A reduced antigen load in vivo, rather than weak inflammation, causes a substantial delay in CD8+ T cell priming against Mycobacterium bovis (bacillus Calmette-Guérin)

Enhancing mucosal immunity by transient microbiota depletion

Tissue resident memory CD8+ T cells (Trm) are poised for immediate reactivation at sites of pathogen entry and provide optimal protection of mucosal surfaces. The intestinal tract represents a portal of entry for many infectious agents; however, to date specific strategies to enhance Trm responses at this site are lacking. Here, we present TMDI (Transient Microbiota Depletion-boosted Immunization), an approach that leverages antibiotic treatment to temporarily restrain microbiota-mediated colonization resistance, and favor intestinal expansion to high densities of an orally-delivered Listeria monocytogenes strain carrying an antigen of choice. By augmenting the local chemotactic gradient as well as the antigenic load, this procedure generates a highly expanded pool of functional, antigen-specific intestinal Trm, ultimately enhancing protection against infectious re-challenge in mice. We propose that TMDI is a useful model to dissect the requirements for optimal Trm responses in the intestine, and also a potential platform to devise novel mucosal vaccination approaches.

Biodegradable Microneedle Patch Delivering Antigenic Peptide-Hyaluronate Conjugate for Cancer Immunotherapy

Antigenic peptide-delivery systems have been extensively investigated to harness the immune system for cancer therapy. Cytotoxic T-cell epitope peptide can induce an antigen-specific CD8⁺ T-cell response, which subsequently inhibits the growth of antigen-bearing tumors. However, there are only a few facile tailored delivery systems of antigenic peptide for effective cancer immunotherapy. Here, we developed a biodegradable microneedle patch delivering a hyaluronate (HA)-antigenic peptide conjugate for prophylactic cancer immunotherapy. Cytotoxic T-cell epitope peptide (SIINFEKL) was conjugated to HA, which was loaded into a biodegradable HA microneedle (MN) patch to efficiently deliver an antigen to the immune system in the skin. HA could act as a transdermal vaccine carrier eliciting strong immune responses by the efficient stimulation of immunocompetent cells. The HA-SIINFEKL conjugates loaded into biodegradable MNs were localized near the MN administration site, exhibiting long-term residence for more than 24 h post-administration. Remarkably, a single transdermal vaccination with the MN patch containing HA-SIINFEKL conjugates resulted in a statistically significant inhibition of tumor growth in B16 melanoma model mice by enhancing antigen-specific cytotoxic T-cell responses.

Intranasal Immunization with DnaK Protein Induces Protective Mucosal Immunity against Tuberculosis in CD4-Depleted Mice

Mycobacterium tuberculosis (Mtb) remains a global health challenge due to the limited efficacy of the Mtb vaccine in current use, Bacillus Calmette-Guérin (BCG). To date, there is no available vaccine for immunocompromised individuals. Thus, there is an urgent need to develop a new vaccine candidate which can induce mucosal immunity in hosts with different immune statuses. DnaK (HSP70) has been shown to induce protective immunity against Mtb infection when administered by DNA vaccine; however, the protection is inferior to that induced by the BCG vaccine. In our study, we vaccinated C57BL/6J mice with DnaK protein alone. Subcutaneous or intranasal vaccination with DnaK generated IFNγ-secreting CD4⁺ T cells in the spleen, but only intranasal vaccination generated IL-17-releasing CD4⁺ T cells in the lungs, even when circulating CD4⁺ T cells were diminished. Furthermore, intranasal vaccination with DnaK generated tissue resident CD4⁺ T cells in the lungs. Vaccination with DnaK alone resulted in protective immunity comparable to BCG vaccination against tuberculosis in mice. Our results demonstrate that intranasal vaccination with DnaK can generate mucosal immunity in immunocompromised or immunocompetent mice and DnaK vaccination can generate protection against Mtb similar to BCG, underscoring its potential utility as an Mtb vaccine candidate in humans.

An Archaeosome-Adjuvanted Vaccine and Checkpoint Inhibitor Therapy Combination Significantly Enhances Protection from Murine Melanoma

Archaeosomes constitute archaeal lipid vesicle vaccine adjuvants that evoke a strong CD8⁺ T cell response to antigenic cargo. Therapeutic treatment of murine B16-ovalbumin (B16-OVA) melanoma with archaeosome-OVA eliminates small subcutaneous solid tumors; however, they eventually resurge despite an increased frequency of circulating and tumor infiltrating OVA-CD8⁺ T cells. Herein, a number of different approaches were evaluated to improve responses, including dose number, interval, and the combination of vaccine with checkpoint inhibitors. Firstly, we found that tumor protection could not be enhanced by repetitive and/or delayed boosting to maximize the CD8⁺ T cell number and/or phenotype. The in vivo cytotoxicity of vaccine-induced OVA-CD8⁺ T cells was impaired in tumor-bearing mice. Additionally, tumor-infiltrating OVA-CD8⁺ T cells had an increased expression of programmed cell death protein-1 (PD-1) compared to other organ compartments, suggesting impaired function. Combination therapy of tumor-bearing mice with the vaccine archaeosome-OVA, and α-CTLA-4 administered concurrently as well as α-PD-1 and an α-PD-L1 antibody administered starting 9 days after tumor challenge given on a Q3Dx4 schedule (days 9, 12, 15 and 18), significantly enhanced survival. Following multi-combination therapy ~70% of mice had rapid tumor recession, with no detectable tumor mass after >80 days in comparison to a median survival of 17-22 days for untreated or experimental groups receiving single therapies. Overall, archaeosomes offer a powerful platform for delivering cancer antigens when used in combination with checkpoint inhibitor immunotherapies.

Poly(I:C)-Encapsulating Nanoparticles Enhance Innate Immune Responses to the Tuberculosis Vaccine Bacille Calmette-Guérin (BCG) via Synergistic Activation of Innate Immune Receptors

The attenuated live vaccine strain bacille Calmette-Guérin (BCG) is currently the only available vaccine against tuberculosis (TB), but is largely ineffective against adult pulmonary TB, the most common disease form. This is in part due to BCG's ability to interfere with the host innate immune response, a feature that might be targeted to enhance the potency of this vaccine. Here, we investigated the ability of chitosan-based nanoparticles (pIC-NPs) containing polyinosinic-polycytidylic acid (poly(I:C)), an inducer of innate immunity via Toll-like receptor 3 (TLR3), to enhance the immunogenicity of BCG in mouse bone marrow derived macrophages (BMDM) in vitro. Incorporation of poly(I:C) into NPs protected it against degradation by ribonucleases and increased its uptake by mouse BMDM. Whereas soluble poly(I:C) was ineffective, pIC-NPs strongly enhanced the proinflammatory immune response of BCG-infected macrophages in a synergistic fashion, as evident by increased production of cytokines and induction of nitric oxide synthesis. Using macrophages from mice deficient in key signaling molecules involved in the pathogen recognition response, we identified combined activation of MyD88- and TRIF-dependent TLR signaling pathways to be essential for the synergistic effect between BCG and NP. Moreover, synergy was strongly dependent on the order of the two stimuli, with TLR activation by BCG functioning as the priming event for the subsequent pIC-NP stimulus, which acted through an auto-/paracrine type I interferon (IFN) feedback loop. Our results provide a foundation for a promising new approach to enhance BCG-vaccine immunogenicity by costimulation with NPs. They also contribute to a molecular understanding of the observed synergistic interaction between the pIC-NPs and BCG vaccine.

Layer-by-layer nanocoating of live Bacille-Calmette-Guérin mycobacteria with poly(I:C) and chitosan enhances pro-inflammatory activation and bactericidal capacity in murine macrophages

Tuberculosis (TB) is a major disease burden globally causing more than 1.5 million deaths per year. The attenuated live vaccine strain Bacille Calmette-Guérin (BCG), although providing protection against childhood TB, is largely ineffective against adult pulmonary TB. A major aim therefore is to increase the potency of the BCG vaccine to generate stronger and more sustained immunity against TB. Here, we investigated the use of layer-by-layer (LbL) nanocoating of the surface of live BCG with several layers of polyinosinic-polycytidylic acid (poly(I:C)), a strong inducer of cell-mediated immunity, and the biodegradable polysaccharide chitosan to enhance BCG immunogenicity. Nanocoating of live BCG did not affect bacterial viability or growth in vitro but induced killing of the BCG in infected mouse bone marrow-derived macrophages and enhanced macrophage production of pro-inflammatory cytokines and expression of surface co-stimulatory molecules relative to uncoated BCG. In addition, poly(I:C) surface-coated BCG, but not BCG alone or together with soluble poly(I:C), induced high production of nitric oxide (NO) and IL-12. These results argue that BCG and surface absorbed poly(I:C) act in a synergistic manner to elicit pro-inflammatory macrophage activation. In conclusion, nanocoating of live BCG with the immunostimulatory agent poly(I:C) may be an appropriate strategy to enhance and modulate host responses to the BCG vaccine.

Improving the Immunogenicity of the Mycobacterium bovis BCG Vaccine by Non-Genetic Bacterial Surface Decoration Using the Avidin-Biotin System

Current strategies to improve the current BCG vaccine attempt to over-express genes encoding specific M. tuberculosis (Mtb) antigens and/or regulators of antigen presentation function, which indeed have the potential to reshape BCG in many ways. However, these approaches often face serious difficulties, in particular the efficiency and stability of gene expression via nucleic acid complementation and safety concerns associated with the introduction of exogenous DNA. As an alternative, we developed a novel non-genetic approach for rapid and efficient display of exogenous proteins on bacterial cell surface. The technology involves expression of proteins of interest in fusion with a mutant version of monomeric avidin that has the feature of reversible binding to biotin. Fusion proteins are then used to decorate the surface of biotinylated BCG. Surface coating of BCG with recombinant proteins was highly reproducible and stable. It also resisted to the freeze-drying shock routinely used in manufacturing conventional BCG. Modifications of BCG surface did not affect its growth in culture media neither its survival within the host cell. Macrophages phagocytized coated BCG bacteria, which efficiently delivered their surface cargo of avidin fusion proteins to MHC class I and class II antigen presentation compartments. Thereafter, chimeric proteins corresponding to a surrogate antigen derived from ovalbumin and the Mtb specific ESAT6 antigen were generated and tested for immunogenicity in vaccinated mice. We found that BCG displaying ovalbumin antigen induces an immune response with a magnitude similar to that induced by BCG genetically expressing the same surrogate antigen. We also found that BCG decorated with Mtb specific antigen ESAT6 successfully induces the expansion of specific T cell responses. This novel technology, therefore, represents a practical and effective alternative to DNA-based gene expression for upgrading the current BCG vaccine.

mRNA Structural constraints on EBNA1 synthesis impact on in vivo antigen presentation and early priming of CD8+ T cells

Recent studies have shown that virally encoded mRNA sequences of genome maintenance proteins from herpesviruses contain clusters of unusual structural elements, G-quadruplexes, which modulate viral protein synthesis. Destabilization of these G-quadruplexes can override the inhibitory effect on self-synthesis of these proteins. Here we show that the purine-rich repetitive mRNA sequence of Epstein-Barr virus encoded nuclear antigen 1 (EBNA1) comprising G-quadruplex structures, limits both the presentation of MHC class I-restricted CD8⁺ T cell epitopes by CD11c⁺ dendritic cells in draining lymph nodes and early priming of antigen-specific CD8⁺ T-cells. Destabilization of the G-quadruplex structures through codon-modification significantly enhanced in vivo antigen presentation and activation of virus-specific T cells. Ex vivo imaging of draining lymph nodes by confocal microscopy revealed enhanced antigen-specific T-cell trafficking and APC-CD8⁺ T-cell interactions in mice primed with viral vectors encoding a codon-modified EBNA1 protein. More importantly, these antigen-specific T cells displayed enhanced expression of the T-box transcription factor and superior polyfunctionality consistent with the qualitative impact of translation efficiency. These results provide an important insight into how viruses exploit mRNA structure to down regulate synthesis of their viral maintenance proteins and delay priming of antigen-specific T cells, thereby establishing a successful latent infection in vivo. Furthermore, targeting EBNA1 mRNA rather than protein by small molecules or antisense oligonucleotides will enhance EBNA1 synthesis and the early priming of effector T cells, to establish a more rapid immune response and prevent persistent infection.

Maintenance proteins of viruses establishing latent infections regulate their synthesis to levels sufficient for maintaining persistent infection but below threshold levels for host immune detection. The Epstein-Barr virus maintenance protein, EBNA1, has recently been shown to contain unusual G-quadruplex structures within its repeat mRNA that reduces its translational efficiency. In this study we assess how modification of the EBNA1 mRNA repeat sequence to destabilize the native G-quadruplex structures and thereby increase translation, impacts on the activation of EBNA1-specific T cells in vivo. Mice primed with viral vectors encoding a more efficiently translated EBNA1 mRNA revealed increased trafficking of EBNA1-specific T cells, an enhanced functional profile and increased expression of transcription factors providing evidence for a potential link between mRNA translational efficiency and antigen presentation in vivo and the resultant impact on the functional programming of effector T cells. These findings suggest a novel approach to therapeutic development through the use of antisense strategies or small molecules targeting EBNA1 mRNA structure.

The use of directed evolution to create a stable and immunogenic recombinant BCG expressing a modified HIV-1 Gag antigen

Numerous features make Mycobacterium bovis BCG an attractive vaccine vector for HIV. It has a good safety profile, it elicits long-lasting cellular immune responses and in addition manufacturing costs are affordable. Despite these advantages it is often difficult to express viral antigens in BCG, which results in genetic instability and low immunogenicity. The aim of this study was to generate stable recombinant BCG (rBCG) that express high levels of HIV antigens, by modification of the HIV genes. A directed evolution process was applied to recombinant mycobacteria that expressed HIV-1 Gag fused to the green fluorescent protein (GFP). Higher growth rates and increased GFP expression were selected for. Through this process a modified Gag antigen was selected. Recombinant BCG that expressed the modified Gag (BCG[pWB106] and BCG[pWB206]) were more stable, produced higher levels of antigen and grew faster than those that expressed the unmodified Gag (BCG[pWB105]). The recombinant BCG that expressed the modified HIV-1 Gag induced 2 to 3 fold higher levels of Gag-specific CD4 T cells than those expressing the unmodified Gag (BCG[pWB105]). Mice primed with 10⁷ CFU BCG[pWB206] and then boosted with MVA-Gag developed Gag-specific CD8 T cells with a frequency of 1343±17 SFU/10⁶ splenocytes, 16 fold greater than the response induced with MVA-Gag alone. Levels of Gag-specific CD4 T cells were approximately 5 fold higher in mice primed with BCG[pWB206] and boosted with MVA-Gag than in those receiving the MVA-Gag boost alone. In addition mice vaccinated with BCG[pWB206] were protected from a surrogate vaccinia virus challenge.

Role for Gr-1+ cells in the control of high-dose Mycobacterium bovis recombinant BCG

Mycobacterium bovis bacillus Calmette-Guérin (BCG) is an attractive target for development as a live vaccine vector delivering transgenic antigens from HIV and other pathogens. Most studies aimed at defining the clearance of BCG have been performed at doses between 10(2) and 10(4) CFU. Interestingly, however, recombinant BCG (rBCG) administered at doses of >10(6) CFU effectively generates antigen-specific T-cell responses and primes for heterologous boost responses. Thus, defining clearance at high doses might aid in the optimization of rBCG as a vector. In this study, we used bioluminescence imaging to examine the kinetics of rBCG transgene expression and clearance in mice immunized with 5 × 10(7) CFU rBCG expressing luciferase. Similar to studies using low-dose rBCG, our results demonstrate that the adaptive immune response is necessary for long-term control of rBCG beginning 9 days after immunizing mice. However, in contrast to these reports, we observed that the majority of mycobacterial antigen was eliminated prior to day 9. By examining knockout and antibody-mediated depletion mouse models, we demonstrate that the rapid clearance of rBCG occurs in the first 24 h and is mediated by Gr-1(+) cells. As Gr-1(+) granulocytes have been described as having no impact on BCG clearance at low doses, our results reveal an unappreciated role for Gr-1(+) neutrophils and inflammatory monocytes in the clearance of high-dose rBCG. This work demonstrates the potential of applying bioluminescence imaging to rBCG in order to gain an understanding of the immune response and increase the efficacy of rBCG as a vaccine vector.

Restoration of innate immune activation accelerates Th1-cell priming and protection following pulmonary mycobacterial infection

The immune mechanisms underlying delayed induction of Th1-type immunity in the lungs following pulmonary mycobacterial infection remain poorly understood. We have herein investigated the underlying immune mechanisms for such delayed responses and whether a selected innate immune-modulating strategy can accelerate Th1-type responses. We have found that, in the early stage of pulmonary infection with attenuated Mycobacterium tuberculosis (M.tb H37Ra), the levels of infection in the lung continue to increase logarithmically until days 14 and 21 postinfection in C57BL/6 mice. The activation of innate immune responses, particularly DCs, in the lung is delayed. This results in a delay in the subsequent downstream immune responses including the migration of antigen-bearing DCs to the draining lymph node (dLN), the Th1-cell priming in dLN, and the recruitment of Th1 cells to the lung. However, single lung mucosal exposure to the TLR agonist FimH postinfection is able to accelerate protective Th1-type immunity via facilitating DC migration to the lung and draining lymph nodes, enhancing DC antigen presentation and Th1-cell priming. These findings hold implications for the development of immunotherapeutic and vaccination strategies and suggest that enhancement of early innate immune activation is a viable option for improving Th1-type immunity against pulmonary mycobacterial diseases.

Increasing the Vaccine Potential of Live M. bovis BCG by Coadministration with Plasmid DNA Encoding a Tuberculosis Prototype Antigen

The attenuated live M. bovis Bacille-Calmette-Guérin (BCG) is still the sole vaccine used against tuberculosis, but confers only variable efficacy against adult pulmonary tuberculosis (TB). Though no clear explanation for this limited efficacy has been given, different hypotheses have been advanced, such as the waning of memory T-cell responses, a reduced antigenic repertoire and the inability to induce effective CD8⁺ T-cell responses, which are known to be essential for latent tuberculosis control. In this study, a new BCG-based vaccination protocol was studied, in which BCG was formulated in combination with a plasmid DNA vaccine. As BCG is routinely administered to neonates, we have evaluated a more realistic approach of a simultaneous intradermal coadministration of BCG with pDNA encoding the prototype antigen, PPE44. Strongly increased T- and B-cell responses were observed with this protocol in C57BL/6 mice when compared to the administration of only BCG or in combination with an empty pDNA vector, as measured by Th1-type spleen cell cytokine secretion, specific IgG antibodies, as well as specific IFN-γ producing/cytolytic-CD8⁺ T-cells. Moreover, we observed a bystander activation induced by the coding plasmid, resulting in increased immune responses against other non-plasmid encoded, but BCG-expressed, antigens. In all, these results provide a proof of concept for a new TB vaccine, based on a BCG-plasmid DNA combination.

Priming with recombinant auxotrophic BCG expressing HIV-1 Gag, RT and Gp120 and boosting with recombinant MVA induces a robust T cell response in mice

In previous studies we have shown that a pantothenate auxotroph of Myocbacterium bovis BCG (BCGΔpanCD) expressing HIV-1 subtype C Gag induced Gag-specific immune responses in mice and Chacma baboons after prime-boost immunization in combination with matched rMVA and VLP vaccines respectively. In this study recombinant BCG (rBCG) expressing HIV-1 subtype C reverse transcriptase and a truncated envelope were constructed using both the wild type BCG Pasteur strain as a vector and the pantothenate auxotroph. Mice were primed with rBCG expressing Gag and RT and boosted with a recombinant MVA, expressing a polyprotein of Gag, RT, Tat and Nef (SAAVI MVA-C). Priming with rBCGΔpanCD expressing Gag or RT rather than the wild type rBCG expressing Gag or RT resulted in higher frequencies of total HIV-specific CD8⁺ T cells and increased numbers of T cells specific to the subdominant Gag and RT epitopes. Increasing the dose of rBCG from 10⁵ cfu to 10⁷ cfu also led to an increase in the frequency of responses to subdominant HIV epitopes. A mix of the individual rBCGΔpanCD vaccines expressing either Gag, RT or the truncated Env primed the immune system for a boost with SAAVI MVA-C and generated five-fold higher numbers of HIV-specific IFN-γ-spot forming cells than mice primed with rBCGΔpanCD containing an empty vector control. Priming with the individual rBCGΔpanCD vaccines or the mix and boosting with SAAVI MVA-C also resulted in the generation of HIV-specific CD4⁺ and CD8⁺ T cells producing IFN-γ and TNF-α and CD4⁺ cells producing IL-2. The rBCG vaccines tested in this study were able to prime the immune system for a boost with rMVA expressing matching antigens, inducing robust, HIV-specific T cell responses to both dominant and subdominant epitopes in the individual proteins when used as individual vaccines or in a mix.

Mechanisms of delayed anti-tuberculosis protection in the lung of parenteral BCG-vaccinated hosts: a critical role of airway luminal T cells

The immune mechanisms underlying unsatisfactory pulmonary mucosal protection by parenteral Bacillus Calmette-Guérin (BCG) immunization remain poorly understood. We found that parenteral BCG immunization failed to elicit airway luminal T cells (ALT) whereas it induced significant T cells in the lung interstitium. After Mycobacterium tuberculosis (M.tb) challenge, ALT remained missing for 10 days. The lack of ALT correlated with lack of lung protection for 14 days post-M.tb challenge. To further investigate the role of ALT, ALT were elicited in BCG-immunized animals by intranasal inoculation of M.tb culture-filtrate (CF) proteins. Installment of ALT by CF restored protection in the early phases of M.tb infection, which was linked to rapid increases in ALT, but not in lung interstitial T cells. Also, adoptive transfer of T cells to the airway lumen of BCG-immunized animals also accelerated protection. This study thus provides novel evidence that unsatisfactory lung protection by parenteral BCG immunization is due to delayed ALT recruitment after pulmonary M.tb exposure.

Escape from the Phagosome: The Explanation for MHC-I Processing of Mycobacterial Antigens?

Mycobacterium tuberculosis (Mtb) is thought to live in an altered phagosomal environment. In this setting, the mechanisms by which mycobacterial antigens access the major histocompatibility class I (MHC-I) processing machinery remain incompletely understood. There is evidence that Mtb antigens can be processed in both endocytic and cytosolic environments, with different mechanisms being proposed for how Mtb antigens can access the cytosol. Recently, electron microscopy was used to demonstrate that Mtb has the potential to escape the phagosome and reside in the cytosol. This was postulated as the primary mechanism by which Mtb antigens enter the MHC-I processing and presentation pathway. In this commentary, we will review data on the escape of Mtb from the cytosol and whether this escape is required for antigen presentation to CD8⁺ T cells.

Rapid multitarget immunomagnetic separation through programmable DNA linker displacement

Immunomagnetic separation has become an essential tool for high-throughput and low-cost isolation of biomolecules and cells from heterogeneous samples. However, as magnetic selection is essentially a "black-and-white" assay, its application has been largely restricted to single-target and single-parameter studies. To address this issue, we have developed an immunomagnetic separation technology that can quickly sort multiple targets in high yield and purity using selectively displaceable DNA linkers. We envision that this technology will be readily adopted for experiments requiring high-throughput selection of multiple targets or further adapted for selection of a single target based on multiple surface epitopes.

Partitioning regulatory mechanisms of within-host malaria dynamics using the effective propagation number

Immune clearance and resource limitation (via red blood cell depletion) shape the peaks and troughs of malaria parasitemia, which in turn affect disease severity and transmission. Quantitatively partitioning the relative roles of these effects through time is challenging. Using data from rodent malaria, we estimated the effective propagation number, which reflects the relative importance of contrasting within-host control mechanisms through time and is sensitive to the inoculating parasite dose. Our analysis showed that the capacity of innate responses to restrict initial parasite growth saturates with parasite dose and that experimentally enhanced innate immunity can affect parasite density indirectly via resource depletion. Such a statistical approach offers a tool to improve targeting of drugs or vaccines for human therapy by revealing the dynamics and interactions of within-host regulatory mechanisms.

Caspase-3 is transiently activated without cell death during early antigen driven expansion of CD8(+) T cells in vivo

Background

CD8⁺ T cell responses develop rapidly during infection and are swiftly reduced during contraction, wherein >90% of primed CD8⁺ T cells are eliminated. The role of apoptotic mechanisms in controlling this rapid proliferation and contraction of CD8⁺ T cells remains unclear. Surprisingly, evidence has shown non-apoptotic activation of caspase-3 to occur during in vitro T-cell proliferation, but the relevance of these mechanisms to in vivo CD8⁺ T cell responses has yet to be examined.

Methods and Findings

We have evaluated the activity of caspase-3, a key downstream inducer of apoptosis, throughout the entirety of a CD8⁺ T cell response. We utilized two infection models that differ in the intensity, onset and duration of antigen-presentation and inflammation. Expression of cleaved caspase-3 in antigen specific CD8⁺ T cells was coupled to the timing and strength of antigen presentation in lymphoid organs. We also observed coordinated activation of additional canonical apoptotic markers, including phosphatidylserine exposure. Limiting dilution analysis directly showed that in the presence of IL7, very little cell death occurred in both caspase-3^hi and caspase-3^low CD8⁺ T cells. The expression of active caspase-3 peaked before effector phenotype (CD62L^low) CD8⁺ T cells emerged, and was undetectable in effector-phenotype cells. In addition, OVA-specific CD8⁺ cells remained active caspase-3^low throughout the contraction phase.

Conclusions

Our results specifically implicate antigen and not inflammation in driving activation of apoptotic mechanisms without cell death in proliferating CD8⁺ T cells. Furthermore, the contraction of CD8⁺ T cell response following expansion is likely not mediated by the key downstream apoptosis inducer, caspase-3.

Regulatory T cells induced by Mycobacterium chelonae sensitization influence murine responses to bacille Calmette-Guerin

The efficacy of live Mycobacterium bovis BCG as a tuberculosis vaccine is highly varied globally. Differential sensitization to environmental mycobacteria prior to BCG vaccination may prime immune effects leading to this variation, but the precise immune mechanisms and cell types involved in this phenomenon are unknown. We hypothesized that pre-vaccination sensitization to environmental mycobacteria induces mycobacterium-specific Tregs that suppress responses to BCG. This was investigated by testing Treg responses following priming of BALB/c mice by i.p. immunization with heat-killed CHE. Such mice produced higher levels of IL-10 before and after intranasal, live BCG administration and had fewer lung inflammatory cells post-BCG, relative to nonsensitized mice. In CHE-sensitized mice, the percentage of splenic CD4+CD25+ cells expressing Foxp3 amongst total lymphocytes was not elevated significantly, but these cells limited nonspecific proliferation of CD4+CD25⁻ effector cells upon coculture and promoted higher expression levels of CD103 and Foxp3 in response to BCG antigen stimulation than CD4+CD25+ cells from nonsensitized mice. In adoptive transfer experiments, naïve, WT mice receiving CD4+CD25+ cells from CHE-sensitized mice and then given live BCG intranasally had significantly elevated lung IL-10 levels, reduced frequencies of lung IL-2-producing cells, and lower lymphocyte numbers in the BAL. Therefore, CHE sensitization induced CD4+CD25+ Tregs with functional, suppressive activity on BCG responses in vitro and in vivo. Treg induction could therefore be one mechanism underlying how environmental mycobacteria priming modulates host responses to the BCG vaccine.

Regulation of antigen presentation by Mycobacterium tuberculosis: a role for Toll-like receptors

Mycobacterium tuberculosis survives in antigen-presenting cells (APCs) such as macrophages and dendritic cells. APCs present antigens in association with major histocompatibility complex (MHC) class II molecules to stimulate CD4(+) T cells, and this process is essential to contain M. tuberculosis infection. Immune evasion allows M. tuberculosis to establish persistent or latent infection in macrophages and results in Toll-like receptor 2 (TLR2)-dependent inhibition of MHC class II transactivator expression, MHC class II molecule expression and antigen presentation. This reduction of antigen presentation might reflect a general mechanism of negative-feedback regulation that prevents excessive T cell-mediated inflammation and that M. tuberculosis has subverted to create a niche for survival in infected macrophages and evasion of recognition by CD4(+) T cells.

IFN-gamma expressed by T cells regulates the persistence of antigen presentation by limiting the survival of dendritic cells

Ag presentation to T cells orchestrates the development of acquired immune response. Although it is considered that Ag presentation may persist at high levels during chronic infections, we have previously reported that in mice infected with bacillus Calmette-Guérin, Ag presentation gets drastically curtailed during the chronic stage of infection despite antigenic persistence. In this report we evaluated the mechanism of this curtailment. Ag presentation declined precipitously as the T cell response developed, and Ag presentation was not curtailed in mice that were deficient in CD8(+) T cells or MHC class II, suggesting that T cells regulate Ag presentation. Curtailment of Ag presentation was reduced in IFN-gamma-deficient mice, but not in mice with a deficiency/mutation in inducible NOS2, perforin, or Fas ligand. In hosts with no T cells (Rag1(-/-)), Ag presentation was not curtailed during the chronic stage of infection. However, adoptive transfer of wild-type, but not IFN-gamma(-/-), CD4(+) and CD8(+) T cells into Rag1-deficient hosts strongly curtailed Ag presentation. Increased persistence of Ag presentation in IFN-gamma-deficient hosts correlated to increased survival of dendritic cells, but not of macrophages, and was not due to increased stimulatory capacity of IFN-gamma-deficient dendritic cells. These results reveal a novel mechanism indicating how IFN-gamma prevents the persistence of Ag presentation, thereby preventing memory T cells from going into exhaustion.

Towards improved understanding of protective mechanisms induced by the BCG vaccine

EVALUATION OF: Ryan AA, Nambiar JK, Wozniak TM et al. Antigen load governs the differential priming of CD8 T cells in response to the bacille Calmette-Guérin vaccine or Mycobacterium tuberculosis infection. J. Immunol. 182(11), 7172-7177 (2009). Mycobacterium tuberculosis is the causative agent of human TB, which is responsible for 26% of all preventable deaths in the developing world. Mycobacterium bovis bacillus Calmette-Guérin (BCG) has been used as a vaccine against TB since 1921, with protection varying from 0 to 80%. Although the reasons for this variability are unclear, protection is thought to be mediated by Th1-type responses. CD8(+) T cells have been shown to play a role in the response to mycobacteria. However, the nature of the CD8(+) response induced by BCG requires further characterization. In the paper being evaluated, it was shown that the antigenic load is important for the induction of CD8(+) T-cell responses and subsequent protective responses. Identification of the factors affecting induction of CD8(+) responses against mycobacteria will facilitate studies to characterize their nature and provide a platform for the development of systems seeking to improve on protection conferred by BCG.

Coxsackievirus B3 inhibits antigen presentation in vivo, exerting a profound and selective effect on the MHC class I pathway

Many viruses encode proteins whose major function is to evade or disable the host T cell response. Nevertheless, most viruses are readily detected by host T cells, and induce relatively strong T cell responses. Herein, we employ transgenic CD4⁺ and CD8⁺ T cells as sensors to evaluate in vitro and in vivo antigen presentation by coxsackievirus B3 (CVB3), and we show that this virus almost completely inhibits antigen presentation via the MHC class I pathway, thereby evading CD8⁺ T cell immunity. In contrast, the presentation of CVB3-encoded MHC class II epitopes is relatively unencumbered, and CVB3 induces in vivo CD4⁺ T cell responses that are, by several criteria, phenotypically normal. The cells display an effector phenotype and mature into multi-functional CVB3-specific memory CD4⁺ T cells that expand dramatically following challenge infection and rapidly differentiate into secondary effector cells capable of secreting multiple cytokines. Our findings have implications for the efficiency of antigen cross-presentation during coxsackievirus infection.

Many viruses—for example, large DNA viruses like smallpox virus and herpesviruses—encode several proteins whose major function is to combat the host's immune response, but these proteins usually battle in vain; in general, the mammalian immune system is sufficiently accomplished to penetrate this viral armor, allowing the infected animal to mount an immune response that can eradicate—or, at least, suppress—the infectious agent. Here, we show that coxsackievirus, a small RNA virus, carries a far more powerful punch than its larger DNA cousins; it almost entirely evades detection by host CD8⁺ T cells, which usually are one of the key components of an antiviral immune response. How does the virus achieve such success? Normally, when a virus infects a cell, certain host proteins capture small fragments of the virus and display them on the cell's surface, allowing them to be detected by the host immune system—usually, by cells called CD8⁺ T cells. We show here that coxsackievirus very effectively prevents these “flags” from reaching the cell surface in a form that can trigger naïve T cells to respond; in effect, the virus renders the cell “invisible” to CD8⁺ T cells, creating a cocoon in which the virus can multiply undisturbed by host immunity.

Selectively reduced intracellular proliferation of Salmonella enterica serovar typhimurium within APCs limits antigen presentation and development of a rapid CD8 T cell response

Ag presentation to CD8(+) T cells commences immediately after infection, which facilitates their rapid expansion and control of pathogen. This paradigm is not followed during infection with virulent Salmonella enterica serovar Typhimurium (ST), an intracellular bacterium that causes mortality in susceptible C57BL/6J mice within 7 days and a chronic infection in resistant mice (129 x 1SvJ). Infection of mice with OVA-expressing ST results in the development of a CD8(+) T cell response that is detectable only after the second week of infection despite the early detectable bacterial burden. The mechanism behind the delayed CD8(+) T cell activation was evaluated, and it was found that dendritic cells/macrophages or mice infected with ST-OVA failed to present Ag to OVA-specific CD8(+) T cells. Lack of early Ag presentation was not rescued when mice or dendritic cells/macrophages were infected with an attenuated aroA mutant of ST or with mutants having defective Salmonella pathogenicity island I/II genes. Although extracellular ST proliferated extensively, the replication of ST was highly muted once inside macrophages. This muted intracellular proliferation of ST resulted in the generation of poor levels of intracellular Ag and peptide-MHC complex on the surface of dendritic cells. Additional experiments revealed that ST did not actively inhibit Ag presentation, rather it inhibited the uptake of another intracellular pathogen, Listeria monocytogenes, thereby causing inhibition of Ag presentation against L. monocytogenes. Taken together, this study reveals a dichotomy in the proliferation of ST and indicates that selectively reduced intracellular proliferation of virulent pathogens may be an important mechanism of immune evasion.

Incorporation of NKT cell-activating glycolipids enhances immunogenicity and vaccine efficacy of Mycobacterium bovis bacillus Calmette-Guerin

The attenuated strain of Mycobacterium bovis known as bacille Calmette-Guérin (BCG) has been widely used as a vaccine for prevention of disease by Mycobacterium tuberculosis, but with relatively little evidence of success. Recent studies suggest that the failure of BCG may be due to its retention of immune evasion mechanisms that delay or prevent the priming of robust protective cell-mediated immunity. In this study, we describe an approach to enhance the immunogenicity of BCG by incorporating glycolipid activators of CD1d-restricted NKT cells, a conserved T cell subset with the potential to augment many types of immune responses. A method was developed for stably incorporating two forms of the NKT cell activator alpha-galactosylceramide into live BCG organisms, and the impact of this on stimulation of T cell responses and protective antimycobacterial immunity was evaluated. We found that live BCG containing relatively small amounts of incorporated alpha-galactosylceramide retained the ability to robustly activate NKT cells. Compared with immunization with unmodified BCG, the glycolipid-modified BCG stimulated increased maturation of dendritic cells and markedly augmented the priming of Ag-specific CD8(+) T cells responses. These effects were correlated with improved protective effects of vaccination in mice challenged with virulent M. tuberculosis. These results support the view that mycobacteria possess mechanisms to avoid stimulation of CD8(+) T cell responses and that such responses contribute significantly to protective immunity against these pathogens. Our findings raise the possibility of a simple modification of BCG that could yield a more effective vaccine for control of tuberculosis.

A prime-boost immunisation regimen using recombinant BCG and Pr55(gag) virus-like particle vaccines based on HIV type 1 subtype C successfully elicits Gag-specific responses in baboons

Mycobacterium bovis BCG is considered an attractive live bacterial vaccine vector. In this study, we investigated the immune response of baboons to a primary vaccination with recombinant BCG (rBCG) constructs expressing the gag gene from a South African HIV-1 subtype C isolate, and a boost with HIV-1 subtype C Pr55(gag) virus-like particles (Gag VLPs). Using an interferon enzyme-linked immunospot assay, we show that although these rBCG induced only a weak or an undetectable HIV-1 Gag-specific response on their own, they efficiently primed for a Gag VLP boost, which strengthened and broadened the immune responses. These responses were predominantly CD8+ T cell-mediated and recognised similar epitopes as those targeted by humans with early HIV-1 subtype C infection. In addition, a Gag-specific humoral response was elicited. These data support the development of HIV-1 vaccines based on rBCG and Pr55(gag) VLPs.

IL-15 protects antigen-specific CD8+ T cell contraction after Mycobacterium bovis bacillus Calmette-Guérin infection

We reported previously that IL-15 plays a critical role in protecting effector CD8(+) T cells from apoptosis during the contraction phase following acute infection with Listeria monocytogenes by inducing antiapoptotic molecules. In the present study, we examined the effects of in vivo administration of rIL-15 on contraction of CD8(+) T cells after chronic infection with Mycobacterium bovis BCG and on the efficacy of BCG vaccination against Mycobacterium tuberculosis infection. Antigen-specific CD8(+) T cells reached an expansion peak at approximately Day 21, followed by a contraction after inoculation with rBCG expressing OVA. In vivo administration of rIL-15 from Days 22 to 42 after BCG inoculation inhibited apoptosis of effector CD8(+) T cells by up-regulating their Bcl-2 expression, resulting in a significant increase of antigen-specific memory CD8(+) T cells producing IFN-gamma. However, the IL-15 treatment did not elicit improved efficacy of BCG vaccination against M. tuberculosis. These results suggest that IL-15 plays a critical role in protecting activated CD8(+) T cells from apoptosis during the contraction phase following BCG inoculation, although IL-15 administration alone at the contraction phase might not be sufficient to protect the efficient memory T cell responses against subsequent infection with M. tuberculosis.

Recombinant Mycobacterium bovis BCG prime-recombinant adenovirus boost vaccination in rhesus monkeys elicits robust polyfunctional simian immunodeficiency virus-specific T-cell responses

While mycobacteria have been proposed as vaccine vectors because of their persistence and safety, little has been done systematically to optimize their immunogenicity in nonhuman primates. We successfully generated recombinant Mycobacterium bovis BCG (rBCG) expressing simian immunodeficiency virus (SIV) Gag and Pol as multigenic, nonintegrating vectors, but rBCG-expressing SIV Env was unstable. A dose and route determination study in rhesus monkeys revealed that intramuscular administration of rBCG was associated with local reactogenicity, whereas intravenous and intradermal administration of 10(6) to 10(8) CFU of rBCG was well tolerated. After single or repeat rBCG inoculations, monkeys developed high-frequency gamma interferon enzyme-linked immunospot responses against BCG purified protein derivative. However, the same animals developed only modest SIV-specific CD8(+) T-cell responses. Nevertheless, high-frequency SIV-specific cellular responses were observed in the rBCG-primed monkeys after boosting with recombinant adenovirus 5 (rAd5) expressing the SIV antigens. These cellular responses were of greater magnitude and more persistent than those generated after vaccination with rAd5 alone. The vaccine-elicited cellular responses were predominantly polyfunctional CD8(+) T cells. These findings support the further exploration of mycobacteria as priming vaccine vectors.

The race between infection and immunity: how do pathogens set the pace?

Infection is often referred to as a race between pathogen and immune response. This metaphor suggests that slower growing pathogens should be more easily controlled. However, a growing body of evidence shows that many chronic infections are caused by failure to control slow growing pathogens. The slow growth of pathogens seems to directly affect the kinetics of the immune response. Compared with the response to fast growing pathogens, the T-cell response to slow pathogens is delayed in its initiation, lymphocyte expansion is slow and the response often fails to clear the pathogen, leading to chronic infection. Understanding the 'rules of the race' for slow growing pathogens has important implications for vaccine design and immune control of many chronic infections.

Pathogen proliferation governs the magnitude but compromises the function of CD8 T cells

CD8+ T cell memory is critical for protection against many intracellular pathogens. However, it is not clear how pathogen virulence influences the development and function of CD8+ T cells. Salmonella typhimurium (ST) is an intracellular bacterium that causes rapid fatality in susceptible mice and chronic infection in resistant strains. We have constructed recombinant mutants of ST, expressing the same immunodominant Ag OVA, but defective in various key virulence genes. We show that the magnitude of CD8+ T cell response correlates directly to the intracellular proliferation of ST. Wild-type ST displayed efficient intracellular proliferation and induced increased numbers of OVA-specific CD8+ T cells upon infection in mice. In contrast, mutants with defective Salmonella pathogenicity island II genes displayed poor intracellular proliferation and induced reduced numbers of OVA-specific CD8+ T cells. However, when functionality of the CD8+ T cell response was measured, mutants of ST induced a more functional response compared with the wild-type ST. Infection with wild-type ST, in contrast to mutants defective in pathogenicity island II genes, induced the generation of mainly effector-memory CD8+ T cells that expressed little IL-2, failed to mediate efficient cytotoxicity, and proliferated poorly in response to Ag challenge in vivo. Taken together, these results indicate that pathogens that proliferate rapidly and chronically in vivo may evoke functionally inferior memory CD8+ T cells which may promote the survival of the pathogen.

Repeated BCG treatment of mouse bladder selectively stimulates small GTPases and HLA antigens and inhibits single-spanning uroplakins

BACKGROUND

Despite being a mainstay for treating superficial bladder carcinoma and a promising agent for interstitial cystitis, the precise mechanism of Bacillus Calmette-Guerin (BCG) remains poorly understood. It is particularly unclear whether BCG is capable of altering gene expression beyond its well-recognized pro-inflammatory effects and how this relates to its therapeutic efficacy. The objective of this study was to determine differentially expressed genes in the mouse bladder following repeated intravesical BCG therapy.

METHODS

Mice were transurethrally instilled with BCG or pyrogen-free on days 1, 7, 14, and 21. Seven days after the last instillation, urothelia along with the submucosa was removed and amplified ds-DNA was prepared from control- and BCG-treated bladder mucosa and used to generate suppression subtractive hybridization (SSH). Plasmids from control- and BCG-specific differentially expressed clones and confirmed by Virtual Northern were then purified and the inserts were sequenced and annotated. Finally, chromatin immune precipitation combined with real-time polymerase chain reaction assay (ChIP/Q-PCR) was used to validate SSH-selected transcripts.

RESULTS

Repeated intravesical BCG treatment induced an up regulation of genes associated with antigen presentation (B2M, HLA-A, HLA-DQA1, HLA-DQB2, HLA-E, HLA-G, IGHG, and IGH) and representatives of two IFNgamma-induced small GTPase families: the GBPs (GBP1, GBP2, and GBP5) and the p47GTPases (IIGTP1, IIGTP2, and TGTP). Genes expressed in saline-treated bladders but down-regulated by BCG included: the single-spanning uroplakins (UPK3a and UPK2), SPRR2G, GSTM5, and RSP 19.

CONCLUSION

Here we introduced a hypothesis-generator approach to determine key genes involved in the urothelium/sumbmucosa responses to BCG therapy. Urinary bladder responds to repeated BCG treatment by up-regulating not only antigen presentation-related genes, but also GBP and p47 small GTPases, both potentially serving to mount a resistance to the replication of the Mycobacterium. It will be of tremendous future interest to determine whether these immune response cascades play a role in the anti-cancer effects exerted by BCG.