Immunization with irradiated autoreactive T cells (T cell vaccination) induces anti-idiotypic T cell responses that preferentially recognize complementarity-determining region 3 sequences, contributing to clonal depletion of autoreactive T cells. However, it remains unknown whether T cell vaccination elicits anti-idiotypic humoral responses and whether the anti-idiotypic Abs play a similar role in the regulatory mechanism induced by T cell vaccination. In this study we examined the occurrence, the reactivity pattern, and the regulatory role of anti-idiotypic Abs elicited by T cell vaccination in patients with multiple sclerosis. We demonstrated for the first time that B cells producing anti-idiotypic Abs could be isolated from vaccinated patients. These EBV-transformed B cell lines were selected for specific reactivity to a 20-mer TCR peptide incorporating a common complementarity-determining region 3 sequence of the immunizing T cell clones. The resulting anti-idiotypic Abs were found to react with the original immunizing T cell clones and exhibit an inhibitory effect on their proliferation. The findings suggest that anti-idiotypic Ab responses can be induced by T cell vaccination in humans and that their regulatory properties are likely to contribute to the suppression of myelin basic protein-reactive T cells in vaccinated patients. The study has important implications in our understanding of the regulatory role of the anti-idiotypic humoral responses induced by T cell vaccination.