Chapter 7: Pathogenesis of Viral Infection

Viral pathogenesis is the process by which viruses produce disease in the host. The factors that determine the viral transmission, multiplication, dissemination, and development of disease in the host involve complex and dynamic interactions between the virus and the susceptible host. Viruses cause disease when they breach the host’s primary physical and natural protective barriers; evade local, tissue, and immune defenses; spread in the body; and destroy cells either directly or via bystander immune and inflammatory responses. Viral pathogenesis comprises of several stages, including (1) transmission and entry of the virus into the host, (2) spread in the host, (3) tropism, (4) virulence and cytopathogenicity, (5) patterns of viral infection and disease, (6) host factors, (7) host defense, and (8) virus-induced immunopathology. The stages of a typical viral infection and its pathogenesis (eg, poliovirus pathogenesis) are shown in Figure 7–1.

Viruses are transmitted via horizontal (common route of transmission: person to person), and vertical (mother-to-child transmission) routes or vector transmission (from mosquitoes, animals; Tables 7–1 and 7–2). Human viruses cause either systemic or localized infections by entering the host through a variety of routes, including direct inoculation as well as respiratory, conjunctival, gastrointestinal, and genitourinary routes (Figure 7–2). In addition, viruses can enter the host through a break in the skin or via mucosal surfaces of various routes such as respiratory, gastrointestinal, and genitourinary tracts. Mother-to-child transmission (vertical transmission) can occur in utero, during delivery (via birth canal), and through breast-feeding.

Zoonotic (animal-to-human) transmission of viral infections can occur from the bite of animals (eg, rabies) or insects (eg, dengue, yellow fever, West Nile) or from inhalation of animal excreta (eg, hantavirus, arenavirus; Table 7–2). In some cases, avian flu virus (bird flu) can be transmitted from birds or poultry to humans, and swine flu virus can also be transmitted to humans.

After virus entry into the host, viruses have variable incubation periods. Incubation period is the time between exposure to the organism and appearance of the first symptoms of the disease. Viruses generally multiply at the site of entry to establish infection in the host. Some of the examples include: respiratory viruses multiplying in the upper respiratory tract just after entry; rabies virus multiplying in the muscle cells after animal bite; and West Nile virus multiplying in Langerhans cells of skin after mosquito bite. Some viruses have short incubation periods (influenza—2-4 days), whereas others have long incubation periods (eg, hepatitis B virus—weeks to several months). Incubation periods of common viral infections are shown in Table 7–3. Communicability of a disease is the ability of the organism to shed in secretions, which may occur early in the incubation period. Some viruses can integrate into the host genome (HIV), survive by slow replication in the presence of an immune response (hepatitis B and C viruses [HBV, HCV]) or stay latent (herpes simplex virus [HSV]). This dormancy or latency is dangerous because the virus may emerge long after the original infection has occurred and potentially infect others.

Viral infections produce either localized infection at the site of entry or disseminated infection spread throughout the body. Localized infections include influenza, parainfluenza, common cold (rhinoviruses, coronaviruses, adenoviruses), gastrointestinal infections (rotaviruses, Norwalk viruses), and skin infections (papillomaviruses). In localized infections, the virus spreads mainly by infecting adjacent or neighboring cells.

Several viruses that cause systemic disease in the host spread from the site of entry to the target tissue, where they cause cell injury after multiplication. Viruses use two major routes to spread and cause systemic infection, that is, hematogenous (via the bloodstream) and neural (via neurons) spread. Some of the viruses that cause systemic or disseminated infection are poliovirus, flavivirus, rabies virus, HBV, HCV, HIV, measles, varicella zoster virus, and others. Pathogenesis of poliovirus can be cited as an example of disseminated infection in which poliovirus is transmitted via the fecal–oral route, and the disease (paralytic poliomyelitis) is caused in the central nervous system (CNS; Figure 7–1). Poliovirus replicates at the sites of entry in the small intestine and spreads to the regional lymph nodes where it multiplies again and enters the bloodstream, resulting in primary viremia. The virus is spread via the bloodstream to other organs (liver, spleen), where it multiplies and enters the bloodstream causing secondary viremia followed by transmission to, and replication in, the CNS and resultant damage to motor neurons. The development of viremia allows the immune system to mount humoral and cell-mediated responses to control the poliovirus infection.

Some of the viruses that are spread by the neural route are HSV, poliovirus, rabies virus, and certain arboviruses, including West Nile virus and St. Louis encephalitis virus. HSV is transmitted in vesicle fluids, saliva, and vaginal secretions and replicated in the mucoepithelial cells, causing primary infection and then traveling via sensory neurons to nerve bundles called ganglia where they establish latent infection. HSV can also travel into the CNS and infect the brain causing herpes encephalitis.

Tropism is the capability of viruses to infect a discrete population of cells within an organ. Cellular or tissue tropism is most often determined by the specific interaction of viral surface proteins (spikes) and cellular receptors on the host cells. Some of the identified cellular receptors for viruses are shown in Table 6–5. However, it should be kept in mind that the presence of a receptor for a virus is not always sufficient for viral infection in the target cells. For example, the presence of CD4 (HIV receptor) alone on target cells does not allow virus entry into these cells, but it requires that target cells also express coreceptors, CXCR4 or CCR5 (chemokine receptors) for efficient viral attachment. Different viruses may use the same cellular molecule as receptors. Some examples are sialic acid residues functioning as important components of the receptor for influenza, corona, and reoviruses. Similarly, heparan sulfate is the receptor for HSV, cytomegalovirus (CMV), and adeno-associated virus (AAV).

Tropism can also be determined by intracellular factors, including host transcription factors and other factors necessary for viral replication. After attachment of viral surface proteins to the cellular receptor, the viral genome-protein complex is released in the cytoplasm followed by transcription, replication, and virus assembly. While enveloped viruses use two mechanisms for entry—fusion and receptor-mediated endocytosis (viropexis), naked capsid viruses use viropexis without membrane–membrane fusion. Influenza virus is tropic to cells that express sialic acid residues containing glycoproteins where the influenza virus attachment protein, hemagglutinin (HA), binds to the receptor, following which the virion is internalized into an endosomal vesicle and the viral envelope membrane fuses with the vesicle’s membrane. For other enveloped viruses, such as HIV, viral envelope gp120 binds to the cellular receptor (CD4) and coreceptor (CXCR4 or CCR5) for attachment, and envelope gp41 fuses the viral envelope with the plasma membrane. Naked capsid viruses, such as poliovirus and hepatitis A virus, use outer capsid spikes to begin attachment to the cellular receptor; the virion is internalized and the viral genome is released in the cytoplasm without membrane–membrane fusion.

Both RNA and DNA viruses undergo genetic changes, including mutation and recombination (see Chapter 6). Viral tropism can be altered in the case of some viruses because of genetic variation in the viral surface proteins. Avian influenza virus (H5N1) does not bind to the receptor of human influenza virus (H1N1), but mutation or reassortment in H5N1 may allow binding of H5N1 to H1N1 receptor (see Chapter 9). Similarly, genetic changes in the variable region 3 (V3 region) of HIV-1 Env gp120 during infection in patients switch the coreceptor requirement from CCR5 to CXCR4. CCR5 is predominantly expressed on macrophages, Langerhans cells, and mucosal T lymphocytes, whereas CXCR4 is mainly expressed on naïve T lymphocytes (see Chapter 18).

Although interaction of the viral surface proteins with the receptors on the host cell plays a critical role in determining the tropism, other factors such as viral gene expression, especially in the case of retroviruses, hepatitis B viruses, and papillomaviruses, contribute to tropism. For example, HBV replicates more efficiently in liver cells, and papillomavirus in skin cells, because of regulation of individual viral promoter transcriptions.

The ability of a virus to cause disease in an infected host is called pathogenicity. Virulence is the relative ability of a virus to cause disease. Viral virulence is, basically, the degree of pathogenicity of a virus. A virus may be of high or low virulence for a particular host. Different strains of the same virus may differ in the degree of pathogenicity. The ability of a virus to cause degenerative changes in cells or cell death is called cytopathogenicity. Viral strains that kill target cells and cause disease are called virulent viruses, but other strains that have mutated and lost their ability to cause cytopathic effects (CPE) and disease are termed as avirulent or attenuated strains. Some attenuated strains can be used as live vaccines. Examples of live attenuated vaccines are MMR (measles, mumps, rubella), smallpox, poliovirus (not used in the United States), and yellow fever.

Three major outcomes can be attributed to a viral infection: (1) abortive infection, in which no progeny virus particles are produced, but the cell may die because early viral functions can occur; (2) lytic infection, in which active virus production is followed by cell death; and (3) persistent infection, in which small numbers of virus particles are produced with little or no CPE. Persistent infections include latent infection, in which viral genetic material remains in host cell without production of virus and may be activated at a later time to produce virus and/or transform the host cell; chronic infection, which involves low level of virus production with little or no CPE; and viral transformation, in which viral infection or viral gene product induces unregulated cellular growth, and cells form tumors in the host. If two closely related viruses infect a host, then infection by the first virus can inhibit the function of the second virus; this is termed interference.

Virulence and cytopathogenicity depend on the nature of viruses and the characteristics of cells such as permissive and nonpermissive cells. A permissive cell permits production of progeny virus particles and/or viral transformation. A nonpermissive cell does not allow virus replication, but it may permit transformation of the cell. Replication of the virus results in alterations of cellular morphology and function as well as antigenicity of the virus. When a lytic virus infects a permissive cell, lots of daughter viruses are produced and this is followed by lysis of the infected cells, called cytopathic effects (CPE) of the virus (Figure 4–9). The features of CPE are morphologic changes of the cell organelles, including nucleus (inclusion bodies, thickening of the nucleus, swelling, nucleolar changes, margination of chromatin), cytoplasm (inclusion bodies, vacuoles), and membranes (cells round up, loss of adherence, cell fusion [syncytia]), followed by cellular lysis (disintegration).

The molecular and genetic determinants of viral virulence are complex. Viral gene products influence pathogenesis and virulence. As previously described, viral surface proteins, both in enveloped and naked capsid viruses, determine tropism and spread, and alterations in these surface proteins may result in changes in tropism, spread, and virulence. However, other regions of the viral genome contribute to pathogenicity and virulence. There is no single master gene or protein that determines virulence. For example, live attenuated vaccine of poliovirus, also called oral polio vaccine (OPV), contains all three serotypes of poliovirus that are attenuated and have markedly reduced neurovirulence compared with wild-type polioviruses. The neurovirulence determinants are located in the 5′ untranslated region of the genome involved in initiation of translation and an internal ribosomal entry site, structural capsid proteins (VP1-VP4), and nonstructural proteins such as viral polymerase.

Some viruses encode a new class of proteins called virokines and viroreceptors, which contribute to viral virulence by mimicking cellular proteins. It is believed that some large DNA viruses, such as poxviruses and herpesviruses, have acquired these genes by recombination from the cells in which they replicate. Virokines are secreted from infected cells and act as cytokines, helping the cells to proliferate and increase virus production. Viroreceptors resemble cytokine receptors and attract cellular cytokines. In addition, some viruses encode proteins that bind antibodies or components of complement pathways to avoid lysis of virus-infected cells. For example, a member of the poxvirus family, vaccinia virus (strain used in smallpox vaccine), encodes a vaccinia complement control protein (VCP) that abrogates the complement-mediated killing of virus-infected cells. Similarly, two glycoproteins of HSV act together as a receptor for the Fc domain of immunoglobulins to avoid antibody-directed cell-mediated cytotoxicity (ADCC).

Not every viral infection results in a disease. Infection involves multiplication of the virus in the host, whereas disease represents a clinically apparent response. Infections are much more common than disease; unapparent infections are termed subclinical, and the individual is referred to as a carrier. Although some primary infections are invariably accompanied by clinical manifestations of the disease (influenza, measles), other infections may propagate and spread for long periods before the extent of problem is recognized (HIV, HBV, and HCV).

Relative susceptibility of a host for a viral infection in terms of severity of the disease depends on several factors such as virulence, molecular and genetic determinants of the virus, and host factors (immune status of the host, age, health, and genetic background). After viral transmission, the virus multiplies in the host; this phase is referred to as the incubation period, which varies for different viruses (Table 7–3). Initial virus replication generally results in viremia, which allows the virus to travel to the target tissues and replicate further to cause cell damage and clinical symptoms. The host immune system plays a pivotal role in determining the course of infection and progression of disease.

Viral infection results in either a lytic or persistent (latent or chronic) infection. Lytic infections are those in which productive virus replication results in cell death because viral replication is not compatible with essential cellular functions. Several viruses interfere with the synthesis of cellular macromolecules and other factors that prevent cellular growth, maintenance, and repair, thus leading to cell death. For example, poliovirus blocks the synthesis of cellular proteins by inhibiting the translation of cellular mRNA and competing for ribosomes. Accumulation of progeny viruses and viral proteins can destroy the structure and function, and enhance the process of apoptosis, resulting in cell death. In enveloped viruses, such as respiratory syncytial virus (RSV), HIV, and HSV, replication of the virus and cell surface expression of the envelope glycoproteins (spikes) cause cell-to-cell spread and formation of multinucleated giant cells (syncytia) causing cell death (cytopathic effect).

Persistent viral infections are those in which the infected cells survive the effect of viral replication. Persistent infections are of two kinds: latent (viral genome without virus production) and chronic (low level of virus production without immune clearance). In addition, some persistent viruses cause oncogenic transformations. Several DNA viruses have the potential to cause oncogenic transformation; some viruses can cause tumors in their natural hosts (human papillomaviruses, HPV; HBV), whereas others can cause tumors in other species or only transform cells in vitro (human adenoviruses, human polyomaviruses). Some RNA viruses, such as retroviruses (human T lymphotropic virus, HTLV) and HCV can cause oncogenic transformation in infected hosts. In these human oncogenic viruses, viral gene products transform the cells either by interfering with the tumor suppressor gene pathways (eg, HPV) or increasing the expression of proto-oncogenes (HTLV).

Based on patterns and levels of detectable infectious virus in the host and the role of immune response in clearing the virus, viral infections can be divided into five categories: (1) acute infection that is cleared by the immune response; (2) acute infection that becomes latent and periodically reactivated; (3) acute infection that becomes chronic; (4) acute infection followed by persistent infection (viral set point) established by immune response and followed by virus overproduction, immune dysfunction, and opportunistic infections; and (5) slow chronic infections. These patterns are shown in Figure 7–3A-E. In acute infection, the virus enters the host, then multiplies at the site of entry and in the target tissue, and this is followed by viremia and cytopathic effects. This type of infection is a lytic infection. The immune system mounts both cellular and humoral responses and successfully eliminates the virus from the host. Examples of acute viral infections followed by clearance of the virus from the host by immune responses are hepatitis A, influenza, parainfluenza, rhino, and corona viruses. After causing acute or lytic infection, some viruses are not eliminated by the immune response but persist in the host either in a noninfectious latent form or an infectious chronic form. Most of the viruses opting to persist in the host have evolved various mechanisms for persistence, including restriction of viral cytopathic effects, infection of immunologically privileged sites, maintenance of viral genomes without full viral gene expression, antigenic variation, suppression of immune components, and transformation of host cells.

In some viral infections, acute infection may result in either asymptomatic or symptomatic disease followed by latent infection in which the viral genome persists without any infectious virus production. This latent virus could be periodically reactivated, with virus shedding at or near the primary infections along with some symptomatic disease, as seen in HSV infections. In this case, productive (lytic) infection takes place in permissive cells (mucoepithelial cells), whereas latent infection occurs in nonpermissive cells (neurons).

In some persistent infections, acute infection causes initial disease, which is followed by a chronic infection in which a low level of infectious virus is continuously produced with little or no damage to the target tissue. Initially, the immune system controls the infection by bringing the viral load lower than seen in acute infection; however, the immune system is unable to eliminate the infection during the acute phase. During chronicity, the virus is maintained via several mechanisms, such as infection of nonpermissive cells, spread to other cell types, antigenic variation, and inability of the immune response to completely eliminate the virus. Examples of viruses that cause this type of infection are HBV and HCV.

In other persistent infections such as HIV, the acute infection results in high viremia and mono-like illness known as “acute retroviral syndrome” followed by a persistent infection in which the immune responses bring down the high viral load to a “viral set point.” The viral set point is maintained because of the robust immune response against the mutating virus for a long time in most infected patients. Because of impairment of the immune system and downregulation of immune components by HIV, the mutating and highly replicating HIV could not be contained by the immune system depletion of CD4+ T lymphocytes, which also offers an opportunity for other pathogens (opportunistic infections) to establish infection and cause full-blown AIDS.

Some unconventional infectious agents cause slow, chronic infection without acute infection such as caused by prions. Prions are infectious protein molecules without any genes, causing slow, chronic infection in humans such as Creutzfeldt-Jacob disease (CJD) and bovine spongiform encephalopathy (BSE, mad cow diseases) (see Chapter 20).

Many DNA and some RNA viruses, especially the retroviruses, can transform normal cells into abnormal cells called tumors (benign or malignant). This process is called viral transformation, and these viruses are oncogenic viruses. Viruses that can either cause tumors in their natural hosts or other species or can transform cells in vitro are considered to have oncogenic potential. Specifically, a tumor is an abnormal growth of cells and is classified as benign or malignant—depending on whether it remains localized or has a tendency to invade or spread by metastasis. Therefore, malignant cells have at least two defects. They fail to respond to controlling signals that normally limit the growth of nonmalignant cells, and they fail to recognize their neighbors and remain in their proper location.

When grown in tissue culture in the laboratory, these tumor cells exhibit a series of properties that correlate with the uncontrolled growth potential associated with the tumor in the organism. They have altered cell morphology and fail to grow in the organized patterns found for normal cells. In addition, they grow to a much higher cell density than do normal cells under conditions of unlimited nutrients and can lose contact inhibition and the requirement for growth on a solid substrate; therefore, they appear unable to enter the resting G0 state. Furthermore, they have lower nutritional and serum requirements than normal cells and can grow indefinitely in cell culture. These transformed or tumor cells often are used as cell lines for the culture or propagation of viruses in the laboratory.

In addition to the listed properties, viral transformation usually, but not always, endows the cells with the capacity to form a tumor when introduced into the appropriate animal. Although the original use of the term transformation referred to the changes occurring in cells grown in the laboratory, current usage often includes the initial events in the animal that lead to the development of a tumor. In recent years, it has become increasingly clear that some, but not all, of these viruses cause cancers in the host species from which they were isolated.

Transformation by DNA Human Viruses

The oncogenic potential of human DNA viruses is summarized in Table 7–4. With the exception of parvoviruses, most DNA virus families have some members capable of causing aberrant cell proliferation under some conditions. For some viruses, transformation or tumor formation has been observed only in species other than their natural host. Apparently, infections of cells from the natural host are so cytocidal that no survivor cells remain to be transformed. In addition, some viruses have been implicated in human tumors without any indication that they can transform cells in culture.

In nearly all cases that have been characterized, viral transformation is the result of the continual expression of one or more viral genes that are directly responsible for the loss of cell growth control. Two targets have been identified that appear to be critical for the transforming potential of these viruses. Adenoviruses, papillomaviruses, and polyomaviruses (simian virus 40) all code for either one or two proteins that interact with the tumor suppressor proteins such as p53 and pRb (for retinoblastoma protein) to block their normal function, which is to exert a tight control over cell-cycle progression. The end result is endless cell cycling and uncontrolled cell growth.

Some viruses integrate into the host chromosome at random sites (with a high efficiency for retroviruses and a very low efficiency for adeno-, polyo-, papilloma- viruses), although the DNAs of papillomaviruses and herpesviruses are found in transformed cells as extrachromosomal DNA. Unlike retroviruses that code for the enzymes necessary for integration, papillomaviruses, polyomaviruses, and adenoviruses may integrate by nonhomologous recombination using enzymes present in the host cell. In summary, two events appear to be necessary for viral transformation: a persistent association of viral genes with the cell, and the expression of certain viral “transforming” proteins.

Transformation by Retroviruses

Two features of the replicative cycle of retroviruses are related to the oncogenic potential of this class of viruses known as oncoretroviruses. First, most retroviruses (exception human immunodeficiency virus, HIV) do not kill the host cell, but rather set up a permanent infection with continual virus production. Second, a DNA copy of the RNA genome is maintained in cell via integration into the host cell DNA by a virally encoded integrase (IN).

Retroviruses are known to transform cells by three different mechanisms; the first and second mechanisms for animal retrovirus and the third mechanism for human retrovirus (HTLV). First, many animal retroviruses have acquired transforming genes called oncogenes. These retroviruses require a helper virus as the insertion of the oncogene replaces a viral gene. More than 30 such oncogenes have now been found since the original oncogene was identified in animal Rous sarcoma virus (called v-src, where the v stands for viral). Because normal cells possess homologs of these genes called protooncogenes (eg, c-src, where c stands for cellular), it is generally thought that viral oncogenes originated from host DNA. It is possible they were picked up by “copy choice” recombination involving packaged cellular mRNAs, as previously described. Because these transforming viruses carry cellular genes, they are sometimes referred to as transducing retroviruses. Most of the viral oncogenes have undergone mutations that make them different from the cellular proto-oncogenes. These changes presumably alter the protein products such that they cause transformation. Although the mechanisms of oncogenesis are not completely understood, it appears that transformation results from inappropriate production of an abnormal protein that interferes with normal signaling processes within the cell, causing uncontrolled cell proliferation. Because tumor formation in vitro by retroviruses carrying an oncogene is efficient and rapid, these viruses are often referred to as acute transforming viruses. Although common in some animal species, this mechanism has not yet been recognized as a cause of any human cancers.

The second mechanism is called insertional mutagenesis and is not dependent on continued production of a viral gene product. Instead, the presence of the viral promoter or enhancer is sufficient to cause the inappropriate expression of a cellular gene residing in the immediate vicinity of the integrated provirus. This mechanism was first recognized in avian B-cell lymphomas caused by an avian leukosis virus, a disease characterized by a very long latent period in birds. Tumor cells from different hosts were found to have a copy of the provirus integrated at the same place in the cellular DNA. The site of the provirus insertion was found to be next to a cellular proto-oncogene called c-myc. The myc gene had previously been identified as a viral oncogene called v-myc. In this case, transformation occurs not because the c-myc gene is altered by mutation, but because the viral promoter adjacent to the gene turns on its expression continuously and the gene product is overproduced. The disease has a long latent period because, although the birds are viremic from early life, the probability of an integration occurring next to the c-myc gene is very low. After such an integration event does occur, however, cell proliferation is rapid and a tumor develops. No human tumors are known to be caused from insertional mutagenesis caused by a retrovirus. However, some human cancers such as Burkitt lymphoma and chronic myelogenous leukemia (CML) are known to occur in which a chromosome translocation has placed an active cellular promoter next to a cellular proto-oncogene. In addition, a few retroviral gene therapy trials were stopped because of the induction of leukemia likely due to retroviral insertion near a proto-oncogene.

The third mechanism was revealed by the discovery of the first human retrovirus, human T lymphotropic virus type 1 (HTLV-1), the causative agent of adult T-cell leukemia and lymphoma (ATLL). HTLV-I sequences are found integrated in the DNA of the leukemic cells, and all tumor cells from a particular individual have the proviral DNA in the same location. This observation indicates that the tumor is a clone derived from a single cell; however, the sites of integration in tumors from different individuals are different. Thus, HTLV-I does not cause malignancy by promoter insertion near a particular cellular gene. Instead, HTLV has a regulatory gene called tax that encodes for Tax protein that transactivates or upregulate not only the transcription of its own proviral DNA, but also the transcription of many cellular genes, including proto-oncogenes. The resulting cellular proteins cooperate to cause uncontrolled cell proliferation. HTLV-I is commonly described as a transactivating retrovirus. The same mechanism is also observed in the second human retrovirus, HTLV-II that causes hairy T cell leukemia.

Transformation by Other RNA Viruses

Hepatitis C virus (HCV) causes chronic infection in more than 80% of infected people. The chronicity in HCV infection increases the risk of cirrhosis of liver and hepatocellular carcinoma (HCC). HCC occurs on average approximately 20 to 30 years after chronic infection but alcohol and drug abuse can accelerate this process. It is thought that the constant inflammation and regeneration of hepatocytes leads to the eventual induction of the tumor and is, therefore, considered indirect oncogenesis. However, several studies suggest that HCV nonstructural proteins, NS3 and NS5A, NS5B, and the HCV core protein may be involved in transformation. These HCV proteins interfere with cellular proteins that are responsible for the regulation of cell cycle control.

Viral infection also depends on host factors. Several viral infections have repeatedly shown a variable range of outcomes from asymptomatic to symptomatic infections and even fatal disease in some cases. Furthermore, host factors probably play an important role in reversion of some of the live attenuated vaccines to a virulent state. Several of the host factors, including immune status, genetic background, age, and nutrition, play important roles in determining the outcome of viral infection. Several innate immune responses (interferons α and β, natural killer cells, mucocilliary responses, and others) and adaptive immune responses (antibody and T-cell responses) influence the outcome of viral infections. Individuals with weak immune systems or those who are immunocompromised or immunosuppressed often have more severe outcomes. Details of immune responses to infection are described in Chapter 2.

Host genetics is one of the most important factors that influence the outcome of viral infections. Several host genes, in addition to viral factors, contribute to the variable outcome of HIV infection in infected individuals; some become rapid progressors. The majority are slow progressors. Elevated levels of β-chemokines such as MIP1-α, MIP1-β, RANTES, which are natural ligands of CCR5 (HIV coreceptor), have been found to be associated with decline in the rate of HIV disease progression. These chemokines are also called HIV-suppressive β-chemokines. Genetic resistance to HIV-1 infection was found in individuals expressing a truncated CCR5 coreceptor, CCR5Δ32. Individuals homozygous for the Δ32 allele seem to have normal life expectancy and are strongly protected (not completely) against HIV infection, whereas the heterozygous Δ32 allele slows the cell-to-cell spread of HIV in infected patients. The Δ32 homozygous allele is found in 1% of Caucasians, predominantly in Northern European populations. Furthermore, long-term progressors also have a high frequency of Δ32CCR5 deletion. Although Δ32CCR5 deletion or antagonists of CCR5 provide some protection against HIV infection, it may cause a higher risk of symptomatic West Nile virus infection and a lower likelihood of clearing HCV. In addition, the human leukocyte antigen (HLA) alleles have been associated with slow disease progression or protection against HIV infection.

Age-related correlation between the host and several viral infections has been observed. Several viruses such as varicella-zoster virus (VZV), mumps, polio, and Epstein-Barr virus (EBV) cause less severe infection in infants as compared with teens or adults, whereas others (rotaviruses, respiratory syncytial virus) result in severe disease in infants. Although the same strain of HIV infects both mothers (adults) and infants, infants develop symptomatic AIDS faster than adults because HIV replicates more efficiently in infants mononuclear cells than adult cells. It appears that age-related increased resistance to viral infections might reflect the maturity of the immune system and other defense mechanisms.

Production of hormones may also influence the outcome of some viral infections. For example, polio, hepatitis A, B, and E, and poxviruses are more severe during pregnancy, suggesting that hormones may influence viral pathogenesis. Polyomaviruses can also be reactivated during pregnancy.

Nutritional state and personal habits of the hosts can also influence viral pathogenesis. Protein deficiency has been shown to be associated with severity of measles infection, most likely owing to weak cellular immunity. Some personal habits, such as smoking, increase the severity in influenza virus infection. In addition, host responses such as fever and inflammation have been suggested to have an important role in combating viral infections.

The two major types of host defenses are nonspecific (innate) and specific (adaptive) immune responses. The innate immune response includes interferons (α, β), natural killer cells, macrophages (phagocytosis), α-defensing, mucociliary clearance, apolipoprotein B RNA editing enzyme (APOBEC3G, an anti-HIV enzyme), and fever among many other factors, whereas the adaptive immune response involves humoral and cell-mediated immunity. Details of specific immune response to infection are described in Chapter 2.

Interferons

Interferons are host-encoded proteins that provide the first line of defense against viral infections. They belong to the class of molecules called cytokines, which are proteins or glycoproteins that are involved in cell-to-cell communication. There are three types of interferon, interferon-α (leukocyte), interferon-β (fibroblast), and interferon-γ (lymphocyte). Interferon-α/β are also called Interferon-I and interferon-γ is referred as Interferon-II. Virus infection of all types of cells stimulates the production and secretion of either interferon-α or interferon-β, which acts on other cells to induce what is called the antiviral state. Unlike specific immunity, the interferons are not specific to a particular kind of virus; however, interferons usually act only on cells of the same species. Other agents such as antigens and mitogens stimulate the production of interferon-γ by lymphoid cells. In this case, the interferon appears to play an important role in the immune system regardless of any role as an antiviral protein (see Chapter 2).

A major signal that leads to the production of interferon by an infected cell appears to be double-stranded RNA (dsRNA). This conclusion is based on the observation that treatment of cells with purified dsRNA or synthetic double-stranded ribopolymers results in the secretion of interferon. Viral infections, in general, lead to the accumulation of significant levels of dsRNA in the cell. DsRNA is known to activate interferon through the activation of specific receptors called toll-like receptors (TLR) or intracellular receptors called retinoic acid-inducible gene 1 (RigI) like receptors (RLR) or melanoma differentiation-associated gene 5 (MDA5)/mitochondrial antiviral signaling protein (MAVS). These receptors via several signaling molecules activate transcription factors interferon regulatory factor 3 (IRF3) and NF-κB leading to interferon production.

Changes in the synthesis of many cellular proteins are characteristic of the antiviral state induced by interferon. However, the cells exhibit only minimal changes in their metabolic or growth properties. The machinery to inhibit virus production is mobilized only on infection. Interferon has multiple effects on cells, and three systems have been extensively studied. The first system involves a protein called Mx, which is induced by interferon and specifically blocks influenza infections by interfering with viral transcription. The second system involves the upregulation of protein kinase R (PKR), which is dependent on dsRNA recognition by PKR which phosphorylates and inactivates one of the subunits of an initiation factor (eIF-2) necessary for protein synthesis. In some cases, viruses have evolved specific mechanisms to block the action of this protein kinase. The third system involves the induction of an enzyme called 2′, 5′-oligoadenylate synthetase, which synthesizes chains of 2′, 5′-oligo (A) up to 10 residues in length. In turn, the 2′, 5′-oligo (A) activates a constitutive ribonuclease, called RNase L, which degrades mRNA. The activities of both protein kinase and 2′, 5′-oligo (A) synthetase requires the presence of dsRNA, the intracellular signal that an infection is occurring. This requirement prevents interferons from having an adverse effect on protein synthesis in uninfected cells.

In the latter two cases, viral infection of a cell that has been exposed to interferon results in a general inhibition of protein synthesis, leading to cell death and no virus production. A cell that was destined to die anyway from a viral infection is sacrificed for the benefit of the entire organism. Virus-induced interferon pathways are shown in Figure 7–4. In addition, interferon prepares uninfected cells to fight viral infections. Presence of interferon induces oligosynthetase and protein kinase but does not activate because there is no viral dsRNA in uninfected cells. Thus, interferon kills only infected cells but not uninfected cells.

Other Host Defenses

Natural killer (NK) cells, like interferons, are also not virus-specific but kill virus-infected cells by secreting perforins (pore-forming proteins) and granzymes (serine proteases), which cause apoptosis of infected cells. NK cell-induced killing of infected cells does not require immune components such as antigen, T-cell receptor, or major histocompatibility complex (MHC). NK cells recognize cells lacking class I MHC, which is downregulated by many viruses. Another important cell type that limits virus infection in a nonspecific manner via phagocytosis is the macrophage, especially alveolar macrophages and macrophages of the reticuloendothelial system. Macrophages also secrete interferon-γ upon activation, leading to further inhibition of virally infected cells. Furthermore, other factors show antiviral activity, especially against HIV infection, including α-defensins, APOBEC3G and BST-2/CD317 (tetherin). α-Defensins are a class of peptides known to have antiviral activity against both enveloped (HSV) and nonenveloped viruses (adenovirus and papillomavirus), and have also been found to interfere with the interaction of HIV Env gp120 with chemokine receptor CXCR4. On the other hand, APOBEC3G is an enzyme that hypermutates retroviral (HIV) DNA by deaminating cytosines in both viral DNA and mRNA, reducing viral infectivity. BST-2 (bone marrow stromal antigen 2) is a type 2 integral membrane protein, which inhibits retroviruses, and other enveloped viruses infection by restricting the release of fully formed progeny virions from infected cells. However, HIV Vif and Vpu proteins antagonize APOBEC3G and BST-2 activities, respectively.

Adaptive immune responses involving humoral (antibody) and cell-mediated (cytotoxic T lymphocytes) immune responses are also described in Chapter 2. These are virus-specific immune responses directed against viral proteins (antigens). Antibody is effective in eliminating cell-free virus, and cytotoxic T lymphocytes (CTL) destroy virus-infected cells. The idea that adaptive or acquired immunity in patients is viral antigen-specific led the way in the development of vaccines against several viral infections. Immunity could be either active, in that it is elicited by exposure to a pathogen or vaccine, or passive, in which it is transferred by immune serum. After viral infection, the first specific immune response is T-cell mediated in which CD8 T cells recognize viral antigen presented by class I MHC and kill virus-infected cells by secreting perforins and granzymes and activating FAS proteins, causing apoptosis. It is important to differentiate that CD8 T-cell killing of virus-infected cells is viral antigen-specific, whereas NK cell killing of infected cells is nonspecific. The second important control is neutralization of the virus in infected hosts by antigen–antibody interactions, preventing the virus from infecting target cells by blocking the virus-receptor interactions. Antibodies are generated against all viral antigens; however, antibody against surface antigens is most effective in eliminating the virus. Antibody in conjunction with complement can also kill virus-infected cells. The evidence that viral infection elicits antibody and CTL that help the clearance of viruses in many cases (acute infection) and control or suppress the viruses in certain cases (persistent infection) has allowed researchers to develop live attenuated vaccines. Live attenuated vaccines activate both arms of the immune system, are very effective in preventing infection, and are long lasting, but can carry a very small risk of reversion. On the other hand, killed or inactivated vaccines (pathogen-killed or inactivated) and subunit vaccines (one or few proteins of the virus) predominantly activate the humoral (antibody) response, may not confer long-lasting immunity, and are also needed in a larger quantity. Several of the live attenuated, killed, and subunit viral vaccines that are currently recommended for use in humans are listed in Table 7–5.

Viral diseases are usually the result of virus-host cell interactions causing either a lytic infection and cell death or persistent infections and cell survival with some cellular dysfunction. However, sometimes both humoral and cellular immune responses against viral infections, especially those causing less cytopathic or persistent infections, mediate inflammation and disease. This could be true in viral infections in which a large number of cells are infected in an individual before the immune response is turned on and in which destruction of these infected cells by immune response may have severe or fatal pathologic outcomes. Specifically, proinflammatory cytokines, antigen–antibody complexes, complement activation pathways, CD4+ T–cell induced delayed hypersensitivity, and CTL-mediated cell killing contribute to virus-induced immunopathology.

The most important mediators of virus-induced immunopathology are the CD8+ CTLs. They release several proinflammatory cytokines, including interferon-γ, tumor necrosis factor alpha (TNF-α), and several interleukins (ILs), which play an important role in clinical manifestations of virus-induced immunopathology. Some selected examples of immune-mediated viral diseases are shown in Table 7–6. After viral infections, interferon-γ and other cytokines are secreted, which stimulate multiple organ systems to cause systemic infection (flu-like symptoms), and then other immune components such as antigen–antibody complex, complement, CTL and proinflammatory cytokines cause cell damage. This may be the case with several viral infections of the CNS and other tissues in which “cytokine storm” causes cell damage rather than direct viral replication (Figure 7–5). Chronic HBV infection provided the first clue that the disease is caused by an indirect mechanism rather than the virus itself because a low level of virus can be present in chronically infected people without any damage to the target tissue (liver) for a long time. However, the circulating hepatitis B surface antigen (HBsAg) can form immune complexes that activate the complement system, causing inflammation and tissue damage. In addition, accumulation of these immune complexes in the kidney results in renal damage. In other viral infections, such as measles and mumps, many symptoms are caused by T–cell induced inflammatory responses as opposed to the direct cytopathic effects of the virus.

An important example of acute antibody-mediated immunopathology is dengue hemorrhagic fever, in which a small percentage of infected patients develop dengue shock syndrome (DSS) with a mortality rate up to 10%. This syndrome mostly occurs in people who are either undergoing a second infection with a different serotype or in infants carrying maternal anti-dengue antibody and undergoing first infection. A nonneutralizing antibody (enhancing antibody) facilitates the adsorption of flaviviruses (dengue and yellow fever viruses) into macrophages through Fc receptors followed by replication, thereby changing the tropism of the virus. The infected macrophages secrete cytokines interferon-γ, TNF-α, and others. In addition, dengue specific CD4+ and CD8+ T lymphocytes secrete similar types of cytokines, resulting in cytokine storm and causing hemorrhage and shock. The circulating immune complex activates the complement pathway, which also contributes to immunopathology.

Virus-initiated autoimmunity, in which a viral infection may induce an autoimmune response because the viral protein resembles a host cell protein, induces a phenomenon called molecular mimicry. Both viral epitope-specific antibody and T lymphocytes may react with cognate epitopes on the host proteins, which may elicit an autoimmune response. Viral proteins, such as the polymerase of hepatitis B, contain sequences similar to the encephalitogenic epitope of myelin basic protein (MBP), which is a major component of myelin sheath in the CNS. Immune responses against an epitope of hepatitis B polymerase induce an immune response against MBP, initiating an autoimmune disease process. Coxsackie virus infection has also been linked to autoimmune responses associated with type 1 diabetes as a result of molecular mimicry between a viral protein and a protein found in islet cells called glutamic acid decarboxylase (GAD).

Viral infections, in several instances, can suppress the immune response. Immunosuppression can be achieved either by direct viral replication or by viral antigens. Some viruses specifically infect and kill immune cells. In some instances, immunosuppression is often associated with antenatal or perinatal infections. Historically, immunosuppression was first described approximately a century ago when patients lost their tuberculin sensitivity during, and weeks after, measles infection. In the last decade, immunosuppression has been the topic of discussion, concern, and treatment in the HIV/AIDS epidemic because HIV specifically infects and destroys the major type of immune cells, CD4+ T lymphocytes. Table 7–7 shows the mechanisms of selected human viruses causing immune suppression. Several mechanisms have been proposed for virus-induced immune suppression: (1) viral replication in a major immune cells (CD4+ helper T lymphocytes) or antigen-presenting cells (dendritic cells or macrophages) leading to apoptosis; (2) viral antigens stimulating proinflammatory cytokines causing cell death; (3) tolerance generated by clonal deletion of T lymphocytes by viral antigens, generally associated with perinatal infections; and (4) expression of viral proteins that destroy infected and uninfected cells such as HIV Env gp120 depleting uninfected and infected CD4+ T lymphocytes.

The extensively studied virus-induced immunosuppression problem is HIV/AIDS, which is a persistent infection. The primary target for HIV is CD4+ T lymphocytes and monocytes/macrophages. However, HIV is highly cytopathic to CD4+ T lymphocytes but not to monocytes/macrophages. Therefore, depletion of CD4+ T lymphocytes in HIV-infected patients results in immunosuppression. The mechanisms of depletion of CD4+ T lymphocytes include direct killing of CD4+ T lymphocytes as a result of HIV replication and also depletion of uninfected CD4+ T lymphocytes by HIV Env gp120-induced syncytia formation and apoptosis. Immunosuppression in HIV-infected patients causes opportunistic infection, whereas several other pathogens establish infection without immune challenge. However, antiretroviral therapy (ART) has significantly reduced the viral load and the risk of opportunistic infections in patients.

Measles is an acute viral infection that produces immunosuppression, which appears during the incubation period and the clinical phase of the disease. Some results of measles-induced immunosuppression include increased susceptibility to other infections, possible aggravation of chronic latent infections such as tuberculosis, and remission of autoimmune diseases. The mechanisms of measles-induced immunosuppression involve infection of several cell types and pathways. However, during measles infection, the function of antigen-presenting cells such as monocytes/macrophages, and CD4 and CD8 T lymphocytes is compromised, which may contribute to immunosuppression. An example of immunosuppression in utero or during infancy is rubella virus infection. Fetal infections that commonly produce congenital rubella (see Chapter 10) cause greatly reduced cellular immune responses to rubella virus antigens even several years after infection. In general, several factors or determinants could be responsible for virus-induced immunosuppression, such as the strain of the virus, dose, or amount of the virus entering the host, route of transmission or virus entry, age and immune status of the host, and other immunologic disorders in the host.

In the past decade, we have gained significant knowledge about how viruses interact with their hosts and cause disease as well as how the hosts, in turn, respond in ways that may be either beneficial or deleterious to their well-being. Our understanding of these processes is as yet incomplete, but the knowledge gained to date has enabled scientists to develop new strategies to deal with these issues. Two approaches that have already resulted in success are: (1) prevention, including development of effective environmental controls, and vaccines for prevention; and (2) development of specific antiviral agents that can cure, mitigate, or temporarily prevent infection. Better approaches to more advantageously manipulate specific and nonspecific host responses to such infections are expected as well. For now, all that can be stated with certainty is that exciting, meaningful progress will continue well into the future.