Plasticity of the T Cell Receptor Repertoire in TCR β-Chain Transgenic Mice

Abstract

The potential αβ T cell receptor (TCR) repertoire in normal mice is extremely large and estimated by M. M. Davis and P. J. Bjorkman (Nature334, 395, 1988) to include 5.2 × 1018different receptor molecules. This tremendous diversity provides the basis for T cell recognition of the universe of antigens including bacterial, viral, and allogeneic epitopes. Expression of a single TCR β-chain transgene should alter the repertoire by limiting the available diversity, therefore, creating holes in the repertoire or producing TCR with decreased affinity. To determine the effect of drastically decreasing the size of the repertoire, we investigated T cell responses in TCR β-chain transgenic mice expressing the Vβ8.2 transgene. Previous results showed that >98% of T cells in these mice express the transgene; thus, the TCR repertoire is reduced by orders of magnitude. We tested the T cell responses of the transgenic mice and nontransgenic littermates to nine different MHC haplotypes in mixed lymphocyte reactions, five protein antigens, and eight immunogenic peptides. Surprisingly, the transgenic mice responded to all antigenic stimuli tested indicating the lack of a hole in the TCR repertoire. Interestingly, however, the response in every case was quantitatively lower than the response by the nontransgenic littermates. In contrast, transgenic and nontransgenic T cells responded equivalently to stimulation with mitogens or to stimulation with immobilized α-TCR mAb indicating that the transgenic T cells had a normal capacity to respond. To differentiate between decreased TCR affinity and decreased precursor frequency, we performed a limiting dilution analysis to the peptide antigens CI:NP and OVA324–339. The results showed approximately a three- to eight-fold decrease in the frequency of transgenic T cells responding to the peptide compared to nontransgenic littermates. We previously showed that the response to cI84–98and PLP could be blocked with anti-Vβ8 mAb indicating that Vβ8.2-bearing T cells are capable of responding to peptide antigen. Analysis of TCR Vα chain expression by PCR and flow cytometry showed similar Vα expression in both the transgenic and the nontransgenic mice. These results demonstrate tremendous plasticity in the TCR repertoire permitting T cell responses by the transgenic mice to all antigens tested. However, the decreased magnitude of the responses may impair the capacity to defend against natural pathogens. Therefore, although the large TCR repertoire of normal mice may not be necessary to producein vitroresponses to many experimental antigens, it may confer survival benefits in natural environments.