Large soluble CD18 complexes with exclusive ICAM-1-binding properties are shed during immune cell migration in inflammation.

The family of heterodimeric CD11/CD18 integrins facilitate leukocyte adhesion and migration in a wide range of normal physiologic responses, as well as in the pathology of inflammatory diseases. Soluble CD18 (sCD18) is found mainly in complexes with hydrodynamic radii of 5 and 7.2 nm, suggesting a compositional difference. Earlier work reported that the complexes include at least part of the CD11a or CD11b chains containing the intercellular adhesion molecule (ICAM)-1 binding domain, and that sCD18 is capable of quantitatively competing with the cell membrane-bound form for ICAM-1 binding. However, it is not clear if the size differences between the sCD18 complexes reflect any functional variance regarding shedding from the cell membrane or binding to ICAM-1. Here, we show evidence that sCD18 found in serum regulates release of the proinflammatory cytokine monocyte chemoattractant protein-1 (MCP-1/CCL2) from fibroblast-like synovial cells. Further, only large sCD18 complexes are capable of binding to ICAM-1. Migrating neutrophils shed large, but not small, sCD18 complexes. Together, these observations explain results measured from patients with rheumatoid arthritis (RA), where large sCD18 complexes dominated in local inflammatory processes involving neutrophil influx into zones of inflammation. Our data points to a previously unappreciated aspect of sCD18 integrin biology as regulators of inflammation in the context of migrating leukocyte. Surprisingly, this regulation is tied to sCD18 complex size, opening new opportunities for therapeutic intervention in serious inflammatory diseases such as arthritis.