Ontogenic development of "natural" and induced plaque-forming cell isotypes in normal mice.

The numbers of cells and background plaque-forming cells (PFC) in the spleen of C3H/HeJ mice increase exponentially during the first 2 weeks after birth, but much slower in bone marrow (BM). IgG1 and IgG2a PFC are the first non-IgM PFC detectable, while IgG3 and IgA PFC appear only around weaning. Adult-type PFC numbers and isotype pattern are present in spleen and BM at 4 and 15 weeks, respectively. Neonatal splenic C3H/Tif B cells produce non-IgM Ig classes in vitro in response to polyclonal activation by lipopolysaccharide or by helper T cells. These responses are of low magnitude during the first 2 weeks of life, but both secreted and membranebound IgG1 and IgG3 isotypes are detectable already a few days after birth, in a pattern that is identical to that typical of T cell-dependent or independent responses of adult cells. These results indicate full maturity of B cells in "switch" abilities already from birth, in spite of a general deficiency in terminal maturation. In addition, they demonstrate the complexity of isotype regulation in "background" antibody production in vivo.