Priming of CBA/J mice with minute doses of protein antigens (Ag) leads to high IgE antibody (Ab) titres in the immune sera of these animals. In contrast priming with large doses elicits only a marginal production of IgE Ab. In vitro restimulation of spleen cells from animals primed with large doses and lacking in vivo IgE Ab leads to a burst of IgE Ab-forming cells. This in vitro anamnestic response is lacking in mice primed with minute doses of Ag. In order to trace the cellular basis of the in vitro IgE memory response we have extended the analysis of the distribution of Ab isotypes to Ag-primed IgG1-deficient delta 5'S gamma 1 mice. The data presented here must be interpreted as followed. Priming of mice with minute doses of Ag leads to a direct switch from IgM to IgE Ab expression in both strains. These animals have high IgE Ab titres without establishing an IgE memory. The direct switch was verified by polymerase chain reaction and Southern blot analysis of switch circle DNA isolated from Ag-specific B cells of CBA/J mice primed with minute doses of Ag. In contrast to immunization with minute doses, priming with large doses of Ag fails to induce in vivo IgE Ab production in CBA/J and delta 5'S gamma 1 mice but establishes a B epsilon memory in CBA/J mice which involves IgG1-bearing intermediate B cells. In vivo these B epsilon memory cells do not enter the status of IgE Ab-producing cells. In vitro they can be released from this anergy and presumed suppression and develop in an anamnestic response into a large population of IgE Ab-forming B cells. This increase in the number of IgE Ab-producing cells after restimulation in vitro is lacking in delta 5'S gamma 1 mice, apparently because of their inability to generate IgG1-expressing precursor cells. The notion of a sequential switch and an IgG1 intermediate B epsilon memory status is also supported by depletion and inhibition experiments. Elimination of IgG1-expressing B cells in CBA/J mice primed with high doses of Ag prevents the IgE Ab burst after in vitro challenge with Ag. The data further suggest that the two switch pathways are not mutually exclusive and that the Ag dose can decide which pathway is preferentially used.

Analysis of the humoral immune response of BALB/c mice to alpha(1-->3) dextran (Dex) reveals novel aspects of T cell-mediated control of 'type 2 thymus-independent' responses against polysaccharide antigens. The IgM and IgG antibody response, dominated by the J558 idiotype (Id), is controlled by Id-specific T cells. These regulatory T cells, for which the T cell clone 178-4 Ts with characterized TCR alpha and beta chain sequences is the prototype, expand in all BALB/c mice upon immunization with Dex. They suppress in a cognate interaction the expansion of J558 Id-bearing B cells, committed for production of IgG antibodies. Furthermore they provide a gate which precludes variability in the VH CDR3 region of IgG antibodies appearing occasionally in the periphery. The VH CDR3 region is the recognition element of 178-4 Ts analogous T cells but contributes little to affinity for the antigen. For recognition by 178-4 Ts cells not even minimal sequence deviations of the J558 Id peptide are allowed. The tight germline programmed complementarity between J558 Id-bearing Dex-specific B and J558 Id-specific 178-4 Ts analogous T cells leaves little room on both sides for ontogenetic variability.

The humoral response against the bacterial polysaccharide antigen a(133) dextran (Dex) is controlled by J558 idiotype-(Id) specific T cells. These T cells of which the cell clone 178±4 Ts is a representative by all relevant criteria, recognize J558 Id-bearing B cells in an I-E d -restricted manner. Costimulation via CD28/B7-1 but not via CD40/CD40L leads to T-cell activation. These T cells do not only suppress B cells producing the immunoglobulin (Ig)G3 isotype but also support the survival and clonal expansion of J558 Id positive B cells both in vivo and in vitro. This T-cell mediated dominance of the J558 idiotype limits the appearance of antibodies carrying other more diverse idiotypes which appear in immunized BALB/c nu/nu mice where no regulatory T cells occur. This T-cell mediated antibody invariance could be a strategy of the immune system responding to highly conserved antigens like polysaccharides, different from those against protein antigens, where diversity is assumed to be the basis for a successful response.

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The isotype expression in the J558 idiotype-associated humoral immune response against alpha(1-->3)-dextran in BALB/c mice is controlled by idiotype-specific T cells which silence in situ B lymphocytes primed and committed to an IgG response. This leads to a restriction of the type II thymus-independent response to the sole production of IgM antibodies. The availability of the T cell receptor (TcR) alpha and beta sequences for such a regulatory T cell clone allows the investigation of the degree of heterogeneity of the TcR usage of these T cells. It is found that all alpha(1-->3)-dextran-primed BALB/c mice use a very similar, possibly identical TcR. This suggests a tight, possibly genetically programmed, interaction between the J558 idiotype-bearing dextran-specific B cells and their idiotype-specific regulatory T cell counterparts.