Behavior of attenuated mycobacteria in organs of neonatal and adult mice

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.

CD36 deficiency attenuates experimental mycobacterial infection

Background

Members of the CD36 scavenger receptor family have been implicated as sensors of microbial products that mediate phagocytosis and inflammation in response to a broad range of pathogens. We investigated the role of CD36 in host response to mycobacterial infection.

Methods

Experimental Mycobacterium bovis Bacillus Calmette-Guérin (BCG) infection in Cd36^+/+ and Cd36^-/- mice, and in vitro co-cultivation of M. tuberculosis, BCG and M. marinum with Cd36^+/+ and Cd36^-/-murine macrophages.

Results

Using an in vivo model of BCG infection in Cd36^+/+ and Cd36^-/- mice, we found that mycobacterial burden in liver and spleen is reduced (83% lower peak splenic colony forming units, p < 0.001), as well as the density of granulomas, and circulating tumor necrosis factor (TNF) levels in Cd36^-/- animals. Intracellular growth of all three mycobacterial species was reduced in Cd36^-/- relative to wild type Cd36^+/+ macrophages in vitro. This difference was not attributable to alterations in mycobacterial uptake, macrophage viability, rate of macrophage apoptosis, production of reactive oxygen and/or nitrogen species, TNF or interleukin-10. Using an in vitro model designed to recapitulate cellular events implicated in mycobacterial infection and dissemination in vivo (i.e., phagocytosis of apoptotic macrophages containing mycobacteria), we demonstrated reduced recovery of viable mycobacteria within Cd36^-/- macrophages.

Conclusions

Together, these data indicate that CD36 deficiency confers resistance to mycobacterial infection. This observation is best explained by reduced intracellular survival of mycobacteria in the Cd36^-/- macrophage and a role for CD36 in the cellular events involved in granuloma formation that promote early bacterial expansion and dissemination.

Characterization of lung gamma delta T cells following intranasal infection with Mycobacterium bovis bacillus Calmette-Guérin

The lungs are considered to have an impaired capacity to contain infection by pathogenic mycobacteria, even in the presence of effective systemic immunity. In an attempt to understand the underlying cellular mechanisms, we characterized the gammadelta T cell population following intranasal infection with Mycobacterium bovis bacillus Calmette-Guérin (BCG). The peak of gammadelta T cell expansion at 7 days postinfection preceded the 30 day peak of alphabeta T cell expansion and bacterial count. The expanded population of gammadelta T cells in the lungs of BCG-infected mice represents an expansion of the resident Vgamma2 T cell subset as well as an influx of Vgamma1 and of four different Vdelta gene-bearing T cell subsets. The gammadelta T cells in the lungs of BCG-infected mice secreted IFN-gamma following in vitro stimulation with ionomycin and PMA and were cytotoxic against BCG-infected peritoneal macrophages as well as against the uninfected J774 macrophage cell line. The cytotoxicity was selectively blocked by anti-gammadelta TCR mAb and strontium ions, suggesting a granule-exocytosis killing pathway. Depletion of gammadelta T cells by injection of specific mAb had no effect on the subsequent developing CD4 T cell response in the lungs of BCG-infected mice, but significantly reduced cytotoxic activity and IFN-gamma production by lung CD8 T cells. Thus, gammadelta T cells in the lungs might help to control mycobacterial infection in the period between innate and classical adaptive immunity and may also play an important regulatory role in the subsequent onset of alphabeta T lymphocytes.

Pulmonary immune responses during primary mycobacterium bovis- Calmette-Guerin bacillus infection in C57Bl/6 mice

Mechanisms of protective immunity to mycobacterial infection in the lung remain poorly defined. In this study, T-cell subset expansion and cytokine expression in bronchoalveolar spaces, lung parenchyma, and mediastinal lymph nodes of mice infected intratracheally with Mycobacterium bovis-Calmette-Guerin bacillus (BCG) were analyzed in parallel with histopathology and bacterial burden. M. bovis-BCG was cleared rapidly from bronchoalveolar spaces without evidence for persistence. In lung parenchyma bacteria grew during the first 4 wk followed by gradual clearance with less than 0.1% of the original inoculum persisting for more than 8 mo. Clearance of M. bovis-BCG from bronchoalveolar lavage was associated with recruitment of both neutrophils and lymphocytes. Lung CD4(+), CD8(+), and gammadelta T-cell receptor-positive T cells expanded maximally by Week 4, and declined by Week 8 to control values despite bacterial persistence. Both CD4(+) and CD8(+) lung T cells produced interferon (IFN)-gamma in response to M. bovis-BCG. Four distinct pathologic states of lung parenchymal infection were noted. Early focal sub-bronchial inflammation with transmigration of cells into airways was followed by diffuse peribronchitis, perivasculitis, and alveolitis with activated macrophages, lymphoblasts, and occasional giant cells. The latter stage corresponded to maximal M. bovis-BCG growth. Resolving infection consisted of small lymphocytes and foamy macrophages, which coincided with decreasing M. bovis-BCG colony-forming units, T-cell infiltration, and IFN-gamma expression. A final quiescent phase consisted of residual lymphoid aggregates and perivasculitis associated with persistent spontaneous IFN-gamma production. Bacterial dissemination to lymph node and spleen occurred by Week 4 and declined in parallel to lung. In contrast to lung, IFN-gamma secretion was detected only late despite early expansion of CD4(+) and CD8(+) T cells. By reverse transcriptase/polymerase chain reaction, IFN-gamma and interleukin (IL)-12 p40 messenger RNA (mRNA) in lung paralleled IFN-gamma protein production. Tumor necrosis factor-alpha, IL-4 and IL-10 mRNA expression was not increased during M. bovis-BCG lung infection. Thus, protective immunity to M. bovis-BCG in the lung evolved differently in air space, lung, and lymph node.

Ontogeny of macrophage-mediated protection against Listeria monocytogenes

We investigated the ontogenic development of macrophage functions which are important in the expression of host defense against infection by Listeria monocytogenes. Macrophage functions, including accumulation in response to local stimuli, chemotaxis in vitro, and intracellular killing, as well as number of macrophages, were examined by using mice 1, 2, 3, 4, and 8 weeks old. The number of peritoneal macrophages was extremely low in younger mice even when their body weights were taken into consideration. Macrophage accumulation in response to infectious stimulus with viable listeria was poor in younger mice and showed an age-dependent development. In younger groups, chemotaxis in vitro was as immature as chemotaxis in vivo. In 1- and 2-week-old mice, macrophages did not show any intracellular killing activity against L. monocytogenes, but killing was observed in mice over 3 weeks of age. These functions developed in an age-dependent manner and reached the 8-week-old adult level after the mice were 4 weeks of age. In adult mice, these macrophage functions were shown to be enhanced after immunization with viable listeria; however, such an immunization-induced enhancement was very poor in the younger groups of mice. Protection judged by mortality and in vivo bacterial growth was weaker in the younger groups against both primary and secondary challenges. In vivo protection against L. monocytogenes seemed to develop in the same age-dependent manner as the development of macrophage functions. These results indicate that age-dependent immaturity of macrophage functions mainly comprises the age-dependent immaturity of protection against L. monocytogenes.

In vitro studies on the mechanism of acquired resistance to tuberculous infection. II. The effects of the culture supernatants of specifically stimulated-sensitized lymphocytes on the growth of tubercle bacilli within macrophages

Immune lymph node cells were obtained from mice immunized with bovine gamma globulin (BGG) in complete Freund's adjuvant or allogeneic MH134 tumor cells. They showed the capacity of conferring bactericidal activity on macrophages infected with Mycobacterium tuberculosis, H37Rv, when they were incubated on macrophage monolayers together with the corresponding antigen, i.e., BGG or solubilized cellular antigen of the tumor cells. However, such capacity was lower than that of tubercle bacilli-immune lymph node cells. Culture supernatants were harvested after incubation of tubercle bacilli-immune, BGG-immune or allogeneic tumor-immune lymph node cells with the corresponding antigen for 24 hr. Macrophages were altered so as to suppress intracellular bacillary growth when macrophage monolayers were exposed to the supernatants for more than 2 days. When normal lymph node cells were incubated on normal macrophage monolayers together with a mitogen such as PHA or concanavalin A, growth of tubercle bacilli within the macrophages was slightly but difinitely suppressed. The mechanism of elicitation of cellular immunity to the infection with tubercle bacilli is discussed on the basis of results presented in this and the preceding paper.

Partial characterization of a factor extracted from sensitized lymphocytes that inhibits the growth of Mycobacterium tuberculosis within macrophages in vitro

Spleen lymphocytes of BCG-immunized mice contain a soluble factor that inhibits in vitro the growth of the H37Rv strain of Mycobacterium tuberculosis within normal peritoneal macrophages. The water-soluble extracts of sensitized lymphocytes, disrupted by freezing and thawing, although less active than the corresponding viable cells retained a significant growth-inhibiting activity. Dialysis against distilled water, lyophilization, exposure to ribonuclease and deoxyribonuclease, and storage at -20 degrees C of the water-soluble extracts did not affect their antimycobacterial activity, whereas extracts heated at 100 degrees C were completely devoid of such an activity. All the inhibiting activity was recovered in the void volume of the column after chromatography on Sephadex G-200. Water-soluble constitutents of sensitized lymphocytes did not affect BCG grown in vitro, and on repeated treatments of tuberculous mice they led to a negligible protection against pulmonary tuberculosis. Preliminary observations seem to indicate that other soluble factors in lymphocytes of BCG-sensitized mice have the capacity to potentiate in vitro the phagocytic activity of normal macrophages.