IL-23 responsive innate-like T cells in spondyloarthritis: the less frequent they are, the more vital they appear.

A key role for the IL-23/IL-17 immune axis in spondyloarthritis (SpA) is supported by cumulative evidence from genetic and translational studies and was recently confirmed in clinical trials. Although initially linked to T helper 17 cells, it is now clear that additional unconventional T cell subpopulations respond towards IL-23, including RORγt(+) CD3(+)CD4(-)CD8(-) T cells, TCRγδ17 cells, KIR3DL2(+)CD4(+) T cells and iNKT17 cells. Although these innate-like T cells are present only at low frequencies and often with a specific tissue distribution, it is proposed that they could play a vital function in the development or progression of SpA-related pathology. In this review, we highlight the emerging knowledge on these specialized IL-23 responsive T cells with regard to their relevance in SpA. Finally, we will discuss these findings in light of novel drugs targeting the IL-23/IL-17 axis, currently being tested in SpA patients.