HLA-B27 Alters the Response to Tumor Necrosis Factor and Promotes Osteoclastogenesis in Bone Marrow Monocytes From HLA-B27-Transgenic Rats

Objective. To determine whether HLA-B27 expression alters the response of bone marrow monocytes from HLA-B27/human beta(2)-microglobulin-transgenic (B27-Tg) rats to tumor necrosis factor alpha (TNF alpha) and, if so, whether this affects the cells involved in bone homeostasis. Methods. Bone marrow monocytes were treated with RANKL or with TNF alpha to promote osteoclast formation. Osteoclasts were quantified by counting. Gene expression was measured using quantitative polymerase chain reaction analysis, and protein was detected by enzyme-linked immunosorbent assay, immunoblotting, or immunofluorescence. Effects of endogenously produced cytokines on osteoclast formation were determined with neutralizing antibodies. Results. TNF alpha treatment enhanced osteoclast formation 2.5-fold in HLA-B27-expressing cells as compared to wild-type or to HLA-B7/human beta(2)-microglobulin-expressing monocytes. TNF alpha induced similar to 4-fold up-regulation of HLA-B27, which was associated with the accumulation of misfolded heavy chains, binding of the endoplasmic reticulum (ER) chaperone BiP, and activation of an ER stress response, which was not seen with HLA-B7. No differences were seen with RANKL-induced osteoclastogenesis. Enhanced interleukin-1 alpha (IL-1 alpha) production from ER-stressed bone marrow monocytes from B27-Tg rats was found to be necessary and sufficient for enhanced osteoclast formation. However, bone marrow monocytes from B27-Tg rats also produced more interferon-beta (IFN beta), which attenuated the effect of IL-1 alpha on osteoclast formation. Conclusion. HLA-B27-induced ER stress alters the response of bone marrow monocytes from B27-Tg rats to TNF alpha, which is associated with enhanced production of IL-1 alpha and IFN beta, cytokines that exhibit opposing effects on osteoclast formation. The altered response of cells expressing HLA-B27 to proinflammatory cytokines suggests that this class I major histocompatibility complex allele may contribute to the pathogenesis of spondyloarthritis and its unique phenotype through downstream effects involving alterations in bone homeostasis.