Correlation between Specific Immunosuppression and Polyclonal B Cell Activation Induced by a Protein Secreted by Streptococcus mutans

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Abstract
The relationship between polyclonal B cell activation and immunosuppressor effects induced by F5''EP-Sm, a non-cytotoxic protein secreted by Streptococcus mutans, was studied in C57BL/6 mice. Mice treated with F5''EP-Sm exhibited a considerable increase in splenic non-specific Ig plaque-forming cells (PFC) compared with untreated mice. The isotypic pattern of non-specific PFC responses favours IgG2a.apprxeq.IgG2b>IgG3>IgG1.apprxeq.IgM, when taken as a ratio between treated and untreated animals. When F5''EP-Sm was administered 2 days before immunization with sheep red blood cells (SRBC), the non-specific PFC production was accompanied by an ephemeral increase in specific PFC against SRBC1 day after immunization, which was quickly replaced by a strong immunosuppression. In contrast, when F5''EP-Sm was injected after priming, there was little or no demonstrable suppression of specific PFC, and the increase of non-specific PFC was much less evident. The kinetic curves representing increase or decrease in relation to controls of specific and non-specific PFC are almost mirror images in each of the isotypes. The in vivo suppressor effect was abrogated in thymectomized mice, although the involvement of the T cell compartment is probably secondary to the B cell mitogen effect, since T-depleted spleen cells proliferate and synthesize non-specific Ig when stimulated in vitro with F5''EP-Sm.