Additional human β2‐microglobulin curbs HLA–B27 misfolding and promotes arthritis and spondylitis without colitis in male HLA–B27–transgenic rats

Objective Ankylosing spondylitis and related spondylarthritides are associated with HLA–B27, and also with intestinal inflammation, by unknown mechanisms. The folded HLA–B27 molecule is a trimer of heavy chain, β2‐microglobulin (β2m), and short peptide. However, B27 heavy chain has an unusual propensity to misfold and trigger the unfolded protein response (UPR). This study was initiated to test the hypothesis that B27 misfolding plays a role in the pathogenesis of spondylarthritis. Methods Rats with high transgene copy numbers of HLA–B27 heavy chain together with human β2m (Huβ2m) spontaneously develop colitis, peripheral arthritis, and occasional spondylitis, and those with lower transgene copy numbers remain healthy. We crossed disease‐prone and healthy HLA–B27/Huβ2m–transgenic rat lines with a healthy line, 283‐2, carrying only the Huβ2m transgene. HLA–B27 assembly was assessed by pulse‐chase analysis of B27 molecules, and UPR triggering was assessed by measuring BiP/Grp78 messenger RNA (mRNA) in splenic concanavalin A blasts. Surface expression of B27 and Huβ2m was determined by flow cytometry. Disease manifestations were identified by clinical observation, histology, and measurement of cytokine mRNA. Results The extra Huβ2m from the 283‐2 line significantly reduced B27 misfolding and UPR triggering. Unexpectedly, however, F1 male offspring of the healthy 21‐3 line crossed with the 283‐2 line showed a high prevalence, severity, and duration of arthritis and spondylitis, in the absence of colitis. The arthropathy showed many features characteristic of human spondylarthritis. Conclusion These results suggest that B27 misfolding is associated with intestinal inflammation, but that neither B27 misfolding nor intestinal inflammation is critical to the development of B27‐associated arthropathy.