1 were also profoundly diminished. This proximal defect impacted mostly on the calcium/NFAT and NF-κB pathways, with only a modest decrease in ERK1/2 phosphorylation. This state was contrasted with T cell clonal anergy in which the RAS/MAPK pathway was preferentially impaired and there was much less inhibition of Zap70 kinase activity. Both hyporesponsive states manifested a block in IκB degradation. These results demonstrate that T cell adaptive tolerance and clonal anergy are distinct biochemical states, possibly providing T cells with two molecular mechanisms to curtail responsiveness in different biological circumstances.