Rat natural killer cells synthesize fibronectin. Possible involvement in the cytotoxic function

The ability of NK cells to synthesize and secrete fibronectin (FN), an extracellular matrix glycoprotein which plays a key role in many biologic processes including cellular adhesion, morphology, cytoskeletal organization, cell migration, and invasiveness, was studied. By using affinity-purified polyclonal antibodies directed against human cellular or plasma FN, the presence of FN was evidentiated on Percoll-purified rat large granular lymphocyte or on large granular lymphocyte tumor cell line (CRC) by flow cytometry and immunoelectron microscopy. Its expression increased after NK cell activation by poly I:C administration. Biochemical analysis by immunoprecipitation and SDS-PAGE indicated that FN was associated to cell surface and secreted in the supernatant in a molecular form similar to that of FN from L929 fibroblasts. In an attempt to understand the role of FN in the NK cell function, we found that an antibody against human plasma FN and its F(ab')₂ fragment inhibited NK cytotoxicity against YAC-1 target at the effector cell level. Inhibition occurred at the postbinding level, because F(ab')₂ anti-FN inhibited induction of phosphatidylinositol hydrolysis by YAC-1 target cells whereas binding to target cells was not affected. The possible role of FN in the NK cytotoxic function is suggested.