Rabbit Model of Tuberculosis

Tuberculosis: a problem with persistence

Mycobacterium tuberculosis is one of most successful pathogens of mankind, infecting one-third of the global population and claiming two million lives every year. The ability of the bacteria to persist in the form of a long-term asymptomatic infection, referred to as latent tuberculosis, is central to the biology of the disease. The persistence of bacteria in superficially normal tissue was recognized soon after the discovery of the tubercle bacillus, and much of our knowledge about persistent populations of M. tuberculosis dates back to the first half of the last century. Recent advances in microbial genetics and host immunity provide an opportunity for renewed investigation of this persistent threat to human health.

The secret lives of the pathogenic mycobacteria

Pathogenic mycobacteria, including the causative agents of tuberculosis and leprosy, are responsible for considerable morbidity and mortality worldwide. A hallmark of these pathogens is their tendency to establish chronic infections that produce similar pathologies in a variety of hosts. During infection, mycobacteria reside in macrophages and induce the formation of granulomas, organized immune complexes of differentiated macrophages, lymphocytes, and other cells. This review summarizes our understanding of Mycobacterium-host cell interactions, the bacterial-granuloma interface, and mechanisms of bacterial virulence and persistence. In addition, we highlight current controversies and unanswered questions in these areas.

Experimental Mycobacterium tuberculosis infection of cynomolgus macaques closely resembles the various manifestations of human M. tuberculosis infection

Nonhuman primates were used to develop an animal model that closely mimics human Mycobacterium tuberculosis infection. Cynomolgus macaques were infected with low doses of virulent M. tuberculosis via bronchoscopic instillation into the lung. All monkeys were successfully infected, based on tuberculin skin test conversion and peripheral immune responses to M. tuberculosis antigens. Progression of infection in the 17 monkeys studied was variable. Active-chronic infection, observed in 50 to 60% of monkeys, was characterized by clear signs of infection or disease on serial thoracic radiographs and in other tests and was typified by eventual progression to advanced disease. Approximately 40% of monkeys did not progress to disease in the 15 to 20 months of study, although they were clearly infected initially. These monkeys had clinical characteristics of latent tuberculosis in humans. Low-dose infection of cynomolgus macaques appears to represent the full spectrum of human M. tuberculosis infection and will be an excellent model for the study of pathogenesis and immunology of this infection. In addition, this model will provide an opportunity to study the latent M. tuberculosis infection observed in approximately 90% of all infected humans.

Determinants of vaccine-induced resistance in animal models of pulmonary tuberculosis

A more effective vaccine will be essential if the global tuberculosis (TB) pandemic is ever to be controlled. A large number of new tuberculosis vaccines have been developed, representing the whole range of modern strategies for vaccine formulation and delivery. There is currently no alternative to testing these new vaccines in experimental animals challenged with virulent Mycobacteriurn tuberculosis in order to assess their protective efficacy. Although such testing is being carried out in several animal species (mice, guinea pigs, rabbits), all rational models include pulmonary challenge with a low dose of virulent mycobacteria. The quantitative measures for TB vaccines include increased survival, amelioration of clinical signs and symptoms (e.g. prevention of weight loss), decreased lesion size, reduction in bacillary loads in the lungs, and prevention of extrapulmonary dissemination and hematogenous reseeding of the lung. Although the ultimate objective of vaccination in humans is to prevent transmission to susceptible contacts, no such measurement is being used in animal studies of new vaccines. The validation of an immunological "correlate of protection" is urgently needed. Candidates for such a correlate include antigen-specific interferon-gamma production by T cells of the memory phenotype (CD45RB(high) or mycobacterial killing by macrophages co-cultured with immune T cells. Additional animal models must be developed for vaccines designed to prevent endogenous reactivation or exogenous reinfection, or to be used as a adjunct to chemotherapy.

Fish monocytes as a model for mycobacterial host-pathogen interactions

Mycobacterium marinum, a relatively rapid-growing fish and human pathogen, has become an important model for the investigation of mycobacterial pathogenesis. M. marinum is closely related to the Mycobacterium tuberculosis complex and causes a disease in fish and amphibians with pathology similar to tuberculosis. We have developed an in vitro model for the study of M. marinum virulence mechanisms using the carp monocytic cell line CLC (carp leukocyte culture). We found that fish monocytes can differentiate between pathogenic and nonpathogenic mycobacterial species. Interestingly, M. marinum enters fish monocytes at a 40- to 60-fold-higher rate than Mycobacterium smegmatis. In addition, M. marinum survives and replicates in fish monocytes while M. smegmatis is killed. We also found that M. marinum inhibits lysosomal fusion in fish monocytes, indicating that these cells may be used to dissect the mechanisms of intracellular trafficking in mycobacteria. We conclude from these observations that monocytic cells from fish, a natural host for M. marinum, provide an extremely valuable model for the identification and characterization of mycobacterial virulence determinants in the laboratory.

Immune responses in tuberculosis: antibodies and CD4-CD8 lymphocytes with vascular adhesion molecules and cytokines (chemokines) cause a rapid antigen-specific cell infiltration at sites of bacillus Calmette-Guérin reinfection

Rabbit primary dermal bacillus Calmette-Guérin (BCG) lesions were compared with reinfection BCG lesions in order to gain insight into how immune responses protect against clinical tuberculosis. As early as 3 hr, a marked infiltration of macrophages and lymphocytes occurred in the reinfection group, while very little cell infiltration occurred in the primary group. It seems that only an antigen-antibody reaction could produce such an immediate pronounced antigen-specific chemotactic effect, because very few lymphocytes are normally present in the skin. Therefore, antibodies hasten the accumulation of an expanded antigen-specific T-lymphocyte population (memory cells) at sites of bacillary lodgement. By 1-2 days, the primary and reinfection BCG lesions differed 400- to 500-fold in size. By 4-5 days, the size of the reinfection lesions had declined, while the size of the primary lesions had increased, so that, grossly, both types of lesion were similar. At 8 days in reinfection lesions and at 12 days in primary lesions, small secondary peaks in size occurred, which were probably caused by cell-mediated immune responses. In rabbits with primary BCG lesions, skin tests with Old Tuberculin were positive at 9 days, accompanied by a rise in the levels of antibodies to the secreted antigen, phosphate-specific transport protein 1, but the levels of antibodies to the constitutive antigens, purified protein derivative and heat-shock protein 65, did not increase appreciably until some time after 23 days. In tissue sections of reinfection BCG lesions, the percentage of mononuclear cells labelled, by in situ hybridization techniques, for the mRNA of monocyte chemoattractant protein 1 (MCP-1), a chemokine, peaked at 3 hr and then was down-regulated, whereas in primary lesions, this percentage was down-regulated only after 2 days. [The percentage in the tissue sections for the mRNAs of interleukins 1beta and 8, as well as the proteins of MCP-1 and tumor necrosis factor alpha (TNF-alpha), followed a somewhat similar time-course to that of MCP-1 mRNA.] A high percentage of mononuclear cells containing the MCP-1 mRNA 'factory' would favour enlargement of the lesions and a low percentage would favour their regression. At 5 days, the percentage of CD4 and CD8 lymphocytes, stained by immunohistochemical techniques, and the amount of microvasculature stained similarly for vascular cell adhesion molecule 1 were higher in the reinfection group, indicating that prior immunization caused a more rapid (antigen-dependent) up-regulation of these factors. Tuberculin reactions resembled early reinfection BCG lesions in almost every factor evaluated herein. In brief, the production of chemokines began soon after BCG reinfection, peaked within a few hours and was markedly down-regulated by 24 hr, a time at which the lesions of reinfection were of maximal size. Therefore, the amount of cell infiltration was tightly controlled, probably by the variety of mechanisms listed herein.

In vivo administration of mycobacterial cord factor (Trehalose 6, 6'-dimycolate) can induce lung and liver granulomas and thymic atrophy in rabbits

Trehalose 6,6'-dimycolate (TDM) is a cell surface molecule of Mycobacterium tuberculosis. TDM induced a loss of body weight and prominent granulomas in the liver and lungs by the intravenous injection of TDM into rabbits. TDM also induced atrophy of the thymus and spleen due to apoptosis. By contrast, sulfolipid (2,3,6, 6'-tetraacyl trehalose 2'-sulfate) induced neither toxicity, nor granuloma formation, nor atrophy of the thymus and spleen. In rabbits the histopathological changes were more dramatic than in mice. The rabbit model may be more sensitive and may provide more information on the beneficial or pathological effects of TDM.

Virulence of Mycobacterium tuberculosis CDC1551 and H37Rv in rabbits evaluated by Lurie's pulmonary tubercle count method

The virulence of the CDC1551 strain of Mycobacterium tuberculosis was compared to that of H37Rv in a rabbit inhalation model. While rabbits that inhaled the two strains produced equal numbers of grossly visible primary tubercles, CDC1551 tubercles were smaller and contained fewer bacilli than H37Rv tubercles. These findings suggest that a miniepidemic near the Kentucky-Tennessee border caused by CDC1551 was due not to increased virulence but to increased transmissibility.

Pulmonary bovine-type tuberculosis in rabbits: bacillary virulence, inhaled dose effects, tuberculin sensitivity, and Mycobacterium vaccae immunotherapy

This report elucidates four aspects of the immunology of pulmonary tuberculosis produced in rabbits: (i) the virulence of bovine-type tubercle bacilli, strain Ravenel S, (ii) systemic factors influencing the generation of visible primary pulmonary tubercles, (iii) differences in tuberculin sensitivity of rabbits and humans, and (iv) the effect of Mycobacterium vaccae immunotherapy on cavitary tuberculosis. Laboratory strain Ravenel S (ATCC 35720) was not fully virulent. Fully virulent strains produce one visible primary pulmonary tubercle for each three bacillary units inhaled. Strain ATCC 35720 produced one such tubercle for each 18 to 107 bacillary units inhaled, indicating that its virulence was reduced by 6- to 36-fold. When a low dose of this Ravenel S strain was inhaled, the host resistance (measured by the number of inhaled bacilli needed to generate one visible primary pulmonary tubercle) was increased at least 3.5-fold compared to the host resistance when a high dose was inhaled. Rabbits and humans differ in the degree and in the maintenance of their dermal sensitivities to tuberculin. Compared to rabbits, humans are 100 times more sensitive to tuberculin. Also, at 33 weeks rabbits with well-controlled cavitary tuberculosis usually showed a decrease in their tuberculin reactions of about 50% from peak values, whereas humans with such well-controlled tuberculosis are thought to maintain strong reactions for many years. These species differences may be due to desensitization to group II mycobacterial antigens in the rabbits because they have a different diet and a different type of digestive tract. M. vaccae immunotherapy of rabbits with cavitary tuberculosis produced no statistically significant effects. Experiments with many more rabbits would be required to prove whether or not such immunotherapy is beneficial.