Binding of IgA to protein-A-containing staphylococci: relationship to subclasses

Among 54 human monoclonal IgA proteins, 5 inhibited the binding of labeled human monoclonal IgM (IgM Se and IgM Ba) to protein-A-containing staphylococci, whereas 48 did not have this ability. One protein (IgA Ha) showed discordant findings in the IgM Se and IgM Ba inhibitions assays. Among the 54 IgA proteins, 49 were of the IgA1 and 5 of the IgA2 subclass. All of the 5 IgA2 proteins inhibited the binding of IgM Se and IgM Ba to staphylococci, whereas the 48 proteins that reacted negatively in the two inhibition assays belonged to the IgA1 subclass. These findings show that the ability to react with staphylococci is a property of the IgA2 subclass. IgM proteins may also be divided into two groups based on different reactivities with protein A. It is proposed to call the two IgM subclasses IgM1 and IgM2, where the latter is defined by ability to react with protein A.