Abstract
BACKGROUND: Dendritic cells (DCs) play a predominant role in antitumor immunity. As professional antigen-presenting cells (APCs), DCs can be functionally matured by TLR2 ligand binding to enhance innate immune response and subsequent T cell-dependent adaptive immunity. DC function is often suppressed by the tumor microenvironment, while current TLR2 agonists exhibit suboptimal stability and diminished efficacy in vivo. Therefore, reactivation of suppressed DCs could be a promising strategy for enhancing the efficacy of cancer immunotherapy. METHODS: To investigate the antitumor immunity induced by the novel Toll-like receptor 2 (TLR2) agonist SUP3 with better stability, we established murine melanoma, colon cancer and breast cancer tumor models. The hematopoietic growth factor Flt3L-dependent dendritic cells (FLDCs) were generated and utilized to examine their capacities of antigen processing and cross-presentation, and migration to the tumor-draining lymph nodes (TdLNs) in response to SUP3 treatment. To further improve the antitumor response of SUP3 by increasing the abundance and activation of DCs, Flt3L was administrated in vivo in combination with immune checkpoint blockade. RESULTS: SUP3 exhibited stronger inhibition of tumor growth and metastasis than classical TLR2 agonist, Pam3. SUP3 could increase cDC1 antigen cross-presentation and TdLN migration, promoting the proliferation, activation and cytotoxicity of antigen-specific cytotoxic T lymphocytes (CTL). SUP3 promoted the intracellular accumulation of antigens and facilitated the process of antigen cross-presentation, the processe regulated by the small GTPase Rab7. SUP3 induced PD-L1 expression by DCs via an interferon-γ-independent pathway. The combination of SUP3 treatment with immune checkpoint blockade by anti-PD-L1 further improved the antitumor response. Moreover, Flt3L increased DC proliferation and infiltration into the tumor tissues that further enhanced the effects of antitumor immunotherapy when used in combination with SUP3 and anti-PD-L1. CONCLUSIONS: This study demonstrated that the modified and more stable TLR2 agonist SUP3 provided an optimal strategy for promoting antitumor immunity via activation of cDC1. SUP3 enhanced antigen cross-presentation by cDC1 and subsequent activation of CTLs. The antitumor effect was further enhanced when SUP3 and Flt3L synergized with PD-L1 blockade. Therefore, reactivation of suppressed DCs in tumor microenvironment would be a promising strategy for designing effective antitumor immunotherapy.