BINDING OF MURINE IgM AND MONOCLONAL IgM/A (κ) HYBRID ANTI‐TYPE III PNEUMOCOCCAL POLYSACCHARIDE (SIII) ANTIBODIES TO STAPHYLOCOCCAL PROTEIN A

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Abstract
Evidence is produced to show that two specific subclasses of IgM antibodies are formed during the primary immune response to Type III pneumococcal polysaccharide (SIII) in mice. The IgM proteins can be divided into two groups based on different reactivities with protein-A. As with human IgM subclasses, it is proposed to call the two IgM subclasses IgM1 and IgM2, where the latter is defined by the ability to react with protein-A of Staphylococcus aursus. Only the IgM molecule which reacted with protein-A produced passive haemolysis in the presence of guinea-pig complement. Results also show that the IgA anti-SIII activity in serum of SIII-immune mice on day 5 of the primary response is due lo hybrid IgM/A() antibody which is undetected by conventional methods for enumerating antibody-forming cells because cells producing IgM/A antibody develop direct plaques with guinea-pig complement. A monoclonal IgM/A() anti-SIII produced from spleen cells 5 days after injection of 10 g SIII plus pertussis vaccine exhibited similar properties to molecules with and -determinants in serum. The IgM/A hybrid antibody reacted strongly with protein-A and produced passive haemolysis of SIII-coated erythrocytes in the pretence of guinea-pig, but not mouse, complement Despite its specificity for the capsular antigen of Type III pneumococci, the IgM/A hybrid antibody conferred only temporary immunity in mice challenged with viable pneumococci. The nature and properties of IgM/A antibodies as well as those of the subclasses of IgM may give important clues lo the genetic regulation and expression of antibody production. These findings may provide an explanation for some of the anomalies in various areas of immunological research.