A novel mucosal vaccine formulation against tuberculosis by exploiting the adjuvant activity of S100A4—a damage-associated molecular pattern molecule

Abstract

Tuberculosis (TB) is a disease that has afflicted humans for much of the recorded history. Mycobacterium tuberculosis, the causative agent of TB, is the world's leading cause of death by a single infectious agent, second only to SARS-CoV-2, which causes the COVID-19 infection during the pandemic. Vaccination is the most cost-effective strategy for preventing infectious diseases. Currently, the only vaccine approved for human use against TB is the bacillus Calmette-Guerin (BCG), which has a protective efficacy against extrapulmonary forms of TB in children. However, BCG is notoriously variable in protecting adolescents and adults, who account for the majority of pulmonary TB transmission. Therefore, new TB vaccine formulations that can elicit improved protective immunity for adults against the pulmonary form of TB are desperately needed. In the first part of my thesis, it is demonstrated that S100A4 effectively augmented durable antigen-specific immune responses to the model vaccine antigen ovalbumin (OVA). S100A4, which could reach the pulmonary tissues and spleen after intranasal instillation, was capable of promoting the migration of dendritic cells and maturation of CD8 T cells. S100A4 facilitated antigen transport to nasal tissues and lymph nodes. Importantly, it is worth noting that S100A4 did not induce inflammation of the olfactory bulb, an organ that transmits smell information from the nose to the brain, which is vulnerable to the inflammatory stimulation by most microbial toxin-based adjuvants. In the second part of my work, a clinically relevant vaccine antigen derived from Mycobacterium tuberculosis, or ESAT-6, was admixed to S100A4 for intranasal immunization. S100A4 remarkably augmented the levels of anti-ESAT-6 IgG in serum and IgA in mucosal sites, including lung exudates, nasal mucosa, and bronchoalveolar lavage fluid (BALF). Furthermore, in both the lungs and the spleen, S100A4 strongly promoted ESAT-6 ­specific expansion of CD4 T cells. Upon reencounter with the immunizing antigen, CD8 T cells from S100A4-treated mouse spleen and lung tissues expressed higher levels of granzyme B, a toxic substance critical for cytotoxicity exhibited by these killer T cells. Both CD4 and CD8 T cells from these tissues expressed increased levels of IFN-γ, TNF-α, and IL-17, cytokines critical for antimicrobial activity. Antigen-reencounter-induced T cell proliferative responses, a key vaccine performance indicator, were augmented in the spleen of S100A4-adjuvanted mice. In the current thesis work, I have sufficiently advanced our understanding of S100A4 as a potential mucosal adjuvant. I have provided compelling experimental evidence demonstrating that S100A4 can induce remarkable Mycobacterium tuberculosis-specific adaptive immune responses after intranasal immunization with a Mycobacterium tuberculosis-derived vaccine antigen with defined mechanisms. Thus, my work has laid a solid foundation for further exploration of the translational value of this molecule as a clinically applicable mucosal adjuvant for infectious diseases including TB.