Airway T cells are a correlate of i.v. Bacille Calmette-Guerin-mediated protection against tuberculosis in rhesus macaques

Bacille Calmette-Guerin (BCG), the only approved Mycobacterium tuberculosis (Mtb) vaccine, provides limited durable protection when administered intradermally. However, recent work revealed that intravenous (i.v.) BCG administration yielded greater protection in macaques. Here, we perform a dose-ranging study of i.v. BCG vaccination in macaques to generate a range of immune responses and define correlates of protection. Seventeen of 34 macaques had no detectable infection after Mtb challenge. Multivariate analysis incorporating longitudinal cellular and humoral immune parameters uncovered an extensive and highly coordinated immune response from the bronchoalveolar lavage (BAL). A minimal signature predicting protection contained four BAL immune features, of which three remained significant after dose correction: frequency of CD4 T cells producing TNF with interferon γ (IFNγ), frequency of those producing TNF with IL-17, and the number of NK cells. Blood immune features were less predictive of protection. We conclude that CD4 T cell immunity and NK cells in the airway correlate with protection following i.v. BCG.

Prevention of tuberculosis in macaques after intravenous BCG immunization

Mycobacterium tuberculosis (Mtb) is the leading cause of death from infection worldwide¹. The only available vaccine, BCG (Bacillus Calmette-Guérin), is given intradermally and has variable efficacy against pulmonary tuberculosis, the major cause of mortality and disease transmission^1,2. Here we show that intravenous administration of BCG profoundly alters the protective outcome of Mtb challenge in non-human primates (Macaca mulatta). Compared with intradermal or aerosol delivery, intravenous immunization induced substantially more antigen-responsive CD4 and CD8 T cell responses in blood, spleen, bronchoalveolar lavage and lung lymph nodes. Moreover, intravenous immunization induced a high frequency of antigen-responsive T cells across all lung parenchymal tissues. Six months after BCG vaccination, macaques were challenged with virulent Mtb. Notably, nine out of ten macaques that received intravenous BCG vaccination were highly protected, with six macaques showing no detectable levels of infection, as determined by positron emission tomography-computed tomography imaging, mycobacterial growth, pathology and granuloma formation. The finding that intravenous BCG prevents or substantially limits Mtb infection in highly susceptible rhesus macaques has important implications for vaccine delivery and clinical development, and provides a model for defining immune correlates and mechanisms of vaccine-elicited protection against tuberculosis.

Conservation of molecular and cellular phenotypes of invariant NKT cells between humans and non-human primates

Invariant NKT (iNKT) cells in both humans and non-human primates are activated by the glycolipid antigen, α-galactosylceramide (α-GalCer). However, the extent to which the molecular mechanisms of antigen recognition and in vivo phenotypes of iNKT cells are conserved among primate species has not been determined. Using an evolutionary genetic approach, we found a lack of diversifying selection in CD1 genes over 45 million years of evolution, which stands in stark contrast to the history of the MHC system for presenting peptide antigens to T cells. The invariant T cell receptor (TCR)-α chain was strictly conserved across all seven primate clades. Invariant NKT cells from rhesus macaques (Macaca mulatta) bind human CD1D-α-GalCer tetramer and are activated by α-GalCer-loaded human CD1D transfectants. The dominant TCR-β chain cloned from a rhesus-derived iNKT cell line is nearly identical to that found in the human iNKT TCR, and transduction of the rhesus iNKT TCR into human Jurkat cells show that it is sufficient for binding human CD1D-α-GalCer tetramer. Finally, we used a 20-color flow cytometry panel to probe tissue phenotypes of iNKT cells in a cohort of rhesus macaques. We discovered several tissue-resident iNKT populations that have not been previously described in non-human primates but are known in humans, such as TCR-γδ iNKTs. These data reveal a diversity of iNKT cell phenotypes despite convergent evolution of the genes required for lipid antigen presentation and recognition in humans and non-human primates.

Boosting BCG with proteins or rAd5 does not enhance protection against tuberculosis in rhesus macaques

Tuberculosis (TB) is the leading cause of death from infection worldwide. The only approved vaccine, BCG, has variable protective efficacy against pulmonary TB, the transmissible form of the disease. Therefore, improving this efficacy is an urgent priority. This study assessed whether heterologous prime-boost vaccine regimens in which BCG priming is boosted with either (i) protein and adjuvant (M72 plus AS01_E or H56 plus CAF01) delivered intramuscularly (IM), or (ii) replication-defective recombinant adenovirus serotype 5 (Ad5) expressing various Mycobacterium tuberculosis (Mtb) antigens (Ad5(TB): M72, ESAT-6/Ag85b, or ESAT-6/Rv1733/Rv2626/RpfD) administered simultaneously by IM and aerosol (AE) routes, could enhance blood- and lung-localized T-cell immunity and improve protection in a nonhuman primate (NHP) model of TB infection. Ad5(TB) vaccines administered by AE/IM routes following BCG priming elicited ~10-30% antigen-specific CD4 and CD8 T-cell multifunctional cytokine responses in bronchoalveolar lavage (BAL) but did not provide additional protection compared to BCG alone. Moreover, AE administration of an Ad5(empty) control vector after BCG priming appeared to diminish protection induced by BCG. Boosting BCG by IM immunization of M72/AS01_E or H56:CAF01 elicited ~0.1-0.3% antigen-specific CD4 cytokine responses in blood with only a transient increase of ~0.5-1% in BAL; these vaccine regimens also failed to enhance BCG-induced protection. Taken together, this study shows that boosting BCG with protein/adjuvant or Ad-based vaccines using these antigens, by IM or IM/AE routes, respectively, do not enhance protection against primary infection compared with BCG alone, in the highly susceptible rhesus macaque model of tuberculosis.

Intravenous Bacille Calmette-Guerin provides protection across a dose spectrum in a Rhesus macaque model of tuberculosis

Our laboratory has demonstrated that intravenous (IV) vaccination with Bacille Calmette-Guerin (BCG; 5x107CFU) provides remarkably robust protection against low-dose Mycobacterium tuberculosis (Mtb) infection in Rhesus macaques (9/10 animals protected [<50 thoracic Mtb CFU]; 100,000-fold reduction compared to intradermal [ID] BCG). IV BCG also resulted in a 100-fold increase in mycobacterium-specific T cells in the bronchoalveolar lavage (BAL) of animals compared to other routes of administration (ID or aerosol). These results led us to hypothesize that significant protection could be observed at lower IV BCG doses. We therefore vaccinated macaques with decreasing doses of BCG (5x107, 1.6x107, 5x106, 1.6x106, 5x105and 1.6 x105CFU) followed by six months of immune monitoring and a three-month low-dose Mtb challenge. As expected, a mycobacterium-specific T cell dose-dependent response was observed in both the BAL (FACS) and PBMCs (IFNgELIspot) prior to infection, as well as in the lung at time of necropsy (FACS). Remarkably, 17/25 animals had no thoracic Mtb CFU across all groups, and at least one animal from each group was protected (30–100% protection). Mycobacterium-specific CD4 T cell numbers in the BAL appear to be a correlate of protection. These results demonstrate that even at low doses IV BCG can be efficacious in preventing tuberculosis with a potential role for T cell-dependent immunity.