The spondyloarthritides (SpA) are a group of rheumatic diseases characterized by ossification and inflammation of entheseal tissue, the region where tendon attaches to bone. Interleukin‐23 (IL‐23) is involved in the pathogenesis of SpA by acting on IL‐23 receptor (IL‐23R) expressed on enthesis‐resident lymphocytes. Upon IL‐23 binding, CD3+CD4−CD8− tissue‐resident lymphocytes secrete IL‐17A and IL‐22, leading to inflammation, bone loss, and ossification. Knowledge about enthesis‐resident lymphocytes remains fragmentary, and the contribution of entheseal γ/δ T cells in particular is not clear. This study was undertaken to investigate the presence of γ/δ T cells in the enthesis.

We used 2‐photon microscopy and flow cytometry to analyze entheseal lymphocytes from C57BL/6, Tcrd‐H2BeGFP, Rorc‐GFP, and IL‐23R‐eGFP mice. To analyze entheseal γ/δ T cells in IL‐23−induced inflammation, Tcrd‐H2BeGFP mice were crossed with mice of the susceptible B10.RIII background. Hydrodynamic injection of IL‐23 minicircle DNA was performed for overexpression of IL‐23 and induction of inflammation. Light‐sheet fluorescence microscopy was used to visualize arthritic inflammation.

Activated Vγ6+CD27− γ/δ T cells were abundant in uninflamed entheseal tissue and constituted the large majority of retinoic acid receptor−related orphan nuclear receptor γt (RORγt)+IL‐23R+ enthesis‐resident lymphocytes. Fetal thymus−dependent γ/δ T cells were the main source of IL‐17A at the enthesis. Under inflammatory conditions, γ/δ T cells increased in number at the Achilles tendon enthesis, aortic root, and adjacent to the ciliary body.

Entheseal γ/δ T cells are derived from fetal thymus and are maintained as self‐renewing tissue‐resident cells. As main IL‐17A producers within tissues exposed to mechanical stress including enthesis, γ/δ T cells are key players in the pathogenesis of IL‐23−induced local inflammation.