Highly Biased CDR3 Usage in Restricted Sets of β Chain Variable Regions During Viral Superantigen 9 Response

Superantigens encoded by the mouse mammary tumor virus can stimulate a large proportion of T cells through interaction with germline-encoded regions of the T cell receptor β chain like the hypervariable region 4 (HV4) loop. However, several lines of evidence suggest that somatically generated determinants in the CDR3 region might influence superantigen responses. We stimulated T cells from donors differing at the BV6S7 allele with vSAG9 to assess the nature and structure of the T cell receptor in amplified T cells and to evaluate the contribution of non-HV4 elements in vSAG recognition. This report demonstrates that vSAG9 stimulation caused the expansion of TCR BV6-expressing T cells, although to varying degrees depending on the BV6 subfamily. The BV6S7 subfamily was preferentially expanded in all donors, but in donors homozygous for the BV6S7*2 allele, a significant number of BV6S5 T cells were amplified and showed a highly biased β chain junctional region (BJ) and CDR3 usage. As CDR3 regions are involved in major histocompatibility complex (MHC)–peptide interaction, such a selection is highly suggestive of an intimate MHC–TCR interaction and would imply that the topology of the MHC-vSAG-TCR complex is similar to the one occurring during conventional antigen recognition.