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PATHOGENESIS OF PRIMARY BILIARY CIRRHOSIS

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PATHOGENESIS OF PRIMARY BILIARY CIRRHOSIS

PBC is a chronic cholestatic liver disease of autoimmune origin.

Enhanced apoptosis in BEC is a critical step in ductular destruction in PBC, and some clues exist on mechanisms by which apoptosis in BEC cause the tissue-specific autoimmune reactivity characteristic of PBC.




GENETICS IN PBC

PBC pathogenesis is multifactorial, with genetic and environmental factors interplaying to determine disease onset and progression.

The lines of evidence for genetic predisposition are:

o       Rate of PBC in monozygote twins is 63%.

o       6% of patients with PBC have a first-degree relative that also suffers from PBC.

o       high female/male disease incidence ratio.

o       women with PBC have a significantly enhanced monosomy x frequency in peripheral white blood cells ( with SS and autoimmune thyroid disease.

o       Controversially variate association with alleles of HLA (DRB108).

ENVIRONMENTAL FACTORS

- Bacterial infection – concept of molecular (epitope) mimicry.

- The cross-reactivity of AMA with prokaryotic antigens for a number of microbes.

- Microbial sequences with high degree of similarity to PDC-E2 212-226 epitope (E. Coli, ……..

- The microbial motif ….. enhances IgM production in peripheral BMC, with CD27+ memory B cell in PBBC patients through toll-like receptor 9 and in B cell.

- Retroviral infection.

- Xenobiotics.

ROLE OF BEC (CHOLANGIOCYTES)

BEC express cell surface adhesion molecules.

Increase expression of ICAM and others, MHC class I and II, TNF α, IFN γ, IL-1, upon stimulation with proinflammatory cytokines.

Up-regulation of VCAM-1 and LFA-1.

Express accessory molecules responsible for the second (co-stimulatory) signal to T cells, CD 80, 86 (B7-1, B7-2).

Susceptibility to apoptosis via perforin/granzyme B pathway and this ….. enhanced …. Interaction of CD95 ( ) CD178 (….) in BEC of PBC.

Glutathiolation of the lysine-….. moiety of PDC-E2 was dramatically reduced by …. AMA.

Apoptoic cells are phagocytized by BECs and could be an endogenous source of auto antigens from BECs.

BECs actively transfer IgAs, and in …. These IgAs have specificity for PDC E2. These specific IgA type AMA can be detected in almost all body fluids of patients with PBC (saliva, urine, bile).

Fukushima and col. Detected deposits of such antibodies with PDC-E2 (or a highly homologous molecule) at the apical surface and in the cytoplasm of BECs and also detected the presence in liver allografts in patients with recurrent PBC after liver transplant.

B CELLS AND AUTOANTIBODIES

The presence of serum AMA and autoreactive B cells strongly endorses the concept of an autoimmune pathogenesis of PBC.

AMA is highly specific for PBC. They are detected against 2-OADC. Among them the major autoantigen is the E2 subunit of PDC.

The epitopes for this antibody to E2 subunit localize to three domains of PDC-E2

o       The inner (and outer) lipoic end domains.

o       E3 binding domain.

o       The catalytic and E2 binding domain.



Reactivity at lower frequency is also found to: 2 OGDC-E2 and BCOADC-E2

A pathogenic role for AMA is uncertain.

Other disease-specific autoantibodies:

o       PBC-specific ANA reactants (….. pore glycoproteins) (gp210, p62) in 30% of patients correlate with disease severity and progression.

o       PBC specific nucleoproteins: sp100 promyelocyte leukemia autoantigen that gives the characteristic fine nuclear dot pattern in …. (20-30%)

o       Anti-centromere antibodies (10%) – significant predictive factor in PBC for the development of ……

T CELLS

Both CD4 and CD8 T lymphocytes can be purified from biopsy of PBC patients and both subsets recognize epitopes of PDC-E2.

The minimal T cell epitope for CD4 T cells is aminoacid residue 163-176.

PDC-E2 163-176 specific CD4 T cell clones recognize other mitochondrial autoantigenes (OGDC-E2, BCOADC-E2).

CD8 T cells …… from peripheral blood of  patients with PBC identify amino-end residues 159-167 and 165-174 of PDC-E2.

There was a greater increase in the numbers of CTLs precursors in blood in early versus advanced stages of PBC and a 10-fold increase in ….. CTLs in the liver compared to the peripheral blood.

Potential role of target cell damage for a CD4-T cell subset – cytokine IL-17 induced by TGFβ and IL6.

Upregulated TGFβ signaling is present in liver in BC and IL6 release is augmented by the actions of hydrophobic bile acid.

A decreased reactivity of CD4+ CD25 high natural …. T cells (Tregs) contribute to a number of human autoimmune diseases, including PBC.

A reduction of Tregs compared with healthy control was detected and the ratio of hepatic Tregs over hepatic CD8+ cells in PBC was lower than that in patients with chronic hepatitis C or autoimmune hepatitis.

INNATE IMMUNITY IN PBC

Is the first line of defense.

Cellular components: dendritic cells, other professional APC, natural killer T cells.

In PBC: marked increase of NK in blood and liver.

The cytotoxic activity and perforin expression of NK cells significantly …. association with increased levels of plasma IL8 and IL8 receptors.

CYTOKINES

In PBC - … Th1 cytokine predominant in serum and liver (INFγ)

BECs of patients with PBC overexpress TNFα and corresponding receptor (paracrine activity).

Dysfunction of TGFβ receptor II (essential for signal transduction of TGFβ which regulates activation of lymphocytes) – PBC-like disease in mice.

ROLE OF REACTIVE OXYGEN SPECIES (ROS)

In damaged bile ducts of PBC glutathione S-transferase expression is markedly reduced, while perinuclear expression of 4-hydroxy….. is increased, reflecting active lipid peroxidation.

Antioxidant substances are reduced in PBC.

Free radicals involve overproduction of proinflammatory cytokines (TNFα, IL-6, IL 1b via enhanced activation of nuclear factor KB.



Taurochenodeoxycolic acid and taurocholic acid cause hepatocyte injury with concomitant generation of hydroperoxyde by mitochondria and induce hepatocyte apoptosis via ROS generation.

An increased bile acid concentration is a feature of at least late …

UCD has extensive ROS scavenging properties and prevents mitochondrial oxidative stress and lipid peroxidation.

PATHOGENIC MECHANISMS

Several theories have been proposed for the etiopathogenesis of the immune mediated tissue injury observed in PBC (Fig. 1)

  1. INITIATION

Microbial infection or chemical xenobiotic modification of the PDC-E2 epitope sequence with tolerance-breaking effects due to molecular mimicry.

  1. PERPETUATION

Tregs are important for the prevention of autoimmunity and maintenance of self-tolerance.

PBC display significant lower frequencies of Tregs which may contribute to the failure in tolerance in PBC.

  1. DESTRUCTION

The concept of ‘upstream’ and ‘downstream’ pathogenetic processes in PBC

‘Upstream’ refers to pathological processes responsible of early BEC loss and resulting initiation of ductopenia and cholestasis.

‘Downstream’ processes are the further cytophatic, pro-inflammatory and fibrotic mechanisms that occur as a consequence of cholestasis and …. sequelae of the homeostatic proliferative BEC response.

“UPSTREAM” PROCESSES IN PBC

The immune model for BEC damage

Altered self and molecular mimicry mechanisms for the breakdown of tolerance to self-PDC with cross reactivity between the lipoic acid co-factor and environmental xenobiotics, between bacterial and self-PDC and between self-PDC and viral protein.

PBC display significant lower Tregs, which may contribute to the failure in tolerance in PBC.

The immunomodulatory approaches to therapy should play a role in the very earliest stage of PBC.

BECs damage through antibody-directed cell cytotoxicity (BEC transcytosis).

The cytopathic model for BEC damage

Non-immune mediated cytopathic processes are responsible for BEC damage.

Direct cytopathic effects of beta-retrovirus.

BEC apoptosis secondary to environmental factors excreted in bile.

BEC senescence (oxidative stress; markers of senescence: β galactosidase, p16, p21)

Primary dysfunction of endothelial cells overproducing ET2-ET2 à proinflammatory cytokines IL1, IL6, MCP2, MCP3 (from Kupffer cells) à insufficient perfusion of peribiliary spaceà ischemia of biliary endothelium à apoptosis of BEC.

Similarities with …..

“DOWNSTREAM” MECHANISM

- Apoptosis of BEC à epithelial cells undergo phenotypic ‘reprogramming’ (process of EMT)

- phenotypic studies demonstrate an intermediate stage in which both epithelial (CK 19.9 E-cadherin) and mezenchymal (S100A4, Vimentin, αSMA) markers are expressed, with the loss of epithelial features and acquisition of a full mesenchymal functional and surface phenotype, loss of BEC and their ‘replacement’ by fibroblasts.

…. can be driven by TGFβ produced by CD1037 regulatory T cells

- The EMP can be reversed by HGF, which is in turn able to reverse liver fibrosis and ameliorate PBC-like bile duct lesions in chronic …..






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