Adoptively transferred
tumor-specific cells can mediate
tumor regression in
cancers refractory to conventional
therapy. Autologous polyclonal
tumor-specific cytotoxic T cells (CTL) generated from peripheral blood and infused into patients with metastatic
melanoma show enhanced persistence, compared to equivalent numbers of more extensively expanded monoclonal CTL, and are associated with complete remissions (CR) in select patients. We applied high-throughput
T cell receptor Vβ sequencing (HTTCS) to identify individual clonotypes within CTL products, track them in vivo post-infusion and then deduce the pre-adoptive transfer (endogenous) frequencies of cells ultimately responsible for
tumor regression. The summed in vivo post-transfer frequencies of the top 25 HTTCS-defined clonotypes originally detected in the infused CTL population were comparable to enumeration by binding of
antigen peptide-HLA multimers, revealing quantitative HTTCS is a reliable, multimer-independent alternative. Surprisingly, the polyclonal CTL products were composed predominantly of clonotypes that were of very low frequency (VLF) in the endogenous samples, often below the limit of HTTCS detection (0.001%). In patients who achieved durable CRs, the composition of transferred CTLs was dominated (57-90%) by cells derived from a single VLF clonotype. Thus, HTTCS now reveals that
tumor-specific CTL enabling long-term
tumor control originate from endogenous VLF populations that exhibit proliferative/survival advantages. Along with results indicating that naïve cell populations are most likely to contain cells that exist at VLF within the repertoire, our results provide a strong rationale for favoring T cells arising from VLF populations and with early-differentiation phenotypes when selecting subset populations for adoptive transfer.