In vitro expression of immunoglobulin M and G subclasses by murine B lymphocytes in response to a polyclonal activator from Actinomyces.

A cell wall extract from the gram-positive bacterium Actinomyces viscosus contains the mitogen AVIS, a potent polyclonal B-cell activator for murine B lymphocytes. Cultures of splenocytes from heterozygous nude mice in the presence of an optimal concentration of AVIS responded by a deoxyribonucleic acid synthesis response, and proliferaction reached maximal levels after 3 to 4 days. There was no requirement for T cells in the deoxyribonucleic acid synthesis, proliferactive, immunoglobulin M (IgM), or IgG responses. Significant numbers of IgM-producing cells were present as early as day 2 of culture, whereas later in the culture periods (days 3 to 6) IgG-producing plasmablasts and plasma cell were observed. In cultures of splenocytes from nude mice stimulated with AVIS for 4 to 5 days, 20 to 25% of the recoverable cells synthesized IgM, and 10% contained only IgG2 or IgG3; 5 to 8% of the cells stained for both IgM and IgG2 or both IgM and IgG3. Fine-structure analysis of AVIS-stimulated splenocytes from heterozygous nude mice after 3 days of culture demonstrated that 20 to 25% of the cells were activated to various degrees. Of most importance, all of the activated cells had the characteristic of B lymphoblasts, plasmablasts, or plasma cells. This is the first demonstration of a polyclonal B-cell activator other than lipopolysaccharide which induces IgG3 synthesis. We suggest that AVIS may be a useful probe for the exploration of the functional activities of subpopulations of B cells.