Circulating staphylococcal antigen in humans and immune rabbits with endocarditis due to Staphylococcus aureus: inhibition of detection by preexisting antibodies.

A radioimmunoassay for staphylococcal antigen that had detected antigenemia in each of 12 nonimmune rabbits with staphylococcal endocarditis detected antigen in sera from one of nine humans and two of eight immune rabbits with Staphylococcus aureus endocarditis. Staphylococcal antigens could be detected at concentrations as low as 0.78 microgram/ml when diluted in normal rabbit serum, compared with 6.25 microgram/ml when diluted in normal human serum and greater than 25 microgram/ml when diluted in immune rabbit or human serum. Low titers of staphylococcal antibody were found in normal rabbit serum compared with immune rabbit and normal or immune human sera. Detection of staphylococcal antigen was inhibited when the antigen was diluted in the IgG and IgM fractions, but not in the albumin fraction, of normal human serum. This study demonstrated that antigenemia can be detected infrequently in patients and immune rabbits with staphylococcal endocarditis; staphylococcal antibodies inhibit detection of antigen, presumably through formation of immune complexes.