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

Abstract

Objective To determine whether HLA-B27 expression alters the response of bone marrow monocytes (BMMo) from HLA-B27/human β2-microglobulin transgenic (B27-Tg) rats to tumor necrosis factor-α (TNFα), and whether this affects cells involved in bone homeostasis. Methods BMMo were treated with receptor activator of NF-κB ligand or TNFα to promote osteoclast formation. Osteoclasts were quantified by counting. Gene expression was measured using quantitative polymerase chain reaction, 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 TNFα enhanced osteoclast formation 2.5-fold in HLA-B27-expressing cells compared to either wild type or HLA-B7/human β2-microglobulin expressing monocytes. TNFα induced approximately 4-fold upregulation of HLA-B27, which was associated with accumulation of misfolded heavy chains, binding of the 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-1α (IL-1α) production from ER stressed B27-Tg BMMo was found to be necessary and sufficient for enhanced osteoclast formation. However, B27-Tg BMMo also produced more interferon-β (IFNβ), which attenuated the effect of IL-1α on osteoclast formation. Conclusions HLA-B27-induced ER stress alters the response of BMMo from B27-Tg rats to TNFα, which is associated with enhanced production of IL-1α and IFNβ, cytokines that exhibit opposing effects on osteoclast formation. The altered response of cells expressing HLA-B27 to pro-inflammatory cytokines suggests that this MHC class I allele may contribute to the pathogenesis of spondyloarthritis and its unique phenotype through downstream effects involving alterations in bone homeostasis.

Hla-B27

“…The latter has, however, not been addressed experimentally. Nevertheless, it is clear that the effects of HLA-B27 are protean, with abnormalities in dendritic cell populations (65) and osteoclasts also described in B27TG rats (66). Lastly, differences in the cecal microbiome of Lewis rats transgenic for HLA-B27 and human β 2 m (including increases in Prevotella spp.…”

“…HLA-B27 transgenic rats exhibit functional alterations in a number of cell populations, which might correlate with misfolding. Both defects in dendritic cell populations and function (65) and enhanced ability to form osteoclasts have been described (66).…”

Review: The Interleukin‐23/Interleukin‐17 Axis in Spondyloarthritis Pathogenesis: Th17 and Beyond

“…Briefly, HLA-B27 misfolding generates ER stress, which can activate pathways such as the unfolded protein response (UPR) (95,96) that restore ER homeostasis or trigger apoptotic cell death (reviewed in (97)). Components of the UPR are required for full expression of cytokines such as IL-6 and TNF-α (98), and when upregulated converge with innate immune signaling pathways to synergistically induce IL-23 and IFN-β (16,99–103), with potential effects on IL-1α as well (104). Concomitant UPR activation and IL-23 overexpression is apparent in myeloid cells isolated from the gut of HLA-B27-Tg rats, suggesting that HLA-B27 misfolding may drive activation of the IL-23/IL-17 axis and gut inflammation in this animal model (16).…”

Autoinflammation and HLA-B27: Beyond Antigen Presentation

“…In HLA-B27 transgenic rats, this occurs in bone marrow macrophages following cytokine stimulation, is independent of antigen presentation, and is associated with the degree of HLA-B27 upregulation 42,43 . The net effect is ER stress resulting in IL-23 43 , IFN-β 44 , and IL-1α 45 production. Attempts to reproduce these experiments in humans have yielded mixed results.…”