Congeneric CBA and CBA/N mice (with and without Lyb5+ B cells, respectively) respond to T cell-dependent antigens by the appearance of antibody-producing cells and accumulation of cells synthesizing nonspecific immunoglobulins. Type 2 cell-independent antigens induce similar changes only in CBA mice having mature B lymphocytes. The data suggest that B lymphocytes are responsible for the appearance of antibody-producing cells and polyclonal activation. Key Words: antibodies; nonspecific immunoglobulins; polyclonal activation; B cell subpopulationContact with antigens stimulates the synthesis of specific antibodies and sharply enhances production of nonspecific immunoglobulins [3]. Induction of the immune response usually requires the presence of T helper cells. Some antigens induce the immune response without T helpers, so-called T cell-independent (TI) antigens. TI can be divided into 2 types: TI-I and TI-2 antigens. TI-1 antigens are polyclonal activators stimulating production of antibodies by mature (Lyb5+) and immature (Lyb5-) B lymphocytes, while TI-2 antigens possess no mitogenic properties and stimulate the production of antibodies only by mature Lyb5+ B lymphocytes [4,6]. The mechanisms of the effect of TI-2 remain unclear. It is unknown whether nonspecific immunoglobulins are synthesized only by mature B lymphocytes or they are also produced by immature B cells. The factors contributing to polyclonal activation are still unstudied.Here we studied the role of Lyb5+ and Lyb5-B lymphocytes in polyclonal activation induced by TI-2 antigens. Laboratory of Biosynthesis of Immunoglobulins, Institute of Viral Preparations, Russian Academy of Medical Sciences, Moscow MATERIALS AND METHODSExperiments were performed on congeneric CBA and CBA/N mice (with and without Lyb5+ B cells, respectively) obtained from Stolbovaya nursery and gifted by B. V. Nikonenko (Institute of Tuberculosis).Influenza virus strain A/PR/8 and water-soluble antigen of sheep erythrocytes obtained as described elsewhere [5] were used as T cell-dependent antigens. Polyvinylpyrrolidone with a molecular weight of 350 kD and dinitrophenyl-Ficoll served as TI-2 antigens.In in vivo experiments, the mice were intravenously injected with 1-2 ~tg polyvinylpyrrolidone, 5 pg influenza virus, or 500-700 ~tg sheep erythrocytes antigen. Antibody-producing (APC) and immunoglobulinsynthesizing (IGSC) cells of splenocyte suspensions were counted on day 4 using ELISPOT [1].During in vitro induction of the immune response, splenocytes from nonimmunized animals were incubated in 96-well plates with 5 pg/ml influenza virus and 10 ng/ml polyvinylpyrrolidone or 30 ng/ml dinitrophenyl-Ficoll in RPMI-1640 medium containing 10% fetal serum and other additives for 4 days in a CO 2 incubator at 37~ The number of APC and IGSC
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