Editors' ChoiceImmunology

Positive Selection

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Science Signaling  04 Sep 2012:
Vol. 5, Issue 240, pp. ec231
DOI: 10.1126/scisignal.2003552

During T cell development, both CD4 and CD8 [cell surface molecules that bind to major histocompatibility complexes (MHCs)] are present on immature thymocytes, which are referred to as “double-positive.” Double-positive thymocytes with a T cell receptor (TCR) that interacts weakly with self peptide bound to MHCs undergo positive selection to become single-positive CD4+ or CD8+ T cells. In contrast, those with a TCR that interacts strongly with self peptide bound to MHCs undergo negative selection, which leads to cell death to prevent the development of self-reactive T cells. Ca2+ signaling and activation of extracellular signal–regulated kinase (ERK) are required for positive selection. To identify other molecules involved in positive selection, Lo et al. used thymocytes from AND mice, which express a TCR that is activated by MCC-I-EK [a complex of moth cytochrome c (MCC) and the MHC class II molecule I-Ek (I-EK)]. Stimulation of double-positive AND thymocytes with MCC-I-EK, which binds strongly to the AND TCR, results in negative selection. In contrast, stimulation with the peptide gp250, which binds weakly to the AND TCR, results in positive selection and increased surface abundance of CD69, a cell surface marker of positively selected and activated T cells. Double-positive AND thymocytes undergoing positive selection with gp250 showed sustained Ca2+ influx as well as ERK activation, whereas Ca2+ influx was transient in double-positive AND thymocytes that were negatively selected with MCC-I-EK. Transcriptional profiling indicated that positively selected thymocytes (but not negatively selected AND thymocytes; mature, single-positive T cells; or peripheral T cells from normal mice) showed increased abundance of the mRNAs encoding SCN4B, the regulatory β subunit of voltage-gated sodium channels (VGSCs), and SCN5A, one of the pore-forming α subunits that can interact with SCN4B. Blocking the pore of VGSCs with tetrodotoxin reduced Ca2+ influx and the development of CD4+, but not CD8+, T cells from positively selected AND thymocytes. Irradiated mice reconstituted with B6 hematopoietic stem cells expressing a short hairpin RNA (shRNA) directed against SCN5A had fewer CD4+ T cells compared with those reconstituted with B6 cells expressing a nontargeting shRNA. Conversely, gain-of-function experiments indicated that ectopic expression of SCN5A and SCN4B in AND T cell hybridomas or peripheral AND T cells (which lack VSGCs) enabled these cells to respond to gp250, as indicated by increased surface abundance of CD69. The authors propose that VGSCs promote Ca2+ influx in double-positive thymocytes in response to positively selecting ligands. Malissen notes in the accompanying commentary that the mechanism by which VGSCs are activated by TCRs and the link between VGSCs and increased Ca2+ influx remain to be determined.

W.-L. Lo, D. L. Donermeyer, P. M. Allen, A voltage-gated sodium channel is essential for the positive selection of CD4+ T cells. Nat. Immunol. 13, 880–887 (2012). [PubMed]

B. Malissen, A voltage-gated sodium channel mediates positive selection of T cells. Nat. Immunol. 13, 810–812 (2012). [PubMed]

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