Neutralization of immune complex inhibition of antibody dependent cellular cytotoxicity in vitro by Staphylococcus-aureus protein A.

Staphylococcus-aureus protein A (SPA) incubated with sera containing immune complexes and effector cells increases antibody dependent cellular cytotoxicity (ADCC)and rosette formation. The enhanced ADCC and rosetting is due to SPA functioning as an IgG Fc cellular receptor. SPA binds to the Fc portion of Igg and has much greater affinity for immune complexes than free IgG. SPA bound immune complexes are blocked from attaching to the Fc cellular receptors via a competitive inhibition which neutralizes the inhibitory effect of these complexes on ADCC and rosette formation.