T- and B-cell epitopes in the secreted Mycobacterium bovis antigen MPB70 in mice

In Vivo Antigen Expression Regulates CD4 T Cell Differentiation and Vaccine Efficacy against Mycobacterium tuberculosis Infection

Tuberculosis, caused by Mtb, constitutes a global health crisis of massive proportions, and the impact of the current coronavirus disease 2019 (COVID-19) pandemic is expected to cause a rise in tuberculosis-related deaths. Improved vaccines are therefore needed more than ever, but a lack of knowledge on protective immunity hampers their development.

New vaccines are urgently needed against Mycobacterium tuberculosis (Mtb), which kills more than 1.4 million people each year. CD4 T cell differentiation is a key determinant of protective immunity against Mtb, but it is not fully understood how host-pathogen interactions shape individual antigen-specific T cell populations and their protective capacity. Here, we investigated the immunodominant Mtb antigen, MPT70, which is upregulated in response to gamma interferon (IFN-γ) or nutrient/oxygen deprivation of in vitro-infected macrophages. Using a murine aerosol infection model, we compared the in vivo expression kinetics of MPT70 to a constitutively expressed antigen, ESAT-6, and analyzed their corresponding CD4 T cell phenotype and vaccine protection. For wild-type Mtb, we found that in vivo expression of MPT70 was delayed compared to ESAT-6. This delayed expression was associated with induction of less differentiated MPT70-specific CD4 T cells but, compared to ESAT-6, also reduced protection after vaccination. In contrast, infection with an MPT70-overexpressing Mtb strain promoted highly differentiated KLRG1⁺CX3CR1⁺ CD4 T cells with limited lung-homing capacity. Importantly, this differentiated phenotype could be prevented by vaccination, and against the overexpressing strain, vaccination with MPT70 conferred protection similar to vaccination with ESAT-6. Together, our data indicate that high in vivo antigen expression drives T cells toward terminal differentiation and that targeted vaccination with adjuvanted protein can counteract this phenomenon by maintaining T cells in a protective less differentiated state. These observations shed new light on host-pathogen interactions and provide guidance on how future Mtb vaccines can be designed to tip the immune balance in favor of the host.

In vivo antigen expression regulates CD4 T cell differentiation and vaccine efficacy against Mycobacterium tuberculosis infection

New vaccines are urgently needed against Mycobacterium tuberculosis (Mtb), which kills more than 1.4 million people each year. CD4 T cell differentiation is a key determinant of protective immunity against Mtb, but it is not fully understood how host-pathogen interactions shape individual antigen-specific T cell populations and their protective capacity. Here, we investigated the immunodominant Mtb antigen, MPT70, which is upregulated in response to IFN-γ or nutrient/oxygen deprivation of in vitro infected macrophages. Using a murine aerosol infection model, we compared the in vivo expression kinetics of MPT70 to a constitutively expressed antigen, ESAT-6, and analysed their corresponding CD4 T cell phenotype and vaccine-protection. For wild-type Mtb, we found that in vivo expression of MPT70 was delayed compared to ESAT-6. This delayed expression was associated with induction of less differentiated MPT70-specific CD4 T cells but, compared to ESAT-6, also reduced protection after vaccination. In contrast, infection with an MPT70-overexpressing Mtb strain promoted highly differentiated KLRG1+CX3CR1+ CD4 T cells with limited lung-homing capacity. Importantly, this differentiated phenotype could be prevented by vaccination and, against the overexpressing strain, vaccination with MPT70 conferred similar protection as ESAT-6. Together our data indicate that high in vivo antigen expression drives T cells towards terminal differentiation and that targeted vaccination with adjuvanted protein can counteract this phenomenon by maintaining T cells in a protective less-differentiated state. These observations shed new light on host-pathogen interactions and provide guidance on how future Mtb vaccines can be designed to tip the immune-balance in favor of the host.

Proteomic characterisation of bovine and avian purified protein derivatives and identification of specific antigens for serodiagnosis of bovine tuberculosis

Background

Bovine purified protein derivative (bPPD) and avian purified protein derivative (aPPD) are widely used for bovine tuberculosis diagnosis. However, little is known about their qualitative and quantitative characteristics, which makes their standardisation difficult. In addition, bPPD can give false-positive tuberculosis results because of sequence homology between Mycobacterium bovis (M. bovis) and M. avium proteins. Thus, the objective of this study was to carry out a proteomic characterisation of bPPD, aPPD and an immunopurified subcomplex from bPPD called P22 in order to identify proteins contributing to cross-reactivity among these three products in tuberculosis diagnosis.

Methods

Trypsin digests of bPPD, aPPD and P22 were analysed by nanoscale liquid chromatography-electrospray ionization tandem mass spectrometry. Mice were immunised with bPPD or aPPD, and their serum was tested by indirect ELISA for reactivity against these preparations as well as against P22.

Results

A total of 456 proteins were identified in bPPD, 1019 in aPPD and 118 in P22; 146 of these proteins were shared by bPPD and aPPD, and 43 were present in all three preparations. Candidate proteins that may cause cross-reactivity between bPPD and aPPD were identified based on protein abundance and antigenic propensity. Serum reactivity experiments indicated that P22 may provide greater specificity than bPPD with similar sensitivity for ELISA-type detection of antibodies against M. tuberculosis complex.

Conclusion

The subpreparation from bPPD called P22 may be an alternative to bPPD for serodiagnosis of bovine tuberculosis, since it shares fewer proteins with aPPD than bPPD does, reducing risk of cross-reactivity with anti-M. avium antibodies.

Electronic supplementary material

The online version of this article (10.1186/s12014-017-9171-z) contains supplementary material, which is available to authorized users.

Reduced expression of antigenic proteins MPB70 and MPB83 in Mycobacterium bovis BCG strains due to a start codon mutation in sigK

Mycobacterium bovis Bacille Calmette-Guerin (BCG) strains are genetically and phenotypically heterogeneous. Expression of the antigenic proteins MPB70 and MPB83 is known to vary considerably across BCG strains; however, the reason for this phenotypic difference has remained unknown. By immunoblot, we separated BCG into high- and low-producing strains. By quantitative reverse transcription polymerase chain reaction (RT-PCR), we determined that transcription of the antigen-encoding genes, mpb70 and mpb83, follows the same strain pattern with mRNA levels reduced over 50-fold in low-producing strains. Transcriptome comparison of the same BCG strains by DNA microarray revealed two gene regions consistently downregulated in low-producing strains compared with high-producing strains, one including mpb70 (Rv2875) and mpb83 (Rv2873) and a second that includes the predicted sigma factor, sigK. DNA sequence analysis revealed a point mutation in the start codon of sigK in all low-producing BCG strains. Complementation of a low-producing strain, BCG Pasteur, with wild-type sigK fully restored MPB70 and MPB83 production. Microarray-based analysis and confirmatory RT-PCR of the complemented strains revealed an upregulation in gene transcription limited to the sigK and the mpb83/mpb70 gene regions. These data demonstrate that a mutation of sigK is responsible for decreased expression of MPB70 and MPB83 in low-producing BCG strains and provide clues into the role of Mycobacterium tuberculosis SigK.

Strategies for improved formulation and delivery of DNA vaccines to veterinary target species

Interest in DNA immunization of animals continues, despite the fact that immune responses induced by DNA vaccines are generally lower than those elicited by conventional vaccines. In attempts to enhance the immune response to DNA vaccines, individuals have tried a variety of immune modulators, cytokines, and costimulatory molecules, but these only boost immune responses marginally. These results clearly demonstrate that the major challenge to improving DNA-based vaccines is to improve the transfection efficiency. Gene gun and electroporation can increase transfection and improve immune responses significantly, but these technologies have not yet advanced to the stage of routine use in livestock. Hopefully, transfection efficiency can be increased further in a user-friendly manner to ensure that the benefits of using DNA vaccines become a reality.

Cellular immune responses induced in cattle by heterologous prime-boost vaccination using recombinant viruses and bacille Calmette-Guérin

The development of novel vaccine strategies to replace or supplement bacille Calmette-Guérin (BCG) is urgently required. Here we study, in cattle, the use of heterologous prime-boost strategies based on vaccination with BCG and the mycobacterial mycolyl transferase Ag85A (Rv3804c) expressed either in recombinant modified vaccinia virus Ankara (MVA85A) or attenuated fowlpox strain FP9 (FP85A). Five different vaccination schedules were tested in the first experiment: MVA85A followed by BCG (group 1); BCG followed by MVA85A (group 2); BCG followed by FP85A and then MVA85A (group 3); MVA85A followed by MVA85A and then FP85A (group 4); and FP85A followed by FP85A and then MVA85A (group 5). Vaccine-induced levels of cellular immunity were assessed by determining interferon-gamma (IFN-gamma) responses in vitro. Prime-boost protocols, using recombinant MVA and BCG in combination (groups 1-3), resulted in significantly higher frequencies of Ag85-specific IFN-gamma-secreting cells than the two viral vectors used in combination (P=0.0055), or BCG used alone (groups 2 and 3, P=0.04). The T-cell repertoires of the calves in all five groups were significantly broader following heterologous booster immunizations than after the primary immunization. In a second experiment, the effects of BCG\MVA85A heterologous prime-boost vaccination were compared with BCG\BCG homologous revaccination. The results suggested a higher Ag85A-specific response with a wider T-cell repertoire in the MVA85A-boosted calves than in the BCG\BCG-vaccinated calves. In conclusion therefore, the present report demonstrates the effectiveness of heterologous prime-boost strategies based on recombinant MVA and BCG to induce strong cellular immune responses in cattle and prioritise such vaccination strategies for rapid assessment of protective efficacy in this natural target species of tuberculosis.

Synthetic peptides identify promiscuous human Th1 cell epitopes of the secreted mycobacterial antigen MPB70

MPB70 is a secreted protein of Mycobacterium bovis and Mycobacterium tuberculosis which stimulates both cellular and humoral immune responses during infection with bovine and human tubercle bacilli. In addition, vaccination with MPB70 has been shown to induce Th1 cell responses and protection in animal models of tuberculosis. The present study was carried out to map the dominant human Th1 cell epitopes of MPB70 in relation to major histocompatibility complex (MHC) class II restriction in healthy subjects showing strong T-cell responses to complex mycobacterial antigens. Peripheral blood mononuclear cells (PBMC) from HLA-DR-typed donors were tested with complex mycobacterial antigens (whole-cell M. tuberculosis and M. tuberculosis culture filtrates), with MPB70 purified from the culture filtrate of M. bovis BCG Tokyo, and with 13 synthetic peptides (25-mers overlapping by 10 residues) covering the sequence of MPB70. The donors that responded to the complex antigens and MPB70 also responded to the cocktail of synthetic MPB70 peptides. Testing of PBMC with individual peptides showed that peptides p5 (amino acids [aa] 61 to 85), p6 (aa 76 to 100), p8 (aa 106 to 130), p12 (aa 166 to 190), and p13 (aa 181 to 193) were most frequently recognized in proliferation and gamma interferon (IFN-gamma) assays. Testing of antigen-specific CD4(+) T-cell lines with the individual peptides of MPB70 confirmed that peptides p8, p12, and p13 contain immunodominant Th1 cell epitopes of MPB70. MHC restriction analysis with HLA-typed donors showed that MPB70 and its immunodominant peptides were presented to T cells promiscuously. The T-cell lines responding to MPB70 and peptides p8, p12, and p13 in IFN-gamma assays mediated antigen-peptide-specific cytotoxic activity against monocytes/macrophages pulsed with the whole-protein antigen or the peptides. In conclusion, the promiscuous recognition of MPB70 and its immunodominant peptide defined epitopes (aa 106 to 130 and 166 to 193) by IFN-gamma-producing Th1 cells supports possible application of this secreted antigen to subunit vaccine design.

DNA injection in combination with electroporation: a novel method for vaccination of farmed ruminants

Injection of plasmid DNA encoding antigens into rodents followed by electroporation improved the immune response when compared with injection without electroporation (Widera et al. J Immunol 2000;164:4635-40; Zucchelli et al. J Virol 2000;74:11598-607; Kadowaki et al. Vaccine 2000;18:2779-88). The present study describes the extension of this technology to farm animals, by injecting plasmid DNA encoding mycobacterial antigens (MPB70, Ag85B and Hsp65) into the muscles of goats and cattle using two different types of electrodes, both allowing DNA delivery at the site of electroporation. The animals were vaccinated under local anaesthesia without any observed immediate or long-term distress or discomfort, or any behavioural signs of muscle damage or pathological changes after the electroporation. DNA-injected and electroporated goats showed increased humoral response after the primary vaccination when compared with nonelectroporated animals. Improved T-cell responses following electroporation were observed in hsp65 DNA-vaccinated cattle. DNA injection with or without electroporation did not compromise the specificity of the tuberculin skin test. In conclusion, a protocol applying in vivo electroporation free of side effects to farmed ruminants was established. In addition, we show that DNA vaccination in combination with electroporation can improve the primary immune responses to the encoded antigens.