Demonstration on protein A of two distinct immunoglobulin-binding sites and their role in the mitogenic activity of Staphylococcus aureus Cowan I on human B cells.

The immunoglobulin-binding capacity of Staphylococcal protein A (SpA) and its role in the triggering of B cell proliferation induced by SpA-containing Staphylococcus aureus Cowan first strain (Cowan Staph) were investigated. Monoclonal lymphocytes from 34% of patients with surface IgM (slgM)-bearing B cell chronic lymphocytic leukemia (CLL) were found capable of binding to SpA. A similar proportion of monoclonal serum IgM proteins also showed the ability to react with SpA, whereas the remaining proportion of either slgM-bearing B cell CLL or monoclonal serum IgM proteins did not reveal any SpA-reactivity. Monomeric subunits as well as F(ab’)2µ fragments obtained from SpA-reactive monoclonal IgM proteins retained SpA-binding capacity, even though it was lower than that of native 19S IgM molecules. The SpA-reacting structure present on IgM molecules was snared by human polyclonal IgG and human F(ab’)2γ and F(ab’)2µ fragments but not by human Fcγ fragments or rabbit polyclonal IgG. The inactivation of tyrosil residues of SpA, which are essential for its Fcγ-binding ability, virtually abolished the capacity of SpA and Cowan Staph to react with rabbit IgG and human Fcγ fragments, but it did not alter the reactivity of either SpA or Cowan Staph with IgM molecules. The mitogenic activity of Cowan Staph on normal human B cells was inhibited by addition to the cell cultures of human polyclonal IgG or SpA-reactive monoclonal IgM proteins, but it was poorly influenced or not by influenced at all by addition of rabbit polyclonal IgG. Moreover, the inactivation of SpA-Fcγ-binding regions on Cowan Staph failed to affect the ability of these bacteria to induce proliferation of normal human B cells. In contrast, the Cowan Staph-induced proliferative response of surface IgG- (slgG) bearing B lymphocytes from a patient with CLL was abrogated either by addition to the cell cultures of very small amounts of rabbit polyclonal IgG or inactivation of the Fcγ-binding regions of SpA present on the bacterial cell wall. These data indicate the existence on SpA molecules of an IgM F(ab’)2-reactive site, distinct from the Fcγ-binding region. They also suggest that, even though the classical SpA-Fcγ interaction can represent a triggering signal for the Cowan Staph-induced proliferation of slgG-bearing B lymphocytes, the binding of SpA present on the bacterial cell wall to slgM is primarily responsible for the mitogenic activity of Cowan Staph on normal human B cells.