Mitogenic activity of two Brucella fractions for murine B lymphocytes

Two lipid A-free fractions extracted from Brucella melitensis (fractions PI and SF) were shown to behave as B-cell non-specific mitogens for murine lymphocytes: they stimulated the uptake of tritiated thymidine by normal unsensitized murine splenic lymphocytes, by spleen cells from nude mice and by T-cell depleted but not by T-cell enriched and B-cell-deprived splenic populations. Since depletion of adherent cells leads to a two- to three-fold depression of PI- or SF-induced mitogenic responses these two fractions were shown to require accessory adherent cell participation for an optimal mitogenicity. Moreover they behaved as polyclonal activators for murine spleen cells. These results are discussed in terms of a possible classification of PI and SF amongst other B-cell mitogens and of the respective role of the peptidoglycan and lipoprotein moieties in B-cell activation by Brucella fractions.