IL-23 and monocyte interactions redirect human Th17.1 cells to an IL-17A^highIFN-γ^high potentially pathogenic profile

IL-23-targeted therapies have demonstrated clinical efficacy in psoriatic arthritis (PsA), primarily through the reduction of IL-17A-driven inflammation. Consequently, most studies have focused on classical, high IL-17A-producing, CCR6⁺CXCR3^-CCR4⁺ Th17 cells. In contrast, the impact of IL‑23 signaling on CCR6⁺CXCR3⁺CCR4^- Th17.1 cells -traditionally characterized by high IFN-γ and low IL-17A production-remains largely unexplored, despite their recognized pathogenicity in autoimmune disease. The aim of this study was to characterize the effects of IL-23 on human memory Th17.1 cells.

Peripheral blood mononuclear cells (PBMCs) from treatment-naive patients with early PsA were stimulated in the presence of IL-23 or anti-IL-23 antibody before quantifying IL-17A and IFN-γ secretion. Additionally, human memory CCR6^- Th1, CCR6⁺ Th17.1 and CCR6+ Th17 cells from healthy donors were sorted and analyzed ex vivo for IL23R, TBX21 and RORC expression using RT-qPCR. Sorted Th17.1, Th17 and Th1 cells were subsequently cultured alone or cocultured with IL-23-producing monocytes. Cultures received either exogenous IL-23 or IL-23 inhibition before analyzing IL-17A and IFN-γ secretion.

As expected, IL-23 stimulation induced IL-17A in PsA PBMCs, whereas IL-23 inhibition reduced IL-17A secretion but did not affect IFN-γ. In healthy donor PBMCs, ex vivo IL-23R expression was restricted to memory CCR6⁺ Th17 and Th17.1 cells. Remarkably, IL-23 stimulation of Th17.1 cells significantly increased IFN-γ without affecting IL-17A secretion. Compared to monoculture, cocultures with monocytes increased IFN-γ secretion in Th17.1 and classical Th1 cells, and IL-17A secretion in Th17 and Th17.1 cells. Additional IL-23 stimulated IL-17A in both Th17 and Th17.1 cocultures, while unexpectedly reducing IFN-γ in Th17.1 cells. In contrast, IL-23 inhibition reduced IL-17A secretion in Th17 and Th17.1 cells without affecting IFN-γ production.

IL-23 exerts distinct effects on Th17.1 compared to Th17 cells. We show a role for IL-23 in IFN-γ upregulation in Th17.1 cells, which is reversed in the presence of monocytes. In co-culture with monocytes, Th17.1 cells progress to a pathogenic IL-17A^highIFN-γ^high profile, which is further elevated by IL-23 stimulation and reduced upon IL-23 neutralization. We show that IL-23 drives Th17.1 plasticity and underscore the therapeutic relevance of neutralizing IL-23 in the suppression of these pathogenic IL-17A^highIFN-γ^high Th17.1 cells.