In the majority of patients, tuberculosis (TB) is typically found as an airborne disease due to droplet nuclei infection. In the immuno-competent host, lungs are most commonly affected in M. Tuberculosis (MTB) infection, with estimates of 80-87% of lung involvement in subjects with active disease. A similar estimate of 70-90% has also been observed in immuno-compromised hosts, such as those with human immuno-deficiency virus infection (HIV) . A person in close contact with a smear-positive patient is at maximum risk of being infected. Studies have shown that the infection rate among close contacts ranges from 25-50%, even in the worst over-crowded and sub-standard conditions, while among those infected only a 10% will progress to disease . Thus, a great portion of close contacts remains uninfected after exposure and is probably due to the host's innate immune system.
The effective antimicrobial action of monocytes/macrophages against MTB infection consists of a well-organized antimicrobial machinery, with a large armamentarium of cytotoxic molecules, including Toll-like receptors, phagosome-lysosome function and the recently described tuberculosis resistance genes that are considered integral parts of this mechanism [3, 4].
Toll-like receptors (TLRs) are a family of proteins that play a key role in the innate immune response to infectious agents through their ability to discriminate conserved microbial structures, known as pathogen-associated molecular patterns (PAMPs), from self. TLRs recognition of PAMPs, such as lipopolysaccharide (LPS), initiates signal transduction through the NF-kB pathway. Nuclear translocation of NF-kB induces transcription of pro-inflammatory cytokine genes essential to mounting a protective immune response and host defence . TLRs' stimulation in macrophages has been shown to promote phagosomal maturation via activation of p38 MAP kinase in a MyD88 dependent signalling pathway [6, 7]. Pathogen associated molecular pattern signatures for pathogenic mycobacteria include, CpG DNA, 19 kDa lipoprotein, lipoarabidomannan and mannosylated phosphotidylinositol (PIM) . TLRs have been reported to participate in the interaction of pathogenic mycobacteria or in their extracellular products in mice and humans. In particular, TLR-2, in association with TLR-1, TLR-6 and TLR-4, has been implicated as a receptor involved in the recognition of mycobacterial antigens and activation of macrophages and dendritic cells (DCs) . Furthermore, TLRs are essential in the killing process of intracellular mycobacteria by human macrophages through the induction of the antimicrobial peptide cathelicidin . In addition, it has recently been shown that TLRs' non-synonymous polymorphisms are significantly associated with tuberculosis disease in humans .
Pathogenic mycobacteria have evolved a unique strategy to survive intracellularly within macrophages. They modulate phagosome maturation to prevent the fusion of endosomes with lysosomes. The block of phagolysosome biogenesis has been proposed to play a central role in the pathogenesis of the mycobacterium . The prolonged survival of M. tuberculosis within their mammalian host cells suggested that these pathogens, apart from producing virulence factors such as phosphatase SapM and protein kinase PknG, have evolved mechanisms to utilise host molecules for their own survival [3, 13]. An analysis of the protein composition of mycobacterial phagosome showed the exclusive presence of a protein that was strongly retained on phagosomes harbouring live mycobacteria [13, 14]. This protein, initially named TACO (for tryptophan-aspartatecontaining coat protein) is now referred as Coronin-1. Coronin-1 is a member of the WD repeatcontaining protein family of coronins. The founding member of this protein family was identified in the amoeba Dictyostelium discoideum and is involved in the regulation of actin-based dynamics .
In mammalian cells up to seven coronin isoforms have been identified and were thought to regulate F-actin-dependent cytoskeleton . Recent data indicate that Coronin-1 is essential for the survival of live mycobacteria into monocytes/macrophages, because this protein arrests the maturation of phagosome to phagolysosome. Coronin-1 is specifically restricting phagosomes containing live mycobacteria from entering the lysosomal pathway by regulating a calcium- dependent signalling process, which activates the calcium-dependent phosphatase calcineurin. This enzyme is responsible for the blocking effect of phagosome-lysosome fusion. In the absence of Coronin-1, calcineurin cannot be activated which results in lysosomal transfer and death of internalized mycobacteria. Similar results were also retrieved with calcineurin blockers, such as cyclosporine A or FK506 . The prolonged retention of Coronin-1 in phagosome during active infection has been attributed to a secreted mycobacterial lipoamide dehydrogenase C (LpdC), which retains Coronin-1 on the phagosomal membrane  and is produced only by live BCG and M. tuberculosis.
Also, recent data have shown that inbred mice expressing the Ipr1 (intracellular pathogen resistance 1) gene in the sst1 locus were protected against infection by M. tuberculosis and M. bovis. The up-regulation of Ipr1 after infection by M. tuberculosis limits the infection in tuberculosis resistant mice by switching the cell death pathway of the infected macrophages from necrosis to apoptosis . Sp110 is the closest human homologue of Ipr1 gene and is located on chromosome 2 (2q37,1). Sp110 is a component of the nuclear body, a multi-protein complex assumed to participate in the regulation of gene transcription. This gene is very important for monocytes' differentiation, apoptosis and activation including the response to pathogens [20, 21]. Sp110 inhibits vesicular stomatitis and influenza virus replication. It also confers resistance to human Foamy virus, while gene polymorphisms or mutations have been associated with susceptibility to hepatitis C, immunodeficiency virus and hepatic veno-occlusive disease [22, 23]. In this respect, Sp110 may also have a role in human tuberculosis infection.
Studies conducted so far, support the idea that intracellular factors like Coronin-1, Sp110 and TLRs play a decisive role in the progression of TB infection, at least in experimental tuberculosis. Hence, it is possible, that the expression of these molecules somehow regulates the intracellular fate of M. tuberculosis through a complicated cascade of up- and down-regulations. However, studies regarding the expression of these intracellular molecules in humans are lacking. We hypothesized that during active infection, because of the turnover of the infected monocytes and the following haematogenous spread of the infection, these markers could be increased in patients with TB.
The purpose of this study was to investigate the expression of these molecules in peripheral blood mononuclear cells and to look for any possible correlations between their level of expression and the disease status. We utilized a real-time PCR assay to measure the expression of mRNA encoding Coronin-1, Sp110 and TLRs in fresh whole blood cells of TB patients, close contacts to patients that had latent infection (Quantiferon positive, QFT (+)), close contacts to patients with no latent infection (Quantiferon negative, QFT (-)) and a negative control population. QFT testing was performed according to the CDC and the literature recommendations in all circumstances in which the TST is currently used, including contact investigations, evaluation of recent immigrants, and sequential-testing surveillance programs for infection control (e.g., those for healthcare workers) [24–26].
We demonstrated striking changes in the expression of Coronin-1, Sp110 and TLR-2, between the TB-infected population (actively and latently infected) compared to the uninfected controls, whereas the levels of the other TLRs did not differ significantly. Our data suggest that Coronin-1, Sp110 and at least Toll-like receptor-2 molecules are involved in the infectious process of tuberculosis. However, their significance and their precise mechanism in human tuberculous infection deserve further investigation.