Role of Gut Inflammation in Altering the Monocyte Compartment and Its Osteoclastogenic Potential in HLA-B27-Transgenic Rats

Objective. To investigate the relationship between intestinal inflammation and the central and peripheral innate immune system in the pathogenesis of HLA-B27-associated spondyloarthritis using an HLA-B27-transgenic (B27-Tg) rat model. Methods. The myeloid compartment of the blood and bone marrow (BM) of B27-Tg rats, as well as HLA-B7-Tg and non-Tg rats as controls, was evaluated by flow cytometry. Plasma from rats was assessed by enzyme-linked immunosorbent assay for levels of CCL2 and interleukin-1 alpha (IL-1 alpha). Rats were treated with antibiotics for 4 weeks, and the myeloid compartment of the blood and BM was evaluated by flow cytometry. The osteoclastogenic potential of BM-derived cells from antibiotic-treated rats, in the presence or absence of tumor necrosis factor (TNF), was evaluated in vitro. Results. B27-Tg rats had substantially higher numbers of circulating Lin-CD172a+CD43(low) monocytes as compared to control animals, and this was significantly correlated with higher levels of plasma CCL2. Antibiotic treatment of B27-Tg rats markedly reduced the severity of ileitis, plasma levels of CCL2 and IL-1 alpha, and number of BM and blood Lin-CD172a+CD43(low) monocytes, a cell subset shown in the present study to have the greatest in vitro osteoclastogenic potential. Antibiotic treatment also prevented the TNF-dependent enhancement of osteoclastogenesis in B27-Tg rats. Conclusion. Microbiota-dependent intestinal inflammation in B27-Tg rats directly drives the systemic inflammatory and bone-erosive potential of the monocyte compartment.