Long-term survivors of melanoma after anti-PD-1 therapy demonstrate on-treatment increase in abundance of 4-1BB+ CD4 T cells in blood.

Anti-PD-1 monotherapy has shown a significant survival benefit for patients with advanced-stage melanoma. In this study, we explored whether long-term clinical benefit is determined by abundance or changes in such abundances of CD4 and CD8 T-cell subsets in blood. Peripheral blood mononuclear cells from 58 patients with advanced-stage melanoma treated with anti-PD-1 monotherapy were collected prior to first, second, and third administration. Overall survival (OS) was categorized into 3 groups: <1 yr (short term); 1-3 yr; and >3 yr (long term). The abundance of different T-cell subsets was determined via multiplex flow cytometry and transcriptomics analysis. Long- and short-term survivors demonstrated no differences in frequencies of CD4 and CD8 T-cell subsets at baseline. However, on-treatment, long-term survivors showed an early increase in the frequency of CD4 T cells expressing 4-1BB, whereas this subset frequency was decreased in short-term survivors. Notably, this early increase in the 4-1BB+ CD4 T-cell frequency was associated with an increase in the frequency of LAG3+TIM3+ CD4 T cells and followed by a later decrease in the frequency of FOXP3+ CD4 T cells. Notably, 4-1BB+ CD4 T cells of long-term survivors demonstrated a distinctively high expression of IL-17. In short, long-term survivors have unique on-treatment changes in the frequency of different CD4 T-cell subsets, particularly those related to co-signaling and regulatory T cells. These results show that CD4 T cell-mediated stimulation and differentiation are associated with long-term benefit of anti-PD-1 treatment, which may guide development of future treatment strategies.