The HIV-1 virus (Fig. 25-1) is composed of a lipid membrane, structural proteins, and glycoproteins that protrude. The viral genome consists of three important structural components—pol, gag, and env. These gene components code for various products (Table 25-1). Long terminal redundancies (LTRs) border these three components. HIV-2 has a different envelope and slightly different core proteins.

Table 25-1

Viral Genome Components

Component	Product
pol	Produces DNA polymerase
pol	Produces endonuclease
gag	Codes for p24 and for proteins such as p17, p9, and p7
env	Codes for two glycoproteins, gp41 and gp120

Figure 25-1 The human immunodeficiency virus.
The envelope is made up of glycoproteins (gp) of 120 kDa and 41 kDa. The main core protein is p24. As an RNA virus, it relies on reverse transcriptase to produce complementary DNA for transcription and translation. (From Peakman M, Vergani D: Basic and clinical immunology, ed 2, London, 2009, Churchill Livingstone.)

Cells infected with HIV can be examined with an electron microscope. The virus may appear as buds of the cell membrane particles. The virion has a double-membrane envelope and an electron-dense laminar crescent or semicircular cores. An intermediate, less electron-dense layer lies between the envelope and core. In a mature, free extracellular virion, the core appears as a bar-shaped nucleoid structure in cross section. This structure appears circular and is frequently located eccentrically. It is composed of structural proteins and glycoproteins that occupy the core and envelope regions of the particle. The virion consists of knoblike structures composed of a protein called glycoprotein (gp) 120, which is anchored to another protein called gp41. Each knob includes three sets of these protein molecules. The core of the virus includes a major structural protein called p25 or p24 encoded for by the gag gene. After human exposure, these and other viral components may induce an antibody response important in serodiagnosis (Table 25-2).

Table 25-2

HIV Proteins of Serodiagnostic Importance

Virus	Protein	Location	Gene
HIV-1	gp41	Envelope (transmembrane protein)	env
	gp160/120	Envelope (external protein)	env
	p24	Core (major structural protein)	gag
HIV-2	gp34	Envelope (transmembrane protein)	env
	gp140	Envelope (external protein)	env
	p26	Core (major structural protein)	gag

Retroviruses contain a single, positive-stranded ribonucleic acid (RNA) with the genetic information of the virus and a special enzyme called reverse transcriptase in their core. Reverse transcriptase enables the virus to convert viral RNA into deoxyribonucleic acid (DNA). This reverses the normal process of transcription in which DNA is converted to RNA—thus, the term retrovirus.

The genomes of all known retroviruses are organized in a similar way. In the provirus, which is formed when complementary DNA (cDNA) synthesis is completed from the retroviral RNA template, viral core protein, envelope protein, and reverse transcriptase are encoded by the gag, env, and pol genes, respectively, whereas viral gene expression is regulated by tat, trs, sor, and 3′orf gene products. The gag gene encodes a polyprotein found at high levels in infected cells and is subsequently cleaved to form p17 and p24, both of which are associated with viral particles. The pol gene encodes for reverse transcriptase, endonuclease, and protease activities. The sor gene stands for small open-reading frame. The sor gene product is a protein that induces antibody production in the natural course of infection. The tat gene also represents a small open-reading frame; the protein product has not been identified to date.

The env gene encodes for a polyprotein that contains numerous glycosylation sites. The glycoprotein gp160 is found on infected cells but is deficient on viral particles; however, gp160 gives rise to two glycoproteins, gp120 and gp41, which are associated with the viral envelope. The encoding genes and gene products, or antigens, of the AIDS virus may induce an antibody response after human exposure (Table 25-3).

Table 25-3

Encoding Genes and Antigens of AIDS Virus

Encoding Gene	Antigen
gag	p55
gag	p24
gag	p17
pol	p66
pol	p51
sor	p24
env	gp160
env	gp120
env	gp41
3′ orf	p27

LTRs, which exist at each end of the proviral genome, play an important role in the control of viral gene expression and the integration of the provirus into the DNA of the hosts. Although a structural similarity exists between the genomes of HIV-1 and HIV-2 (HTLV-IV), the nucleotide sequence homology is limited. There is a nucleotide sequence homology of only 60% between the gag genes and 30% to 40% between the remainder of the genes of HIV-1 and HIV-2.

Viral Replication

The replication of HIV is complicated and involves several steps (Fig. 25-2). The HIV life cycle is that of a retrovirus (Box 25-1). Retroviruses are so named because they reverse the normal flow of genetic information. In body cells, the genetic material is DNA. When genes are expressed, DNA is first transcribed into messenger RNA (mRNA), which then serves as the template for the production of proteins. The genes of a retrovirus are encoded in RNA; before they can be expressed, the RNA must be converted into DNA. Only then are the viral genes transcribed and translated into proteins in the usual sequence.

Box 25-1 Summary of HIV-1 Life Cycle

1. The virus attaches to the CD4 membrane receptor and sheds its protein coat, exposing its RNA core.

2. Reverse transcriptase converts viral RNA into proviral DNA.

3. The proviral DNA is integrated into the genome (genetic complement of host cell).

4. New virus particles are produced as the result of normal cellular activities of transcription and translation. Once the viral genome is integrated into host cell DNA, the potential for viral production always exists and the viral infection of new cells can continue.

5. New particles bud from the cell membrane.

Figure 25-2 HIV replication cycle
Steps in the HIV replication cycle
1. Fusion of the HIV cell to the host cell surface; 2. HIV RNA, reverse transcriptase, integrase, and other viral proteins enter the host cell; 3. viral DNA is formed by reverse transcription; 4. viral DNA is transported across the nucleus and integrates into the host DNA; 5. new viral RNA is used as genomic RNA and to make viral proteins; 6. new viral RNA and proteins move to the cell surface and a new, immature, HIV virus forms; 7. the virus matures by protease releasing individual HIV proteins. (Courtesy National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Md.)

Target Cells

The infectious process begins when the gp120 protein on the viral envelope binds to the protein receptor, called CD4, located on the surface of a target cell. HIV-1 has a marked preference for the CD4+ subset of T lymphocytes (Fig. 25-3). In addition to T lymphocytes, macrophages, peripheral blood monocytes, and cells in the lymph nodes, skin, and other organs also express measurable amounts of CD4 and can be infected by HIV-1. About 5% of the B lymphocytes may express CD4 and may be susceptible to HIV-1 infection. Macrophages may play an important role in spreading HIV infection in the body, both to other cells and to the target organs of HIV. Monocyte-macrophages enable HIV-1 to enter the immune-protected domain of the central nervous system (CNS), including the brain and spinal cord.

Figure 25-3 Mechanisms of HIV entry into a cell.
In the model, depicted sequential conformational changes in gp120 and gp41 promote fusion of the HIV-1 and host cell membranes. The fusion peptide of activated gp41 contains hydrophobic amino acid residues that promote insertion into the host cell plasma membrane lipid bilayer. (Adapted from Abbas AK, Lichtman AH, Pillai S: Cellular and molecular immunology, ed 6, Philadelphia, 2007, Saunders.)

Fusion of the virus to the membrane of a host cell enables the viral RNA and reverse transcriptase to invade the cytoplasm of the cell. However, CD4 receptors are not sufficient for HIV envelope fusion with the T4 cell membrane or for HIV penetration or entry into the interior of the cell. Chemokine coreceptors to CD4, which HIV uses to enter a host cell after binding to it, have been identified. Beta chemokine receptors are cell surface proteins that bind small peptides. They are classified into three groups, depending on the location of the amino acid cysteine (C) in the peptide. These receptors are identified by the individual chemokine(s) that bind(s) to them. In essence, the reference to a specific chemokine also identifies its receptor. The first example of a coreceptor was CXCKR-4 (FUSIN R-4). Other coreceptors include CCKR-2 (R-2), CCKR-3 (R-3), and CC-CKR-5 (R-5). Current research involves exploring ways to block or fill the chemokine receptors with a harmless molecule, thus blocking the binding site of the HIV on the host cell.

Although some cells do not produce detectable amounts of CD4, they contain low levels of mRNA encoding the CD4 protein, which indicates that they do produce some CD4. Because these cells can be infected by HIV in culture, the expression of only minimal CD4 or an alternate receptor molecule may be sufficient for HIV infection to occur. These cell types include certain brain cells, neuroglial cells, a variety of malignant brain tumor cells, and cells derived from bowel cancers. Cells of the gastrointestinal system do not produce appreciable amounts of CD4, although chromaffin cells sometimes appear to be infected by HIV in vivo.

Replication

Retroviruses carry a single, positive-stranded RNA and use reverse transcriptase to convert viral RNA into DNA. The life cycle of the HIV-1 virus consists of five phases (see Box 25-1):

1. The virus attaches and penetrates target cells (e.g., lymphocytes) that express the CD4 receptor. After penetration, the virus loses its protein coat, exposing the RNA core.

2. Reverse transcriptase converts viral RNA into proviral DNA.

3. The proviral DNA is integrated into the genome (genetic complement of the host cell).

4. New virus particles are produced as a result of normal cellular activities of transcription and translation.

5. These new particles bud from the cell membrane.

Once the viral genome is integrated into host cell DNA, the potential for viral production always exists and the viral infection of new cells can continue.

Immunologic activation of CD4+ cells latently infected with HIV induces the production of multiple viral particles, leading to cell death. The extensive destruction of cells leads to the gradual depletion of CD4+ lymphocytes. Progressive defects in the immune system include a severe B cell failure, defects in monocyte function, and defects in granulocyte function.

Epidemiology

HIV causes a chronic infection that leads to a progressive disease. Without treatment, most persons with HIV develop AIDS, which results in substantial morbidity and premature death.

• CD4+ T-lymphocyte count of <200 cells/µL or CD4+ T-lymphocyte percentage of total lymphocytes of <14 or documentation of an AIDS-defining condition (see Appendix A). Documentation of an AIDS-defining condition supersedes a CD4+ T-lymphocyte count of >200 cells/µL and a CD4+ T-lymphocyte percentage of total lymphocytes of >14. Definitive diagnostic methods for these conditions are available in Appendix C of the 1993 revised HIV classification system and the expanded AIDS case definition (2) and from the National Notifiable Diseases Surveillance System (available at http://www.cdc.gov/epo/dphsi/casedef/case_definitions.htm).

HIV Infection, Stage Unknown

• No information available on CD4+ T-lymphocyte count or percentage and no information available on AIDS-defining conditions. (Every effort should be made to report CD4+ T-lymphocyte counts or percentages and the presence of AIDS-defining conditions at the time of diagnosis.)

2008 Surveillance Case Definition for HIV Infection Among Children Aged <18 Months

The 2008 case definition of HIV infection among children aged <18 months replaces the definition published in 1999 and applies to all variants of HIV (e.g., HIV-1 or HIV-2). The 2008 definition is intended for public health surveillance only and not as a guide for clinical diagnosis.

Criteria for Definitive or Presumptive HIV Infection

A child aged <18 months is categorized for surveillance purposes as definitively or presumptively HIV infected if born to an HIV-infected mother and if the laboratory criterion or at least one of the other criteria is met.

Laboratory Criterion for Definitive HIV Infection

A child aged <18 months is categorized for surveillance purposes as definitively HIV infected if born to an HIV-infected mother and the following laboratory criterion is met.

• Positive results on two separate specimens (not including cord blood) from one or more of the following HIV virologic (non-antibody) tests:– HIV nucleic acid (DNA or RNA) detection– HIV p24 antigen test, including neutralization assay, for a child aged >1 month– HIV isolation (viral culture)

Laboratory Criterion for Presumptive HIV Infection

A child aged <18 months is categorized for surveillance purposes as presumptively HIV infected if (1) born to an HIV-infected mother, (2) the criterion for definitively HIV infected is not met, and (3) the following laboratory criterion is met.

• Positive results on one specimen (not including cord blood) from the listed HIV virologic tests (HIV nucleic acid detection test; HIV p24 antigen test, including neutralization assay, for a child aged >1 month; or HIV isolation [viral culture] for definitively HIV infected) and no subsequent negative results from HIV virologic or HIV antibody tests.

Other Criteria (for Cases that Do Not Meet Laboratory Criteria for Definitive or Presumptive HIV Infection)

• HIV infection diagnosed by a physician or qualified medical-care provider based on the laboratory criteria and documented in a medical record. Oral reports of prior laboratory test results are not acceptable.

• When test results regarding HIV infection status are not available, documentation of a condition that meets the criteria in the 1987 pediatric surveillance case definition for AIDS.

Criteria for Uninfected With HIV, Definitive or Presumptive

A child aged <18 months born to an HIV-infected mother is categorized for surveillance purposes as either definitively or presumptively uninfected with HIV if (1) the criteria for definitive or presumptive HIV infection are not met and (2) at least one of the laboratory criteria or other criteria are met.

Laboratory Criteria for Uninfected With HIV, Definitive

A child aged <18 months born to an HIV-infected mother is categorized for surveillance purposes as definitively uninfected with HIV if (1) the criteria for definitive or presumptive HIV infection are not met and (2) at least one of the laboratory criteria or other criteria are met.^¶

• At least two negative HIV DNA or RNA virologic tests from separate specimens, both of which were obtained at age ≥1 month and one of which was obtained at age ≥4 months.

• At least two negative HIV antibody tests from separate specimens obtained at age >6 months.

and

• No other laboratory or clinical evidence of HIV infection (i.e., no positive results from virologic tests [if tests were performed] and no current or previous AIDS-defining condition)

Laboratory Criteria for Uninfected With HIV, Presumptive

A child aged <18 months born to an HIV-infected mother is categorized for surveillance purposes as presumptively uninfected with HIV if (1) the criteria for definitively uninfected with HIV are not met and (2) at least one of the laboratory criteria are met.

• Two negative RNA or DNA virologic tests, from separate specimens, both of which were obtained at age ≥2 weeks and one of which was obtained at age ≥4 weeks.^#

• One negative RNA or a DNA virologic test from a specimen obtained at age ≥8 weeks.

• One negative HIV antibody test from a specimen obtained at age ≥6 months.

• One positive HIV virologic test followed by at least two negative tests from separate specimens, one of which is a virologic test from a specimen obtained at age ≥8 weeks or an HIV antibody test from a specimen obtained at age ≥6 months.

and

• No other laboratory or clinical evidence of HIV infection (i.e., no subsequent positive results from virologic tests if tests were performed, and no AIDS-defining condition for which no other underlying condition indicative of immunosuppression exists).

Other Criteria (for Cases that Do Not Meet Laboratory Criteria for Uninfected With HIV, Definitive or Presumptive)

• Determination of uninfected with HIV by a physician or qualified medical-care provider based on the laboratory criteria and who has noted the HIV diagnostic test results in the medical record. Oral reports of prior laboratory test results are not acceptable.

and

• No other laboratory or clinical evidence of HIV infection (i.e., no positive results from virologic tests [if tests were performed] and no AIDS-defining condition for which no other underlying condition indicative of immunosuppression exists).

Criteria for Indeterminate HIV Infection

A child aged <18 months born to an HIV-infected mother is categorized as having perinatal exposure with an indeterminate HIV infection status if the criteria for infected with HIV and uninfected with HIV are not met.

2008 Surveillance Case Definitions for HIV Infection and AIDS Among Children Aged 18 Months to <13 Years

The 2008 laboratory criteria for reportable HIV infection among persons aged 18 months to <13 years exclude confirmation of HIV infection through the diagnosis of AIDS-defining conditions alone. Laboratory-confirmed evidence of HIV infection is now required for all reported cases of HIV infection among children aged 18 months to <13 years.

Criteria for HIV Infection

Children aged 18 months to <13 years are categorized as HIV infected for surveillance purposes if at least one of laboratory criteria or the other criterion is met.^∗∗

Laboratory Criteria

• Positive result from a screening test for HIV antibody (e.g., reactive EIA), confirmed by a positive result from a supplemental test for HIV antibody (e.g., Western blot or indirect immunofluorescence assay).

• Positive result or a detectable quantity by any of the following HIV virologic (non-antibody) tests^††:– HIV nucleic acid (DNA or RNA) detection (e.g., PCR)– HIV p24 antigen test, including neutralization assay– HIV isolation (viral culture)

Other Criterion (for Cases that Do Not Meet Laboratory Criteria)

Criteria for AIDS

Children aged 18 months to <13 years are categorized for surveillance purposes as having AIDS if the criteria for HIV infection are met and at least one of the AIDS-defining conditions has been documented.

^∗Rapid tests are EIAs that do not have to be repeated but require a confirmatory test if reactive. Most conventional EIAs require a repeatedly reactive EIA that is confirmed by a positive result with a supplemental test for HIV antibody. Standard laboratory testing procedures should always be followed.

^†For HIV screening, HIV virologic (non-antibody) tests should not be used in lieu of approved HIV antibody screening tests. A negative result (i.e., undetectable or nonreactive) from an HIV virologic test (e.g., viral RNA nucleic acid test) does not rule out the diagnosis of HIV infection.

^‡Qualified medical-care providers might differ by jurisdiction and might include physicians, nurse practitioners, physician assistants, or nurse midwives.

^§An original or copy of the laboratory report is preferred; however, in the rare instance the laboratory report is not available, a description of the laboratory report results by a physician or qualified medical-care provider documented in the medical record is acceptable for surveillance purposes. Every effort should be made to obtain a copy of the laboratory report for documentation in the medical record.

HIV nucleic acid (DNA or RNA) detection tests are the virologic methods of choice for the diagnosis of exclusion of infection in children aged <18 months. Although HIV culture can be used, culture is less standardized and less sensitive than nucleic acid detection tests. The use of p24 antigen testing to exclude infection in children aged <18 months is not recommended because of poor sensitivity, especially in the presence of HIV antibody. Commercial tests for RNA and DNA detection have become widely available. Quantitative RNA tests have been approved by the Food and Drug Administration (FDA) for monitoring HIV infection, and qualitative RNA tests have been approved to aid diagnosis. The quantitative and qualitative RNA tests meet FDA standards for high analytic and clinical sensitivity and specificity (14-16). All available tests detect the subtypes of group M and strains of group O. HIV-2 can be diagnosed with HIV-2 DNA PCR. HIV RNA tests sometimes do not detect HIV-2 because the viral loads in some HIV-2–infected persons are below detectable levels. Because of the possibility of mutation or recombination involving the sequences detected by a particular test, occasionally, virus might not be detected in a specimen from an HIV-2 infected individual. If HIV-2 infection seems likely but results are negative, testing with a different assay might be advisable.

^¶Suspected cases of HIV infection among children aged <18 months who are born to a documented HIV-uninfected mother should be assessed on a case-by-case basis by the appropriate health care and public health specialists.

^#If specimens for both negative RNA or DNA virologic tests are obtained at age ≥4 weeks, specimens should be obtained on separate days.

^∗∗Children aged 18 months to <13 years with perinatal exposure to HIV are categorized as uninfected with HIV if the criteria for uninfected with HIV among children aged <18 months are met.

^††For HIV screening among children aged 18 months to <13 years infected through exposure other than perinatal exposure, HIV virologic (non-antibody) tests should not be used in lieu of approved HIV antibody screening tests. A negative result (i.e., undetectable or nonreactive) by an HIV virologic test (e.g., viral RNA nucleic acid test) does not rule out the diagnosis of HIV infection.

Adapted from Centers for Disease Control and Prevention: Revised Surveillance Case Definitions for HIV Infection Among Adults, Adolescents, and Children Aged <18 Months and for HIV Infection and AIDS Among Children Aged 18 Months to <13 Years—United States, 2008. MMWR 57(RR10);1-8, December 5, 2008.

Infectious Patterns

Acquired immunodeficiency syndrome is present worldwide. In some countries and regions (e.g., sub-Saharan Africa, Thailand, India), more than 90% of HIV-1 infections are acquired through heterosexual transmission, in contrast to 10% or less in the United States and Western Europe. HIV-1 and HIV-2 are distinct but related viruses, and both can cause AIDS.

HIV-1

HIV-1 is responsible for the main AIDS epidemic. By analyzing genome sequences of representative strains, HIV-1 has been divided into four groups: group M (for major), including at least nine subtypes, three sub-subtypes of A, and two sub-subtypes of F (A1, A2, A3, B, C, D, F1, F2, G, H, J, and K); group O (for outlier); group N (for non-M, non-O), and group P.

The progression of the natural history and immunopathogenesis of HIV-1 infection can be demonstrated in six discrete stages (Fig. 25-4). These stages are based on the sequential appearance in plasma of HIV-1 viral RNA, the gag p24 protein antigen, antibodies that bind to fixed viral proteins. No matter how HIV-1 was acquired, the timing of the appearance of viral and other markers of infection is generally uniform and follows an orderly pattern.