Channel Catfish NK-like Cells Are Armed with IgM Via a Putative FcmicroR

Two-color flow cytometry demonstrated that 4-8% of channel catfish PBL are positive for both F and G IgL chain isotypes, suggesting that they passively acquire serum IgM via a putative FcmicroR. These cells show spontaneous killing toward allogeneic targets, and in vitro stimulation of PBL with allogeneic cells results in an increase of double IgL chain positive cells with a concomitant increase in nonspecific cytotoxicity. Long-term cultures of alloantigen-stimulated PBL contain both sIgM(+) and sIgM(-) cytotoxic cells that transcribe message for the catfish homolog of the FcepsilonR gamma chain, but not for Igmicro and TCR-alpha,-beta, or -gamma chains. Immunoprecipitation of lysates from sIgM(+) NK-like cells with anti-IgM co-immunoprecipitated a putative FcmicroR of approximately 64 kDa. Finally, removal of IgM from sIgM(+) NK-like cells and replacement with anti-hapten antibody enabled antibody-armed effectors to kill haptenated targets that were refractory to killing by effectors armed with normal IgM. This is the first report suggesting that teleost NK-like cells express a putative FcmicroR which participates in antibody-dependent cell-mediated cytotoxicity.