Innate PLZF+CD4+ αβ T Cells Develop and Expand in the Absence of Itk

Abstract

T cell development in the thymus produces multiple lineages of cells, including innate T cells. Studies in mice harboring alterations in TCR signaling proteins or transcriptional regulators have revealed an expanded population of CD4+ innate T cells in the thymus that produce IL-4 and express the transcription factor promyelocytic leukemia zinc finger (PLZF). In these mice, IL-4 produced by the CD4+PLZF+ T cell population leads to the conversion of conventional CD8+ thymocytes into innate CD8+ T cells resembling memory T cells expressing eomesodermin. The expression of PLZF, the signature invariant NKT cell transcription factor, in these innate CD4+ T cells suggests that they might be a subset of αβ or γδ TCR+ NKT cells or mucosal-associated invariant T (MAIT) cells. To address these possibilities, we characterized the CD4+PLZF+ innate T cells in itk−/− mice. We show that itk−/− innate PLZF+CD4+ T cells are not CD1d-dependent NKT cells, MR1-dependent MAIT cells, or γδ T cells. Furthermore, although the itk−/− innate PLZF+CD4+ T cells express αβ TCRs, neither β2-microglobulin–dependent MHC class I nor any MHC class II molecules are required for their development. In contrast to invariant NKT cells and MAIT cells, this population has a highly diverse TCRα-chain repertoire. Analysis of peripheral tissues indicates that itk−/− innate PLZF+CD4+ T cells preferentially home to spleen and mesenteric lymph nodes owing to increased expression of gut-homing receptors, and that their expansion is regulated by commensal gut flora. These data support the conclusion that itk−/− innate PLZF+CD4+ T cells are a novel subset of innate T cells.