Routemaps for Highly Effective Tuberculosis Vaccination

Abstract

There is no highly effective tuberculosis vaccine. Darrah et al. (2020) and Tait et al. (2019) are setting new benchmarks for protection against infection and pulmonary disease by changing the route of vaccine delivery and by using a protein subunit vaccine with a potent adjuvant.

Tuberculosis (TB) remains the deadliest infectious disease, and a successful vaccine that provides sterilizing immunity or prevents lung disease is critical for TB control (World Health Organization, 2019). Live attenuated BCG (Mycobacterium bovis bacillus Calmette-Guérin) is the only licensed TB vaccine to date and is given to infants intradermally shortly after birth. However, it offers only partial and variable protection against pulmonary TB (Fine, 1995). Since BCG’s first introduction in 1921, remarkably little progress has been made toward an effective TB vaccine. A key reason is the complete lack of meaningful clinical immune correlates of protection against TB disease. Fortunately, over the past several years there have been advances in TB vaccine development (McShane, 2019), and it has become evident that reliable pre-clinical animal models of TB, specifically non-human primates (NHPs), are critical for improving TB vaccination strategies (Flynn et al., 2015; Hansen et al., 2018,).

Along these lines, a recent NHP study by Darrah et al. (2020) published in Nature shows that administration of BCG by the intravenous route can prevent infection and disease in the majority of animals. Meanwhile, in a recent phase II clinical trial by Tait et al. (2019) published in the New England Journal of Medicine, individuals with latent TB received a protein antigen-adjuvant vaccine that resulted in significant and durable protection against pulmonary TB disease. Thus, both studies lead the way for improved TB vaccination strategies and the hunt for immune correlates of protection against TB.

T cell-dependent immunity is required for control of pulmonary Mycobacterium tuberculosis (Mtb) infection, and BCG shares T cell antigens with Mtb. Standard BCG vaccination affords only variable protection, and insights gained from recent clinical and pre-clinical data could lead to a more universally effective BCG vaccine (Nemes et al.,2018; McShane, 2019). However, it remains unclear whether the dose and route of BCG currently used is optimal to elicit lung-specific T cell responses and whether modulation of dose and delivery route could alter clinical efficacy. Darrah et al. (2020) employed a pre-exposure vaccination approach in rhesus macaques (Macaca mulatta) to ask whether simply changing the route of BCG delivery to induce higher T cell responses in the lung could provide superior protection in comparison to traditional intradermal administration. They compared the protective capacity of different BCG inoculation routes and doses against virulent Mtb challenge six months after vaccination: (1) standard-dose intradermal injection, (2) high-dose intradermal injection, (3) intravenous (i.v.) injection, (4) aerosol delivery, and (5) combined aerosol and high-dose intradermal vaccination. BCG given intradermally by the standard route or by the mucosal route via aerosol provided only modest protection against pulmonary TB disease. In stark contrast, i.v. administration of BCG resulted in sterilizing protection against subsequent Mtb infection in six out of ten animals (Figure 1). Importantly, Mtb infection was cleared in protected animals even before Mtb-specific adaptive immunity was generated, demonstrating that rapid sterilizing immunity against Mtb is achievable. Inaddition, nineout of ten intravenously immunized animals showed little signs of lung disease, as measured by positron emission tomography-computed tomography. Only one animal remained unprotected after i.v. BCG inoculation, making identification of immune correlates of protection difficult. In fact, the primary determinant of Mtb control was the inoculation route itself. To better understand how the route influenced innate and adaptive immune cells, extensive analyses characterizing immune responses in blood and airways were performed in all animals. In comparison with standard intradermal delivery, i.v. BCG vaccination resulted in a massive recruitment of innate and adaptive immune cells to the lung airways as early as two weeks after immunization. BCG-specific responses by CD4⁺ and CD8⁺ T cells remained highly elevated in the lung tissue and airways for at least six months after immunization, a time point when animals were challenged with virulent Mtb. Through extensive functional, phenotypic, and gene-expression profiling of BCG-responsive T cells, Darrah et al. (2020) showed a selective increase in differentiated “memory” T cells in i.v.-injected animals. Moreover, the vast majority of responding CD4⁺ T cells in i.v. BCG-vaccinated animals were located in the lung parenchyma, likely a consequence of delivering high doses of BCG to the lung via the i.v. route. Recruitment of antigen-specific cells directly to the site of infection is thought to provide more rapid activation of memory T cells that results in superior protection. A recent study showing that a different method of delivery of BCG to the lung by direct endobronchial inoculation can protect against repeated Mtb challenge supports this hypothesis (Dijkman et al., 2019). Although the findings of Darrah et al. (2020) implicate increased lung tissue-resident memory T cells as potential drivers of the profound protective efficacy of i.v. BCG, other mechanisms of immunity could be involved as well. I.v. BCG-vaccinated animals also showed higher IgG, IgA, and IgM antibody responses in both the airways and plasma than did all other routes of immunization, suggesting a possible role for antibody responses. Recent evidence from mice suggests that i.v. BCG induces enhanced innate immune activation in form of “trained immunity,” thus innate immune cells activated either directly by BCG-derived toll-like receptor (TLR) agonists or indirectly through i.v. BCG-induced inflammatory responses likely also contribute to protection (Kaufmann et al., 2018). It also remains unclear how durable immune-mediated protection after i.v. BCG vaccination is. Clinically, standard intradermal BCG immunization is given shortly after birth, whereas pulmonary TB disease is often acquired years later during adolescence and adulthood. Regardless, and despite an unknown mechanism of protection, Darrah et al. (2020) convincingly establish that the route of BCG administration profoundly modulates protective immunity against TB, with the i.v. delivery route providing unprecedented sterilizing protection against TB. Although an i.v. route for BCG vaccination of infants is unlikely to be easily translated into clinical settings in TB endemic countries, there are important biological implications from this pre-clinical study. First, Darrah et al. (2020) provide proof of concept that rapid sterilizing immunity against Mtb is achievable through vaccination, because some i.v. BCG-vaccinated animals cleared Mtb infection even before adaptive Mtb-specific immunity was generated. Second, immune mechanisms of protection against Mtb can now be functionally dissected in a relevant pre-clinical model with key questions centering around whether i.v. immunization induces a qualitatively or quantitatively different immune response in comparison with standard intradermal immunization.

Figure 1. Recent Pre-clinical and Clinical TB Vaccination Studies That Resulted in Improved Efficacy.

Darrah et al. (2020) show in rhesus macaques that intravenous BCG administration is profoundly superior in inducing sterilizing immunity in comparison with standard intradermal vaccination, with six out of ten animals protected. Tait et al. (2019) report 26 out of 1,663 latently infected participants in the placebo group developed TB disease, whereas 13 out of 1,626 in the M72/AS01_E group developed TB disease. Red indicates presence of bacteria and tuberculosis lung disease (humans and macaques), and orange signifies reduced bacterial burdens and disease in macaques, whereas black denotes no clinical active disease in humans and sterilizing immunity in macaques.

In a seminal study in humans, Tait el al. report the final analysis of a randomized, double-blind, placebo-controlled, phase IIb trial of the new subunit vaccine formulation M72/AS01_E in regard to efficacy, safety, and immunogenicity (Tait et al., 2019, clinicaltrials.gov number NCT01755598). This experimental vaccine consists of a recombinant fusion protein of the Mtb antigens Mtb32A and Mtb39A and the adjuvant AS01_E, a liposome-based adjuvant composed of the saponin QS-21 and the TLR4 agonist MPL (monophosphoryl lipid A) that is already used successfully as an adjuvant in the commercial vaccine Shingrix (GlaxoSmith-Kline). Between 2014 and 2015, the study enrolled a total of 3,575 latently Mtb-infected, HIV-negative adolescents and adults in Kenya, South Africa, and Zambia. After randomization, M72/AS01_E or placebo was delivered intramuscularly, and individuals were monitored for development of active pulmonary TB disease. Remarkably, after three years of follow-up, the vaccine proved 49.7% effective in preventing bacteriologically confirmed TB, whereas serious adverse events, potential immune-mediated diseases, and deaths occurred with similar frequencies in both the vaccine and placebo group (Figure 1). In contrast to preliminary results (Van Der Meeren et al., 2018), vaccine efficacy was not differentially affected by age or sex in the final analysis reported by Tait et al. (2019). To assess immunogenicity, humoral and cell-mediated immune responses were measured in a subgroup of 300 participants. Vaccinated individuals generated anti-M72 IgG antibodies and antigen-specific polyfunctional CD4⁺ T cells producing IFN-γ, IL-2, and TNF-α that stayed elevated for three years after vaccination. At the same time, polyfunctional CD4⁺ T cell frequencies remained unchanged in the placebo group and antigen-specific CD8⁺ T cell responses undetected. This suggests that durable humoral and CD4⁺ T cell responses to the vaccine afford partial protection in individuals that were latently infected. This raises the important issue if M72/AS01_E will show efficacy in individuals unexposed to TB or if protection is based on the boosting of pre-existing responses in latently infected people. Whether the potent and sustained nature of the M72-specific CD4⁺ T cell response and the potent immune stimulatory properties of AS01 are associated with the ≈50% protection against TB disease afforded by M72/AS01_E can now be addressed in dedicated studies seeking to identify immune correlates of protection or risk. Thus, the study of Tait et al. (2019) greatly advances the development of a post-exposure vaccine that could prevent disease and meet the attributes recommended by the World Health Organization (WHO) for new TB vaccines.

Looking past practical considerations like the safe implementation of an intravenously-given immunization, the highlighted studies demonstrate that highly effective vaccine-based protection is both biologically feasible and achievable while providing a path toward the identification of meaningful immunological correlates of protection against Mtb. This is important because understanding exactly how protective immunity is achieved provides roadmaps for the design and development of a universally effective TB vaccine.