**4.6 Factors of** *Mycobacterium tuberculosis* **virulence**

The *M. tuberculosis* virulence depends upon various metabolic processes that provide for successful infection process. The pathogen survival in host cells directly depends on avoiding host defence mechanisms and getting nutrient substances from host tissues for the existence and reproduction. A considerable damage of host tissues and organs at later stages of infection is also needed as it facilitates dissemination of the infection. We will consider the factors that directly affect the host organism in order to suppress host defence mechanisms. One of such immune response modulators is the ESAT-6 protein (Sorensen et al., 1995) encoded by the *esxA* gene and secreted by the type VII secretion system (T7SS) ESX-1 (Abdallah et al., 2007; Simeone et al., 2009). ESAT-6 is one of the immune modulation key factors suggested to be involved in lysis of phagolysosome membranes and macrophage outer membranes thus facilitating spread of mycobacteria from one host cell to another (de Jonge et al., 2007; Kinhikar et al., 2010). The *in vivo* expression of the *esx* cluster genes is under constant control and can be both decreased and increased depending on the conditions (Fontan et al., 2008; Rohde et al., 2007; Schnappinger et al., 2003; Tailleux et al., 2008).

One more important system of immune response modulation is the expression of the PE/PPE family proteins that possess immunogenic properties (Bottai & Brosch, 2009; Sampson, 2011; Voskuil et al., 2004). The level of their expression is also dependent upon specific conditions (Azhikina et al., 2010; Fontan et al., 2008; Schnappinger et al., 2003; Tailleux et al., 2008; Voskuil et al., 2004).

### **4.7 Transcription regulation**

The adaptation to changing conditions of the environment is mainly underlain by the activity of signal and regulatory systems of the bacterium, and therefore by changing expression of transcription regulatory systems' genes. Among genes of the regulatory systems, 13 genes of sigma factors (Manganelli et al., 2004; Rodrigue et al., 2006) are of special interest. Their differential expression was repeatedly observed in *in vivo* experiments. For instance, *sigH* was upregulated in artificial granulomas in mice and in human macrophages (Karakousis et al., 2004), whereas *sigB* and *sigE* were also upregulated

The cell wall and inner plasma membrane are components of the complex cell envelope of mycobacteria. The inner plasma membrane contains a lot of transport systems. Some genes of these systems were observed to be upregulated, among them the *irtA/B* genes encoding carboxymycobactin transporters (Li et al., 2010; Schnappinger et al., 2003; Tailleux et al., 2008), the *sugI*, *Rv2040c* and *Rv2041c* genes of carbohydrate transporters (Azhikina et al., 2010; Schnappinger et al., 2003; Tailleux et al., 2008), and the *narK2* gene of the nitrate transporter protein (Azhikina et al., 2010; Garton et al., 2008; Tailleux et al., 2008; Talaat et

Enhanced expression of the genes of siderophore-synthesis enzymes, mycobactin and carboxymycobactin (*mbtA-J*, *mbtL-N*), could be explained by limited access to iron (Azhikina

The *M. tuberculosis* virulence depends upon various metabolic processes that provide for successful infection process. The pathogen survival in host cells directly depends on avoiding host defence mechanisms and getting nutrient substances from host tissues for the existence and reproduction. A considerable damage of host tissues and organs at later stages of infection is also needed as it facilitates dissemination of the infection. We will consider the factors that directly affect the host organism in order to suppress host defence mechanisms. One of such immune response modulators is the ESAT-6 protein (Sorensen et al., 1995) encoded by the *esxA* gene and secreted by the type VII secretion system (T7SS) ESX-1 (Abdallah et al., 2007; Simeone et al., 2009). ESAT-6 is one of the immune modulation key factors suggested to be involved in lysis of phagolysosome membranes and macrophage outer membranes thus facilitating spread of mycobacteria from one host cell to another (de Jonge et al., 2007; Kinhikar et al., 2010). The *in vivo* expression of the *esx* cluster genes is under constant control and can be both decreased and increased depending on the conditions (Fontan et al., 2008; Rohde et al., 2007; Schnappinger et al., 2003; Tailleux et al.,

One more important system of immune response modulation is the expression of the PE/PPE family proteins that possess immunogenic properties (Bottai & Brosch, 2009; Sampson, 2011; Voskuil et al., 2004). The level of their expression is also dependent upon specific conditions (Azhikina et al., 2010; Fontan et al., 2008; Schnappinger et al., 2003;

The adaptation to changing conditions of the environment is mainly underlain by the activity of signal and regulatory systems of the bacterium, and therefore by changing expression of transcription regulatory systems' genes. Among genes of the regulatory systems, 13 genes of sigma factors (Manganelli et al., 2004; Rodrigue et al., 2006) are of special interest. Their differential expression was repeatedly observed in *in vivo* experiments. For instance, *sigH* was upregulated in artificial granulomas in mice and in human macrophages (Karakousis et al., 2004), whereas *sigB* and *sigE* were also upregulated

**4.5 Cell wall, membrane, and transport** 

et al., 2010; Schnappinger et al., 2003; Tailleux et al., 2008).

**4.6 Factors of** *Mycobacterium tuberculosis* **virulence** 

al., 2007).

2008).

Tailleux et al., 2008; Voskuil et al., 2004).

**4.7 Transcription regulation** 

in mouse phagosomes (Rohde et al., 2007), as well as in artificial mouse granulomas and in the mouse lung, respectively (Karakousis et al., 2004; Talaat et al., 2004).

Other transcription regulatory genes whose upregulation was observed in *in vivo* experiments are *whiB3* (Fontan et al., 2008; Rohde et al., 2007), *ethR*, *ideR*, *kstR* and *relA*  (Fontan et al., 2008; Schnappinger et al., 2003; Tailleux et al., 2008). In addition, *M. tuberculosis* has 12 two-component regulatory systems (Tucker et al., 2007). Two of them, *phoPR* (Gonzalo-Asensio et al., 2008) and *dosRS* (*devRS*) (Park et al., 2003), were studied more thoroughly than others. It was shown that functional activity of the phoPR system supports the *M. tuberculosis* virulence by regulating the metabolism of complex lipids and the work of the ESX secretion systems (Gonzalo-Asensio et al., 2008). The positive transcription regulator phoP was observed to induce genes under its control in a low-acid (pH 6.4) environment of mouse macrophage phagosomes (Rohde et al., 2007). Not less important is the two component regulatory system dosRS responsible for expression regulation of about 50 genes (Park et al., 2003). Enhanced expression of these genes was repeatedly observed in the course of mycobacterial infection of macrophages (Fontan et al., 2008; Rohde et al., 2007; Schnappinger et al., 2003; Tailleux et al., 2008). Moreover, *dosRS*  regulon genes were expressed practically in all other conditions: in *M. tuberculosis* from artificial mouse granulomas, samples of mouse lung tissues and surgical samples of human lung, as well as in samples of sputum and in some experiments *in vitro* (Garton et al., 2008; Homolka et al., 2010; Karakousis et al., 2004; Li et al., 2010; Talaat et al., 2007; Timm et al., 2003). The functional role of this regulon is still not quite clear, but its activity was suggested to be important for *M. tuberculosis* adaptation to variations in the redox status during the infection process (Bacon et al., 2004; Bacon & Marsh, 2007; Rustad et al., 2009).
