Vaccine-induced tissue-resident memory T cells and mucosal immunity to respiratory pathogens

Abstract

Respiratory infections cause considerable morbidity and mortality globally, in particular in the most vulnerable individuals in our societies. Whooping cough (pertussis) is a respiratory disease caused by the Gram-negative bacterium Bordetella pertussis. Despite high immunization rates with acellular pertussis (aP) vaccines in high-income countries, there has been a resurgence in pertussis in many of these countries in recent years. This has been attributed to a failure of the aP vaccines to induce the appropriate immune responses that protect against B. pertussis infection in the nasopharyngeal cavity. There is strong evidence to suggest that Th17 and IL-17-secreting tissue-resident memory T (TRM) cells in the nasal mucosa are key to protection against B. pertussis infection in the upper respiratory tract. However, current aP vaccines generate potent circulating IgG and Th2-skewed responses but suppress induction of respiratory IL-17+ TRM cells. Similar to aP vaccines, current COVID-19 vaccines prevent severe disease, hospitalizations, and deaths, but do not prevent infection of the upper respiratory tract or transmission of SARS-CoV-2. This has been attributed to their failure to induce strong mucosal immunity, in particular secretory IgA (sIgA) in the nasopharyngeal cavity. The key aims of this study were: to elucidate the mechanisms of aP vaccine-induced immune suppression; to explore the use of novel adjuvants and the nasal immunization route to overcome the shortcomings of current parenterally-delivered aP vaccines; and to apply these immunization strategies to experimental COVID-19 vaccines, with the goal of generating strong mucosal immunity and protection against respiratory infection with SARS-CoV-2.

The results of this study showed that immunization of mice with commercial aP vaccines induced antigen-specific IL-10 in peripheral lymphoid organs. This IL-10 was produced by a heterogenous population of antigen-specific CD4+ and CD8+ regulatory T (Treg) cells, which suppressed B. pertussis-specific Th17 cells in vitro. Furthermore, disruption of IL 10 signalling in vivo, at the time of aP immunization or during B. pertussis challenge of aP-immunized mice partially reversed the suppression of IL-17+ TRM cell recruitment to the nasal tissue, and enhanced bacterial clearance from the nasal mucosa.

The adjuvant LP-GMP, comprising the toll-like receptor 2 (TLR2) agonist, LP1569, and the stimulator of interferon genes (STING) agonist, cyclic-di-GMP (c-di-GMP), potently induces Th1 and Th17-polarizing cytokines from dendritic cells (DCs). The present study showed that adding LP-GMP to a commercial alum-adjuvanted vaccine enhanced induction of IL-17+ TRM cells and protection against B. pertussis infection in the nasal mucosa. The benefits of adding LP-GMP to the aP vaccine were augmented by delivering the vaccine by intranasal (i.n.) route. Furthermore, immunization with an i.n.-delivered experimental aP (EaP) vaccine, formulated with 3 B. pertussis antigens and LP-GMP, without alum, promoted recruitment of antigen-specific IL 17+ TRM cells and Siglec-F+ neutrophils to the nasal tissue, and enhanced bacterial clearance from the nasal mucosa following challenge with B. pertussis.

This study revealed that LP-GMP was also an effective adjuvant when used in experimental COVID-19 vaccines formulated with the SARS-CoV-2 spike trimer, S1 monomer or nucleocapsid (N) proteins delivered by the i.n route. Transgenic K18-hACE2 mice immunized twice with the spike trimer and LP-GMP delivered by i.n. or intramuscular (i.m.) route, or i.m. prime followed by i.n. boost, were protected against disease and infection following challenge with lethal doses of ancestral and Delta strains of SARS-CoV-2. However, the strongest and most durable mucosal IgG and IgA, and CD4+ and CD8+ TRM cell responses were observed when the vaccine was delivered by i.n. immunization or by i.n. boosting following i.m. priming.

Collectively, this study revealed that the failure of aP vaccines to protect against infection with B. pertussis in the nose can be at least partially attributed to the induction of IL-10-secreting CD4+ and CD8+ Treg cells by this parenterally delivered vaccine adjuvanted with alum. These vaccine-induced IL-10+ Treg cells suppressed the induction of protective Th17 responses and recruitment of TRM cells to the respiratory tissues, thereby contributing to the failure of aP vaccines to protect against infection with B. pertussis in the nose. Intranasal immunization with aP vaccines adjuvanted with LP-GMP, with or without alum, overcame the suppression of CD4+ TRM responses and protected against B. pertussis infection in the nose. Furthermore, LP-GMP was shown to be an effective adjuvant in a nasally delivered COVID-19 vaccine, generating robust mucosal antibody and TRM responses and protecting against SARS-CoV-2 infection. These findings demonstrate that i.n. immunization with subunit vaccines formulated with strong mucosal adjuvants is an effective strategy for generating protective immunity against bacterial and viral respiratory pathogens.