Augmentation of the antibody response by hapten help. I. Dominant genetic control

Self versus non-self carrier in primary and secondary anti-TNP B cell response: I. Specificity of help

Humoral anti-hapten responses are supposed to require carrier-specific help. Yet, "TNP-specific" help can be activated with TNP coupled to syngeneic lymphocytes. To further clarify the role of hapten-specific vs. carrier-specific helper T cells (TH) in the humoral anti-TNP response, BALB/c mice were immunized with TNP coupled to cellular or soluble self or non-self carriers, and we analyzed primary and secondary B cell responses as well as the apparent specificity of help. TNP bound to syngeneic red blood cells (sRBC), syngeneic albumin (sA) and syngeneic polyclonal or monoclonal immunoglobulin (sIg/smIg) and TNP coupled to non-self carriers initiated equivalent primary anti-TNP responses. On the other hand, a secondary anti-TNP response was only obtained with heterologous carriers or with smIg. Even with heterologous carriers or with smIg, the magnitude of the secondary anti-TNP response exceeded only slightly the amplitudes of a primary anti-TNP response. Furthermore, if animals were challenged shortly after priming, they appeared un/-hyporesponsive. In vitro analysis revealed that in the primary as well as in the secondary anti-hapten response, TNP-specific TH were involved, i.e., the primary response to sIg, sA and sRBC was exclusively due to TNP-specific help, the response after priming with TNP-smIg or TNP-heterologous carrier was due to the additive effects of carrier- and TNP-specific help. Since "carrier-specific" help with smIg was independent of the antibody specificity as well as of the Ig class, we suppose that smIg activated idiotype-specific TH, which functioned like "carrier" TH. Two mechanisms were responsible for the low magnitude of the secondary anti-hapten response: competition for carrier-specific help (using heterologous carriers), and antibody (AB)-dependent suppression.

Trinitrobenzenesulphonic acid-induced unresponsiveness at the systemic level

To investigate the mechanism underlying B-cell tolerance, which is still discussed as being the consequence of (functional) clonal deletion or suppression, limiting dilution (LD) analysis of the frequencies of B cells as well as regulatory cells after tolerance induction with trinitrobenzenesulphonic acid (TNBS) was performed. It was shown that the frequency of functionally active hapten-specific B cells was decreased to less than 10% of the frequency in untreated BALB/c mice. After an immunogenic challenge, trinitrophenyl (TNP)-specific B cells of tolerized mice expanded, but did not reach the level of TNP-specific B cells in untreated mice. The expansion of TNP-specific B cells in TNBS-tolerized mice after challenge with TNP-horse red blood cells (HRBC) as observed in LD cultures was in contrast to the absence of anti-TNP plaque-forming cells (PFC) in freshly harvested spleen cells (SC) and the non-detectability of anti-TNP antibodies (AB). Hence, the functional deletion (= anergy) of B cells in vivo appears to be sustained by regulatory cells. Analysis of the regulatory compartment revealed that tolerance induction resulted in transient augmentation of TNP-specific helper T cells (TH), continuously elevated levels of suppressor T cells (TS), and a low level of contrasuppressor T cells (TCS). But, contrary to non-tolerized mice, TCS of tolerized mice were rather refractory to stimulation with TNP-HRBC. Hence, we would like to hypothesize that clonal anergy of B cells leads to inappropriate activation of TCS, whose nominal antigens are antibodies. This in turn sustains the persistence of high levels of TS, i.e. tolerance would be maintained by interruption of feedback activation of regulatory cells via effector cells/molecules.

Regulation of carrier-specific response via a haptenic epitope

Influences of haptenic epitopes on the regulation of carrier-specific responses are of importance in vaccination and heterogenization protocols. This situation was modelled by analysing the primary and secondary anti-horse red blood cell (HRBC) response under the influence of a haptenic epitope (trinitrophenol, TNP). Primary and secondary anti-HRBC responses were diminished in the presence of TNP. The primary response against the carrier was delayed in the presence of TNP and primary and secondary anti-carrier responses vanished more rapidly in response to TNP-HRBC than in response to HRBC. When frequencies of HRBC-specific B cells were determined under limiting dilution (LD) conditions, a lower frequency of HRBC-specific B cells was discovered in the presence of TNP-HRBC as compared to HRBC. Accordingly, after priming with HRBC, the frequency of B cells was increased by a factor of 49, while after priming with TNP-HRBC the frequency was only increased by a factor of 11. Furthermore, haptenic epitopes influenced the anti-carrier response via regulatory elements. Under physiological conditions, hapten-specific help was insufficient and hapten-specific B cells competed for carrier-specific helper T cells (TH). Suppression was more efficient in hapten-carrier-primed mice than in carrier-primed mice and in line with this observation, an increased frequency of suppressor T cells (Ts) was observed in LD cultures when TNP-HRBC instead of HRBC were used as antigen. It is concluded that haptenic epitopes are relevant for the anti-carrier response, since hapten-specific B cells compete for carrier-specific help, hapten-specific suppression down-regulates carrier-specific help, and the hapten may hide immunogenic epitopes of the carrier, resulting in incomplete activation of the B-cell repertoire.