Effect of anti-CTLA-4 antibody treatment on T-cell repertoire evolution in treated cancer patients.
3020 Background: CTL-associated antigen 4 (CTLA-4) is an immune checkpoint expressed by T cells. While treatment with anti-CTLA-4 antibody can induce clinical responses in advanced cancer patients, its effects on the breadth of the T cell response is unknown. Methods: We used a sequencing-based method, LymphoSIGHT, to assess T cell repertoire diversity in 46 patients with metastatic castration resistant prostate cancer or metastatic melanoma. Peripheral blood mononuclear cells were obtained from patients prior to and during treatment with anti-CTLA-4 antibody. mRNA was amplified using locus-specific primer sets for T cell receptor (TCR) beta, and the amplified product was sequenced. Sequence reads were used to quantitate absolute TCR frequencies using standardized clonotype determination algorithms with normalization by spiked reference TCR sequences. Following clonotype quantitation, repertoire differences between serial samples were assessed by the Morisita index, a statistical measure of population dispersion. Results: 97 paired samples were assessed, of which 46 (47%) had increases and 22 (23%) had decreases in TCR diversity by more than 2-fold. By comparison, none of 9 untreated sample pairs underwent more than a 2-fold change in diversity (P = 0.005, Fisher’s exact test, two tailed). TCR repertoire differences between monthly samples were markedly higher than for time-matched controls. After the first treatment, median Morisita index between samples was 0.197 for treated samples versus 0.039 for untreated (P = 0.0005, Mann-Whitney U test). The median number of clones that significantly changed in abundance was 421 for treated versus 45 for controls. In patients with multiple time points, this rapid clonotype evolution continued through treatment. Despite this global turnover in repertoire, a subset of high frequency clones, including CMV-specific T cells, remained relatively constant over the course of the study. Conclusions: CTLA-4 blockade increases the global rate of T cell clonotype turnover and influences TCR diversity. This evolution of the TCR repertoire may reflect a mechanism by which CTLA-4 blockade enhances tumor-specific T cells over time.