Background: Despite the immunogenicity and safety profile of dendritic cell (DC) vaccines, the importance of vaccine-induced antigen-specific T cell responses is unclear across clinical trials, and therapeutic efficacy remains low with limited clinical responses. Our comprehensive characterization of T cell responses, cell-intrinsic and soluble immune checkpoint molecules and immune-related gene expression profiles reveal novel insights into CD8+ T cells specific for melanoma-associated antigens (MAA) from patients who received autologous DC engineered to express three full length melanoma antigens: tyrosinase, MART-1 and MAGE-A6 [1].

Methods: MAA-specific T cell responses were examined by standardized IFNγ ELISPOT assays at baseline, day 43 (post DC vaccines) and d89 (post observation or IFNα).  Luminex was used to detect serum checkpoint and costimulatory molecules, and whole blood flow cytometry was used to quantify PBMC subsets. Targeted mRNA and protein expression analyses in circulating lymphocytes and melanoma tumor samples were performed using NanoString nCounter platform (RUO).

Results: The majority of the 35 patients were successfully vaccinated, and the total vaccine-induced T cell responses were higher among those exhibiting a favorable clinical outcome. Patients who received checkpoint blockade prior to DC vaccination had higher baseline MA-specific CD8 T cell responses but no evidence for improved functional responses to the vaccine. Patients who showed the best clinical responses had low PD-1 expression on MA-specific T cells before and after DC vaccination, however blockade of PD-1 during antigen presentation by DC had minimal functional impact on PD-1high MA-specific T cells. Molecular profiling in circulating lymphocytes and tumor biopsies identified critical immunoregulatory pathways, including CTLA-4 and PD-1.  High immune checkpoint, type I interferon response and T-cell exhaustion gene expression networks correlated with inferior clinical outcomes. Interestingly, B cell receptor pathways negatively correlated with PD-1 expression, while gene signatures downstream of T cell receptor activation and IL-2 signaling were negatively correlated with CTLA-4 expression. Serum levels of PD-1 and PD-L2 were inversely correlated and post-vaccine serum levels PD-L2 correlated with decreased circulating T­reg and favorable outcome in patients, suggesting that it may serve as a biomarker of clinical response.

Conclusions: Collectively, our study shows that specific checkpoint molecular pathways are critical for vaccine outcomes and for the activation of anti-tumor responses in melanoma patients. DC-vaccine immunization followed by immune checkpoint blockade may be an optimal sequential therapy to improve antitumor immunity in melanoma.