2562 PART 10 Disorders of the Gastrointestinal System

may be initially episodic. However, in contrast to BRIC, Byler’s disease

progresses to malnutrition, growth retardation, and end-stage liver disease during childhood. This disorder is also a consequence of an FIC1

mutation. The functional relationship of the FIC1 protein to the pathogenesis of cholestasis in these disorders is unknown. Two other types

of PFIC (types 2 and 3) have been described. PFIC type 2 is associated

with a mutation in the protein originally named sister of P-glycoprotein,

now known as bile salt excretory protein (BSEP, ABCB11), which is

the major bile canalicular exporter of bile acids. As noted above, some

mutations of this protein are associated with BRIC type 2, rather than

the PFIC type 2 phenotype. PFIC type 3 has been associated with a

mutation of MDR3 (ABCB4), a protein that is essential for normal

hepatocellular excretion of phospholipids across the bile canaliculus.

Although all three types of PFIC have similar clinical phenotypes, only

type 3 is associated with high serum levels of γ-glutamyl transferase

(GGT) activity. In contrast, activity of this enzyme is normal or only

mildly elevated in symptomatic BRIC and PFIC types 1 and 2. Interestingly, mutations in FIC1 or BSEP are not found in approximately onethird of patients with clinical PFIC and normal GGT. Recent studies

have shown that patients with mutations in NR1H4, the gene encoding

the farnesoid X receptor (FXR), a nuclear hormone receptor activated

by bile acids, have a syndrome identical to PFIC2 with absent expression of BSEP. Mutations in tight junction protein 2 (TJP2) have also

been associated with severe cholestasis with normal GGT levels, likely

due to disruption of tight junctions at the bile canaliculus.

■ FURTHER READING

Bull LN, Thompson RJ: Progressive familial intrahepatic cholestasis.

Clin Liver Dis 22:657, 2018.

Canu G et al: Gilbert and Crigler Najjar syndromes: An update of the

UDP-glucuronosyltransferase 1A1 (UGT1A1) gene mutation database.

Blood Cells Mol Dis 50:273, 2013.

Gomez-Ospina N et al: Mutations in the nuclear bile acid receptor

FXR cause progressive familial intrahepatic cholestasis. Nat Commun

7:10713, 2016.

Hansen TW: Biology of bilirubin photoisomers. Clin Perinatol 43:277,

2016.

Lamola AA: A pharmacologic view of phototherapy. Clin Perinatol

43:259, 2016.

Memon N et al: Inherited disorders of bilirubin clearance. Pediatr Res

79:378, 2016.

Sambrotta M et al: Mutations in TJP2 cause progressive cholestatic

liver disease. Nat Genet 46:326, 2014.

Soroka CJ, Boyer JL: Biosynthesis and trafficking of the bile salt

export pump, BSEP: Therapeutic implications of BSEP mutations.

Mol Aspects Med 37:3, 2014.

van de Steeg E et al: Complete OATP1B1 and OATP1B3 deficiency

causes human Rotor syndrome by interrupting conjugated bilirubin

reuptake into the liver. J Clin Invest 122:519, 2012.

van Wessel DBE et al: Genotype correlates with the natural history of

severe bile salt export pump deficiency. J Hepatol 73:84, 2020.

Wolkoff AW: Organic anion uptake by hepatocytes. Compr Physiol

4:1715, 2014.

339 Acute Viral Hepatitis

Jules L. Dienstag

Acute viral hepatitis is a systemic infection affecting the liver predominantly. Almost all cases of acute viral hepatitis are caused by one of five

viral agents: hepatitis A virus (HAV), hepatitis B virus (HBV), hepatitis

C virus (HCV), the HBV-associated delta agent or hepatitis D virus

(HDV), and hepatitis E virus (HEV). All these human hepatitis viruses

are RNA viruses, except for hepatitis B, which is a DNA virus but

replicates like a retrovirus. Although these agents can be distinguished

by their molecular and antigenic properties, all types of viral hepatitis

produce clinically similar illnesses. These range from asymptomatic

and inapparent to fulminant and fatal acute infections common to all

types, on the one hand, and from subclinical persistent infections to

rapidly progressive chronic liver disease with cirrhosis and even hepatocellular carcinoma, common to the bloodborne types (HBV, HCV,

and HDV), on the other.

■ VIROLOGY AND ETIOLOGY

Hepatitis A HAV is a nonenveloped 27-nm, heat-, acid-, and etherresistant, single-stranded, positive-sense RNA virus in the Hepatovirus

genus of the picornavirus family (Fig. 339-1). Quasi-enveloped virus

particles encased in host plasma membrane–derived membranous vesicles circulate in the bloodstream. The virion contains four structural

capsid polypeptides, designated VP1–VP4, as well as six nonstructural

proteins, which are cleaved posttranslationally from the polyprotein

product of a 7500-nucleotide genome. Inactivation of viral activity can

be achieved by boiling for 1 min, by contact with formaldehyde and

chlorine, or by ultraviolet irradiation. Despite nucleotide sequence

variation of up to 20% among isolates of HAV and despite the recognition of six genotypes (three of which affect humans), all strains of

this virus are immunologically indistinguishable and belong to one

serotype. Human HAV can infect and cause hepatitis in chimpanzees, tamarins (marmosets), and several monkey species. Recently, a

hepatotropic Hepatovirus related to, and likely to have shared common evolutionary ancestry with, human HAV has been identified in

several species of harbor seals, albeit without histologic evidence for

liver injury or inflammation; HAV-like hepatoviruses have also been

identified in small mammals, including bats and rodents. Hepatitis

A has an incubation period of ~3–4 weeks. Its replication is limited

to the liver, but the virus is present in the liver, bile, stools, and blood

during the late incubation period and acute preicteric/presymptomatic

phase of illness. Despite slightly longer persistence of virus in the liver,

fecal shedding, viremia, and infectivity diminish rapidly once jaundice

becomes apparent. Detection of HAV RNA by sensitive reverse transcription polymerase chain reaction assays has been reported to persist

at low levels in stool, the liver, and serum for up to several months after

acute illness; however, this does not correlate with persistent infectivity,

probably because of the presence of neutralizing antibody. HAV can be

cultivated reproducibly in vitro and in primate models.

Antibodies to HAV (anti-HAV) can be detected during acute illness when serum aminotransferase activity is elevated and fecal HAV

shedding is still occurring. This early antibody response is predominantly of the IgM class and persists for several (~3) months, rarely for

6–12 months. During convalescence, however, anti-HAV of the IgG class

becomes the predominant antibody (Fig. 339-2). Therefore, the diagnosis of hepatitis A is made during acute illness by demonstrating antiHAV of the IgM class. After acute illness, anti-HAV of the IgG class

remains detectable indefinitely, and patients with serum anti-HAV

are immune to reinfection. Neutralizing antibody activity parallels the

appearance of anti-HAV, and the IgG anti-HAV present in immune

globulin accounts for the protection it affords against HAV infection.

Hepatitis B HBV is a DNA virus with a remarkably compact

genomic structure; despite its small, circular, 3200-bp size, HBV DNA

codes for four sets of viral products with a complex, multiparticle

structure. HBV achieves its genomic economy by relying on an efficient

strategy of encoding proteins from four overlapping genes: S, C, P, and

X (Fig. 339-3), as detailed below. Once thought to be unique among

viruses, HBV is now recognized as one of a family of animal viruses,

hepadnaviruses (hepatotropic DNA viruses), and is classified as hepadnavirus type 1. Similar viruses infect certain species of woodchucks,

ground and tree squirrels, and Pekin ducks, to mention the most carefully characterized; genetic evidence of ancient HBV-like virus forbears

has been found in fossils of ancient birds, and an HBV-like virus has

been identified in contemporary fish. Studies of ancient HBV genomes

date an association between HBV and human beings back as long

as 21,000 years ago; primate HBV-like viruses date back millions of years,

2563Acute Viral Hepatitis CHAPTER 339

suggesting that HBV predated the emergence of modern humans.

Like HBV, all have the same distinctive three morphologic forms,

have counterparts to the envelope and nucleocapsid virus antigens

of HBV, replicate in the liver but exist in extrahepatic sites, contain

their own endogenous DNA polymerase, have partially double-strand

and partially single-strand genomes, are associated with acute and

chronic hepatitis and hepatocellular carcinoma, and rely on a replicative strategy unique among DNA viruses but typical of retroviruses.

Entry of HBV into hepatocytes is mediated by binding to the sodium

taurocholate cotransporting polypeptide receptor. Instead of DNA replication directly from a DNA template, hepadnaviruses rely on reverse

transcription (effected by the DNA polymerase) of minus-strand DNA

from a “pregenomic” RNA intermediate. Then, plus-strand DNA

is transcribed from the minus-strand DNA template by the DNAdependent DNA polymerase and converted in the hepatocyte nucleus

to a covalently closed circular DNA, which serves as a template for

messenger RNA and pregenomic RNA. Viral proteins are translated by

the messenger RNA, and the proteins and genome are packaged into

virions and secreted from the hepatocyte. Although HBV is difficult to

cultivate in vitro in the conventional sense from clinical material, several cell lines have been transfected with HBV DNA. Such transfected

cells support in vitro replication of the intact virus and its component

proteins.

VIRAL PROTEINS AND PARTICLES Of the three particulate forms

of HBV (Table 339-1), the most numerous are the 22-nm particles,

which appear as spherical or long filamentous forms; these are antigenically indistinguishable from the outer surface or envelope protein

of HBV and are thought to represent excess viral envelope protein.

Outnumbered in serum by a factor of 100 or 1000 to 1 compared with

the spheres and tubules are large, 42-nm, double-shelled spherical

particles, which represent the intact hepatitis B virion (Fig. 339-1). The

envelope protein expressed on the outer surface of the virion and on

the smaller spherical and tubular structures is referred to as hepatitis

B surface antigen (HBsAg). The concentration of HBsAg and virus particles in

the blood may reach 500 μg/mL and 10

trillion particles per milliliter, respectively. The envelope protein, HBsAg, is

the product of the S gene of HBV.

Envelope HBsAg subdeterminants

include a common group-reactive antigen,

a, shared by all HBsAg isolates and one

of several subtype-specific antigens—d

or y, w or r—as well as other specificities. Hepatitis B isolates fall into one

of at least 8 subtypes and 10 genotypes

(A–J). Geographic distribution of genotypes and subtypes varies; genotypes A

(corresponding to subtype adw) and D

(ayw) predominate in the United States

and Europe, whereas genotypes B (adw)

and C (adr) predominate in Asia; however, these geographic distinctions have been blunted by recent-decade

migration across continents. Clinical course and outcome are independent of subtype, but genotype B appears to be associated with less

rapidly progressive liver disease and cirrhosis and a lower likelihood,

or delayed appearance, of hepatocellular carcinoma than genotype C or

D. Patients with genotype A are more likely to clear circulating viremia

and achieve hepatitis B e antigen (HBeAg) and HBsAg seroconversion,

both spontaneously and in response to antiviral therapy. In addition,

“precore” mutations are favored by certain genotypes (see below).

Upstream of the S gene are the pre-S genes (Fig. 339-3), which code

for pre-S gene products, including receptors on the HBV surface for

polymerized human serum albumin and for hepatocyte membrane

proteins. The pre-S region actually consists of both pre-S1 and pre-S2.

Depending on where translation is initiated, three potential HBsAg

gene products are synthesized. The protein product of the S gene is

HBsAg (major protein), the product of the S region plus the adjacent

pre-S2 region is the middle protein, and the product of the pre-S1 plus

pre-S2 plus S regions is the large protein. Compared with the smaller

spherical and tubular particles of HBV, complete 42-nm virions are

enriched in the large protein. Both pre-S proteins and their respective

antibodies can be detected during HBV infection, and the period of

pre-S antigenemia appears to coincide with other markers of virus replication, as detailed below; however, pre-S proteins have little clinical

relevance and are not included in routine serologic testing repertoires.

The intact 42-nm virion contains a 27-nm nucleocapsid core particle. Nucleocapsid proteins are coded for by the C gene. The antigen

expressed on the surface of the nucleocapsid core is hepatitis B core

antigen (HBcAg), and its corresponding antibody is anti-HBc. A third

HBV antigen is HBeAg, a soluble, nonparticulate, nucleocapsid protein

that is immunologically distinct from intact HBcAg but is a product

of the same C gene. The C gene has two initiation codons: a precore

and a core region (Fig. 339-3). If translation is initiated at the precore

region, the protein product is HBeAg, which has a signal peptide that

binds it to the smooth endoplasmic reticulum, the secretory apparatus

of the cell, leading to its secretion into the circulation. If translation

begins at the core region, HBcAg is the protein product; it has no signal

peptide and is not secreted, but it assembles into nucleocapsid particles,

which bind to and incorporate RNA, and which, ultimately, contain

HBV DNA. Also packaged within the nucleocapsid core is a DNA

polymerase, which directs replication and repair of HBV DNA. When

packaging within viral proteins is complete, synthesis of the incomplete

plus strand stops; this accounts for the single-strand gap and for differences in the size of the gap. HBcAg particles remain in the hepatocyte,

where they are readily detectable by immunohistochemical staining

and are exported after encapsidation by an envelope of HBsAg. Therefore, naked core particles do not circulate in the serum. The secreted

nucleocapsid protein, HBeAg, provides a convenient, readily detectable, qualitative marker of HBV replication and relative infectivity.

HBsAg-positive serum containing HBeAg is more likely to be

highly infectious and to be associated with the presence of hepatitis B

FIGURE 339-1 Electron micrographs of hepatitis A virus particles and serum from a patient with hepatitis B. Left:

27-nm hepatitis A virus particles purified from stool of a patient with acute hepatitis A and aggregated by antibody to

hepatitis A virus. Right: Concentrated serum from a patient with hepatitis B, demonstrating the 42-nm virions, tubular

forms, and spherical 22-nm particles of hepatitis B surface antigen. 132,000×. (Hepatitis D resembles 42-nm virions of

hepatitis B but is smaller, 35–37 nm; hepatitis E resembles hepatitis A virus but is slightly larger, 32–34 nm; hepatitis C

has been visualized as a 55-nm particle.)

IgG Anti-HAV

IgM Anti-HAV

Jaundice

ALT

Fecal HAV

0 4 8 12 16 20

Weeks after exposure

FIGURE 339-2 Scheme of typical clinical and laboratory features of hepatitis A

virus (HAV). ALT, alanine aminotransferase.

2564 PART 10 Disorders of the Gastrointestinal System

virions (and detectable HBV DNA, see below) than HBeAg-negative

or anti-HBe-positive serum. For example, HBsAg-positive mothers

who are HBeAg-positive almost invariably (>90%) transmit hepatitis

B infection to their offspring, whereas HBsAg-positive mothers with

anti-HBe rarely (10–15%) infect their offspring.

Early during the course of acute hepatitis B, HBeAg appears transiently; its disappearance may be a harbinger of clinical improvement

and resolution of infection. Persistence of HBeAg in serum beyond the

first 3 months of acute infection may be predictive of the development

of chronic infection, and the presence of HBeAg during chronic

hepatitis B tends to be associated with ongoing viral replication, infectivity, and inflammatory liver injury (except during the early decades

after perinatally acquired HBV infection; see below).

The third and largest of the HBV genes, the P gene (Fig. 339-3),

codes for HBV DNA polymerase; as noted above, this enzyme has

both DNA-dependent DNA polymerase and RNA-dependent reverse

transcriptase activities. The fourth gene, X, codes for a small, nonparticulate protein, hepatitis B x antigen (HBxAg), that is capable

of transactivating the transcription of both viral and cellular genes

(Fig. 339-3). In the cytoplasm, HBxAg effects calcium release (possibly

from mitochondria), which activates signal-transduction pathways

that lead to stimulation of HBV reverse transcription and HBV DNA

replication. Such transactivation may enhance the replication of

HBV, leading to the clinical association observed between the expression of HBxAg and antibodies to it in patients with severe chronic

Pre-S2

Pre-S1

Pre-C

C

X

S P

FIGURE 339-3 Compact genomic structure of hepatitis B virus (HBV). This structure,

with overlapping genes, permits HBV to code for multiple proteins. The S gene

codes for the “major” envelope protein, HBsAg. Pre-S1 and pre-S2, upstream of

S, combine with S to code for two larger proteins, “middle” protein, the product of

pre-S2 + S, and “large” protein, the product of pre-S1 + pre-S2 + S. The largest gene,

P, codes for DNA polymerase. The C gene codes for two nucleocapsid proteins,

HBeAg, a soluble, secreted protein (initiation from the pre-C region of the gene),

and HBcAg, the intracellular core protein (initiation after pre-C). The X gene codes

for HBxAg, which can transactivate the transcription of cellular and viral genes; its

clinical relevance is not known, but it may contribute to carcinogenesis by binding

to p53.

TABLE 339-1 Nomenclature and Features of Hepatitis Viruses

HEPATITIS

TYPE

VIRUS

PARTICLE, nm MORPHOLOGY GENOMEa CLASSIFICATION ANTIGEN(S) ANTIBODIES REMARKS

HAV 27 Icosahedral

nonenveloped

7.5-kb

RNA,

linear, ss, +

Hepatovirus HAV Anti-HAV Early fecal shedding

Diagnosis: IgM anti-HAV

Previous infection: IgG anti-HAV

HBV 42

27

22

Double-shelled

virion (surface and

core) spherical

Nucleocapsid core

Spherical and

filamentous;

represents excess

virus coat material

3.2-kb

DNA,

circular,

ss/ds

Hepadnavirus HBsAg

HBcAg

HBeAg

HBcAg

HBeAg

HBsAg

Anti-HBs

Anti-HBc

Anti-HBe

Anti-HBc

Anti-HBe

Anti-HBs

Bloodborne virus; carrier state

Acute diagnosis: HBsAg, IgM anti-HBc

Chronic diagnosis: IgG anti-HBc, HBsAg

Markers of replication: HBeAg, HBV DNA

Liver, lymphocytes, other organs

Nucleocapsid contains DNA and DNA

polymerase; present in hepatocyte nucleus;

HBcAg does not circulate; HBeAg (soluble,

nonparticulate) and HBV DNA circulate—

correlate with infectivity and complete virions

HBsAg detectable in >95% of patients with

acute hepatitis B; found in serum, body fluids,

hepatocyte cytoplasm; anti-HBs appears

following infection—protective antibody

HCV 55 Enveloped 9.4-kb

RNA,

linear, ss, +

Hepacivirus HCV core

antigen

Anti-HCV Bloodborne agent, formerly labeled non-A,

non-B hepatitis

Acute diagnosis: anti-HCV, HCV RNA

Chronic diagnosis: anti-HCV, HCV RNA;

cytoplasmic location in hepatocytes

HDV 35–37 Enveloped hybrid

particle with

HBsAg coat and

HDV core

1.7-kb

RNA,

circular,

ss, –

Resembles viroids

and plant satellite

viruses (genus

Deltavirus)

HBsAg

HDAg

Anti-HBs

Anti-HDV

Defective RNA virus, requires helper function

of HBV (hepadnaviruses); HDV antigen (HDAg)

present in hepatocyte nucleus

Diagnosis: anti-HDV, HDV RNA; HBV/HDV

co-infection—IgM anti-HBc and anti-HDV; HDV

superinfection—IgG anti-HBc and anti-HDV

HEV 32–34 Nonenveloped

icosahedral

7.6-kb

RNA,

linear, ss, +

Orthohepevirus HEV antigen Anti-HEV Agent of enterically transmitted hepatitis; rare in

the United States; occurs in Asia, Mediterranean

countries, Central America

Diagnosis: IgM/IgG anti-HEV (assays not

routinely available); virus in stool, bile,

hepatocyte cytoplasm

a

ss, single-strand; ss/ds, partially single-strand, partially double-strand; −, minus-strand; +, plus-strand.

Note: See text for abbreviations.

2565Acute Viral Hepatitis CHAPTER 339

hepatitis and hepatocellular carcinoma. The transactivating activity

can enhance the transcription and replication of other viruses besides

HBV, such as HIV. Cellular processes transactivated by X include the

human interferon-γ gene and class I major histocompatibility genes;

potentially, these effects could contribute to enhanced susceptibility of

HBV-infected hepatocytes to cytolytic T cells. The expression of X can

also induce programmed cell death (apoptosis). The clinical relevance

of HBxAg is limited, however, and testing for it is not part of routine

clinical practice.

SEROLOGIC AND VIROLOGIC MARKERS After a person is infected with

HBV, the first virologic marker detectable in serum within 1–12 weeks,

usually between 8 and 12 weeks, is HBsAg (Fig. 339-4). Circulating

HBsAg precedes elevations of serum aminotransferase activity and

clinical symptoms by 2–6 weeks and remains detectable during the

entire icteric or symptomatic phase of acute hepatitis B and beyond. In

typical cases, HBsAg becomes undetectable 1–2 months after the onset

of jaundice and rarely persists beyond 6 months. After HBsAg disappears, antibody to HBsAg (anti-HBs) becomes detectable in serum and

remains detectable indefinitely thereafter. Because HBcAg is intracellular and, when in the serum, sequestered within an HBsAg coat, naked

core particles do not circulate in serum, and therefore, HBcAg is not

detectable routinely in the serum of patients with HBV infection. By

contrast, anti-HBc is readily demonstrable in serum, beginning within

the first 1–2 weeks after the appearance of HBsAg and preceding

detectable levels of anti-HBs by weeks to months. Because variability exists in the time of appearance of anti-HBs after HBV infection,

occasionally a gap of several weeks or longer may separate the disappearance of HBsAg and the appearance of anti-HBs. During this

“gap” or “window” period, anti-HBc may represent the only serologic

evidence of current or recent HBV infection, and blood containing

anti-HBc in the absence of HBsAg and anti-HBs has been implicated

in transfusion-associated hepatitis B. In part because the sensitivity of

immunoassays for HBsAg and anti-HBs has increased, however, this

window period is rarely encountered. In some persons, years after HBV

infection, anti-HBc may persist in the circulation longer than anti-HBs.

Therefore, isolated anti-HBc does not necessarily indicate active virus

replication; most instances of isolated anti-HBc represent hepatitis B

infection in the remote past. Rarely, however, isolated anti-HBc represents low-level hepatitis B viremia, with HBsAg below the detection

threshold, and occasionally, isolated anti-HBc represents a crossreacting or false-positive immunologic specificity. Recent and remote

HBV infections can be distinguished by determination of the immunoglobulin class of anti-HBc. Anti-HBc of the IgM class (IgM anti-HBc)

predominates during the first 6 months after acute infection, whereas

IgG anti-HBc is the predominant class of anti-HBc beyond 6 months.

Therefore, patients with current or recent acute hepatitis B, including

those in the anti-HBc window, have IgM anti-HBc in their serum. In

patients who have recovered from hepatitis B in the remote past as well

as those with chronic HBV infection, anti-HBc is predominantly of the

IgG class. Infrequently, in ≤1–5% of patients with acute HBV infection,

levels of HBsAg are too low to be detected; in such cases, the presence

of IgM anti-HBc establishes the diagnosis of acute hepatitis B. When

isolated anti-HBc occurs in the rare patient with chronic hepatitis B

whose HBsAg level is below the sensitivity threshold of contemporary

immunoassays (a low-level carrier), anti-HBc is of the IgG class. Generally, in persons who have recovered from hepatitis B, anti-HBs and

anti-HBc persist indefinitely.

The temporal association between the appearance of anti-HBs and

resolution of HBV infection as well as the observation that persons

with anti-HBs in serum are protected against reinfection with HBV

suggests that anti-HBs is the protective antibody. Therefore, strategies

for prevention of HBV infection are based on providing susceptible

persons with circulating anti-HBs (see below). Occasionally, in ~10%

of patients with chronic hepatitis B, low-level, low-affinity anti-HBs

can be detected. This antibody is directed against a subtype determinant different from that represented by the patient’s HBsAg; its

presence is thought to reflect the stimulation of a related clone of

antibody-forming cells, but it has no clinical relevance and does not

signal imminent clearance of hepatitis B. These patients with HBsAg

and such nonneutralizing anti-HBs should be categorized as having

chronic HBV infection.

The other readily detectable serologic marker of HBV infection,

HBeAg, appears concurrently with or shortly after HBsAg. Its appearance coincides temporally with high levels of virus replication and

reflects the presence of circulating intact virions and detectable HBV

DNA (with the notable exception of patients with precore mutations

who cannot synthesize HBeAg—see “Molecular Variants”). Pre-S1

and pre-S2 proteins are also expressed during periods of peak replication, but assays for these gene products are not routinely available.

In self-limited HBV infections, HBeAg becomes undetectable shortly

after peak elevations in aminotransferase activity, before the disappearance of HBsAg, and anti-HBe then becomes detectable, coinciding with a period of relatively lower infectivity (Fig. 339-4). Because

markers of HBV replication appear transiently during acute infection,

testing for such markers is of little clinical utility in typical cases of

acute HBV infection. In contrast, markers of HBV replication provide

valuable information in patients with protracted infections.

Departing from the pattern typical of acute HBV infections, in

chronic HBV infection, HBsAg remains detectable beyond 6 months,

anti-HBc is primarily of the IgG class, and anti-HBs is either undetectable or detectable at low levels (see “Laboratory Features”) (Fig. 339-5).

0 100

Jaundice

ALT

HBeAg Anti-HBe

IgG Anti-HBc

HBsAg

IgM Anti-HBc

Anti-HBs

4 8 12 16 20 24 28 32 36 52

Weeks after exposure

FIGURE 339-4 Scheme of typical clinical and laboratory features of acute hepatitis

B. ALT, alanine aminotransferase.

0

ALT

1 2 3 4 5

HBsAg

HBeAg Anti-HBe

HBV DNA

6 12 24 36 48 60 120

Months after exposure

Anti-HBc

IgM anti-HBc

FIGURE 339-5 Scheme of typical laboratory features of wild-type chronic

hepatitis B. HBeAg and hepatitis B virus (HBV) DNA can be detected in serum

during the relatively replicative phase of chronic infection, which is associated

with infectivity and liver injury. Seroconversion from the replicative phase to the

relatively nonreplicative phase occurs at a rate of ~10% per year and is heralded by

an acute hepatitis–like elevation of alanine aminotransferase (ALT) activity; during

the nonreplicative phase, infectivity and liver injury are limited. In HBeAg-negative

chronic hepatitis B associated with mutations in the precore region of the HBV

genome, replicative chronic hepatitis B occurs in the absence of HBeAg.

2566 PART 10 Disorders of the Gastrointestinal System

During early chronic HBV infection, HBV DNA can be detected both

in serum and in hepatocyte nuclei, where it is present in free or episomal form. This relatively highly replicative stage of HBV infection

is the time of maximal infectivity and liver injury; HBeAg is a qualitative marker and HBV DNA a quantitative marker of this replicative

phase, during which all three forms of HBV circulate, including intact

virions. Over time, the relatively replicative phase of chronic HBV

infection gives way to a relatively nonreplicative phase. This occurs at

a rate of ~10% per year and is accompanied by seroconversion from

HBeAg to anti-HBe. In many cases, this seroconversion coincides with

a transient, usually mild, acute hepatitis-like elevation in aminotransferase activity, believed to reflect cell-mediated immune clearance of

virus-infected hepatocytes. In this relatively nonreplicative phase of

chronic infection, when HBV DNA is demonstrable in hepatocyte

nuclei, it tends to be integrated into the host genome. In this phase,

only spherical and tubular forms of HBV, not intact virions, circulate,

and liver injury tends to subside. Most such patients would be characterized as inactive HBV carriers. In reality, the designations replicative

and nonreplicative are only relative; even in the so-called nonreplicative

phase, HBV replication can be detected at levels of approximately ≤103

virions/mL with highly sensitive amplification probes such as the polymerase chain reaction (PCR); below this replication threshold, liver

injury and infectivity of HBV are limited to negligible. Still, the distinctions are pathophysiologically and clinically meaningful. Occasionally,

nonreplicative HBV infection converts back to replicative infection.

Such spontaneous reactivations are accompanied by reexpression of

HBeAg and HBV DNA, and sometimes of IgM anti-HBc, as well as

by exacerbations of liver injury. Because high-titer IgM anti-HBc can

reappear during acute exacerbations of chronic hepatitis B, relying on

IgM anti-HBc versus IgG anti-HBc to distinguish between acute and

chronic hepatitis B infection, respectively, may not always be reliable;

in such cases, patient history and additional follow-up monitoring over

time are invaluable in helping to distinguish de novo acute hepatitis B

infection from acute exacerbation of chronic hepatitis B infection.

MOLECULAR VARIANTS Variation occurs throughout the HBV

genome, and clinical isolates of HBV that do not express typical viral

proteins have been attributed to mutations in individual or even

multiple gene locations. For example, variants have been described

that lack nucleocapsid proteins (commonly), envelope proteins (very

rarely), or both. Two categories of naturally occurring HBV variants

have attracted the most attention. One of these was identified initially in Mediterranean countries among patients with severe chronic

HBV infection and detectable HBV DNA, but with anti-HBe instead

of HBeAg. These patients were found to be infected with an HBV

mutant that contained an alteration in the precore region, rendering

the virus incapable of encoding HBeAg. Although several potential

mutation sites exist in the pre-C region, the region of the C gene

necessary for the expression of HBeAg (see “Virology and Etiology”),

the most commonly encountered in such patients is a single base

substitution, from G to A in the second to last codon of the pre-C

gene at nucleotide 1896. This substitution results in the replacement

of the TGG tryptophan codon by a stop codon (TAG), which prevents

the translation of HBeAg. Another mutation, in the core-promoter

region, prevents transcription of the coding region for HBeAg and

yields an HBeAg-negative phenotype. Patients with such mutations

in the precore region and who are unable to secrete HBeAg may have

severe liver disease that progresses more rapidly to cirrhosis, or alternatively, they are identified clinically later in the course of the natural

history of chronic hepatitis B, when the disease is more advanced. Both

“wild-type” HBV and precore-mutant HBV can coexist in the same

patient, or mutant HBV may arise late during wild-type HBV infection. In addition, clusters of fulminant hepatitis B in Israel and Japan

were attributed to common-source infection with a precore mutant.

Fulminant hepatitis B in North America and western Europe, however, occurs in patients infected with wild-type HBV, in the absence

of precore mutants, and both precore mutants and other mutations

throughout the HBV genome occur commonly, even in patients with

typical, self-limited, milder forms of HBV infection. HBeAg-negative

chronic hepatitis with mutations in the precore region is now the most

frequently encountered form of hepatitis B in Mediterranean countries

and in Europe. In the United States, where HBV genotype A (less prone

to G1896A mutation) is prevalent, precore-mutant HBV is much less

common; however, as a result of immigration from Asia and Europe,

the proportion of HBeAg-negative hepatitis B–infected individuals has

increased in the United States, and they now represent ~30–40% of

patients with chronic hepatitis B. Characteristic of such HBeAg-negative chronic hepatitis B are lower levels of HBV DNA (usually

≤105

 IU/mL) and one of several patterns of aminotransferase activity—

persistent elevations, periodic fluctuations above the normal range,

and periodic fluctuations between the normal and elevated range.

The second important category of HBV mutants consists of escape

mutants, in which a single amino acid substitution, from glycine to

arginine, occurs at position 145 of the immunodominant a determinant common to all HBsAg subtypes. This HBsAg alteration leads to

a critical conformational change that results in a loss of neutralizing

activity by anti-HBs. This specific HBV/a mutant has been observed

in two situations, active and passive immunization, in which humoral

immunologic pressure may favor evolutionary change (“escape”) in

the virus—in a small number of hepatitis B vaccine recipients who

acquired HBV infection despite the prior appearance of neutralizing

anti-HBs and in HBV-infected liver transplant recipients treated with

a high-potency human monoclonal anti-HBs preparation. Although

such mutants have not been recognized frequently, their existence

raises a concern that may complicate vaccination strategies and serologic diagnosis.

Different types of mutations emerge during antiviral therapy of

chronic hepatitis B with nucleoside analogues; such YMDD and

similar mutations in the polymerase motif of HBV are described in

Chap. 341.

EXTRAHEPATIC SITES Hepatitis B antigens and HBV DNA have

been identified in extrahepatic sites, including the lymph nodes, bone

marrow, circulating lymphocytes, spleen, and pancreas. Although the

virus does not appear to be associated with tissue injury in any of

these extrahepatic sites, its presence in these “remote” reservoirs has

been invoked (but is not necessary) to explain the recurrence of HBV

infection after orthotopic liver transplantation. The clinical relevance

of such extrahepatic HBV is limited.

Hepatitis D The delta hepatitis agent, or HDV, the only member

of the genus Deltavirus, is a defective RNA virus that co-infects with

and requires the helper function of HBV (or other hepadnaviruses)

for its replication and expression. Slightly smaller than HBV, HDV

is a formalin-sensitive, 35- to 37-nm virus with a hybrid structure.

Its nucleocapsid expresses HDV antigen (HDAg), which bears no

antigenic homology with any of the HBV antigens, and contains the

virus genome. The HDV core is “encapsidated” by an outer envelope

of HBsAg, indistinguishable from that of HBV except in its relative

compositions of major, middle, and large HBsAg component proteins.

The genome is a small, 1700-nucleotide, circular, single-strand RNA

of negative polarity that is nonhomologous with HBV DNA (except

for a small area of the polymerase gene) but that has features and

the rolling circle model of replication common to genomes of plant

satellite viruses or viroids. HDV RNA contains many areas of internal

complementarity; therefore, it can fold on itself by internal base pairing

to form an unusual, very stable, rod-like structure that contains a very

stable, self-cleaving and self-ligating ribozyme. HDV RNA requires

host RNA polymerase II for its replication in the hepatocyte nucleus

via RNA-directed RNA synthesis by transcription of genomic RNA

to a complementary antigenomic (plus strand) RNA; the antigenomic

RNA, in turn, serves as a template for subsequent genomic RNA synthesis effected by host RNA polymerase I. HDV RNA has only one

open reading frame, and HDAg, a product of the antigenomic strand,

is the only known HDV protein; HDAg exists in two forms: a small,

195-amino-acid species, which plays a role in facilitating HDV RNA

replication, and a large, 214-amino-acid species, which appears to

suppress replication but is required for assembly of the antigen into

virions. HDV antigens have been shown to bind directly to RNA

2567Acute Viral Hepatitis CHAPTER 339

polymerase II, resulting in stimulation of transcription. Viral assembly

requires farnesylation of the large HDAg for ribonucleoprotein anchoring to HBsAg. Both HBV and HDV enter hepatocytes via the sodium

taurocholate cotransporting polypeptide receptor. Although complete

hepatitis D virions and liver injury require the cooperative helper function of HBV, intracellular replication of HDV RNA can occur without

HBV. Genomic heterogeneity among HDV isolates has been described.

Although pathophysiologic and clinical consequences of this genetic

diversity have not been defined definitively, preliminarily, genotype

2 has been linked to milder disease and genotype 3 to severe acute

disease. The clinical spectrum of hepatitis D is common to all eight

genotypes identified, the predominant of which is genotype 1.

HDV can either infect a person simultaneously with HBV (coinfection) or superinfect a person already infected with HBV (superinfection); when HDV infection is transmitted from a donor with one

HBsAg subtype to an HBsAg-positive recipient with a different subtype, HDV assumes the HBsAg subtype of the recipient, rather than

the donor. Because HDV relies absolutely on HBV, the duration of

HDV infection is determined by the duration of (and cannot outlast)

HBV infection. HDV replication tends to suppress HBV replication;

therefore, patients with hepatitis D tend to have lower levels of HBV

replication. HDV antigen is expressed primarily in hepatocyte nuclei

and is occasionally detectable in serum. During acute HDV infection,

anti-HDV of the IgM class predominates, and 30–40 days may elapse

after symptoms appear before anti-HDV can be detected. In selflimited infection, anti-HDV is low-titer and transient, rarely remaining detectable beyond the clearance of HBsAg and HDV antigen. In

chronic HDV infection, anti-HDV circulates in high titer, and both

IgM and IgG anti-HDV can be detected. HDV antigen in the liver and

HDV RNA in serum and liver can be detected during HDV replication.

The recent report that, in vitro, HDV can assemble infectious virus

particles with envelope glycoproteins from other viruses, both hepatotropic and nonhepatotropic, raises the possibility that HDV can replicate without hepadnaviruses; however, to date, co-infections in nature

with other viruses have not been observed.

Hepatitis C Hepatitis C virus, which, before its identification,

was labeled “non-A, non-B hepatitis,” is a linear, single-strand, positive-sense, 9600-nucleotide RNA virus, the genome of which is similar

in organization to that of flaviviruses and pestiviruses; HCV is the only

member of the genus Hepacivirus in the family Flaviviridae. The HCV

genome contains a single, large open reading frame (ORF) (gene) that

codes for a virus polyprotein of ~3000 amino acids, which is cleaved

after translation to yield 10 viral proteins. The 5′ end of the genome

consists of an untranslated region (containing an internal ribosomal

entry site [IRES]) adjacent to the genes for three structural proteins,

the nucleocapsid core protein, C, and two envelope glycoproteins, E1

and E2. The 5′ untranslated region and core gene are highly conserved

among genotypes, but the envelope proteins are coded for by the

hypervariable region, which varies from isolate to isolate and may allow

the virus to evade host immunologic containment directed at accessible

virus-envelope proteins. The 3′ end of the genome also includes an

untranslated region and contains the genes for seven nonstructural

(NS) proteins: p7, NS2, NS3, NS4A, NS4B, NS5A, and NS5B. p7 is a

membrane ion channel protein necessary for efficient assembly and

release of HCV. The NS2 cysteine protease cleaves NS3 from NS2, and

the NS3-4A serine protease cleaves all the downstream proteins from

the polyprotein. Important NS proteins involved in virus replication

include the NS3 helicase; NS3-4A serine protease; the multifunctional

membrane-associated phosphoprotein NS5A, an essential component

of the viral replication membranous web (along with NS4B); and the

NS5B RNA-dependent RNA polymerase (Fig. 339-6). Because HCV

does not replicate via a DNA intermediate, it does not integrate into

the host genome. Because HCV tends to circulate in relatively low titer,

103

−107

 virions/mL, visualization of the 50- to 80-nm virus particles

remains difficult. Still, the replication rate of HCV is very high, 1012

virions per day; its half-life is 2.7 h. The chimpanzee is a helpful but

cumbersome animal model. Although a robust, reproducible, small

animal model is lacking, HCV replication has been documented in

an immunodeficient mouse model containing explants of human liver

and in transgenic mouse and rat models; in addition, an HCV-related

rat Hepacivirus has been reported to be a useful surrogate model.

Although in vitro replication is difficult, replicons in hepatocellular

carcinoma–derived cell lines support replication of genetically manipulated, truncated, or full-length HCV RNA (but not intact virions);

infectious pseudotyped retroviral HCV particles have been shown to

yield functioning envelope proteins. In 2005, complete replication of

HCV and intact 55-nm virions were described in cell culture systems.

HCV entry into the hepatocyte occurs via the non-liver-specific CD81

receptor and the liver-specific tight junction protein claudin-1. A growing list of additional host receptors to which HCV binds on cell entry

includes occludin, low-density lipoprotein receptors, glycosaminoglycans, scavenger receptor B1, and epidermal growth factor receptor,

among others. Relying on the same assembly and secretion pathway as

low-density and very-low-density lipoproteins, HCV is a lipoviroparticle and masquerades as a lipoprotein, which may limit its visibility to

the adaptive immune system and explain its ability to evade immune

containment and clearance. After viral entry and uncoating, translation

is initiated by the IRES on the endoplasmic reticulum membrane, and

the HCV polyprotein is cleaved during translation and posttranslationally by host cellular proteases as well as HCV NS2-3 and NS3-4A proteases. Host cofactors involved in HCV replication include cyclophilin

A, which binds to NS5A and yields conformational changes required

for viral replication, and liver-specific host microRNA miR-122.

At least six distinct major genotypes (and a minor genotype 7), as

well as >50 subtypes within genotypes, of HCV have been identified by

nucleotide sequencing. Genotypes differ from one another in sequence

homology by ≥30%, and subtypes differ by ~20%. Because divergence of HCV isolates within a genotype or subtype and within the same

host may vary insufficiently to define a

distinct genotype, these intragenotypic

differences are referred to as quasispecies

and differ in sequence homology by

only a few percent. The genotypic and

quasispecies diversity of HCV, resulting from its high mutation rate, interferes with effective humoral immunity.

Neutralizing antibodies to HCV have

been demonstrated, but they tend to

be short-lived, and HCV infection does

not induce lasting immunity against

reinfection with different virus isolates

or even the same virus isolate. Thus,

neither heterologous nor homologous

immunity appears to develop commonly

after acute HCV infection. Some HCV

genotypes are distributed worldwide,

500 1000 1500 2000 2500 3000

AA

Envelope

Core glycoproteins

Conserved

region

Hypervariable

region

Serine

protease

Helicase

5' 3'

p7 NS4A

RNA-dependent

RNA polymerase

C E1 E2 NS2 NS3 NS4B NS5A NS5B

FIGURE 339-6 Organization of the hepatitis C virus genome and its associated, 3000-amino-acid (AA) proteins. The

three structural genes at the 5′ end are the core region, C, which codes for the nucleocapsid, and the envelope

regions, E1 and E2, which code for envelope glycoproteins. The 5′ untranslated region and the C region are highly

conserved among isolates, whereas the envelope domain E2 contains the hypervariable region. At the 3′ end are seven

nonstructural (NS) regions—p7, a membrane protein adjacent to the structural proteins that appears to function as an

ion channel; NS2, which codes for a cysteine protease; NS3, which codes for a serine protease and an RNA helicase;

NS4 and NS4B; NS5A, a multifunctional membrane-associated phosphoprotein, an essential component of the viral

replication membranous web; and NS5B, which codes for an RNA-dependent RNA polymerase. After translation of the

entire polyprotein, individual proteins are cleaved by both host and viral proteases.