Soluble 4A11 antigen: a novel mediator of leukocyte-endothelial adhesion and monocyte recruitment in rheumatoid arthritis

The rheumatoid arthritis (RA) synovial pannus is characterized by a number of processes, including monocyte recruitment, leukocyte–endothelial adhesion, and angiogenesis. We have generated a novel monoclonal antibody (mAb) (mAb 4A11), which detects the blood group antigens Lewis y and H-5-2 (collectively termed the 4A11 antigen). We have shown that this antigen, in soluble form, as well as a glucose analog of H-5-2 (H-2g), are potent mediators of angiogenesis. The 4A11 antigen is selectively expressed in skin, lymphoid organs, the thymus and synovium, and is upregulated on RA synovium compared with normal synovium, suggesting a role in leukocyte recruitment and homing. We have also shown that H-2g significantly upregulates cell surface expression of the adhesion molecule intercellular adhesion molecule-1 (ICAM-1) on human dermal microvascular endothelial cells (HMVECs), but not vascular cellular adhesion molecule-1 or E-selectin. In accord with this, H-2g induces ICAM-1-mediated leukocyte–endothelial adhesion. Moreover, H-2g or the 4A11 antigen activate the endothelial janus kinase 2 (JAK2)/STAT3 pathway, as demonstrated by inhibition by chemical inhibitors of these pathways as well as antisense oligodeoxynucleotides directed against these signaling intermediates. H-2g may also signal via the Erk1/2 and PI3kinase-Akt pathways. Moreover, H-2g induced endothelial NF-κB activation, and blockade of PI3kinase or JAK2 inhibited H-2g-induced endothelial NF-κB activation. These results suggest that the H-2g and the 4A11 antigen are mediators of leukocyte–endothelial adhesion via ICAM-1.

We next sought to determine whether H-2g could induce monocyte recruitment. We performed monocyte chemotaxis in vitro in modified Boyden chambers. H-2g induced monocyte chemotaxis in the picomolar or greater range. In vivo, injection of H-2g intraperitoneally into the periotoneum of mice resulted in monocyte recruitment. The migration of monocytes in vitro, as with induction of HMVEC ICAM-1 expression, appeared to occur via the mitogen-activated kinase and PI3kinase pathways, but also via the Src pathway. These results suggest that H-2g and the 4A11 antigen are potent mediators of leukocyte–endothelial adhesion and monocyte recruitment, and that efforts aimed at targeting this antigen or its signaling pathways may be useful in the treatment of RA.