The effects of injection of unrelated antigens on the sedimentation velocity and buoyant density characteristics of specific “virgin” or “memory” adoptive transfer AFC-progenitors in unprimed or long-term hapten-primed adult mouse spleen were examined to determine if the physical properties of these B cells could be altered by nonspecific stimulation. Spleen cells from normal unprimed adult CBA mice (as a source of virgin B cells) or from long-term hapten-primed CBA (as a source of memory B cells) were separated and the fractions were tested for NIP-specific AFC-progenitor activity with an adoptive immune assay system that gave a direct linear measure of B cell activity. If the donor mice were injected 1 day previously with POL or HRC, the NIP-specific virgin IgM AFC-progenitor activity shifted from medium sized, medium-light density lymphocytes to larger, much lighter cells. The same 1-day nonspecific stimulation had little effect on the physical properties of either IgM or IgG memory AFC-progenitors, although a small increase in the size of such cells was observed 2 days post immunization with unrelated antigens. Nonspecific stimulation caused identical physical changes in the virgin AFC-progenitors of athymic nude mice, but only partially altered the physical properties of virgin AFC-progenitors in neonatal mice. In the latter case, a normal transformation occurred if adult, radioresistant macrophages were injected with the stimulating antigens.

These results indicate that a particular subset of virgin B cells responds nonspecifically to antigenic injections, becoming larger cells of lighter density. The mechanism of this effect appears to involve mature macrophages, but not functional T lymphocytes. Memory B cells do not readily respond in this way.

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