The Immunoglobulin M Response to Pneumococcal Polysaccharide Vaccine Is Sufficient for Conferring Immunity

In mice, pneumococcal polysaccharide (PPS) vaccines generate antigen-specific immunoglobulin M (IgM) and immunoglobulins G1, G2, and G3. Antibody and complement-dependent opsonophagocytosis correlates with the protection induced by PPS vaccines in vivo. Since IgM is a very efficient immunoglobulin isotype in activating the complement system, we evaluated whether anti-PPS IgM alone is sufficient to confer protective immunity to Streptococcus pneumoniae. We found that immunization of wild-type and activation-induced cytidine deaminase–deficient mice capable of producing only IgM with Pneumovax 23 generated comparable anti-PPS IgM and resistance to lethal systemic challenge with S pneumoniae. These data suggest that an IgM response to PPS vaccines is sufficient for conferring immunity.