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Movement of IgG receptors on the lymphocyte surface induced by protein A of Staphylococcus aureus.

Fluorescent protein A of Staphylococcus aureus (F1-SpA) acts on the IgG receptors of guinea-pig lymphocytes (GPL) and human lymphoid cells (Seraphina) to induce a redistribution pattern (multiple spots, patches, expelled material, caps) similar to the membrane staining of some living cells with anti-IgG. No ring staining was observed, which implies that F1-SpA acts as a multivalent cross-linking agent on IgG receptors. A prozone effect dependent on F1-SpA concentration was observed. 
 
F1-SpA staining was not abolished 2 hours after trypsinization of the cells, nor was it completely inhibited by pretreatment of the cells with non-fluorescent SpA. A low percentage of cap-like stained cells was recorded even at 4° or in the presence of sodium azide. 
 
Seventeen per cent of GPL are specifically stained with F1-SpA whereas with fluorescent anti-guinea-pig gamma-globulin serum 45 per cent of the cells are fluorescent. After immunization of the animals with sheep red blood cells (SRBC) 31 per cent of the GPL became positive with F1-SpA. 
 
Forty per cent of Seraphina cells were stained, whether fluorescent anti-human gamma chain serum or F1-SpA were used. It was also shown that the Fab fragment of an anti-human gamma chain preparation partially inhibited the specific staining of the Seraphina cells with F1-SpA. This suggests that both reagents have a common site of action (IgG receptors). 
 
F1-SpA did not stain human lymphoid cells which bear IgM receptors (Daudi cells); SpA reacts only with the Fc region of IgG. However, by reacting Daudi cells with an anti-IgM serum (containing IgG antibodies) and afterwards with F1-SpA, specific staining was achieved. F1-SpA is therefore recommended as a fluorescent reagent for indirect immunofluorescent staining in which IgG antibodies are used.