Overexpression of Adenylyl Cyclase Encoded by the Mycobacterium tuberculosis Rv2212 Gene Confers Improved Fitness, Accelerated Recovery from Dormancy and Enhanced Virulence in Mice

Earlier we demonstrated that the adenylyl cyclase (AC) encoded by the MSMEG_4279 gene plays a key role in the resuscitation and growth of dormant Mycobacterium smegmatis and that overexpression of this gene leads to an increase in intracellular cAMP concentration and prevents the transition of M. smegmatis from active growth to dormancy in an extended stationary phase accompanied by medium acidification. We surmised that the homologous Rv2212 gene of M. tuberculosis (Mtb), the main cAMP producer, plays similar physiological roles by supporting, under these conditions, the active state and reactivation of dormant bacteria. To test this hypothesis, we established Mtb strain overexpressing Rv2212 and compared its in vitro and in vivo growth characteristics with a control strain. In vitro, the AC-overexpressing pMindRv2212 strain demonstrated faster growth in a liquid medium, prolonged capacity to form CFUs and a significant delay or even prevention of transition toward dormancy. AC-overexpressing cells exhibited easier recovery from dormancy. In vivo, AC-overexpressing bacteria demonstrated significantly higher growth rates (virulence) in the lungs and spleens of infected mice compared to the control strain, and, unlike the latter, killed mice in the TB-resistant strain before month 8 of infection. Even in the absence of selecting hygromycin B, all pMindRv2212 CFUs retained the Rv2212 insert during in vivo growth, strongly suggesting that AC overexpression is beneficial for bacteria. Taken together, our results indicate that cAMP supports the maintenance of Mtb cells vitality under unfavorable conditions in vitro and their virulence in vivo.

Host genetics in susceptibility to and severity of mycobacterial diseases

The genetic analysis of susceptibility to infections has proven to be extremely useful for identification of key cells, molecules, pathways, and genes involved in the battle between two genomes - the essence of the infectious process. This is particularly true for tuberculosis and other mycobacterial infections which traditionally attracted much attention from both immunologists and geneticists. In this short review, we observe results of genetic studies performed in human populations and in animal models and compare relative input of forward and reverse genetic approaches in our knowledge about genetic control of and immune responses to mycobacterial infections.

"Suppressor Factor" of Neutrophils: A Short Story of a Long-Term Misconception

A large body of evidence obtained during the last decade has demonstrated that neutrophils suppress T cell proliferation in different models of inflammation and cell interaction. The commonly used method for assessing cell proliferation and proliferation inhibition is measuring [3H]thymidine incorporation into cells. Earlier, we observed inhibition of [3H]thymidine uptake in experiments on neutrophil-mediated regulation of T cell response in tuberculosis immunity. Here, we used different types of proliferating cells to analyze the nature of the soluble "neutrophil factor" by a variety of methods (dialysis, HPLC, mass spectrometry, and NMR) and unambiguously demonstrated that neutrophils do not synthesize a specific factor inhibiting cell proliferation, but secrete high concentrations of extracellular thymidine that competitively inhibit [3H]thymidine incorporation. Although the physiological significance of thymidine secretion by neutrophils remains unknown, this phenomenon should be carefully considered when designing test systems for studying cell-cell interactions.

Formation of compact aggregates of B-lymphocytes in lung tissue during mycobacterial infection in mice depends on TNF production by these cells and is not an element of the host's immunological protection

Tumor necrosis factor (TNF) plays a pivotal role in the early control of Mycobacterium tuberculosis and M. avium infections by a host. It was previously shown that both phagocyte-derived and T-cell-derived TNF productions are critical for protective immunity against M. tuberculosis, but the role of TNF produced by B-cells remained unclear. By comparing mice with B-cell-specific TNF deletion to littermate control mice, here we show that TNF production by B-lymphocytes is essential for the formation of infection-specific aggregates of B-cells in the lung. It is likely that these compact foci represent a pathogenic feature of inflammatory response rather than an element of protective immunity, since the capacity to form aggregates has no influence on the severity of M. tuberculosis- and M. avium-triggered diseases.

I/St mice hypersusceptible to Mycobacterium tuberculosis are resistant to M. avium

We previously demonstrated that mice of the I/St strain are extremely susceptible to Mycobacterium tuberculosis, as well as to the taxonomically distant intracellular bacteria Chlamydia pneumoniae and Salmonella enterica. To broaden our knowledge about the control of susceptibility to intracellular pathogens, we studied the infection caused by Mycobacterium avium virulent strain 724 in a panel of inbred mouse strains and found that I/St mice are resistant to M. avium. By comparing I/St mice with B6 mice, we demonstrated that (i) B6 mice are much more susceptible to infection caused by M. avium in terms of bacterial multiplication in the lung tissue and severity of lung pathology; (ii) in B6 mice but not in I/St mice infection leads to prolonged leukocyte infiltration of the lung tissue, development of necrotic lung granulomata, and lethality; and (iii) the unfavorable infectious course in B6 mice is accompanied by elevated production of gamma interferon, tumor necrosis factor alpha, and especially interleukin-12 in the lungs. Importantly, M. avium-resistant I/St mice carry a functional r allele of the Slc11a1 (formerly Nramp1) gene, while B6 mice have the Slc11a1(s) genotype. Segregation genetic analysis of (I/St x B6) F2 hybrids demonstrated that susceptibility or resistance to infection caused by M. avium largely depended upon the Slc11a1 genotype and that other genetic traits had a relatively weak influence. This close-to-monogenic pattern differs sharply from the host control of many other intracellular bacterial infections, for which the involvement of numerous quantitative trait loci has been ubiquitously observed.

Lung cell responses to M. tuberculosis in genetically susceptible and resistant mice following intratracheal challenge

One approach to study the role of distinct cellular mechanisms in susceptibility/resistance to tuberculosis (TB) is to compare parameters of response to infection in the lungs of mouse strains exhibiting genetically determined differences in TB susceptibility/severity. Interstrain differences in antimycobacterial macrophage reactions, T cell responses & inflammation in the lungs of TB-susceptible I/St, TB-resistant A/Sn and (I/St x A/Sn)F1 mice were analysed following intratracheal inoculation of 103 CFUs of M. tuberculosis H37Rv. The antimycobacterial responses in the lungs of susceptible I/St mice were characterized by: (i) increased inflammatory infiltration by all major immune cell subsets; (ii) decreased type 1 cytokine production; (iii) impaired antimycobacterial activity of lung macrophages; (iv) unusually high proliferation of lung T lymphocytes. Differences in several parameters of anti-TB immunity between susceptible and resistant mice corresponded well to the polygenic pattern of TB control previously established in this mouse model. Importantly, lung macrophages isolated from noninfected mice were unable to respond to IFN-gamma by increasing their mycobactericidal function, but between weeks 3 and 5 of the infection this capacity developed in all mice. However, by this time point susceptible but not resistant mice demonstrated a pronounced decrease in IFN-gamma production by lung cells. This chain of events may explain the inability of I/St mice to control both early and chronic TB infection.

Comparative analysis of T lymphocytes recovered from the lungs of mice genetically susceptible, resistant, and hyperresistant to Mycobacterium tuberculosis-triggered disease

Genetic control of susceptibility to tuberculosis (TB) is being intensively studied, and immune responses to mycobacteria are considerably well characterized. However, it remains largely unknown which parameters of response distinguish resistant and susceptible TB phenotypes. Mice of I/St and A/Sn inbred strains and (A/Sn x I/St)F(1) hybrids were previously categorized as, respectively, susceptible, resistant, and hyperresistant to Mycobacterium tuberculosis-triggered disease. In the present work we compared parameters of lung T cell activation and response following M. tuberculosis challenge. In all mice, the disease progression was accompanied by a marked accumulation in the lungs of activated CD4(+) (CD44(high)/CD45RB(low)) and CD8(+) (CD44(high)/CD45RB(+)) T cells capable of secreting IFN-gamma and of activating macrophages for NO production and mycobacterial growth inhibition. However, significantly more CD8(+) T cells were accumulated in the lungs of resistant A/Sn and F(1) compared with I/St mice. About 80% A/Sn and F(1) CD8(+) cells expressed CD44(high)/CD45RB(+) phenotype, while about 40% I/St CD8(+) cells did not express CD45RB marker at week 5 of infection. In contrast, in susceptible I/St mice lung CD4(+) cells proliferated much more strongly in response to mycobacterial sonicate, and a higher proportion of these cells expressed CD95 and underwent apoptosis compared with A/Sn cells. Unseparated lung cells and T cells of I/St origin produced more IL-5 and IL-10, respectively, whereas their A/Sn and F1 counterparts produced more IFN-gamma following infection. F(1) cells overall expressed an intermediate phenotype between the two parental strains. Such a more balanced type of immune reactivity could be linked to a better TB defense.

Characterization of T cell clones derived from lymph nodes and lungs of Pseudomonas aeruginosa-susceptible and resistant mice following immunization with heat-killed bacteria

Pseudomonas aeruginosa-resistant BALB/c and susceptible C57Bl/6 (B6) mice were immunized with heat-killed Pseudomonas either in the foot pad or via the trachea, and panels of Pseudomonas-specific T cell clones were developed from lymph nodes and lungs. All clones from either strain, whether of lymph node or lung origin, were CD3+CD4+CD8-TCRalphabeta+. The efficacy of cloning from lymph node cells was comparable between BALB/c and B6 mice. All lymph node BALB/c clones proliferated in response to Pseudomonas antigen in a dose-dependent manner, and this response was MHC class II-restricted. Vigorous proliferation by a considerable proportion of B6 T cell clones occurred in the absence of specific antigen. Lymph node clones from either strain could be categorized as either Th1 or Th0 on the basis of interferon-gamma (IFN-gamma)/IL-4 production. In either mouse strain the efficacy of cloning from lung tissue was substantially lower than from lymph nodes, but the efficacy of cloning from BALB/c compared with B6 lungs was higher. Four lung T cell clones from BALB/c and two from B6 mice were expanded for further analyses, and an interstrain difference was observed in cytokine production. Both B6 lung T cell clones were Th1-like and produced IFN-gamma but not IL-4 and IL-10, whereas four BALB/c lung T cell clones were Th2-like and produced IL-4 and IL-10 but not IFN-gamma. These observations suggest that differences in the CD4+ Th response in the lung may contribute to differences among inbred mouse strains in the level of resistance to bronchopulmonary Pseudomonas infection.

A new approach to obtain Lyb5-specific antiserum

A new approach to obtain antiserum specific for Lyb5.2 allotype, was developed. Mice of DBA/2 inbred strain, bearing Lyb5.1 allotype, were made unresponsive to cellular antigens of Lyb5-CBA/N mice by specific tolerance induction. These tolerant recipients were further immunized with suspensions of splenocytes from CBA mice which contain Lyb5.2 + B lymphocytes. Thus obtained Lyb5.2-specific antiserum was cytotoxic for 25-30% of CBA or BALB/c (both Lyb5.2 + allotype) but not for CBA/N spleen cells. When splenocytes of BALB/c mice preimmunized with polyvinylpyrrolidone (T-independent type 2 antigen) were treated with this antiserum, 50% decrease in numbers of antibody-forming and antigen-induced non-specific immunoglobulin-forming cells was documented. We conclude that, during humoral immune response to T-independent type 2 antigen, not only antibody-forming cells but also non-specific immunoglobulin-forming cells are recruited from the Lyb5 + B cell pool.

In vitro and in vivo T cell responses in mice during bronchopulmonary infection with mucoid Pseudomonas aeruginosa

In vitro and in vivo T cell responses were determined during the course of bronchopulmonary infection with mucoid Pseudomonas aeruginosa. T cell responses were compared in two inbred mouse strains, namely BALB/c mice, which are resistant to the establishment of chronic bronchopulmonary Ps. aeruginosa infection, and C57Bl/6 mice, which have high numbers of bacteria in the lungs through 14 days post-infection. Unseparated lung cells and lung T cells from BALB/c mice exhibited significantly higher in vitro proliferative responses to both heat-killed Ps. aeruginosa and concanavalin A (Con A) than cells from C57Bl/6 mice through 20 days post-intratracheal infection with 10(4) colony-forming units (CFU) Ps. aeruginosa. Proliferation of unseparated lung cells but not lung T cells from BALB/c mice infected 6 days previously with 10(5) CFU Ps. aeruginosa was suppressed in response to Con A; these cells were unresponsive to specific antigen. Suppression of lymphocyte proliferation in the lungs of C57Bl/6 mice infected with 10(4) CFU Ps. aeruginosa and in BALB/c mice infected with 10(5) CFU was found to be mediated by adherent lung cells via the production of nitric oxide and prostaglandins. Determination of in vivo T cell-mediated responses in infected mice demonstrated that resistant BALB/c mice had high DTH and low Pseudomonas-specific antibody responses, while C57Bl/6 mice had low DTH and high antibody levels, in particular, IgG2b and IgM.