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During most immune responses, T cells help antigen-specific B cells to make antibodies against the antigen. One of the contributions of T cells to antibody production is the induction of isotype switching from IgM to IgG, which is the most abundant isotype in blood serum during recall responses. Other features of memory responses are faster kinetics and higher titers of antibody in the serum. What causes a primary immune response to be different from a secondary is not yet very clear and, particularly, the influence of precursor frequencies of T and B cells on memory responses still remains to be answered. To address this issue, a transgenic (tg) mouse line (ADA) was developed; it expresses the beta chain (V beta 2) of a major histocompatibility complex class II-restricted T cell receptor (TcR) specific for the glycoprotein (G) of vesicular stomatitis virus (VSV) serotype Indiana (VSV-IND). These mice exhibit an increased precursor frequency of VSV-specific CD4+ T cells that leads to enhanced neutralizing IgG production against VSV in vivo in unprimed mice. The data indicate that increased frequency of naive specific helper T cells alone may account for features of a memory phenotype such as high titer of antibodies and isotype switching.

T cell tolerance is usually established by clonal deletion of self-specific T cells in the thymus, or some times, in the periphery. Alternatively, tolerance may also be achieved by induction of clonal T cell unresponsiveness by a poorly understood mechanism called "anergy". We found that transgenic mice expressing a soluble form of vesicular stomatitis virus (VSV) glycoprotein (G) predominantly in liver and kidney exhibited normal B cell responses. VSV-G-specific T help-independent neutralizing IgM responses were within normal ranges, but no T help-dependent neutralizing IgG antibodies were generated upon immunization with recombinant VSV-G protein and recombinant vaccinia virus expressing VSV-G. This demonstrated absence of B cell tolerance but presence of T helper cell unresponsiveness. After adoptive transfer of transgenic spleen cells into thymectomized immuno-incompetent hosts, the unresponsive T helper cells regained function and switched the neutralizing IgM response to IgG, comparably to control T helper cells, within 7 days. Conversely, when naive non-transgenic spleen cells were transferred into transgenic mice, VSV-G-specific T helper cells became unresponsive within 3-4 days. These results suggest that VSV-G-specific T helper cells are rendered unresponsive within a few days in the VSV-G transgenic host also outside of the thymus and that this unresponsiveness was reversed by transfer into antigen-free recipients.

The influence of antigen epitope density and order on B cell induction and antibody production was assessed with the glycoprotein of vesicular stomatitis virus serotype Indiana [VSV-G (IND)]. VSV-G (IND) can be found in a highly repetitive form the envelope of VSV-IND and in a poorly organized form on the surface of infected cells. In VSV-G (IND) transgenic mice, B cells were unresponsive to the poorly organized VSV-G (IND) present as self antigen but responded promptly to the same antigen presented in the highly organized form. Thus, antigen organization influences B cell tolerance.

Cytotoxic T lymphocyte (CTL)-mediated cytolysis is induced via the interaction of the specific T-cell antigen receptor and the peptidic viral antigen associated with the major histocompatibility complex class I antigen. Here we demonstrate in vitro that lymphocytic choriomeningitis virus (LCMV) can escape the cytotoxic activity of LCMV-specific cloned CTLs by single amino acid changes within the recognized T-cell epitope defined by residues 275-289 of the LCMV glycoprotein [LCMV-GP-(275-289)]. LCMV-infected fibroblasts at a multiplicity of infection of 10(-3) exposed to virus-specific CTL at an effector-to-target cell ratio of 4:1 4 hr after infection was optimal for virus mutant selection. The selections were carried out with three LCMV-GP-(275-289)-specific CTL clones expressing T-cell antigen receptors containing the identical variable gene segments V alpha 4 and V beta 10 but different junctional regions; selection was also possible with LCMV-GP-(275-289)-specific cytotoxic polyclonal T cells. The most common escape mutation was an amino acid change of asparagine (AAT) to aspartic acid (GAT) at position 280; an additional mutation was glycine (GGT) to aspartic acid (GAT) at position 282. The results presented show that relevant point mutations within the T-cell epitope of LCMV-GP-(275-289) occur frequently and that they are selectable in vitro by CTLs.

We have studied the lymphocytic choriomeningitis virus (LCMV)-specific cytotoxic T cell response in transgenic mice expressing either the T cell receptor (TcR) alpha (V alpha 2/J alpha TA31) or the corresponding TcR beta (V beta 8.1/D beta/J beta 2.4) chain originally isolated from the LCMV glycoprotein specific (residues 32-42), H-2Db-restricted T cell clone P14. The expression of single transgenic TcR chains did not influence the corresponding endogenous TcR V gene usage in unstimulated T cells indicating that one particular TcR alpha or beta chain can randomly pair with different V beta or V alpha chains without any obvious bias. However, upon infection with LCMV, reactive cytotoxic T lymphocytes (CTL) from P14 beta-transgenic mice were predominantly V alpha 2+ whereas CTL from P14 alpha-transgenic mice preferentially expressed V beta 8.1 and unexpectedly also V beta 8.3 (but not V beta 8.2). Correspondingly, the LCMV-specific CTL response in both alpha and beta TcR-transgenic mice was strongly biased to the original P14 T cell epitope (LCMV glycoprotein residues 32-42). Sequence analysis of a large panel of LCMV-reactive "half-transgenic" TcR from P14 single receptor chain-transgenic mice revealed a highly conserved VJ alpha and a more diverse VDJ beta junctional region. This report demonstrates that the antigen specificity of the studied TcR depends on the specific combination of both TcR alpha and beta chains which implies that amino acids located in the TcR V alpha and V beta segments as well as in the junctional region are involved in binding of the viral antigenic fragment.

Clonal deletion in the thymus plays a major part in T-cell tolerance to self antigens. But the mechanism of negative selection, its fine specificity and the threshold of affinity and avidity remains unknown. We have now examined these aspects of negative selection with mice expressing a transgenic T-cell receptor with specificity for lymphocytic choriomeningitis virus (LCMV) glycoprotein in association with the class I H-2Db molecule. These mice were rendered tolerant to LCMV by neonatal infection with mutant LCMVs bearing point mutations in the T-cell epitope recognized by the transgenic T-cell receptor. Variant LCMVs were also tested for their ability to elicit antiviral responses in transgenic mice in vivo and in vitro. Comparison in vivo revealed that a low-avidity receptor interaction, which was unable to induce effector T cells in the periphery, was still sufficient for clonal deletion in the thymus.