Incubation of antigen with normal mouse peritoneal exudate cells in vitro and subsequent reinjection of the washed cells into syngeneic mice resulted in increased antibody titers as compared to mice injected with antigen alone. Several of the variables influencing this system were studied with and without the stimulus of complex homoribopolynucleotides (poly A:U or poly I:C) as adjuvants to determine the cellular site of action of the latter. It was found that addition of poly A:U or poly I:C caused a further rise in circulating antibody levels which correlated with increased RNA synthesis, suggesting that the macrophage was one cell affected by this adjuvant. Actinomycin D was found to inhibit the rise in titer induced by PEC and this inhibition could be overcome by poly A:U. Injection of the polynucleotides 18 hr before antigen resulted in depression of circulating antibody levels, and poly A:U or poly I:C injected 18 hr before harvesting PEC and incubation with antigen also inhibited the capacity of the PEC to increase antibody levels.

A 4S RNA-rich fraction was purified after treatment with phenol of PEC exposed to antigen in vitro, and under the stimulus of poly A:U this RNA was capable of inducing specific antibody titers and rosette-forming cells on injection into mice. Antigen contamination of Pronase-treated RNA, active biologically, was below 10–11 g as determined isotopically.

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