Immune responses in animal models

Spectinamides are effective partner agents for the treatment of tuberculosis in multiple mouse infection models

Objectives: New drug regimens employing combinations of existing and experimental antimicrobial agents are needed to shorten treatment of tuberculosis (TB) in humans. The spectinamides are narrow-spectrum semisynthetic analogues of spectinomycin, modified to avoid intrinsic efflux by Mycobacterium tuberculosis. Spectinamides, including lead 1599, have been previously shown to exhibit a promising therapeutic profile in mice as single agents. Here we explore the in vivo activity of lead spectinamides when combined with other agents.

Methods: The efficacy of 1599 or 1810 was tested in combination in three increasingly advanced TB mouse models. Mice were infected by aerosol and allowed to establish acute or chronic infection, followed by treatment (≤4 weeks) with the spectinamides alone or in two- and three-drug combination regimens with existing and novel therapeutic agents. Bacteria were enumerated from lungs by plating for cfu.

Results: Herein we show the following: (i) 1599 exhibits additive or synergistic activity with most of the first-line agents; (ii) 1599 in combination with rifampicin and pyrazinamide or with bedaquiline and pyrazinamide promotes significantly improved efficacy in the high-dose aerosol model; (iii) 1599 enhances efficacy of rifampicin or pyrazinamide in chronically infected BALB/c mice; and (iv) 1599 is synergistic when administered in combination with rifampicin and pyrazinamide in the C3HeB/FeJ mouse model showing caseous necrotic pulmonary lesions.

Conclusions: Spectinamides were effective partner agents for multiple anti-TB agents including bedaquiline, rifampicin and pyrazinamide. None of these in vivo synergistic interactions was predicted from in vitro MIC chequerboard assays. These data support further development of the spectinamides as combination partners with existing and experimental anti-TB agents.

Limited activity of clofazimine as a single drug in a mouse model of tuberculosis exhibiting caseous necrotic granulomas

New drugs and drugs with a novel mechanism of action are desperately needed to shorten the duration of tuberculosis treatment, to prevent the emergence of drug resistance, and to treat multiple-drug-resistant strains of Mycobacterium tuberculosis. Recently, there has been renewed interest in clofazimine (CFZ). In this study, we utilized the C3HeB/FeJ mouse model, possessing highly organized, hypoxic pulmonary granulomas with caseous necrosis, to evaluate CFZ monotherapy in comparison to results with BALB/c mice, which form only multifocal, coalescing cellular aggregates devoid of caseous necrosis. While CFZ treatment was highly effective in BALB/c mice, its activity was attenuated in the lungs of C3HeB/FeJ mice. This lack of efficacy was directly related to the pathological progression of disease in these mice, since administration of CFZ prior to the formation of hypoxic, necrotic granulomas reconstituted bactericidal activity in this mouse strain. These results support the continued use of mouse models of tuberculosis infection which exhibit a granulomatous response in the lungs that more closely resembles the pathology found in human disease.

Human CD8+ T cells from TB pleurisy respond to four immunodominant epitopes in Mtb CFP10 restricted by HLA-B alleles

CD8⁺ T cells are essential for host defense to Mycobacterium tuberculosis (Mtb) infection and identification of CD8⁺ T cell epitopes from Mtb is of importance for the development of effective peptide-based diagnostics and vaccines. We previously demonstrated that the secreted 10-KDa culture filtrate protein (CFP10) from Mtb is a potent CD8⁺ T cell antigen but the repertoire and dominance pattern of human CD8 epitopes for CFP10 remained poorly characterized. In the present study, we undertook to define immunodominant CD8 epitopes involved in CFP10 using a panel of CFP10-derived 13–15 amino acid (aa) peptides overlapping by 11 aa. Four peptides in CFP10 were observed to induce significant CD8⁺ T cell responses and we further determined the size of the epitopes involved in each individual peptide tested. Four 9 aa CD8 epitopes were finally identified and deleting a single amino acid from the N or C terminus of either peptide markedly reduced IFN-γ production, suggesting that they are minimum of CD8 epitopes. In the individuals tested, each epitope represented a single immunodominant response in CD8⁺ T cells. The epitope-specific CD8⁺ T cells displayed effector or effector memory phenotypes and could upregulate the expression of CD107a/b upon antigen stimulation. In addition, we found that epitope-specific CD8⁺ T cells shared biased usage of T cell receptor (TCR) variable region of β chain (Vβ) 12, 9, 7.2 or Vβ4 chains. As judged from HLA-typing results and using bioinformatics technology for prediction of MHC binding affinity, we found that the epitope-specific CD8⁺ T cells are all restricted by HLA-B alleles. Our findings suggest that the four epitopes in CFP10 recognized by CD8⁺ T cells might be of importance for the development of Mtb peptide-based vaccines and for improved diagnosis of TB in humans.

Evaluation of a mouse model of necrotic granuloma formation using C3HeB/FeJ mice for testing of drugs against Mycobacterium tuberculosis

Persistence of Mycobacterium tuberculosis remains a significant challenge for the effective treatment of tuberculosis in humans. In animals that develop necrotic lung lesions following infection with M. tuberculosis, drug-tolerant bacilli are present and persist in an extracellular microenvironment within the necrotic cores. In this study, we examined the efficacy of drug treatment in C3HeB/FeJ (Kramnik) mice that develop lesions with liquefactive necrosis, in comparison to BALB/c mice that develop nonnecrotic lesions following aerosol challenge. To accomplish this, Kramnik and BALB/c mice were infected by aerosol with M. tuberculosis and treated for 7 to 8 weeks with monotherapy using drugs with different modes of action. The efficacy of drug therapy was quantified by enumeration of bacterial load. The progression of disease and location and distribution of bacilli within lesions were visualized using various staining techniques. In the late stages of infection, Kramnik mice developed fibrous encapsulated lung lesions with central liquefactive necrosis containing abundant extracellular bacilli, whereas BALB/c mice formed nonnecrotic lesions with primarily intracellular bacilli. Necrotic lesions in Kramnik mice showed evidence of hypoxia by pimonidazole staining. Kramnik mice were significantly more refractory to drug therapy, especially for pyrazinamide. Metronidazole showed no bactericidal activity in either model. There were significantly higher numbers of drug-resistant colonies isolated from the Kramnik mice compared to BALB/c mice. These results suggest that the Kramnik mouse model will be a valuable model to test antituberculosis drugs, especially against bacilli that persist within necrotic lesions.

Design and evaluation of an aerosol infection chamber for small animals

In this report, we describe the design of an aerosol exposure chamber to reproducibly produce uniformly distributed clouds of droplet nuclei. The device can deliver desired number of bacilli (20-2000) in lungs of mice. All safety measures to handle infectious bacteria have been incorporated in the design and it is controlled remotely by a personal computer. It is an indispensable device to study the protective efficacy of vaccine candidates against Mycobacterium tuberculosis infection. This device would also be useful to study immunization and drug delivery by nasal route in experimental animals.

Human dendritic cells presenting adenovirally expressed antigen elicit Mycobacterium tuberculosis--specific CD8+ T cells

Previous studies in murine and human models have suggested an important role for CD8+ T cells in host defense to Mycobacterium tuberculosis (Mtb). Consequently, a successful tuberculosis vaccine may require the elicitation of sustained CD4+ and CD8+ T cell responses. We tested the hypothesis that the potent CD4+ T cell antigen Mtb39 is also a CD8+ T cell antigen. A recombinant adenovirus-expressing Mtb39 (adenoMtb39) was used to infect monocyte-derived dendritic cells. Using interferon-gamma enzyme-linked immunospot, Mtb39-specific CD8+ T lymphocytes were detected in three healthy individuals with latent tuberculosis infection who also had strong anti-Mtb39-specific CD4+ T cell responses. An Mtb39-specific CD8+ T cell line was generated using Mtb39-expressing dendritic cells. Mtb39-specific T cell clones were obtained by limiting dilution cloning. All seven T cell clones obtained were HLA-B44 restricted. Using a panel of synthetic overlapping peptides representative of Mtb39, the peptide epitope was identified for two clones. Furthermore, all T cell clones recognized Mtb-infected dendritic cells and were cytolytic. We conclude that infection of dendritic cells with adenoviral vectors expressing Mtb proteins allows for measurement of antigen-specific CD8+ T cell responses from peripheral blood mononuclear cells. The technique will be useful in defining CD8+ T cell antigens and in measuring immunogenicity of tuberculosis vaccines.

Animal models of mycobacteria infection

This unit describes the infection of mice and guinea pigs with mycobacteria via the aerogenic route, or of mice via the intravenous route, as well as necropsy methods for the determination of mycobacterial loads within target organs. Additionally, methods for cultivating mycobacteria and preparing stocks are described. The protocols outlined are primarily used for M. tuberculosis, but can also be used for the study of other opportunistic Mycobacterial species.

In Vivo Immunomodulation Following Intradermal Injection with DNA Encoding IL-18

IL-18, a recently identified cytokine synthesized by different cell types, including Kupffer cells, activated macrophages, and keratinocytes, induces IFN-γ production by T cells and NK cells. The cDNA encoding IL-18 with its natural signal peptide was cloned under control of the CMV promoter and injected into the skin of mice. A single intradermal injection of this construction led to efficient in vivo expression of IL-18 in cutaneous dermal cells and induced IFN-γ mRNA production, indicating that it was produced in a biologically active form. In addition, a massive cellular infiltrate was observed in the skin 2 days after injection. When the mice were subsequently infected with Mycobacterium bovis bacillus Calmette-Guérin (BCG), they produced lower levels of anti-BCG Abs than control animals. However, in contrast to their lowered humoral immune response, the mice produced higher amounts of Ag-specific IFN-γ after in vitro restimulation, as compared with the controls. Therefore, injection of DNA encoding IL-18 into the skin modulates both Ag-specific humoral and T cell responses upon mycobacterial infection. It increases the Th1 type response, which may be particularly useful for the development of new immunotherapeutic or immunoprotective approaches against infections by intracellular parasites, such as mycobacteria.

Molecular Cloning and Immunologic Reactivity of a Novel Low Molecular Mass Antigen of Mycobacterium tuberculosis

Polypeptide Ags present in the culture filtrate of Mycobacterium tuberculosis were purified and evaluated for their ability to stimulate PBMC from purified protein derivative (PPD)-positive healthy donors. One such Ag, which elicited strong proliferation and IFN-γ production, was further characterized. The N-terminal amino acid sequence of this polypeptide was determined and used to design oligonucleotides for screening a recombinant M. tuberculosis genomic DNA library. The gene (Mtb 8.4) corresponding to the identified polypeptide was cloned, sequenced, and expressed in Escherichia coli. The predicted m.w. of the recombinant protein without its signal peptide was 8.4 kDa. By Southern analysis, the DNA encoding this mycobacterial protein was found in the M. tuberculosis substrains H37Rv, H37Ra, Erdman, and “C” strain, as well as in certain other mycobacterial species, including Mycobacterium avium and Mycobacterium bovis BCG (bacillus Calmette-Guérin, Pasteur). The Mtb 8.4 gene appears to be absent from the environmental mycobacterial species examined thus far, including Mycobacterium smegmatis, Mycobacterium gordonae, Mycobacterium chelonae, Mycobacterium fortuitum, and Mycobacterium scrofulaceum. Recombinant Mtb 8.4 Ag induced significant proliferation as well as production of IFN-γ, IL-10, and TNF-α, but not IL-5, from human PBMC isolated from PPD-positive healthy donors. Mtb 8.4 did not stimulate PBMC from PPD-negative donors. Furthermore, immunogenicity studies in mice indicate that Mtb 8.4 elicits a Th1 cytokine profile, which is considered important for protective immunity to tuberculosis. Collectively, these results demonstrate that Mtb 8.4 is an immunodominant T cell Ag of M. tuberculosis.

ATP-induced killing of mycobacteria by human macrophages is mediated by purinergic P2Z(P2X7) receptors

The death of BCG-infected human macrophages induced in vitro by ligation of surface CD95 (Fas), CD69, or complement-mediated lysis was shown not to result in the death of intracellular mycobacteria, whereas exposure to extracellular ATP initiated both macrophage death and killed the intracellular bacteria. ATP acted via P2Z receptors because these effects were mimicked by benzoylbenzoic ATP (a known agonist of P2Z receptors) and blocked by oxidized ATP, DIDS, suramin, amiloride, and KN62 (known inhibitors of P2Z-mediated responses). ATP-mediated bacterial killing was independent of reactive nitrogen and oxygen intermediates and of actinomycin D or cycloheximide inhibition. ATP-induced macrophage cell death, BCG killing, and lucifer yellow dye incorporation were minimal in 2 out of 19 healthy donors. The results suggest possible genetic heterogeneity of this mechanism of mycobacterial killing associated with P2Z-mediated pore formation.