Lymph node targeting of BCG vaccines amplifies CD4 and CD8 T-cell responses and protection against Mycobacterium tuberculosis

A century of BCG vaccination: Immune mechanisms, animal models, non-traditional routes and implications for COVID-19

Bacillus Calmette-Guerin (BCG) has been used as a vaccine against tuberculosis since 1921 and remains the only currently approved vaccine for this infection. The recent discovery that BCG protects against initial infection, and not just against progression from latent to active disease, has significant implications for ongoing research into the immune mechanisms that are relevant to generate a solid host defense against Mycobacterium tuberculosis (Mtb). In this review, we first explore the different components of immunity that are augmented after BCG vaccination. Next, we summarize current efforts to improve the efficacy of BCG through the development of recombinant strains, heterologous prime-boost approaches and the deployment of non-traditional routes. These efforts have included the development of new recombinant BCG strains, and various strategies for expression of important antigens such as those deleted during the M. bovis attenuation process or antigens that are present only in Mtb. BCG is typically administered via the intradermal route, raising questions about whether this could account for its apparent failure to generate long-lasting immunological memory in the lungs and the inconsistent level of protection against pulmonary tuberculosis in adults. Recent years have seen a resurgence of interest in the mucosal and intravenous delivery routes as they have been shown to induce a better immune response both in the systemic and mucosal compartments. Finally, we discuss the potential benefits of the ability of BCG to confer trained immunity in a non-specific manner by broadly stimulating a host immunity resulting in a generalized survival benefit in neonates and the elderly, while potentially offering benefits for the control of new and emerging infectious diseases such as COVID-19. Given that BCG will likely continue to be widely used well into the future, it remains of critical importance to better understand the immune responses driven by it and how to leverage these for the design of improved vaccination strategies against tuberculosis.

Spatial delivery of immune cues to lymph nodes to define therapeutic outcomes in cancer vaccination

Recently approved cancer immunotherapies - including CAR-T cells and cancer vaccination, - show great promise. However, these technologies are hindered by the complexity and cost of isolating and engineering patient cells ex vivo. Lymph nodes (LNs) are key tissues that integrate immune signals to coordinate adaptive immunity. Directly controlling the signals and local environment in LNs could enable potent and safe immunotherapies without cell isolation, engineering, and reinfusion. Here we employ intra-LN (i.LN.) injection of immune signal-loaded biomaterial depots to directly control cancer vaccine deposition, revealing how the combination and geographic distribution of signals in and between LNs impact anti-tumor response. We show in healthy and diseased mice that relative proximity of antigen and adjuvant in LNs - and to tumors - defines unique local and systemic characteristics of innate and adaptive response. These factors ultimately control survival in mouse models of lymphoma and melanoma. Of note, with appropriate geographic signal distributions, a single i.LN. vaccine treatment confers near-complete survival to tumor challenge and re-challenge 100 days later, without additional treatments. These data inform design criteria for immunotherapies that leverage biomaterials for loco-regional LN therapy to generate responses that are systemic and specific, without systemically exposing patients to potent or immunotoxic drugs.

Photochemically-Mediated Inflammation and Cross-Presentation of Mycobacterium bovis BCG Proteins Stimulates Strong CD4 and CD8 T-Cell Responses in Mice

Conventional vaccines are very efficient in the prevention of bacterial infections caused by extracellular pathogens due to effective stimulation of pathogen-specific antibodies. In contrast, considering that intracellular surveillance by antibodies is not possible, they are typically less effective in preventing or treating infections caused by intracellular pathogens such as Mycobacterium tuberculosis. The objective of the current study was to use so-called photochemical internalization (PCI) to deliver a live bacterial vaccine to the cytosol of antigen-presenting cells (APCs) for the purpose of stimulating major histocompatibility complex (MHC) I-restricted CD8 T-cell responses. For this purpose, Mycobacterium bovis BCG (BCG) was combined with the photosensitiser tetraphenyl chlorine disulfonate (TPCS2a) and injected intradermally into mice. TPCS2a was then activated by illumination of the injection site with light of defined energy. Antigen-specific CD4 and CD8 T-cell responses were monitored in blood, spleen, and lymph nodes at different time points thereafter using flow cytometry, ELISA and ELISPOT. Finally, APCs were infected and PCI-treated in vitro for analysis of their activation of T cells in vitro or in vivo after autologous vaccination of mice. Combination of BCG with PCI induced stronger BCG-specific CD4 and CD8 T-cell responses than treatment with BCG only or with BCG and TPCS2a without light. The overall T-cell responses were multifunctional as characterized by the production of IFN-γ, TNF-α, IL-2 and IL-17. Importantly, PCI induced cross-presentation of BCG proteins for stimulation of antigen-specific CD8 T-cells that were particularly producing IFN-γ and TNF-α. PCI further facilitated antigen presentation by causing up-regulation of MHC and co-stimulatory proteins on the surface of APCs as well as their production of TNF-α and IL-1β in vivo. Furthermore, PCI-based vaccination also caused local inflammation at the site of vaccination, showing strong infiltration of immune cells, which could contribute to the stimulation of antigen-specific immune responses. This study is the first to demonstrate that a live microbial vaccine can be combined with a photochemical compound and light for cross presentation of antigens to CD8 T cells. Moreover, the results revealed that PCI treatment strongly improved the immunogenicity of M. bovis BCG.

Lymph nodes-The neglected battlefield in tuberculosis

Lymph nodes, particularly thoracic lymph nodes, are among the most common sites of extrapulmonary tuberculosis (TB). However, Mycobacterium tuberculosis (Mtb) infection in these organs is understudied. Aside from being sites of initiation of the adaptive immune system, lymph nodes also serve as niches of Mtb growth and persistence. Mtb infection results in granuloma formation that disrupts and—if it becomes large enough—replaces the normal architecture of the lymph node that is vital to its function. In preclinical models, successful TB vaccines appear to prevent spread of Mtb from the lungs to the lymph nodes. Reactivation of latent TB can start in the lymph nodes resulting in dissemination of the bacteria to the lungs and other organs. Involvement of the lymph nodes may improve Bacille Calmette-Guerin (BCG) vaccine efficacy. Lastly, drug penetration to the lymph nodes is poor compared to blood, lung tissue, and lung granulomas. Future studies on evaluating the efficacy of vaccines and anti-TB drug treatments should include consideration of the effects on thoracic lymph nodes and not just the lungs.

Deficiency of the AIM2-ASC Signal Uncovers the STING-Driven Overreactive Response of Type I IFN and Reciprocal Depression of Protective IFN-γ Immunity in Mycobacterial Infection

The nucleic acids of Mycobacterium tuberculosis can be detected by intracellular DNA sensors, such as cyclic GMP-AMP synthase and absent in melanoma 2 (AIM2), which results in the release of type I IFN and the proinflammatory cytokine IL-1β. However, whether cross-talk occurs between AIM2-IL-1β and cyclic GMP-AMP synthase-type I IFN signaling upon M. tuberculosis infection in vivo is unclear. In this article, we demonstrate that mycobacterial infection of AIM2^-/- mice reciprocally induces overreactive IFN-β and depressive IFN-γ responses, leading to higher infection burdens and more severe pathology. We also describe the underlying mechanism whereby activated apoptosis-associated speck-like protein interacts with a key adaptor, known as stimulator of IFN genes (STING), and inhibits the interaction between STING and downstream TANK-binding kinase 1 in bone marrow-derived macrophages and bone marrow-derived dendritic cells, consequently reducing the induction of type I IFN. Of note, apoptosis-associated speck-like protein expression is inversely correlated with IFN-β levels in PBMCs from tuberculosis patients. These data demonstrate that the AIM2-IL-1β signaling pathway negatively regulates the STING-type I IFN signaling pathway by impeding the association between STING and TANK-binding kinase 1, which protects the host from M. tuberculosis infection. This finding has potential clinical significance.

Enhanced effect of BCG vaccine against pulmonary Mycobacterium tuberculosis infection in mice with lung Th17 response to mycobacterial heparin-binding hemagglutinin adhesin antigen

Although the BCG vaccine can prevent tuberculosis (TB) in infants, its ability to prevent adult pulmonary TB is reportedly limited. Therefore, development of a novel effective vaccine against pulmonary TB has become an international research priority. We have previously reported that intranasal vaccination of mice with a mycobacterial heparin-binding hemagglutinin adhesin (HBHA) plus mucosal adjuvant cholera toxin (CT) enhances production of IFN-γ and anti-HBHA antibody and suppresses extrapulmonary bacterial dissemination after intranasal infection with BCG. In the present study, the effects of intranasal HBHA + CT vaccine on murine pulmonary Mycobacterium tuberculosis (Mtb) infection were examined. Intranasal HBHA + CT vaccination alone failed to reduce the bacterial burden in the infected lung. However, a combination vaccine consisting of s.c. BCG priming and an intranasal HBHA + CT booster significantly enhanced protective immunity against pulmonary Mtb infection on day 14 compared with BCG vaccine alone. Further, it was found that intranasal HBHA + CT vaccine enhanced not only IFN-γ but also IL-17A production by HBHA-specific T cells in the lung after pulmonary Mtb infection. Therefore, this combination vaccine may be a good candidate for a new vaccine strategy against pulmonary TB.

Immunological evaluation of colonic delivered Hepatitis B surface antigen loaded TLR-4 agonist modified solid fat nanoparticles

Hepatitis B is one of the leading liver diseases and remains a major global health problem. Currently available vaccines provide protection but often results in weaker/minimal mucosal immunity. Thus the present study is devoted to the development and in-vivo exploration of the colonically delivered biomimetic nanoparticles which capably enhance humoral as well as cellular immune response. In present work, Hepatitis B surface antigen (HBsAg) entrapped nanoparticles containing Monophosphoryl lipid A (MPLA) (HB+L-NP) were prepared by solvent evaporation method and characterized for particle size (~210nm), shape, zeta potential (-24mV±0.68), entrapment efficiency (58.45±1.68%), in-vitro release and antigen integrity. Dose escalation study was done to confirm prophylactic immune response following defined doses of prepared nanoparticulate formulations with or without MPLA. Intramuscular administered alum based marketed HBsAg (Genevac B) was used as standard (10μg) and were able to induce significant systemic (IgG) but remarkably low mucosal immune (IgA) response. Notably, HB+L-NP (0.5ml-10μg) induced strong systemic and robust mucosal immunity (510 and 470 mIU/ml respectively, p<0.001) from which mucosal was more significant due to the involvement of Common Mucosal Immune System (CMIS). Likewise, significant cellular immune response was elicited by HB+L-NP through T-cell activation (mixed Th1 and Th2) as confirmed by significantly increased cytokines level (IL-2 and Interferon-γ) in spleen homogenates. This study supports that delivery of HBsAg to the colon may open new vista in designing oral vaccines later being one of most accepted route for potential vaccines in future.

Do schistosome vaccine trials in mice have an intrinsic flaw that generates spurious protection data?

The laboratory mouse has been widely used to test the efficacy of schistosome vaccines and a long list of candidates has emerged from this work, many of them abundant internal proteins. These antigens do not have an additive effect when co-administered, or delivered as SWAP homogenate, a quarter of which comprises multiple candidates; the observed protection has an apparent ceiling of 40-50%. We contend that the low level of maturation of penetrating cercariae (~32% for Schistosoma mansoni) is a major limitation of the model since 68/100 parasites fail to mature in naïve mice due to natural causes. The pulmonary capillary bed is the obstacle encountered by schistosomula en route to the portal system. The fragility of pulmonary capillaries and their susceptibility to a cytokine-induced vascular leak syndrome have been documented. During lung transit schistosomula burst into the alveolar spaces, and possess only a limited capacity to re-enter tissues. The acquired immunity elicited by the radiation-attenuated (RA) cercarial vaccine relies on a pulmonary inflammatory response, involving cytokines such as IFNγ and TNFα, to deflect additional parasites into the alveoli. A principal difference between antigen vaccine protocols and the RA vaccine is the short interval between the last antigen boost and cercarial challenge of mice (often two weeks). Thus, after antigen vaccination, challenge parasites will reach the lungs when both activated T cells and cytokine levels are maximal in the circulation. We propose that "protection" in this situation is the result of physiological effects on the pulmonary blood vessels, increasing the proportion of parasites that enter the alveoli. This hypothesis will explain why internal antigens, which are unlikely to interact with the immune response in a living schistosomulum, plus a variety of heterologous proteins, can reduce the level of maturation in a non-antigen-specific way. These proteins are "successful" precisely because they have not been selected for immunological silence. The same arguments apply to vaccine experiments with S. japonicum in the mouse model; this schistosome species seems a more robust parasite, even harder to eliminate by acquired immune responses. We propose a number of ways in which our conclusions may be tested.

Intralymphatic immunotherapy

Gold Standard allergen-specific immunotherapy is associated with low efficacy because it requires either many subcutaneous injections of allergen or even more numerous sublingual allergen administrations to achieve amelioration of symptoms. Intralymphatic vaccination can maximize immunogenicity and hence efficacy. We and others have demonstrated that as few as three low dose intralymphatic allergen administrations are sufficient to effectively alleviate symptoms. Results of recent prospective and controlled trials suggest that this strategy may be an effective form of allergen immunotherapy.

Harnessing biomaterials to engineer the lymph node microenvironment for immunity or tolerance

Nanoparticles, microparticles, and other biomaterials are advantageous in vaccination because these materials provide opportunities to modulate specific characteristics of immune responses. This idea of “tuning” immune responses has recently been used to combat infectious diseases and cancer, and to induce tolerance during organ transplants or autoimmune disease. Lymph nodes and other secondary lymphoid organs such as the spleen play crucial roles in determining if and how these responses develop following vaccination or immunotherapy. Thus, by manipulating the local microenvironments within these immunological command centers, the nature of systemic immune response can be controlled. This review provides recent examples that harness the interactions between biomaterials and lymph nodes or other secondary lymphoid organs to generate immunity or promote tolerance. These strategies draw on mechanical properties, surface chemistry, stability, and targeting to alter the interactions of cells, signals, and vaccine components in lymph nodes. While there are still many unanswered questions surrounding how best to design biomaterial-based vaccines to promote specific structures or functions in lymph nodes, features such as controlled release and targeting will help pave the way for the next generation of vaccines and immunotherapies that generate immune responses tuned for specific applications.

Intralymphatic immunotherapy and vaccination in mice

Vaccines are typically injected subcutaneously or intramuscularly for stimulation of immune responses. The success of this requires efficient drainage of vaccine to lymph nodes where antigen presenting cells can interact with lymphocytes for generation of the wanted immune responses. The strength and the type of immune responses induced also depend on the density or frequency of interactions as well as the microenvironment, especially the content of cytokines. As only a minute fraction of peripherally injected vaccines reaches the lymph nodes, vaccinations of mice and humans were performed by direct injection of vaccine into inguinal lymph nodes, i.e. intralymphatic injection. In man, the procedure is guided by ultrasound. In mice, a small (5-10 mm) incision is made in the inguinal region of anesthetized animals, the lymph node is localized and immobilized with forceps, and a volume of 10-20 μl of the vaccine is injected under visual control. The incision is closed with a single stitch using surgical sutures. Mice were vaccinated with plasmid DNA, RNA, peptide, protein, particles, and bacteria as well as adjuvants, and strong improvement of immune responses against all type of vaccines was observed. The intralymphatic method of vaccination is especially appropriate in situations where conventional vaccination produces insufficient immunity or where the amount of available vaccine is limited.

Protection against tuberculosis with homologous or heterologous protein/vector vaccine approaches is not dependent on CD8+ T cells

Considerable effort has been directed to develop Mycobacterium tuberculosis vaccines to boost bacille Calmette-Guérin or for those who cannot be immunized with bacille Calmette-Guérin. We hypothesized that CD4(+) and CD8(+) T cell responses with a heterologous prime/boost vaccine approach could induce long-lived vaccine efficacy against M. tuberculosis in C57BL/6 mice. We produced an adenovirus vector expressing ID93 (Ad5-ID93) for induction of CD8 T cells to use with our candidate tuberculosis vaccine, ID93/glucopyranosyl lipid adjuvant (GLA)-stable emulsion (SE), which induces potent Th1 CD4 T cells. Ad5-ID93 generates ID93-specific CD8(+) T cell responses and induces protection against M. tuberculosis. When Ad5-ID93 is administered in a prime-boost strategy with ID93/GLA-SE, both CD4(+) and CD8(+) T cells are generated and provide protection against M. tuberculosis. In a MHC class I-deficient mouse model, all groups including the Ad5-ID93 group elicited an Ag-specific CD4(+) T cell response and significantly fewer Ag-specific CD8(+) T cells, but were still protected against M. tuberculosis, suggesting that CD4(+) Th1 T cells could compensate for the loss of CD8(+) T cells. Lastly, the order of the heterologous immunizations was critical. Long-lived vaccine protection was observed only when Ad5-ID93 was given as the boost following an ID93/GLA-SE prime. The homologous ID93/GLA-SE prime/boost regimen also induced long-lived protection. One of the correlates of protection between these two approaches was an increase in the total number of ID93-specific IFN-γ-producing CD4(+) T cells 6 mo following the last immunization. Our findings provide insight into the development of vaccines not only for tuberculosis, but other diseases requiring T cell immunity.