Flagellins as Vaccine Adjuvants and Cancer Immunotherapy: Recent Advances and Future Prospects

Abstract

Flagellin, an essential structural protein of bacterial flagella, has emerged as a potent modulator of both specific and nonspecific immunity, demonstrating significant potential as a vaccine adjuvant and carrier. By inducing the release of pro-inflammatory cytokines like IL-1β, TNF-α, IL-6, IL-8, and IL-12, flagellin activates the innate immune system, enhancing antigen-specific adaptive immune responses mediated by tumour-specific type 1 helper T cells and cytotoxic T cells, thus positioning it as a valuable adjuvant or complementary therapy for various cancers and infectious diseases. This review explores recent strategies, innovations, and clinical applications of flagellin-based immunotherapies, particularly in the context of infectious diseases and cancers. Flagellin from Salmonella typhimurium has been extensively studied as a vaccine adjuvant for diseases like HIV, influenza, dengue, West Nile virus, poultry cholera, and bursal diseases and shows promise in treating lung metastasis, melanoma, colon, and prostate cancers. It has also proven effective against multidrug-resistant bacteria, including Pseudomonas aeruginosa and S. typhimurium . Notably, S. typhimurium flagellin-based vaccines for influenza have progressed to clinical trials. Additionally, flagellins from S. typhi , S. enteritidis , P. aeruginosa , and Escherichia coli are being evaluated as vaccine candidates for plague, malaria, and infections caused by P. aeruginosa and E. coli . In cancer therapy, flagellin-based treatments, especially when combined with tumour antigens, have exhibited the ability to enhance anti-tumour immunity and improve patient outcomes. Other flagellin-based vaccines derived from S. Dublin, S. munchen, and Vibrio vulnificus have been employed in the treatment of prostate, lung, liver, breast, cervical, and colorectal cancers, as well as lymphoma, melanoma, and radiation-induced mucositis. Mobilan, a recombinant non-replicating adenovirus vector expressing Salmonella flagellin, is currently in a phase Ib clinical trial for prostate cancer. Overall, bacterial flagellin treatments are generally safe, well-tolerated, and associated with minimal side effects, making them a promising option for managing infectious diseases and cancers.

Keywords: adjuvants; cancer; flagellin; immunotherapy; infectious diseases; vaccine.

FIGURE 1

Mode of action of flagellin. The binding of flagellin to TLR5 on the extracellular surface triggers the formation of TLR5 homodimers. This interaction brings in the adaptor protein MyD88, which activates two key signalling pathways: The IkB and MAPK pathways. These pathways lead to the activation of transcription factors NF‐κB and AP‐1, respectively. As a result, cytokines such as TNF‐α, IL‐6, IL‐8, and IL‐12 are produced, all of which play a role in type 1 immunity (1). Besides homodimer signalling, flagellin can also interact with TLR5/TLR4 heterodimers, activating the IRF3 pathway that results in the transcription of the IFN‐β gene (2a). The production of IFN‐β then activates the JAK–STAT signalling pathway, which enhances the synthesis of NO (2b). Additionally, flagellin can be introduced into the host cell cytoplasm via bacterial type III secretion systems. Once inside, intracellular flagellin is detected by NAIP5, leading to the formation of an inflammasome complex with NLRC4. This inflammasome activates caspase‐1 by converting it from its inactive form, procaspase‐1, into its active state. The active caspase‐1 then processes the precursors of IL‐1β (pro‐IL‐1β) and IL‐18 (pro‐IL‐18) into their active forms, IL‐1β and IL‐18, which are essential for the host defence mechanism (3).

FIGURE 2

Mechanism of action of flagellin adjuvant‐based vaccine. Injection of flagellin adjuvant‐ based vaccines establishes a depot at the injection site, which allows for the gradual release of antigens, improving their uptake and presentation by APCs (1). This mechanism draws immune cells like DCs, macrophages, and NK cells, triggers localised inflammation and activates these cells to function as APCs. Once activated, APCs release cytokines such as TNF‐α, IL‐6, IL‐12, IL‐1β, and various chemokines linked to immune responses. This surge of cytokines fosters type 1 immunity, enhancing the functions of tumour‐specific CTLs that identify cancer antigens and destroy cancer cells by releasing cytotoxic substances like granzymes, perforins, TNF‐α, and IFN‐γ (2). Furthermore, activated APCs, especially DCs, travel to draining lymph nodes, where they present antigens to CD4⁺ and CD8⁺ T cells (3). Presenting antigens to CD8⁺ T cells stimulate cellular immunity, leading to the proliferation of CTLs and the cytotoxic actions of CD8⁺ T cells, which ultimately result in the elimination of cancer cells (4a). On the other hand, presenting antigens to CD4⁺ T cells trigger humoral immunity by activating B cells, which then produce tumour‐specific antibodies (4b).