Heterogeneity of mouse B lymphocytes: studies using in vitro response to lipopolysaccharide.

When murine lymphoid cells from spleen and lymph nodes were cultured with LPS, all the transformed blasts observed were B cells bearing surface IgM which could be classified into two types depending upon whether they contained intracellular IgM (i mu +) or not (i mu -). At the ultrastructural level, the two categories of blasts appeared to correspond to cells having a well-developed ER or primarily free ribosomes. A striking observation was that the proportion of i mu + blasts among transformed cells was 5-10 times lower in cultures of lymph node than in cultures of spleen cells. The same observations were made on LPS cultures of "purified" B lymphocytes. Prolongation of the culture period did not modify these results. This indicated that the differences in the response to LPS reflected the existence of at least two pre-existing subpopulations of B cells unequally distributed in peripheral lymphoid organs. Analysis of the surface antigens and receptors (MBLA, surface Ig classes, Fc and C receptors) of the B cells from spleen and lymph nodes did show some differences between these two lymphoid organs. Among the transformed cells, Fc receptors were present mainly on i mu - cells and C receptors on i mu + cells; however, no strict correlation could be made between the i mu + or i mu - nature of these cells and their surface characteristics. The possibility that precursors of i mu + and i mu - blasts might represent virgin and memory B cells or B cell populations differing in their requirement for helper factors released by stimulated T cells was explored by using cultures of spleen and lymph node cells (1) obtained from germ-free mice, which are expected to have few memory cells, or (2) performed in the presence of the supernatant of mixed lymphocyte cultures, in conditions where these supernatants are known to contain factors released by activated T cells. Neither condition modified the degree of transformation or Ig content of blasts.