Background/Purpose : Ankylosing spondylitis (AS) is an inflammatory arthritis of the spine, which has a major impact on function and quality of life. AS is known to have a sex-bias with a M:F ratio of 3:1. In addition, females have a delayed onset and reduced radiographic severity compared to males. Genetic and immunologic studies have implicated the Th17-axis in AS pathogenesis, and recent clinical trials suggest efficacy of anti-IL-17A therapy. Prior studies have demonstrated a suppressive effect of vitamin D₃ on Th17 cells. In the present study we examine whether there is a sex-bias in the Th17-axis, and its possible relationship to vitamin D metabolism in AS.

Methods: Serum IL-6 and IL-17A were measured by ELISA and 25(OH) vitamin D₃ by mass-spectrometry in a cohort of 39 male AS patients, 36 female AS patients and 34 age- and sex-matched healthy controls. Whole blood gene expression for vitamin D₃-associated and Th17-associated genes was measured by RT-PCR. Th17 cells were measured by flow cytometry of peripheral blood mononuclear cells (PBMC) in a second, overlapping cohort of 14 female AS patients, 24 male AS patients and 30 age- and sex-matched healthy controls. Data was analyzed by Mann-Whitney tests and correlations with Spearman tests.

Results: AS patients had an elevated Th17-axis when compared to healthy controls as demonstrated elevated IL-6 (p<0.01), IL-17A (p=0.06) and Th17 cell levels (p<0.05). When stratified for sex, the elevated Th17-axis was restricted to male patients, as exemplified by higher Th17 cell levels in male AS vs female AS (Figure 1). A trend was seen for lower serum 25(OH) vitamin D₃ in male AS patients and healthy controls relative to their respective female counterparts. Gene expression of VDR and CYP27B1 (vitamin D₃ activating enzyme) were equivalent in male and female AS patients, whereas CYP24A1 (vitamin D₃ degrading enzyme) expression was significantly elevated in male AS patients. This was not seen in male vs female healthy controls. In male AS patients, serum 25(OH) vitamin D₃ was inversely proportional to whole blood IL23R expression (r=-0.43, p<0.05) and Th17 cell level (r=-0.014, p=0.085).

Conclusion: This is the first demonstration that elevated levels of Th17 cells in AS are restricted to male patients, which could inform targeted therapy with anti-IL17 agents. This may be due to a sex-related alteration in the biochemistry of vitamin D₃ which functions as an important inhibitory factor to the Th17 axis. This work demonstrates a biological basis for the observed sex-bias in incidence and in disease expression in AS. 

Figure 1: Male AS patients have higher circulating Th17 levels than female AS patients and healthy controls (HC). Results displayed as scatter plot with mean and analyzed by Mann-Whitney test.