Rheumatoid arthritis (RA) is characterized by infiltration of leukocytes, including monocyte/ macrophages, into synovial tissue (ST), but factors mediating the ingress of these cells are poorly understood. Vascular cell adhesion molecule 1 (VCAM-1) plays an important role in adhesion of leukocytes to the vasculature. This study was undertaken to test the hypothesis that soluble VCAM-1 (sVCAM-1) might mediate chemotaxis of monocytes in RA.
Chemotaxis assays were performed using a modified Boyden chamber to determine the effects of sVCAM-1 on and the role of very late activation antigen 4 (VLA-4) in peripheral blood (PB) monocyte migration. Synovial fluids (SF) were immunodepleted of sVCAM-1 to identify a role for sVCAM-1 in RA. Immunohistochemistry and flow cytometry analyses were performed to show the expression of VLA-4 in ST, SF, and PB. Tyrosine phosphorylation was studied by Western blot analysis on PB monocyte lysates in the presence of signaling inhibitors.
Soluble VCAM-1 induced monocyte migration in the nM range, in a concentration-dependent manner. Anti-VLA-4 significantly inhibited sVCAM-1-induced monocyte migration, suggesting that sVCAM-1 acts in part via a VLA-4-dependent mechanism. In RA SF, incubation with anti-VCAM-1 resulted in a reduction in the ability to induce monocyte migration (mean 28%). VLA-4 immunolocalized to RA ST, SF, or PB, monocytes, macrophages, and lymphocytes. Soluble VCAM-1 stimulated tyrosine phosphorylation in monocytes, and pertussis toxin, chelerythrine chloride, and staurosporine significantly reduced sVCAM-1-mediated monocyte chemotaxis, suggesting that signaling pathways via G proteins and protein kinase C are required for sVCAM-1-mediated monocyte migration.
These results demonstrate a novel function for sVCAM-1 as a monocyte chemotactic agent in RA and suggest a new potential target for modulating monocyte ingress into inflamed RA ST.