Ch23 (4)

Clinical Immunology & Serology
A Laboratory Perspective, Third Edition
Copyright © 2010 F.A. Davis CompanyCopyright © 2010 F.A. Davis Company
Laboratory Diagnosis of HIV
Infection
Chapter Twenty-Three

Copyright © 2010 F.A. Davis Company
Laboratory Diagnosis of HIV Infection
 Transmission of HIV occurs by one of three
major routes: (1) intimate sexual contact,
(2) contact with blood or other body fluids,
or (3) perinatally, from infected mother to
infant.
 HIV belongs to the genus Lentivirinae of the
virus family Retroviridae.

 HIV is classified as a retrovirus, because it
contains ribonucleic acid (RNA) as its nucleic
acid and a unique enzyme, reverse
transcriptase, that transcribes the viral RNA
into DNA.
 Figure 23-1 shows the structure of the HIV
virion.

 The genome of HIV includes three main
structural genes— gag, env, and pol—and
a number of regulatory genes.
 Figure 23-2 shows the relative locations of the
major HIV genes and indicates their gene
products.

 Other genes in the HIV genome code for
products that have regulatory or accessory
functions in controlling viral replication and
infectivity.
 Table 23-1 summarizes the major HIV-1
genes, their products, and their functions.

 HIV-2 has gag, env, pol, and
regulatory/accessory genes that have similar
functions to those seen in HIV-1.
 The homology between the genomes of the
two viruses is approximately 50 percent.
 HIV-1 and HIV-2 can most easily be
distinguished on the basis of antigenic
differences in their env proteins.

 The first step in the reproductive cycle of
HIV is attachment of the virus to a susceptible
host cell.
 This interaction is mediated through the
host-cell CD4 antigen, which serves as a
receptor for the virus by binding the gp120
glycoprotein on the outer envelope of HIV.

 T helper cells are the main target for HIV
infection, because they express high
numbers of CD4 molecules on their cell
surface and bind the virus with high affinity.
 Other cells, such as macrophages,
monocytes, dendritic cells, Langerhans cells,
and microglial brain cells, can also be infected
with HIV, because they have some surface
CD4.

 Entry of HIV into the host cells to which it
has attached requires an additional binding
step involving coreceptors that promote fusion
of the HIV envelope with the plasma cell
membrane.
 These coreceptors belong to a family of
chemokine receptors, whose main function is
to direct white blood cells to sites of
inflammation.

 After fusion occurs, the viral particle is taken
into the cell, and uncoating of the particle
exposes the viral genome.
 Action of the enzyme reverse transcriptase
produces complementary DNA from the
viral RNA.
 The viral DNA becomes integrated into the
host cell’s genome and is called a provirus
(see Fig. 23-3).

 Expression of the viral genes is induced
when the infected host cell is activated by
binding to antigen or by exposure to cytokines.
 A latent period may occur before expression of
the viral genes takes place.
 Viral DNA within the cell nucleus is then
transcribed into genomic RNA and
messenger RNA (mRNA), which are
transported to the cytoplasm.

 Translation of mRNA occurs, with production
of viral proteins and assembly of viral
particles.
 The intact virions bud out from the host cell
membrane and acquire their envelope during
the process.
 These viruses can then proceed to infect
additional host cells.

 Because viral replication occurs very rapidly,
and the reverse transcriptase enzyme lacks
proofreading activity, genetic mutations
commonly occur, producing distinct
isolates that exhibit antigenic variation.
 These isolates can vary in their susceptibility
to the host’s immune responses.

 The initial viral replication can be detected in
the laboratory by the presence of increased
levels of p24 antigen and viral RNA in the
host’s bloodstream.
 As the virus replicates, some of the viral
proteins produced within host cells form
complexes with MHC class I antigens and
are transported to the cell surface, where they
stimulate lymphocyte responses.

 B lymphocytes are stimulated to produce
antibodies to HIV, which can usually be
detected 6 weeks after primary infection.
 The first antibodies to be detected are directed
against the gag proteins such as p24, followed
by production of antibodies to the envelope,
pol, and regulatory proteins.

 CD8+ cytotoxic T lymphocytes, also known
as cytolytic T cells (CTLs), appear within
weeks of HIV infection and are associated with
a decline in the amount of HIV in the blood
during acute infection.
 While antibodies can attach only to virions
circulating freely outside of host cells, CTLs
can attack host cells harboring viruses
internally.

 After the CTLs bind to HIV-infected host cells,
cytolytic enzymes are released from their
granules and destroy the target cells.
 CTL can also suppress replication and
spreading of HIV by producing cytokines like
interferon-γ, which have antiviral activity.

 Although the humoral and cell-mediated
immune responses of the host usually reduce
the level of HIV replication, they are generally
not sufficient to completely eliminate the
virus.
 This is because HIV has developed several
mechanisms by which it can escape immune
responses.

 CTL and antibody responses to HIV are
hindered by the virus’s ability to undergo rapid
genetic mutations, generating escape
mutants with altered antigens toward which
the host’s initial immune responses are
ineffective.
 In addition, HIV can down-regulate the
production of MHC class I molecules on the
surface of the host cells it infects.

 HIV can also be harbored as a silent
provirus for long periods by numerous cells in
the body.
 In this proviral state, HIV is protected from
attack by the immune system until cell
activation stimulates the virus to multiply and
display its viral antigens.

 CD4+ T helper cells are most severely
affected, and a decrease in this cell
population is the hallmark feature of HIV.
 CD4+ T helper cells are killed or rendered
nonfunctional as a result of the HIV infection.

 These effects on CD4+ T helper cells occur
from a variety of mechanisms, including loss
of plasma membrane integrity due to viral
budding, destruction by HIV-specific CTL, and
viral induction of apoptosis.
 In addition to reducing T-cell numbers, HIV
also causes abnormalities in T helper cell
function and impairment of memory T helper
cell responses.

 Although the manifestations of the disease
vary in individual patients, the infection
progresses through a clinical course that
begins with primary, or acute, infection,
followed by a period of clinical latency and
eventually culminating in AIDS.
 In the first stage, high levels of circulating
virus, or viremia, exist, and HIV begins to
disseminate to the lymphoid organs.

 As the immune system becomes activated, an
acute retroviral syndrome may develop,
with symptoms that resemble influenza or
infectious mononucleosis.
 As HIV-specific immune responses develop,
they begin to curtail replication of the virus,
and patients enter a period of clinical latency.
 The period of clinical latency may vary in
length, with a median length of 10 years.

 According to this definition, HIV-infected
individuals are classified as having AIDS if
they have an absolute CD4 T lymphocyte
count of less than 200/μL or certain
opportunistic infections or malignancies
indicative of AIDS (see Table 23-3).
 In addition to opportunistic infections and
malignancies, HIV-infected individuals often
demonstrate various neurological symptoms.


 According to this definition, adults,
adolescents, and children aged 18 months or
older are considered to be HIV-infected if they
meet the previously published clinical criteria
or if they demonstrate positive test results on
screening and confirmatory tests for HIV
antibody or on an HIV viral test (i.e., HIV
nucleic acid detection, HIV p24 antigen test, or
HIV isolation in culture).

 Treatment of HIV infection involves
supportive care of the infections and
malignancies and administration of
antiretroviral drugs to suppress the virus’s
replication.
 These drugs block various steps of the HIV
replication cycle.

 The development of an effective HIV vaccine has
been very difficult for many reasons, including the
ability of HIV to rapidly mutate and escape
immune recognition, the capability of HIV to
persist despite vigorous immune responses of the
host, genetic variability in HIV clades, the need to
induce mucosal immunity, the need to induce
potent CTL and antibody responses, and the lack
of an ideal animal model.

 The enumeration of CD4 T cells in the
peripheral blood has played a central role in
evaluating the degree of immune suppression
in HIV-infected patients for several years.
 In untreated patients, there is a progressive
decline in the number of CD4 T cells during
the course of infection (see Fig. 23-4).

 The CDC criteria utilize CD4 T-cell counts
to classify patients into various stages of
HIV infection, with those whose counts are
below 200/μL classified as having AIDS.
 In addition, CD4 T-cell counts are used
routinely to monitor the effectiveness of
antiretroviral therapy.

 It is recommended that CD4 T-cell
measurements be performed every 3–6
months in HIV-infected patients to guide
physicians in determining when antiviral
therapy should be initiated, whether a change
in therapy is necessary, and if prophylactic
drugs for certain opportunistic infections
should be administered.

 According to published guidelines,
antiretroviral therapy should be initiated in
patients whose CD4 T-cell count is less than
350/μL; therapy should be changed if CD4 T-
cell counts decline more than 25 percent.
 The gold standard for enumerating CD4 T
cells is immunophenotyping with data
analysis by flow cytometry.

Absolute numbers of CD4 T cells are
calculated according to the following equation
 Absolute # CD4 T cells = WBC count x %
Lymphocytes x % CD4 T cells
 The absolute CD4 T-cell count is then
compared to the reference range, which is
typically from 500 to 1300 cells/μL peripheral
blood.

 The standard screening method for HIV
antibody has been the ELISA, and the
standard confirmatory test is the Western
blot.
 Succeeding generations of ELISA tests exhibit
increasing sensitivity and specificity, the ability
to detect antibodies to both HIV-1 and HIV-2,
and to detect the p24 antigen.

 While ELISAs have a high level of
sensitivity and specificity, they may
sometimes give erroneous results.
 False-positive and false-negative results may
be obtained for a variety of reasons.

 False-negative results may be due to
collection of the sample prior to
seroconversion, to administration of
immunosuppressive therapy or replacement
transfusion, to conditions of defective antibody
synthesis such as hypogammaglobulinemia, or
to technical errors attributed to improper
handling of kit reagents.

 False-positive results may occur because of
several factors, including heat inactivation of
serum prior to testing, repeated
freezing/thawing of specimens, presence of
autoreactive antibodies, history of multiple
pregnancies, severe hepatic disease, passive
immunoglobulin administration, recent
exposure to certain vaccines, and certain
malignancies.

 In low-risk populations, ELISA tests have a
low positive predictive value, or probability
that the patient truly has the disease if the test
result is positive.
 According to a commonly accepted testing
algorithm established by the CDC, when a
sample screened for HIV antibody by ELISA
yields a positive result, it should be retested in
duplicate by the same ELISA test.

 If two out of the three specimens are
reactive by ELISA, then the results must be
confirmed by a more specific method,
usually Western blot.
 Repeatedly reactive units of blood are not
used for transfusion, regardless of results of
confirmatory testing.

 Rapid screening tests are ideal for use in
resource-limited settings in developing
nations and in situations in which fast
notification of test results is desired.
 For example, rapid results are important in
guiding decisions to begin prophylactic therapy
with antiretroviral drugs following occupational
exposures.

 Other situations in which rapid tests are
very beneficial include testing women whose
HIV status is unknown during labor and
delivery and testing patients in sexually
transmitted disease clinics or emergency
departments who are unlikely to make a return
visit for their test results.

 While each test has unique features, all are
lateral flow or flow-through immunoassays that
produce a colorimetric reaction in the case of
a positive result.
 A reactive result is reported as a
preliminary positive and, like the standard
ELISA, requires confirmation by a more
specific test like the Western blot because of
the possibility of false-positive results

 Western blot kits are prepared commercially
as nitrocellulose or nylon strips containing
individual HIV proteins that have been
separated by polyacrylamide gel
electrophoresis and blotted onto the test
membrane.
 The testing laboratory then reacts the test strip
with patient serum.

 During the incubation period, any HIV
antibodies present will bind to their
corresponding antigen on the test membrane,
and unbound antibody is removed by washing.
 Next, an antihuman immunoglobulin with an
enzyme label (i.e., the conjugate) is added
directly to the test strip and binds to specific
HIV antibodies from the patient sample.

 Unbound conjugate is removed by washing,
and bound conjugate is detected after adding
the appropriate substrate, which produces a
chromogenic reaction.
 Colored bands appear in the positions where
antigen-specific HIV antibodies are present.
 Separate HIV-1- and HIV-2-specific Western
blot tests must be used to test for antibodies to
each virus.

 A negative test result is reported if either no
bands are present or if none of the bands
present correspond to the molecular weights
of any of the known viral proteins.
 According to these criteria, a result should
be reported as positive if at least two of the
following three bands are present: p24,
gp41, and gp120/gp160 (see Fig. 23-5).

Figure 23-5

 Specimens that have some of the
characteristic bands present but do not meet
the criteria for a positive test result are
considered to be indeterminate.
 This result may occur if the test serum is
collected in the early phase of seroconversion
or if the serum contains antibodies that cross-
react with some of the immunoblot antigens,
producing false-positive results.

 False positives may be caused by
antibodies produced to contaminants from the
cells used to culture HIV to prepare the
antigens for the test; to autoantibodies,
including those directed against HLA, nuclear,
mitochondrial, or T-cell antigens; or to
antibodies produced after vaccinations.

 If the pattern converts to positive at a later
time, it can be concluded that the first
specimen was obtained during the early phase
of seroconversion.
 Failure of an indeterminate test pattern to
convert to positive after a few weeks
strongly suggests that the pattern is due to a
false-positive test rather than HIV infection.

 The p24 antigen is detected by a solid-
phase antigen capture enzyme
immunoassay.
 All positive results should be confirmed by a
neutralization assay.
 This is accomplished by preincubating the
patient specimen with human anti-HIV-1
antibody prior to performing the p24 antigen
assay.

 If p24 antigen is present, the neutralizing
antibody will form immune complexes and will
prevent the antigen from binding to the HIV
antibody on the solid support.
 In a positive test, absorbance should
decreased by at least 50 percent.

 Because p24 antigen becomes undetectable
as the host produces antibody and binds the
antigen in immune complexes, the p24 antigen
test cannot replace the ELISA test for HIV
antibody as the primary screening test for HIV-
1 infection.

 Quantitative tests for HIV nucleic acid, also
known as viral load tests, are used routinely
to help predict disease progression, predict
response to antiretroviral therapy, and monitor
effects of the therapy.
 Viral load testing is performed by nucleic acid
amplification methods.

 The optimal goal of therapy is to reach
undetectable levels of HIV RNA (i.e., 50–80
copies/mL, depending on the assay).
 Patients who fail to achieve a significant decrease
in viral load after receiving antiretroviral drugs
may not be adhering to the appropriate drug
administration schedule or may have problems
absorbing the drugs or may have developed viral
resistance to the drugs..

 The U.S. Department of Health and Human
Services recommends that plasma HIV RNA
testing be performed before antiretroviral
therapy begins, to obtain a baseline value.
 Testing should then be done 2–8 weeks after
the initiation of therapy and every 3–4 months
thereafter to determine the effectiveness of the
therapy.

 A change in viral load is considered to be
significant if there is at least a threefold or 0.5
log increase or decrease in the number of
copies/mL.
 Initiation of antiretroviral therapy or a change
in the therapy protocol is recommended for
patients whose HIV RNA levels and CD4 T-cell
counts reach critical values (200–350
cells/mm3).

 Two types of laboratory methods can be
used to test for drug resistance: genotype
resistance assays and phenotype resistance
assays.
 Genotype resistance assays detect
mutations in the reverse transcriptase and
protease genes of HIV.

 In these tests, RNA is isolated from patient
plasma, the desired genes are amplified by
RT-PCR, and the products are analyzed for
mutations associated with drug resistance by
automated DNA sequencing or hybridization.
 Phenotype resistance assays determine the
ability of clinical isolates of HIV to grow in the
presence of antiretroviral drugs.

 These assays are technically complex and are
performed only by highly specialized reference
laboratories.
 The major advantage of phenotypic assays is
that they measure drug susceptibility directly,
on the basis of all mutations present in the
patient’s isolate.

 serological tests are not reliable in detecting
HIV infection in children younger than 18
months of age because of placental passage
of IgG antibodies from an infected mother to
her child.
 A child born to an HIV-positive mother may
test positive for HIV antibody during the first
18 months of life even though the child is not
infected.

 Because of the difficulties with serological
testing, HIV infection in infants is best
diagnosed using molecular methods.
 A qualitative HIV-1 DNA PCR test is the
preferred method for this purpose.
 Alternatively, quantitative HIV RNA assays
may be used to diagnose HIV infection in
infants and young children.

 Testing for p24 antigen is not recommended,
as it has a low sensitivity in infants.
 Increased emphasis on screening pregnant
women for HIV infection should also help
in the identification of HIV-positive infants.
 Infected infants have a better prognosis when
HAART is started early and can benefit from
treatment with prophylactic drugs for
opportunistic infections.

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