Interleukin 12 and interleukin 23 play key pathogenic roles in inflammatory and proliferative pathways in giant cell arteritis.

Objectives: The pathogenesis of giant cell arteritis (GCA) remains unclear. T1 and T17 pathways are implicated, but the proximal initiators and effector cytokines are unknown. Our aim was to assess the role of interleukin 12 (IL-12) and interleukin 23 (IL-23) in GCA pathogenesis.

Methods: IL-12 and IL-23 expression were quantified by immunohistochemistry in temporal artery biopsies (TABs). Temporal artery (TA) explant, peripheral blood mononuclear cell (PBMC) and myofibroblast outgrowth culture models were established. PBMCs and TA explants were cultured for 24 hours in the presence or absence of IL-12 (50 ng/mL) or IL-23 (10 ng/mL). Gene expression in TA was quantified by real-time PCR and cytokine secretion by ELISA. Myofibroblast outgrowths were quantified following 28-day culture.

Results: Immunohistochemistry demonstrated increased expression of interleukin 12p35 (IL-12p35) and interleukin 23p19 (IL-23p19) in biopsy-positive TAs, localised to inflammatory cells. IL-12p35 TA expression was significantly increased in those with cranial ischaemic complications (p=0.026) and large vessel vasculitis (p=0.006). IL-23p19 TA expression was increased in those with two or more relapses (p=0.007). In PBMC cultures, exogenous IL-12 significantly increased interleukin 6 (IL-6) (p=0.009), interleukin 22 (IL-22) (p=0.003) and interferon γ (IFN-γ) (p=0.0001) and decreased interleukin 8 (IL-8) (p=0.0006) secretion, while exogenous IL-23 significantly increased IL-6 (p=0.029), IL-22 (p=0.001), interleukin 17A (IL-17A) (p=0.0003) and interleukin 17F (IL-17F) (p=0.012) secretion. In ex vivo TA explants, IL-23 significantly increased gene expression of IL-8 (p=0.0001) and CCL-20 (p=0.027) and protein expression of IL-6 (p=0.002) and IL-8 (p=0.004). IL-12 (p=0.0005) and IL-23 (p<0.0001) stimulation increased the quantity of myofibroblast outgrowths from TABs.

Conclusion: IL-12 and IL-23 play central and distinct roles in stimulating inflammatory and proliferative pathways relevant to GCA pathogenesis.