THEMIS: Two Models, Different Thresholds

Abstract

THEMIS, a recently identified T-lineage-restricted protein, is the founding member of a large metazoan protein family. Gene inactivation studies have revealed a critical requirement for THEMIS during thymocyte positive selection, implicating THEMIS in signaling downstream of the T cell antigen receptor (TCR), but the mechanistic underpinnings of THEMIS function have remained elusive. A previous model posited that THEMIS prevents thymocytes from inappropriately crossing the positive/negative selection threshold by dampening TCR signaling. However, new data suggest an alternative model where THEMIS enhances TCR signaling enabling thymocytes to reach the threshold for positive selection, avoiding death by neglect. We review the data supporting each model and conclude that the preponderance of evidence favors an enhancing function for THEMIS in TCR signaling.

Figure 1. Themis expression during T cell development

A. Top, Schematic of T cell development showing the main stages of maturation defined by expression of the CD4 and CD8 co-receptors. The stage at which positive and negative selection occur and the point where thymocyte development is partially blocked in Themis^−/− mice are shown. Bottom, Expression of Themis and Ptpn6 during thymocyte development. Results are from RT-PCR of cDNA from different thymocyte populations isolated by cell sorting. B. Domain schematic of the five mammalian CABIT proteins showing location of the globular CABIT module(s) (purple), proline-rich sequence (PRS; pink), conserved helical segment (green), conserved CABIT core sequence (yellow), and Sterile alpha motif (SAM) domain (blue).

Figure 2. Two models of Themis function in thymocytes

THEMIS expression is high in CD4⁺CD8⁺ (DP) thymocytes when positive selection occurs. THEMIS and SHP-1 bind to the cytosolic adapter GRB2 and are recruited by GRB2 to the scaffolding adapter LAT following TCR stimulation. A. Model 1. Left, in Themis^+/+ thymocytes, THEMIS facilitates recruitment of SHP-1 to LAT via GRB2 and/or enhances SHP-1 PTP activity. TCR/co-receptor signals are attenuated by SHP-1 so that signals initiated by low affinity positively selecting TCR/self-pMHC interactions are appropriate for positive selection and do not induce negative selection. Right, in the absence of THEMIS (Themis^−/− thymocytes), SHP-1 activity is reduced and/or SHP-1 recruitment to LAT is impaired. Positively selecting TCR/self-pMHC interactions transduce strong signals that trigger negative selection. B. Model 2. Left, the THEMIS CABIT modules bind to the SHP-1 phosphatase domain blocking access to substrate and promoting or stabilizing oxidation of the SHP-1 catalytic cysteine by Reactive Oxygen Species (ROS) thereby inhibiting SHP-1 phosphatase activity. Inhibition of SHP-1 enables low affinity TCR interactions to generate signals sufficient for positive selection. Right, in the absence of THEMIS (Themis^−/− thymocytes), SHP-1 PTP activity is not attenuated and low affinity TCR/self-pMHC interactions that normally are sufficient for positive selection fail to induce positive selection resulting in cell death by non-selection (also known as ‘death by neglect’). The sequences in SHP-1 responsible for binding to the GRB2 SH3 domain have not been identified; therefore, the SHP-1:GRB2 interaction depicted is speculative. pY, phospho-tyrosine.