BALB/c mice immunized with bacterial levan (BL) produce a vigorous antibody response that fails to include antibodies expressing the idiotype of the beta 2 leads to 6 fructosan-binding myeloma protein ABPC48 (A48). Treatment of newborn BALB/c mice at 1 d of age with 0.1-10 microgram of either the A48 myeloma protein or monoclonal proteins that share idiotopes with the A48 family, followed by immunization with BL 2-4 wk later, produces an anti-BL response that is dominated by the A48Id. Various degrees of activation of the A48Id BL response were observed by injecting mice with A48 monoclonal protein only up until 3 wk of age. Activation of the A48Id clones by treating with A48 monoclonal protein was ineffective in mice who were older than 4 wk. Elicitation of an A48Id BL response required specific antigenic stimulation with either beta 2 leads to 6 or beta 2 leads to 1 fructosan epitopes, because it does not occur after injection with TNP-Ficoll in spite of the A48 treatment. The expansion of A48Id clones in mice treated at birth with A48 monoclonal protein is associated with an increase in A48Id-specific helper T cells. The binding specificity of these cells was demonstrated by infusing them into nu/nu BALB/c mice and observing that they rendered help that enalbed the animal to mount an anti-TNP response after immunization only with A48-TNP, but not with MOPC384-TNP conjugates. The helper activity of these cells is sensitive to the effects of treatment with anti-Lyt-1.2 antibodies plus complement. A predominantly A48Id BL-specific response can be transferred into lethally irradiated mice by infusing them with purified T and B cells from A48-treated mice. The transfer of this response can be ablated by treating the T cells with anti-Lyt-1.2 antibodies plus complement. These results indicate that A48Id-specific helper cells possess the ability to select the A48Id-bearing B cell precursors for expression, thus exerting a fine-tuning effect on the idiotypic expression of the anti-BL repertoire. We propose that this idiotype-induced idiotype response, which can be, in principal, induced by idiotypes provided by the mother, plays an important role in the expansion of precursors of antibody-forming cells during embryonic as well as postnatal life.

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