To determine how in vivo modulation of CD44 function by antibodies influences disease severity in a murine model of rheumatoid arthritis.

Mice with proteoglycan (PG)–induced arthritis were subjected to systemic treatment with 3 different monoclonal antibodies against CD44. Joint swelling and serum levels of hyaluronan (HA) and soluble CD44 (sCD44) were monitored. Inflammatory leukocyte infiltration in the joints, cell surface CD44 expression, and leukocyte adhesion to HA were compared. The effects of anti‐CD44 treatment on the immune status of arthritic animals were also determined.

Antibody IRAWB14, which enhances HA binding, aggravated the inflammatory symptoms, while KM201, which blocks ligand binding, reduced the severity of arthritis. The most effective suppression of inflammation was noted upon treatment with antibody IM7, whose epitope lies outside the HA binding domain of CD44. Serum levels of sCD44 increased, and HA levels decreased, in response to IM7. KM201 and IM7 treatment reduced, but IRAWB14 treatment enhanced, the adhesion of leukocytes to HA. However, these antibodies had little effect on PG‐specific immune responses.

Each antibody acted in vivo by virtue of its combined effects on CD44–HA binding and CD44 shedding. The dramatic reduction in arthritis severity effected by IM7 treatment was associated with extensive shedding of cell surface CD44 molecules. Loss of CD44 appears to be a major factor in preventing CD44‐ and HA‐dependent cell–matrix interactions at the inflammatory site. Our study indicates a critical role for CD44 in the pathology of joint inflammation and reveals a unique mechanism of receptor down‐regulation, which can be used therapeutically.