In recent years, tumor vaccines designed based on DCs have been extensively studied. DC vaccines loaded with tumor antigens are promising approaches for cancer immunotherapy due to their biology and their ability in naive, activated, and memory T cells activation (5). Studies have shown that DC vaccines enhance antitumor immune responses and effectively control tumor growth in various mouse tumor models with good safety and tolerability (10, 35, 36). However, despite extensive efforts, tumor regression in DC vaccine-treated cancer patients is rare and limited progress has been made in cancer treatment (6). Therefore, new strategies and approaches are urgently needed to improve the efficacy of tumor vaccines and thus improve the poor efficacy in the clinic. A fusion protein targeting MSLN was found to promote tumor-specific T cell responses by increasing tumor antigen presentation and cross-presentation via DC in vitro and enhanced tumor cell immunogenicity in vivo (37). In this study, we designed a novel fusion peptide immunogen including human MSLN, PD-L1 immune checkpoint molecule, GM-CSF sequence and T helper epitope sequence assembly as therapeutic tumor vaccine (MSLN-PDL1-GMCSF). Our results showed that the novel PD-L1-containing MSLN targeting vaccine was able to activate a MSLN and PD-L1-specific T cell immune responses in immunized mice, inhibit the growth of MSLN+ and PD-L1+ tumor cells. Meanwhile, cytokine GM-CSF component in the vaccine design as an adjuvant might elicit function in enhancing DC recruitment, activation and cross-presentation.
The immunosuppressive tumor microenvironment leading to inefficient antigen presentation is further complicated by the problem. The immune checkpoint molecule PD-1 is expressed on T cells and its binding with PD-L1 on tumor cells leads to downregulation of proliferative responses, decreased cytokine secretion and T cell unresponsiveness or apoptosis (38–40). To date, there are five antibodies against PD-1 or PD-L1 that have been approved by the FDA and the European Medicines Agency for use in cancer immunotherapy (41). Clinical trials have shown that immune checkpoint regulation therapies have good potential, although in many cases the effect is limited, especially in solid tumors with low response rates, probably because the majority of cancer patients are resistant to PD-1/PD-L1 blockade (42). The induced anti-PD-L1 antibodies can kill PD-L1-expressing tumor cells through multiple mechanisms, including inhibition of PD-L1 interaction with PD-1 on CTLs, antibody-dependent cellular cytotoxicity (ADCC), and complement-dependent cytotoxicity (CDC) (14, 38, 43). Our results showed that this novel PD-L1-containing MSLN vaccine can induce low-dose, durable anti-PD-L1 antibodies and elicit effective tumor-specific CTL responses in vivo. As shown in the Fig. 6, compared with DC vaccine loaded with PD-L1 alone, the MSLN-PDL1-GMCSF vaccine inhibited the growth of lung cancer cells more effectively, and obtained a more pronounced survival benefit, which probably because blocking the PD-1 and PD-L1 axis restores the anti-tumor effect of T cells in the tumor microenvironment, thus reversing the "cold" state of the tumor.
Several clinical trials are underway to evaluate the efficacy of DC-based vaccines in combination with radiotherapy, chemotherapy, immune checkpoint inhibitors, and adoptive cell therapy for the treatment of various cancers (18, 35, 42). Teng, C. et al. demonstrated that inhibition of the PD-1/PD-L1 immune checkpoint during DC vaccination has shown better therapeutic efficacy than DC vaccination alone in an established mice model of in-situ liver tumor (16). However, in contrast to preclinical data, clinical data on the combination of immune checkpoint inhibitors and DCs vaccination are limited. Recent clinical data showed that 39 patients with metastatic melanoma who received ipilimumab plus DCs vaccine had an overall survival rate of only 38% (44). In this study, we further evaluated the therapeutic efficacy of this MSLN-PDL1-GMCSF vaccine in combination with PD-1 immune checkpoint inhibitors in an established mice model of lung cancer. The results showed that by inducing a stronger anti-tumor cytotoxic T-cell response, the combination of MSLN-PDL1-GMCSF vaccine and PD-1 antibody significantly prolonged overall survival, reduced tumor volume and increased tumor cell apoptosis compared to treatment alone. Thus, the combination therapy of this novel vaccine with PD-1 blockade may have great potential as a new treatment strategy for lung cancer and warrants further clinical trials to evaluate the efficacy in lung cancer and other solid tumor patients.
In summary, provided here for the first time is a novel PD-L1-containing MSLN therapeutic vaccine with the ability to induce persistent anti-PD-L1 antibody and CTL responses. It provides a new effective immunotherapeutic strategy for various solid tumors expressing high MSLN and PD-L1 level by combining this MSLN-PDL1-GMCSF vaccine and PD-1 blockade. This work, together with our recent reported PDL1-Vax, lays out a new strategy to overcome the problem of immune tolerance, and promotes the development of drugs targeting MSLN as the relevant antigen, which have theoretical and application values in the development of tumor immunotherapy in human. The combination of therapeutic vaccination with PD-1 blockade therapy may prove to be critical for improving the management and clinical outcomes of patients who do not respond or whose disease eventually progresses.