Vaccination against tuberculosis: is BCG more sinned against than sinner?

Oral delivery of Mycobacterium bovis BCG in a lipid formulation induces resistance to pulmonary tuberculosis in mice

A lipid-based formulation has been developed for oral delivery of Mycobacterium bovis bacille Calmette-Guérin (BCG) vaccine. The formulated M. bovis BCG was fed to BALB/c mice to test for immune responses and protection against M. bovis infection. The immune responses included antigen-specific cytokine responses, spleen cell proliferation, and lymphocyte-mediated macrophage inhibition of M. bovis. Oral delivery of formulated M. bovis BCG to mice induced strong splenic gamma interferon levels and macrophage inhibition of virulent M. bovis compared with results with nonformulated M. bovis BCG. Formulated oral M. bovis BCG significantly reduced the bacterial burden in the spleen and lungs of mice following aerosol challenge with virulent M. bovis. Our data suggest that oral delivery of formulated M. bovis BCG is an effective means of inducing protective immune responses against tuberculosis. Lipid-based, orally delivered mycobacterial vaccines may be a safe and practical method of controlling tuberculosis in humans and animals.

Optimal models to evaluate the protective efficacy of tuberculosis vaccines

BCG has been used widely as a vaccine to prevent tuberculosis (TB) for 80 years, yet there is still considerable controversy about its efficacy. Many experimental variables have obscured the true efficacy of BCG. The absence of appropriate animal models for the study of protective efficacy and the lack of in vitro correlates of protective immunity have impeded progress. Laboratory animal studies, which have contributed to understanding the pathogenesis, heritability of resistance and immunology of TB, have failed to identify the immunological pathways necessary for protective immunity. In recent years, cattle and deer, which are naturally susceptible to TB, have been used to study protective immunity in vaccinated animals, challenged with virulent bacteria. A deer TB infection model has been developed that can measure protection against TB infection or the development of disease. Data from this model show that, providing live BCG is administered in a short interval prime-boost protocol, significant protection against infection and disease can be obtained. Single dose vaccine provides suboptimal protection that attenuates pathology but does not prevent infection. Low dose BCG vaccine (10(4)cfu), administered in a prime-boost protocol, sufficient to prevent infection, does not cause conversion to delayed type hypersensitivity or produce unacceptable side-effects. Immune memory for protection against infection persists at optimal levels for at least 12 months post vaccination. Used optimally, BCG produces good levels of protection against TB and improved protocols or its use should be explored, before attempts are made to replace it with new-generation vaccines. It is now possible to integrate the fundamental information obtained from laboratory animals with studies of functional immune protection in target host species. Justification for the use of TB vaccines for domestic livestock under field conditions, must be underpinned by scientific evidence that they provide acceptable levels of protection long term.

Mycobacterium bovis-infected cervine alveolar macrophages secrete lymphoreactive lipid antigens

Tuberculosis is caused by intracellular bacteria belonging to the genus Mycobacterium, including M. tuberculosis and M. bovis. Alveolar macrophages (AMs) are the primary host cell for inhaled mycobacteria. However, little is known about the mechanisms by which infected AMs can process and present mycobacterial antigens to primed lymphocytes and how these responses may affect ensuing protection in the host. In the present study, we sought to determine whether AMs from a naturally susceptible host for Mycobacterium bovis (red deer) could produce and secrete soluble immunoreactive antigens following mycobacterial infection in vitro. Confluent monolayers of deer AMs were infected with either heat-killed or live virulent M. bovis or M. bovis BCG at a multiplicity of infection of 5:1 and cultured for 48 h. Culture supernatants were collected, concentrated, and tested for the presence of mycobacterial antigens in a lymphocyte proliferation assay by using peripheral blood mononuclear cells from M. bovis-sensitized or naive deer. Supernatants derived from macrophages which had been infected with live bacilli stimulated the proliferation of antigen-sensitized, but not naive, lymphocytes. Supernatants derived from uninoculated AMs or AMs inoculated with heat-killed bacilli failed to stimulate lymphocyte proliferation. The lymphoproliferative activity was retained following lipid extraction of the supernatants, which were free of amino groups as determined by thin-layer chromatography. These results demonstrate that mycobacteria which are actively growing within AMs produce lipids which are secreted into the extracellular milieu and that these lipids are recognized by lymphocytes from mycobacterium-primed hosts. We suggest that mycobacterial lipids are released from AMs following aerosol infection in vivo and that they play an important role in the early immune response to tuberculosis.

Tuberculosis in deer: perceptions, problems and progress

Since the emergence of deer farming as an alternative farming enterprise over the past 30 years, there has been an increasing awareness of the potential threat posed by tuberculosis (TB) to domesticated deer. TB, caused by Mycobacterium bovis, has been found in deer in every country involved with deer farming. Different types of TB control policies, which vary from whole-herd depopulation to selective testing and slaughter of reactor animals, have been implemented. Extensive research has been carried out, incorporating modern microbiological and immunological concepts and advanced molecular methodologies, to find new solutions for the eradication of TB from domesticated deer. This work has resulted in valuable new insights into the aetiology, transmission, pathogenesis, diagnosis, prevention and heritability of resistance to M. bovis infection in ruminants. This knowledge has complemented the existing literature database on bovine and human TB and will provide new strategies for improved diagnosis, vaccination and selective breeding to control TB, which should be relevant for human, domestic livestock and wildlife populations.

MHC class II-restricted, CD4(+) T-cell proliferative responses of peripheral blood mononuclear cells from Mycobacterium bovis-infected white-tailed deer

White-tailed deer are significant wildlife reservoirs of Mycobacterium bovis for cattle, predators, and, potentially, humans. Infection of cattle with M. bovis stimulates an antigen-specific T-cell response, with both CD4(+) and CD8(+) cells implicated in protective immunity. Few studies, however, have examined lymphocyte subset responses to experimental M. bovis infection of white-tailed deer. In this study, a flow cytometric proliferation assay was used to determine the relative contribution of individual peripheral blood mononuclear cell subsets of M. bovis-infected white-tailed deer in the recall response to M. bovis antigen. Naive deer were challenged with M. bovis by cohabitation with infected deer. These M. bovis-challenged deer developed significant in vivo (delayed-type hypersensitivity) and in vitro (proliferative) responses to M. bovis purified protein derivative (PPD). At necropsy, typical tuberculous lesions containing M. bovis were detected within lungs and lung-associated lymph nodes of infected deer. The predominant subset of lymphocytes that proliferated in response to in vitro stimulation with PPD was the CD4(+) subset. Minimal proliferative responses were detected from CD8(+), gamma delta TCR(+), and B-cells. Addition of monoclonal antibodies specific for MHC II antigens, but not MHC I or CD1 antigens, abrogated the proliferative response. Together, these findings indicate that while CD4(+) cells from infected deer proliferate in the recall response to M. bovis antigens, this response is not sufficient to clear M. bovis and immunologic intervention may require stimulation of alternate subsets of lymphocytes.

Veterinary tuberculosis vaccine development

Tuberculosis caused by Mycobacterium bovis in domestic livestock and wildlife is a significant problem in many countries worldwide. Wildlife reservoirs of tuberculosis confound programs for tuberculosis eradication from domestic livestock. Successful vaccination against tuberculosis in domestic animals or wildlife could contribute to tuberculosis eradication. Bacille Calmette-Guérin (BCG) has been used as the prototype vaccine for domestic livestock and wildlife. The majority of studies have been carried out with BCG-vaccinated animals challenged experimentally with M. bovis. Although protection against disease has been evident in all these studies, protection against infection has rarely occurred. Results obtained with BCG vaccination of cattle, deer, ferrets, opossums, and rabbits are presented here and highlight the need for appropriate animal models for vaccination and control of the variables that influence the efficacy of BCG vaccine. Refinement of the existing animal models is essential for the advancement of tuberculosis vaccine research of relevance to animals and humans.

Vaccine protocols to optimise the protective efficacy of BCG

SETTING

A deer model has been developed to study protection produced with BCG vaccination, against infection and the development of pathology, following experimental intratonsilar infection with virulent Mycobacterium bovis.

OBJECTIVE

To determine how the dose of vaccine, the route of vaccination, the viability of the vaccine and exposure to glucocorticoids at the time of vaccination, may affect the protective efficacy of BCG vaccines.

DESIGN

Deer were vaccinated with BCG and later challenged with virulent M. bovis via the tonsilar route. Protection against infection and development of disease was evaluated at necropsy six months after challenge with M. bovis, by histological examination and microbial culture.

RESULTS

Significant protection against infection and disease were obtained following boosting with two low doses (5 x 10(4) cfu) or moderate doses (5 x 10(7) cfu) of live (freshly cultured and lyophilized) BCG. Inferior levels of protection were obtained with high dose (5 x 10(8) cfu) of live BCG. Similar levels of protection were found with vaccines given subcutaneously or via the tonsilar route. Killed vaccine in a mineral-oil adjuvant did not evoke protective immunity and treatment with dexamethasone prior to vaccination with live BCG ablated its efficacy. Protection against infection did not correlate with skin test delayed type hypersensitivity (DTH) or lymphocyte transformation to tuberculin.

CONCLUSIONS

Two doses of live BCG gave significant protection against experimental infection and disease caused by virulent M. bovis. Single dose vaccine protected against disease but not infection. Vaccines administered at a dosage which did not evoke DTH, provided protection against tuberculosis infection and disease.

Activation of an interleukin-4 mRNA-producing population of peripheral blood mononuclear cells after infection with Mycobacterium bovis or vaccination with killed, but not live, BCG

This study examines the expression of mRNA for the Th2 cytokine, interleukin-4 (IL-4). Peripheral blood mononuclear cells from deer infected with Mycobacterium bovis or vaccinated with live or killed M. bovis bacillus Calmette-Guérin (BCG) were cultured with mycobacterial antigens. IL-4 mRNA production was assayed using the polymerase chain reaction. Elevated levels of IL-4 mRNA were detected in response to at least one antigen preparation in all animals infected with M. bovis as compared with none of the non-infected control animals. After a primary immunization, elevated levels of IL-4 mRNA were detected in only a proportion of vaccinated animals and this did not correlate with whether the vaccine was live BCG or killed BCG in oil. After boosting, all the animals vaccinated with killed BCG in oil exhibited elevated IL-4 mRNA production whereas none of the animals vaccinated with live BCG showed elevated levels. The data suggest that IL-4 is turned off during the immune response to live BCG, that boosting of low-dose live BCG vaccine may be required to 'imprint' this signal and that this may be important in the development of protective immunity to tuberculosis. Killed BCG in adjuvant is not protective and as with experimental infection with virulent M. bovis it failed to switch off the IL-4 response. IL-4 may be useful as a diagnostic tool and as an in vitro marker of vaccine efficacy.

T cell responses to Mycobacterium bovis in red deer, a large animal model for tuberculosis

Red deer (Cervus elaphus) represent an appropriate large animal model to study the immunology of tuberculosis, being naturally susceptible to Mycobacterium bovis infection. Cell-mediated immune responses were investigated in deer displaying protective- or disease-type reactions, following immunization with M. bovis bacille Calmette-Guerin (BCG) or infection with virulent M. bovis, respectively. T cell responses were measured as antigen-dependent cell proliferation and production of T cell growth factor (TCGF) following in vitro stimulation with M. bovis antigens (live or heat-killed BCG, or PPD). T cells from immunized deer proliferated less in response to soluble denatured culture antigen (purified protein derivative, PPD) than to particulate BCG, although there were no differences in the magnitude of these responses between the two groups of animals. Cells derived from immunized deer produced less TCGF than cells from infected deer when stimulated with PPD in vitro, although responses to BCG antigens were similar between the two groups. The majority of TCGF activity was neutralized by anti-IL-2 antibodies, regardless of the animal group or source of antigen used for in vitro stimulation. After 7 days in vitro culture with antigen, blast cells staining positively for alpha beta (CD4, CD8) and gamma delta T cell receptors were recorded. The majority of blasts were CD4+, although in immunized deer fewer CD4+ blasts were produced following in vitro stimulation with PPD than with BCG antigens. These results, together with previous reports from our laboratory, represent the only detailed examinations of T cell responses to M. bovis in this naturally-susceptible ruminant species.

Strategies for the development of a vaccine against ringworm

Resolution of lesions and subsequent protection against ringworm is primarily associated with the development of a cell-mediated immune (CMI) response, in which stimulation of Type-1 lymphocytes and cytokines such as interleukin-2 (IL-2), IL-12 and interferon gamma are significant. Type-2 lymphocyte activation and antibody formation seem a feature of chronic disease states, rather than protection, and are antagonistic to a Type-1 cell response. Initial studies on ringworm vaccines should be directed at identifying and characterizing dermatophyte antigens elaborated during spore germination and early hyphal growth, and the method of their presentation which best potentiates Type-1 cell-associated events, and primes the recipient for a subsequent CMI response.

The cloning and sequencing of cervine interleukin 10

We report the cloning and sequencing of the cervine interleukin-10 gene. Specific cDNA was amplified by PCR using primers based on the bovine sequence. This was cloned into pGEM 5Zf and several clones were sequenced. The 762 nucleotide product coded for a 179 amino acid protein which was 86% homologous with its bovine and 77% homologous with its human counterparts. There is a strongly hydrophobic signal sequence consisting of the first 20 amino acids and a potential glycosylation site at amino acids 134-136. There are three regions, comprising 34% of the protein, which show complete homology between the cervine, bovine and human sequences. The transcription of the gene was shown by Northern Blotting where a single, 1.8kb, mRNA transcript was detected 4-8 hours after activation of peripheral blood mononuclear cells with mitogen.

Cloning and expression of the cervine interleukin 4 gene

Interleukin 4 (IL-4), a cytokine produced by the T helper 2 subset of T lymphocytes is involved in up regulating antibody responses. A source of recombinant cervine IL-4 would be useful for studying the immune response of deer to tuberculosis. We report here the cloning, sequencing and expression of recombinant cervine IL-4. To achieve this mRNA was isolated from red deer (Cervus elaphus) mononuclear leucocytes. First strand cDNA was synthesized from the mRNA and the IL-4 cDNA was amplified, cloned and sequenced. The IL-4 cDNA is 408 bp in length and the deduced amino acid sequence is 92% homologous with the published bovine IL-4 amino acid sequence. IL-4 was expressed using the baculovirus expression system in Spodoptera frugiperda Sf9 cells. Northern blot, SDS-PAGE analysis and bioassay were used to confirm the expression of IL-4.