Expression of CD44 and L-selectin in the innate immune system is required for severe joint inflammation in the proteoglycan-induced murine model of rheumatoid arthritis.

Proteoglycan (PG)-induced arthritis, a murine model of rheumatoid arthritis, is characterized by autoimmunity against mouse cartilage PG and chronic joint inflammation. L-selectin (CD62L) and CD44 are major adhesion molecules on leukocytes that regulate their homing to lymph nodes and entry into inflamed tissues. In the present study, we studied the requirement for CD44 and CD62L expression for mediating lymphocyte homing, thus permitting the development of autoimmunity vs mediating the entry of leukocytes into the joints, thus allowing inflammation in PG-induced arthritis. We immunized wild-type, CD44 knockout (KO), CD62L KO, and double (CD44/CD62L) KO BALB/c mice with PG and monitored the effects of gene deficiencies on PG-specific immunity, arthritis severity, leukocyte trafficking, and the ability of lymphocytes to adoptively transfer disease to syngeneic SCID mice. Single and double KO mice demonstrated reduced PG-specific spleen cell proliferation, but the production of Th cytokines and autoantibodies was comparable in KO and wild-type mice. KO leukocytes had reduced ability to adhere tightly to the synovial endothelium in arthritic joints. This diminished leukocyte adhesion correlated with the magnitude of granulocyte (neutrophil) influx and the severity of inflammation, which were both reduced in the joints of KO mice. However, transfer of spleen cells from mildly arthritic KO donors to SCID hosts resulted in development of severe arthritis. Our results indicate that CD44 and CD62L expression in the cells of the innate immune system (granulocytes) is important for their efficient influx into the joints and also suggest that granulocytes play a crucial role in arthritis progression.