B2M and arthritic joint disease: HLA-B27-transgenic rat studies on β2 microglobulin (β2m),25 a noncovalent part of the MHC-I complex, has proven that additional β2m reduces HLA-B27 misfolding and promotes arthritis and spondylitis, implying that B27 misfolding is associated with intestinal inflammation.26 This result suggested that abnormal β2m can coordinate with HLA-B27 in AS development, which may be explained by protein misfolding theories and will be discussed later in the pathogenesis section.