An extract from adult Ascaris suum was fractionated on a Sephadex G-200 column and individual elution fractions assessed for their ability to induce or suppress an IgE antibody response in mice immunized simultaneously with a heterologous antigen (ovalbumin). The results showed that, whilst fractions eluted in the first peak suppressed the anti-ovalbumin IgE antibody production, those eluted in the third peak induced a significant anti-Ascaris IgE response. The mixture of the two kinds of fractions resulted in suppression of anti-Ascaris IgE antibodies. The apparent molecular weight (MW) of the first and third peaks was 530,000 and 29,000 daltons, respectively. After delipidation, the extract still retained most of its biological activities. SDS-PAGE of this material showed selective loss of high-MW components when compared to the whole extract. No bands were observed in SDS-PAGE of peak I in contrast to peak III, which displayed several bands. Immunoblots of all these samples showed at least 3 bands above MW 150,000, which reacted with anti-peak I anti-serum, in the unfractionated extracts and in peak I, but not in peak III. When the extract was fractionated by affinity chromatography using anti-peak I antibodies as ligands, the immunogenic components were present in the effluent, whereas the suppressive components were recovered in the eluate.

A comparative study of the effects of antigenic competition on the IgE, IgG1 and IgG2 antibody production was performed in mice immunized with two unrelated antigens. These antigens were injected as a mixture or in different sites. In addition, the groups of mice received one or three boosters of one or both antigens. The passive cutaneous anaphylaxis titres obtained in several bleedings showed a suppression of IgG1 and IgG2 antibody responses only in the groups injected with the two antigens in the same site, independently of antigenic boosters. The IgE antibody response was diminished when the antigens were given in a mixture, but this effect disappeared after a second booster. In contrast, groups injected with antigens separately started to show a suppressed response only at this time.

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An adoptive cell transfer system was utilized to evaluate the site of action of the suppressive mechanism involved in antigenic competition in IgE antibody production. Carrier-primed (OA) and hapten-primed (DNP-KLH) spleen cells were transferred to syngeneic irradiated recipients that were challenged with a heterologous conjugate (DNP-OA). To study the effect of antigenic competition on T and B cells, donor mice of one or the other cell type received in addition the competitor antigen (Asc) at immunization. The adoptive secondary IgE anti-DNP antibody response was suppressed in both situations. This effect could not be attributed to transfer of Asc-primed cells. Irradiation of donor mice before immunization with the two antigens abrogated the suppressive effect. These results indicate that both Th and B cells primed to the test antigen were affected by antigenic competition.

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