Young mice of dextran high responder strains were found to be complete nonresponders to the alpha-1-6 epitope of dextran during 30-40 days after birth. They also failed to respond to thymus-dependent dextran-protein conjugates. Cells from young and adult mice were activated equally well to polyclonal antibody synthesis by the polyclonal B-cell-activating property of dextran. There was no age difference in the immune response to haptens conjugated to dextran, indicating that dextran can function as an efficient carrier also in young mice. Unresponsiveness could not be attributed to suppressor T cells or to a suppressive environment in young animals, as shown by transfer experiments, in which living or irradiated cells from young and adult mice were admixed in various ways and transferred to irradiated recipients of different ages. Cells from young mice did not affect response of adult cells (and the reverse), nor did the age of the irradiated recipient influence the response. When lymphocytes from young and adult mice were polyclonally activated in vitro by lipopolysaccharide, only cells from young mice failed to synthesize antibodies against the alpha-1-6 epitope of dextran, although they produced antibodies of all other specificities tested for. It was concluded that young animals fail to express immunoglobulins directed against the alpha-1-6 epitope during the first 30-40 days after birth. Since the mice possess the VH gene coding for antibodies against this particular epitope, it was concluded that the timing of V gene expression is regulated during development, possibly at the V-C gene translocation level.

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