Ontogenic development of “natural” and induced plaque-forming cell isotypes in normal mice

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Abstract
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). IgGl 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 membrane‐bound 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.