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The xid gene in mice controls a recessive defect resulting in the absence of a late maturing subset of B cells. Whereas the responsiveness pattern of these mice have been clearly defined in terms of their ability or inability to make antibodies to certain classes of thymus-independent antigens, there are conflicting reports in regard to affinity maturation of the antibody response to thymus-dependent antigens. To resolve this controversial issue, the two major isotypes of the IgG response, namely IgG1 and IgG2a were examined with a highly sensitive radioimmunoassay that measures both the magnitude and affinity of the anti-2,4-dinitrophenyl antibody of each isotype in individual serum samples. It was found that the xid gene reduced the amount but not affinity of the IgG1 antibody produced, whereas it impaired the whole IgG2a responses severely. In fact, mice carrying the defective gene were unable to mount a secondary IgG2a response, measured either quantitatively or qualitatively in terms of increased affinity. To test the possibility that Lyb3, an isogenic B cell-triggering receptor lacking in xid-mutant mice, plays a direct role in the maturation of the immune response, the antibody profile in normal mice immunized eigher with antigen alone or in combination with anti-Lyb3 receptor substantially elevated and accelerated the primary IgG2a response, whereas it had little effect on the IgG1 response.

Antibody responses to commonly used antigens are regulated by an epitope- specific system composed of Igh-restricted elements responsible for controlling the isotype and allotype responses mounted to each of the epitopes on the antigen. Because these elements can be independently induced to either suppress or support antibody production, this system as a whole provides an effector mechanism capable of selectively controlling the amount, affinity, isotype representation, and epitope-specificity of an antibody response. Sequential immunizations with a carrier molecule and a hapten conjugated to that carrier (carrier/hapten-carrier immunization) induce suppression for IgG responses to the hapten. IgG(2a), IgG(2b), and IgG(3) responses are easily suppressed, whereas IgG(1) responses tend to be more resistant. Once induced, suppression tends to be maintained; however, repeated stimulation with the hapten (on any carrier) eventually induces antibody production, first for IgG(1) and later for the more suppressible isotypes (IgG(2a), IgG(2b), IgG(3)). Antibody production, once initiated, also tends to be maintained. Ongoing IgG antihapten responses in animals primed with a hapten-carrier conjugate can be suppressed by subsequent carrier/hapten-carrier immunization (using a different carrier molecule); however, the suppression induced under these circumstances is substantially weaker, i.e., it mainly affects the more suppressible isotypes and is only strong enough to detect clearly in about one-half the immunized animals. Thus, the initiation of antibody production impairs the subsequent induction of suppression, and the initial induction of suppression tends to prevent subsequent initiation of antibody production. This reciprocal relationship defines a bistable regulatory mechanism, i.e., one that tends to maintain its initially induced state but is capable of shifting to the alternate state when stimulatory conditions so dictate. The operation of such a mechanism permits conditions surrounding the first immunization with an epitope (hapten) to strongly influence but not absolutely determine which and how many of the anti-epitope memory B cells generated by that immunization will subsequently be expressed. Thus, epitope- specific regulation, although subordinate to mechanisms that control memory B cell development (as opposed to expression), plays a key role in determining the magnitude, affinity, and isotype representation of anamnestic (memory) responses produced in response to previously encountered epitopes.

The expression of human immunoglobulin (Ig) chains in human-mouse heterohybridomas was investigated on the days after cell fusion. Heterohybridomas were made by fusion of mediastinal lymphocytes from lung carcinoma patients and murine (BALB/c) myeloma P3U1. Most (more than 76%) of heterohybridomas easily lost the light chain rather than the heavy chain expression in the early stages after cell fusion. Frequency of loss of kappa-chain expression was predominant, whereas the expression of heavy chain and lambda-chain was rather stable in comparison with that of kappa-chain. These were also confirmed at the clonal level, demonstrating that only lambda-chain but not kappa-chain producers were successfully enriched. In fact some heterohybridomas producing human Ig of lambda-chain type established were found to be quite stable for more than 2 years without any recloning procedures.