To investigate the functional implications of CD44 splice variant expression in fibroblast-like synoviocytes (FLS) obtained from patients with rheumatoid arthritis (RA).
FLS were isolated from synovial tissue obtained from both diseased and nondiseased joints. The expression of splice variants containing exons v3 and v6 was analyzed using immunocytochemistry with exon-specific antibodies and reverse transcription-polymerase chain reaction followed by Southern blotting. The invasive capacity of the cells was studied in a transwell invasion assay.
FLS obtained from RA joints expressed various CD44 splicing combinations containing the variant exons v3 and/or v6. These cells were highly invasive, whereas cells from normal tissues, which lacked expression of CD44 splice variants, were not. Variant exons CD44v3 and CD44v6 were instrumental in matrix invasion in vitro, with cells enriched for CD44v3 and v6 exhibiting greater invasion and antibodies that specifically recognize CD44v3 and v6 abrogating this capacity to invade. Invasive cells showed a reduced expression of CD44v7/8, and antibodies against this epitope had no significant effect on cellular infiltration of the matrix. The antibodies had no effect on cell migration into the porous section of the transwell.
FLS obtained from patients with RA express CD44 splice variants and are highly invasive, whereas cells obtained from healthy tissue do not express these variants and are not invasive. Expression of the epitopes CD44v3 and CD44v6 is instrumental in the invasive capacity but not in cell migration. This finding highlights a functional implication for the expression of CD44 splice variants at the level of matrix degradation.