Mycobacterial Dormancy Systems and Host Responses in Tuberculosis

Abstract

Tuberculosis (TB) caused by the intracellular pathogen, Mycobacterium tuberculosis (Mtb), claims more than 1.5 million lives worldwide annually. Despite promulgation of multipronged strategies to prevent and control TB, there is no significant downfall occurring in the number of new cases, and adding to this is the relapse of the disease due to the emergence of antibiotic resistance and the ability of Mtb to remain dormant after primary infection. The pathology of Mtb is complex and largely attributed to immune-evading strategies that this pathogen adopts to establish primary infection, its persistence in the host, and reactivation of pathogenicity under favorable conditions. In this review, we present various biochemical, immunological, and genetic strategies unleashed by Mtb inside the host for its survival. The bacterium enables itself to establish a niche by evading immune recognition via resorting to masking, establishment of dormancy by manipulating immune receptor responses, altering innate immune cell fate, enhancing granuloma formation, and developing antibiotic tolerance. Besides these, the regulatory entities, such as DosR and its regulon, encompassing various putative effector proteins play a vital role in maintaining the dormant nature of this pathogen. Further, reactivation of Mtb allows relapse of the disease and is favored by the genes of the Rtf family and the conditions that suppress the immune system of the host. Identification of target genes and characterizing the function of their respective antigens involved in primary infection, dormancy, and reactivation would likely provide vital clues to design novel drugs and/or vaccines for the control of dormant TB.

Keywords: DosR regulon; Mycobacterium tuberculosis; alveolar macrophages; antibiotic resistance; dormancy; granuloma; latency.

Figure 1

I. TLR signaling modulation. The Mycobacterium tuberculosis (Mtb) cell wall components interact with TLR-2 and modulate the host cell signaling via p38 MAPK, resulting in activation of NF-κB and synthesis of C/ERB that binds to class II transactivator (CIITA) promoter and inhibits CIITA production leading to decreased expression of MHC-II, thus inhibiting antigen presentation. II. Immune evasion: prolonged signaling by cell wall components induces the anti-inflammatory cytokines, TGF-β, interleukin (IL)-10, and IL-4 (Th2-dependent manner), which inhibits IL-12. IL-12 is required for the production of interferon-γ, iNOS, and NO, a major defense of the host against Mtb. The intracellular pathogen secretes SucB, AhpC, and AhpD, which catalyzes the breakdown of reactive nitric intermediates (RNIs). III. Phagosome maturation inhibition: mannosylated-LAM (ManLAM) activates GDI via p38MAPK leading to the inhibition of Rab5 activity that is required for the recruitment of early endosomal autoantigen1 (EEA1); it also inhibits the increase in cytosolic Ca²⁺ flux required for the hvps34 activity. pknG prevents phagolysosomal fusion and sapM, ptpA inactivates the phosphatidylinositol 3-phosphate (PI3P) and VPS33B through dephosphorylation. Phosphatidylinositol mannoside (PIM) mediates the early endosomal fusion through which bacilli gains access to nutrients such as iron required for its survival.

Figure 2

Dynamics of granuloma formation, maintenance, and reactivation: Mycobacterium tuberculosis primarily harbors lungs and infects alveolar macrophages and establishes its niche. The host defense sets in to counteract the actions Mtb. In this process, multiple possibilities exist where there could be (A) active infection/clearance: macrophages (Mφ) and Th1 cells secrete tumor necrosis factor (TNF)-α, IFN-γ that recruits other immune cells like neutrophils, dendritic cells (DCs), and B-cells that might clear the infection or bacilli may multiply leading to primary infection. However, some of the bacilli might escape the host’s immune actions and enter into dormancy. (B) Solid granuloma: solid granuloma is composed of macrophages, lymphocytes (B-cells and T-cells), DCs, and neutrophils. The solid granuloma is usually encircled by fibroblasts. During latent infection, Mtb encourages the immune system to form granuloma by manipulating host immune responses for its survival. Some of the Mtb survival strategies include stimulation of macrophages and T-cells to secrete large doses of TNF-α, chemokines such as CCL-2, CCL-12, and CCL-13, which are crutial for the recruitment of other immune cells and maintenance of granuloma. Extensive calcification of granuloma by Mtb leads to prevention of apoptosis. Secretion of Mtb antigens such as Rv0386 (adenylyl cyclase) produce cAMP which signal the synthesis of matrix metalloproteinases (MMPs) that are involved in the maintenance of granuloma by unknown mechanism. (C) Reactivation: Mtb reactivates and exits from the granuloma when immune system compromises and bacilli spread to new sites of infection. Poor nutrition, decrease in the number of T-cells, HIV coinfection are some of the contributing factor for reactivation.

Figure 3

Reactivation of latent bacilli: Mycobacterium tuberculosis (Mtb) persisting under hostile conditions in granuloma reactivates when favorable conditions prevail such as availability of nutrients, re-aeration, and immune compromization of the host leading to activation of different set of genes that are required for the re-growth of the bacilli. During reactivation, Mtb mainly expresses Clp protease gene Regulator (ClpgR) and its inducible genes Clp Proteases. Clp proteases play a vital role in the maintenance of protein turnover, degradation of misfolded or accumulated proteins, helping the bacteria to shift to metabolic phase. On the other hand, Mtb expresses a family of five resuscitation-promoting factors (RPF A to E) that are essential for the lysis of granulomatous cell wall. Certain Universal stress proteins (like Rv2005c) are also upregulated during resuscitation which might play an essential role for the re-growth.