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Crosslinking of glucocorticoid-induced TNF family-related receptor (GITR) with agonist antibodies restores cancer immunity by enhancing effector T cell (Teff) responses while interfering with intra-tumor regulatory T cell (Treg) stability and/or accumulation. However, how anti-GITR antibody infusion changes T cell receptor (TCR) repertoire of Teffs and Tregs engaged in anti-tumor immune response is unclear. Here, we used a transgenic mouse model (TCRmini) where T cells express naturally generated but limited TCR repertoire to trace the fate of individual T cells recognizing B16 melanoma in tumor-bearing mice, treated or non-treated with an anti-GITR monoclonal antibody DTA-1. Analysis of TCRs of CD4+ T cells from these mice revealed that the TCR repertoire of dominant tumor-reactive Teff clones remained rather similar in treated and non-treated mice. In contrast, both tumor-associated and peripheral TCR repertoire of Tregs, which were mostly distinct from that of Teffs, underwent DTA-1 mediated remodeling characterized by depletion of dominant clones and an emergence of more diverse, low-frequency clones bearing increased numbers of TCRs shared with Teffs. We conclude that the DTA-1 infusion eliminates activated Tregs engaged in the initial maintenance of tolerogenic niche for tumor growth, but over time, it favors tumor replenishment by Tregs expressing an array of TCRs able to compete with Teffs for recognition of the same tumor antigens which may prevent its complete eradication. Electronic supplementary material The online version of this article (doi:10.1007/s00005-017-0479-1) contains supplementary material, which is available to authorized users.