The Death Receptors

role in tuberculosis

pathogenesis !! MASOUD KEIKHA

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Tuberculosis

Tuberculosis (TB) remains one of today's global health challenges, ranking as the

second leading infectious cause of death and one of the most burden-inflicting

diseases in the worldwide.

The 2018 WHO Global Tuberculosis Report estimated a worldwide incidence of

10.7 million new TB-cases and 1.6 million deaths in 2017.

In 2018, an estimated 1 million children became ill with TB and 230,000 children

died of TB (including children with HIV associated TB).

Globally, TB incidence is falling at about 2% per year.

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Tuberculosis and its problems

About one-quarter of the world's population has latent TB, which means people

have been infected by TB bacteria but are not (yet) ill with the disease and cannot

transmit the disease.

People infected with TB bacteria have a 5–15% lifetime risk of falling ill with TB.

WHO estimates that there were 558,000 new cases with resistance to rifampicin –

the most effective first-line drug, of which 82% had MDR-TB.

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HIV weakens the immune system, increasing the risk of TB in people with HIV.

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Tuberculosis and its problems

Since 1921, the BCG vaccine, a viable attenuated strain of Mycobacterium bovis bacillus Calmette–Guérin, has been the only available vaccine against

tuberculosis.

its protective efficacy remains controversial between 0–80% for TB in adults.

Suppurative lymphadenitis, osteomyelitis/osteitis, and disseminated BCG vaccine

mediated infection are severe complications.

Disseminated BCG infection is a systemic adverse reaction which usually occurs

in people with impaired immunity. Therefore, BCG should never be given to HIV-

positive patients or infants born to mothers with HIV or suspected of being HIV-

positive.

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Novel therapeutic option for TB

The pathogenesis mechanism of TB is stays

pathless completely, which creates a barrier in

finding an efficient diagnosis and treatment

method. To this purpose, the scrutiny of

pathogenesis at the transcriptomic level can

untangle the intricate pathways of disease.

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Immunology of tuberculosis

Mtb commonly enters the host through mucosal surfaces, usually via the lung after

inhalation of infectious droplets from an infected individual.

Less frequently, it enters the host via the gut following the consumption of

contaminated milk or traumatic inoculation into the skin.

there are four possible clinical outcomes:

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Immunology of tuberculosis

(1) aborted infection: Mtb can be immediately cleared by the

host innate immune responses.

(2) primary active TB: the pathogens grow and multiply after infection, eventually

causing TB.

(3) LTBI: the bacteria can enter to a dormant state and may never cause infection.

(4) reactivation: when the host immune system becomes weakened, the latent

bacteria reactivate and cause active TB.

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Evasion of immunity response by Mycobacterium

tuberculosis

Provoke T-regulatory cells during extension of infection duration.

Inhibits the Maturation of Phagolysosomes

Inhibits the Acidification of Phagolysosomes

Inhibits Oxidative Stress

Inhibits Autophagy

Formation of Granulomas

Inhibition of Apoptosis

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Apoptosis

Apoptosis is morphologically characterized by cell shrinking, fragmentation of the

nucleus, formation of membrane blebs, and apoptotic bodies.

Cellular apoptosis are the loss of mitochondrial outer membrane permeability

(MOMP), cleavage and activation of members of the caspase protease family,

fragmentation of chromosomal DNA, and externalization of phosphatidylserine at

the cell membrane.

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Receptors death and its related protein

DRs including

-Toll-like receptors (TLRs)

-Fas-associated death domain (FADD)

-caspases

-rest in peace 1 (RIPK1)

-rest in peace 3 (RIPK3)

-PD-1

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TLR signaling pathway

The Toll-like receptor (TLR) family plays an instructive role in innate immune

responses against microbial pathogens.

TLRs recognize specific molecular patterns found in a broad range of microbial

pathogens (PAMPs) such as bacteria and viruses, triggering inflammatory and

antiviral responses and dendritic cell maturation, which result in the eradication of

invading pathogens.

TLRs interact with different combinations of adapter proteins and activate various

transcription factors such as nuclear factor (NF-kB), activating protein-1 (AP-1)

and interferon regulatory factors (IRF-1), driving a specific immune response.

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Fas/FasL

Fas or FasR, also known as apoptosis antigen 1 (APO-1 or APT), cluster of

differentiation 95 (CD95) or tumor necrosis factor receptor superfamily member 6

(TNFRSF6) is a protein that in humans is encoded by the FAS gene.

Fas ligand (FasL or CD95L) is a type-II transmembrane protein that belongs to the

tumor necrosis factor (TNF) family. Its binding with its receptor induces

apoptosis. Fas ligand/receptor interactions play an important role in the regulation

of the immune system and the progression of cancer.

Defective Fas-mediated apoptosis may lead to oncogenesis as well as drug

resistance in existing tumors.

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Caspases

Caspases (cysteine-aspartic proteases) are a family of protease enzymes playing

essential roles in programmed cell death (including apoptosis, pyroptosis and

necroptosis) and inflammation.

There are other identified roles of caspases such as cell proliferation, tumour

suppression, cell differentiation, neural development and axon guidance and

ageing.

inflammatory caspase-1 has been implicated in causing autoimmune diseases;

drugs blocking the activation of Caspase-1 have been used to improve the health

of patients.

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RIPK1

Receptor-interacting serine/threonine-protein kinase 1 (RIPK1) is an enzyme

that in humans is encoded by the RIPK1 gene, which is located on chromosome 6.

RIPK1 plays a role in apoptosis and necroptosis. Some of the cell survival

pathways RIPK1 participates in include NF-κB, Akt, and SAPK/JNK Signaling.

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PD-1

programmed cell death-1 (PD-1), a member of the CD28 superfamily of Tcell

regulators, PD ligand 1 (PD-L1) have been identified as a possible target for

immunotherapy in cancer-therapy.

it promotes apoptosis (programmed cell death) of antigen-specific T-cells in lymph

nodes. Second, it reduces apoptosis in regulatory T cells.

PD-1 inhibitors, a new class of drugs that block PD-1, activate the immune system

to attack tumors and are used to treat certain types of cancers.

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Tuberculosis infection and apoptosis

Kornfeld et al have shown that Naïve primary macrophages are resistant to TNF

cytotoxicity but become primed for TNF death signals when infected with

attenuated strains of M. tuberculosis and related mycobacteria.

M. tuberculosis-induced apoptosis in primary macrophages in vitro is mediated by

TNF. There is evidence for the involvement of caspase 9 and caspase 3 in this

process.

This suggests that programmed cell death of the host macrophage not only

eliminates a preferred growth niche for M. tuberculosis but also activates a unique

microbicidal mechanism.

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Kornfeld et al., 2004. in THP-1 cell lines 26

Tuberculosis inhibits apoptosis 27

First of all, Keane et al. showed that virulent species of mycobacteria (M.

tuberculosis H37Rv, M. tuberculosis Erdman and Mycobacte-rium bovis) induced

considerably less apoptosis in primary human alveolar macrophages than

nonvirulent mycobacterial species (M. bovis Bacille Calmette-Guérin, M.

tuberculosis H37Ra and Mycobacterium kansasii).

the mycobacterial gene nuoG, which encodes one sub-unit of the type I NADH

dehydrogenase in M. tuberculosis; which is inhibits apoptosis.

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The Mtb nuoG gene is part of a 14-gene operon (NDH-1) encoding

for a type I NADH dehydrogenase whose function is important for

the capacity of Mtb to inhibit host cell apoptosis.

NDH-1, is important in Mtb-mediated activation of iNOS and

NF_kB signaling pathway in the host macrophages.

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Two other M. tuberculosis proteins, SecA2 and protein kinase E (PknE) have been

implicated in the inhibition of host cell apoptosis.

The superoxide dismutase A (SodA), an enzyme that catalyzes conversion of

superoxide anions to hydrogen peroxide. The knockdown of superoxide dismutase

A (SodA) resulted in increased apoptosis of host cells in mouse lungs and

decreased virulence of the bacteria (Edwards et al., 2011).

The secA2 (SodA is secreted via the SecA2 secretion system in Mtb) deletion

mutants of M. tuberculosis induce more apoptosis upon macrophage infection than

wild-type M. tuberculosis.

Keane et al. showed that M. tuberculosis H37Rv-infected cells can inhibit TNF-α-

mediated cell death, while those infected with M. tuberculosis H37Ra cannot.

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Tuberculosis infection and

apoptosis

EsxA (ESAT-6), is involved in the up-regulation of MMP9 on epithelial cells

surrounding the granuloma, leading to recruitment of uninfected macrophages

during primary stage of infection.

The Mtb-induced secretion of soluble host cell TNF-receptor 2 to inhibit TNF

signaling caused by ESAT-6.

Mtb infection leads to a reduction of Fas expression at the cell surface to inhibit T-

cell-mediated killing (Oddo et al).

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The serine/threonine protein kinase E (PknE) of Mtb is up-regulated by an

increase in Reactive Oxygen Intermediates (ROIs) and Reactive Nitrogen

Intermediates (RNIs), and without PknE there is an decrease in the expression of

pro-apoptotic members of the Bcl-2 family (Bax and Bid).

The first operon (Rv3654c-Rv3660c) encompasses four genes that have homology

with type IV pili; where it binds to and cleaves the protein-associated splice factor

(PSF), which is important for splicing of caspase 8 pre-mRNA.

Rv3364c enters the host cell cytosol and binds to and inactivates the host cell

protease cathepsin G.

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Caspase 3 & 8 34

PD1 & PDl-1 expression level in TB

patients

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Application

The uptake of Mtb antigens via phagocytosis of apoptotic bodies

by DCs allowed for the presentation of extracellular antigens to CD8+ T cells, a

process defined as cross-presentation.

The apoptotic bodies purified from mycobacteria-infected cells can be used to

vaccinate mice and induce a protective immune response (Winau et al. 2006).

The ESX-5 system is crucial for the secretion of the Mtb PE/PPE family of

proteins; Interestingly, the PE_PGRS33 protein is transported to host cell

mitochondria and induces programmed necrosis when expressed ectopically in

mammalian cells which is considered as novel TB-vaccine (Abdallah et al. 2006,

2009).

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This BCG-LLO strain also induces increased host cell apoptosis compared with

wild type BCG (Grode et al. 2005).

This vaccine is now in phase IIa human clinical trials.

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Metformin

Metformin, marketed under the trade name Glucophage among

others, is the first-line medication for the treatment of diabetes.

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Metformin & Apoptosis 39

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