γδ T cells: firefighters or fire boosters in the front lines of inflammatory responses

Summary: Intradermal inoculation of cloned self‐reactive αβ T cells into the footpads of mice induced cutaneous graft‐versus‐host disease (GVHD), and after recovery from GVHD, the epidermis became resistant to subsequent attempts to induce GVHD. Resistance to GVHD was not induced in the epidermis of T‐cell receptor δ‐deficient (TCRδ −/− ) mice that lacked γδ T cells bearing canonical Vγ5/Vδ1 + γδTCRs, known as dendritic epidermal T cells (DETCs), and resistance was restored by reconstitution of these mutant mice with precursors of Vγ5 + DETCs. Pulmonary infection by Cryptococcus neoformans induced an increase of γδ T cells in the lung, and in comparison with wildtype mice, TCRδ −/− mice eliminated C. neoformans more rapidly and synthesized more interferon‐γ in the lung. In the mouse small intestine, the absence of γδ T cells is associated with a reduction in epithelial cell turnover and downregulation of the expression of major histocompatibility complex class II molecules. The protective role of γδ T cells was verified in a dextran sodium sulfate‐induced inflammatory bowel disease (IBD) model, whereas in a spontaneous model of IBD, γδ T cells were involved in the exacerbation of colitis in TCRα −/− mice. Taken together, in addition to the homeostatic regulation of epithelial tissues, γδ T cells appear to play a pivotal role in the modification of inflammatory responses induced in many organs containing epithelia.