CARACTERIZACIÓN DE LA MATERIA PRIMA

1.3.1. Prevention of infection

One of the principal functions of the biliary tree is to prevent bacterial colonisation and possible death from systemic sepsis. The ability to avoid septic episodes is hampered by the need for bilio-enteric continuity in order for the digestive functions of bile. There is a well recognised association between chronic irritation (often initiated by infection), metaplastic change of epithelium and malignant transformation of cells. In the bladder, chronic infestation by schistosomiasis leads to the development of squamous metaplasia and thence squamous cell carcinoma. Metaplasia-carcinoma and dysplasia-carcinoma sequences have been described in the biliary tree, and although the exact causes o f cholangiocarcinoma remain unknown there are certain associations with chronic infection {vide infra). Recurrent Oriental pyogenic cholangitis is a predisposing factor in cholangiocarcinoma as is liver fluke infestation. In HIV-positive individuals, squamous metaplasia has been reported in chronic cryptosporidial infection (Kline et al. 1993). These organisms were found adjacent to areas of metaplasia and AIDS patients are known to develop biliary lymphoma, Karposi’s sarcoma and sclerosing cholangitis.

The biliary system has a number of mechanisms to prevent its colonisation by bacteria. Human bile is normally sterile. Bacteria are constantly present in the small intestine and transiently in the portal vein from the intestine. Both sites o f entry, by reflux up the biliary tract and via haematogenous spread, have been studied but the former has attracted more attention. Most studies o f biliary organisms show a preponderance of enteric organisms such as coliforms. Streptococci species, and

Chapter 1 - Introduction

Klebsiella species. Anaerobes in the biliary tract have been found in chronic biliary disease and in bilio-enteric anastomoses.

Therefore one of the principal functions of the biliary tract is to prevent infection whilst still allowing communication with colonised bowel. Some o f this function is dictated by structural adaptation with tight junctions and sphincter mechanisms whilst others are more physiological.

1.3.2. Anatomical features that prevent infection;

Tight junctions exist between hepatocytes preventing entry o f bacteria from sinusoidal blood into bile. The permeability o f these junctions is regulated by bile acids, oestrogen and intra-biliary pressure (LaGarde et al. 1981; Forker, 1969; Easter et al. 1983). Increased biliary pressure distorts these junctions, while oestrogen may exert its effects by inhibiting the cannalicular active transport systems and reducing the junctional elements present (thus perhaps leading to the cholestasis in pregnancy or contraceptive pill use). The exact mechanism by which bile acids exert their influence is unknown.

The sphincter o f Oddi acts as a mechanical barrier to the influx o f bacteria. Colonisation, as evidenced by positive culture, is more common in patients with ductal stones or carcinoma o f the ampulla with intermittent obstruction to bile floy/. It also occurs after operative procedures with disruption o f the sphincter such as endoscopic sphincterotomy or surgical choledochotomy.

Bilio-enteric anastomoses, with disruption of the Sphincter o f Oddi and reflux of bacteria into the biliary tree, have been studied in an animal model; N-nitrosobis (2- oxopropyl)amine administered to Syrian hamsters resulted in increased proliferation

Chapter 1 - Introduction

of biliary epithelium in two groups undergoing cholecystoduodenostomy or cholecystoileostomy. The duodenal anastomosis group developed intra and extra- hepatic bile duct cancers whereas the ileostomy group developed only intra-hepatic cancers.

1.3.3. Physiological prevention of infection

Bile flow constantly flushes the biliary tree in a similar way to urinary flushing. Flow is approximately 50% dependent on bile salts. In biliary obstruction bile salts and bile flow are reduced. Lipopolysaccharide components from the outer membrane o f gram- negative bacteria exert a cholestatic effect on unobstructed liver (a bile salt independent effect). Reductions in biliary flow may predispose to infections (Utili et al. 1976).

Extra-hepatic ducts are lined with mucus, which in the intestine congregates bacteria and retains secretory IgA. Bile mucus has been implicated in the development of gallstones which in turn are associated strongly with gallbladder cancer. The exact role of bile mucus in protection against epithelial colonisation and prevention of infection is unknown.

In-vitro experiments have shown that, at physiological levels, bile salts are inhibitory to bacterial proliferation especially E. coli, Klebsiella spp. and Enterococci spp (Stewart et al. 1986). This is related to their hydrophobicity and detergent properties, with the more potent bile salts being those with fewer hydroxyl groups and only a - hydroxylation. However this is complicated by the natural phenomenon o f micelle formation in concentrated bile and alteration of bile salt properties.

Chapter 1 - Introduction

KupfFer cells, located in liver sinusoids, make up 30% of sinusoidal cells and 80% of fixed body macrophages. They filter toxic substances, endotoxins and bacteria that arrive in the portal circulation. In chronic liver disease and biliary obstruction their function is altered in a number of ways including:-

a) opsonisation is impaired because o f reduced fibronectin production by hepatocytes and endothelial cells.

b) increased bile salt levels impair phagocytosis and binding of polysaccharides c) Kupffer cell populations are reduced.

(Laskin, 1990; Manifold et al. 1983)

Secretory IgA is found in bile ducts but its function is unclear. Its secretion is impaired in biliary obstruction with a proportionate increase in serum IgA and endotoxin. IgA is not involved in opsonisation, complement fixation or antibody- mediated cell cytotoxicity. It does bind luminal antigens and increases the impermeability o f the intestinal wall, and may have the same or similar functions in the biliary tree (Fubara and Freter, 1973).

Chapter 1 - Introduction