Constant TCR triggering suggests that the TCR expressed on intestinal intraepithelial γδ T cells is functional in vivo

Intestinal intraepithelial lymphocytes carrying the gamma delta TCR (gamma delta iIEL) are involved in the maintenance of epithelial integrity. gamma delta iIEL have an activated phenotype, characterized by CD69 expression and increased cell size compared with systemic T lymphocytes. As an additional activation marker, the majority of gamma delta iIEL express the CD8 alpha alpha homodimer. However, our knowledge about cognate ligands for most gamma delta TCR remains fragmentary and recent advances show that gamma delta T cells including iIEL may be directly activated by cytokines or through NK-receptors, TLR and other pattern recognition receptors. We therefore asked whether the TCR of gamma delta iIEL was functional beyond its role during thymic selection. Using TcrdH2BeGFP (Tcrd, T-cell receptor delta locus; H2B, histone 2B) reporter mice to identify gamma delta T cells, we measured their intracellular free calcium concentration in response to TCR-crosslinking. In contrast to systemic gamma delta T cells, CD8 alpha alpha(+) gamma delta iIEL showed high basal calcium levels and were refractory to TCR-dependent calcium-flux induction; however, they readily produced CC chemokine ligand 4 (CCL4) and IFN-gamma upon TCR triggering in vitro. Notably, in vivo blocking of the gamma delta TCR with specific mAb led to a decrease of basal calcium levels in CD8 alpha(+) gamma delta iIEL. This suggests that the gamma delta TCR of CD8 alpha(+) gamma delta iIEL is constantly being triggered and therefore functional in vivo.