clinical microbiology

1. AIDS & Lentiviruses
Term: Spring 2020
Date:5th Nov, 2020
Lecturer: Mr. Kassim A. Jim’ale
Somali National University
Mogadishu, Somalia

2. Introduction
 There are different Human immunodeficiency virus
(HIV) types
 These viruses are derived from primate lentiviruses
 These viruses are the etiologic agents of AIDS
 The illness was first described in 1981
 HIV-1 was isolated by the end of 1983
 Since then, AIDS has become a worldwide epidemic,
expanding in scope and magnitude
 HIV infections have affected different populations and
geographic regions

3. Introduction
 Millions are now infected worldwide
 Once infected, individuals remain infected for life
 Within a decade, if left untreated:
 The vast majority of HIV-infected individuals develop fatal
opportunistic infections as a result of HIV-induced deficiencies
in the immune system
 AIDS is one of the most important public health problems
worldwide at the start of the 21st century
 The development of highly active antiretroviral therapy
(HAART) for chronic suppression of HIV replication and
prevention of AIDS has been a major achievement in HIV
medicine

4. Properties of Lentiviruses
 Virion: Spherical, 80–100 nm in diameter, cylindric core
 Genome: Single-stranded RNA, linear, positive-sense, 9–10 kb
 Proteins:
 Envelope glycoprotein undergoes antigenic variation
 Reverse transcriptase enzyme contained inside virions
 Protease required for production of infectious virus
 Envelope: Present
 Replication:
 Reverse transcriptase makes DNA copy from genomic RNA
 Provirus DNA is template for viral RNA
 Genetic variability is common
 Maturation: Particles bud from plasma membrane

5. Properties of Lentiviruses
 Outstanding characteristics:
 Members are nononcogenic and may be cytocidal
 Infect cells of the immune system
 Proviruses remain permanently associated with cells
 Viral expression is restricted in some cells in vivo
 Cause slowly progressive, chronic diseases
 Replication is usually species-specific
 Group includes the causative agents of AIDS

6. Structure & Composition
 HIV is a retrovirus, a member of the Lentivirus
genus
 It exhibits many of the physicochemical features
typical of the family
 The unique morphologic characteristic of HIV is:
 A cylindrical nucleoid in the mature virion
 The diagnostic bar-shaped nucleoid is visible in
electron micrographs in those extracellular particles
that happen to be sectioned at the appropriate angle

7. Structure & Composition

8. Classification
 Lentiviruses have been isolated from many species
including more than two dozen different African
nonhuman primate species
 There are two distinct types of human AIDS viruses:
 HIV-1 and HIV-2
 The two types are distinguished on the basis of:
 Genome organization
 Phylogenetic (evolutionary) relationships with other primate
lentiviruses
 Sequence divergence between HIV-1 and HIV-2 exceeds
50%

9. Origin of AIDS
 HIV in humans originated from cross-species
infections by simian viruses in rural Africa
 Probably due to direct human contact with infected
primate blood
 Current evidence is that the primate counterparts of
HIV-1 and HIV-2 were transmitted to humans on
multiple (at least seven) different occasions

10. Origin of AIDS
 Sequence evolution analyses place the introduction of
SIVcpz into humans that gave rise to HIV-1 group M at
about 1930, although some estimates push the date back
to about 1908
 Presumably, such transmissions occurred repeatedly
over the ages, but particular social, economic, and
behavioral changes that occurred in the mid 20th
century provided circumstances that allowed these virus
infections to expand, become well-established in
humans, and reach epidemic proportions

11. Disinfection & Inactivation
 HIV is completely inactivated by treatment for 10 minutes at
room temperature with any of the following:
 The virus is also inactivated by extremes of pH (pH 1.0, pH
13.0)
 When HIV is present in clotted or unclotted blood in a needle
or syringe, exposure to undiluted bleach for at least 30
seconds is necessary for inactivation
bleach 50% ethanol
35% isopropanol 1% Nonidet P40
0.5% Lysol 0.5% paraformaldehyde
0.3% hydrogen peroxide

12. Disinfection & Inactivation
 The virus is not inactivated by 2.5% Tween 20
 Although paraformaldehyde inactivates virus free in
solution, it is not known if it penetrates tissues sufficiently
to inactivate all virus that might be present in cultured cells
or tissue specimens
 HIV is readily inactivated in liquids or 10% serum by
heating at 56°C for 10 minutes, but dried proteinaceous
material affords marked protection
 Lyophilized blood products would need to be heated at
68°C for 72 hours to ensure inactivation of contaminating

13. Animal Lentivirus Systems
 Insights into the biologic characteristics of lentivirus infections
have been gained from experimental infections, including
sheep with visna virus
 Natural disease patterns vary among species, but certain
common features are recognized
1. Viruses are transmitted by exchange of body fluids
2. Virus persists indefinitely in infected hosts, though it may be
present at very low levels
3. Viruses have high mutation rates, and different mutants will
be selected under different conditions (host factors, immune
responses, tissue types)
1. Infected hosts contain "swarms" of closely related viral
genomes, known as quasi species

14. Animal Lentivirus Systems
4. Virus infection progresses slowly through specific
stages
i. Cells in the macrophage lineage play central roles in the
infection
ii. Lentiviruses differ from other retroviruses in that they can
infect nondividing, terminally differentiated cells
iii. However, those cells must be activated before viral
replication ensues and progeny virus is produced
iv. Virus is cell-associated in monocytes and macrophages, but
only about one cell per million is infected
v. Monocytes carry the virus around the body in a form that the
immune system cannot recognize, seeding other tissues
vi. Lymphocyte-tropic strains of virus tend to cause highly
productive infections, whereas replication of macrophage-

15. Animal Lentivirus Systems
5. It may take years for disease to develop
i. Infected hosts usually make antibodies, but they do not
clear the infection, so virus persists lifelong
ii. New antigenic variants periodically arise in infected
hosts, with most mutations occurring in envelope
glycoproteins
iii. Clinical symptoms may develop at any time from 3
months to many years after infection
iv. The exceptions to long incubation periods for lentivirus
disease include AIDS in children, infectious anemia
in horses, and encephalitis in young goats

16. Animal Lentivirus Systems
 Host factors important in pathogenesis of disease include:
A. Age (the young are at greater risk)
B. Stress (may trigger disease)
C. Genetics (certain breeds of animals are more susceptible)
D. Concurrent infections (may exacerbate disease or facilitate
virus transmission)
 Simian lentiviruses share molecular and biologic
characteristics with HIV and cause an AIDS-like disease in
rhesus macaques
 The SIV model is important for understanding disease
pathogenesis and developing vaccine and treatment
strategies

17. Virus Receptors
 All primate lentiviruses use as a receptor:
 The CD4 molecule, which is expressed on macrophages and T
lymphocytes
 A second coreceptor in addition to CD4 is necessary for HIV-1 to gain
entry to cells
 The second receptor is required for fusion of the virus with the cell
membrane
 The virus first binds to CD4 and then to the coreceptor
 These interactions cause conformational changes in the viral
envelope, activating the gp41 fusion peptide and triggering
membrane fusion
 Chemokine receptors serve as HIV-1 second receptors

18. Virus Receptors
 The chemokine receptors used by HIV for cell entry
are found on:
 Lymphocytes, Macrophages, and Thymocytes as well
as on Neurons and cells in the colon and cervix
 The requirement for a coreceptor for HIV fusion
with cells provided new targets for antiviral
therapeutic strategies
 The first HIV entry inhibitor licensed in the United
States in 2003

19. HIV Infections in Humans: Pathogenesis
& Pathology
Overview of Course of HIV Infection
 The typical course of untreated HIV infection spans about a decade
 Stages include:
1. The primary infection
2. Dissemination of virus to lymphoid organs
3. Clinical latency
4. Elevated HIV expression
5. Clinical disease
6. Death
 The duration between primary infection and progression to clinical
disease averages about 10 years
 In untreated cases, death usually occurs within 2 years after the
onset of clinical symptoms

20. HIV Infections in Humans: Pathogenesis
& Pathology
 Following primary infection, there is a 4 to 11day period between
mucosal infection and initial viremia
 The viremia is detectable for about 8–12 weeks
 Virus is widely disseminated throughout the body during this time, and
the lymphoid organs become seeded
 An acute mononucleosis-like syndrome develops in many patients
(50–75%) 3–6 weeks after primary infection
 There is a significant drop in numbers of circulating CD4 T cells at this
early time
 An immune response to HIV occurs 1 week to 3 months after infection
A. Plasma viremia drops
B. Levels of CD4 cells rebound
 However, the immune response is unable to clear the infection
completely, and HIV-infected cells persist in the lymph nodes

21. HIV Infections in Humans: Pathogenesis
& Pathology
 This period of clinical latency may last for as long as 10 years
 During this time, there is a high level of ongoing viral replication
 It is estimated that 10 billion HIV particles are produced and
destroyed each day
 The half-life of the virus in plasma is about 6 hours, and the virus
life cycle (from the time of infection of a cell to the production of new
progeny that infect the next cell) averages 2.6 days
 CD4 T lymphocytes, major targets responsible for virus production,
appear to have similar high turnover rates
 Once productively infected, the half-life of a CD4 lymphocyte is about
1.6 days
 Because of this rapid viral proliferation and the inherent error rate of
the HIV reverse transcriptase, it is estimated that every nucleotide
of the HIV genome probably mutates on a daily basis

22. HIV Infections in Humans: Pathogenesis
& Pathology
 Eventually, the patient will develop constitutional
symptoms and clinically apparent disease, such as
opportunistic infections or neoplasms
 Higher levels of virus are readily detectable in the plasma
during the advanced stages of infection
 HIV found in patients with late-stage disease is usually
much more virulent and cytopathic than the strains of
virus found early in infection
 Often, a shift from monocyte-tropic or macrophage-
tropic (M-tropic) strains of HIV-1 to lymphocyte-tropic
(T-tropic) variants accompanies progression to AIDS

23. HIV Infections in Humans: Pathogenesis
& Pathology

24. CD4 T Lymphocytes, Memory Cells, &
Latency
 The cardinal feature of HIV infection is the depletion of T
helper-inducer lymphocytes
 —the result of HIV replication in this population of lymphocytes
as well as of the death of uninfected T cells by indirect
mechanisms
 The T-cells express the CD4 phenotypic marker on their
surface
 The CD4 molecule is the major receptor for HIV
 It has a high affinity for the viral envelope
 The HIV coreceptor on lymphocytes is the CXCR4
chemokine receptor

25. CD4 T Lymphocytes, Memory Cells, &
Latency
 Early in infection, primary HIV isolates are M-tropic
 However, all strains of HIV infect primary CD4 T lymphocytes
 As the infection progresses, the dominant M-tropic viruses are replaced by T-
tropic viruses
 The consequences of CD4 T cell dysfunction caused by HIV infection are
devastating
 This is because the CD4 T lymphocyte plays a critical role in the human immune
response
 It is responsible directly or indirectly for induction of a wide array of lymphoid and
nonlymphoid cell functions
 These effects include:
 Activation of macrophages
 Induction of functions of cytotoxic T cells, natural killer cells, and B cells
 Secretion of a variety of soluble factors that induce growth and differentiation of

26. CD4 T Lymphocytes, Memory Cells, &
Latency
 At any given time, only a small fraction of CD4 T cells are productively
infected
 Many infected T cells are killed, but a fraction survives and reverts to a
resting memory state
 There is little or no virus gene expression in the memory cells, and they
provide a long-term, stable latent reservoir for the virus
 Less than 1 cell per million resting CD4 T cells harbor latent HIV-1
provirus in patients on successful antiretroviral therapy
 Even after 10 years of treatment, patients show very little change in
the size of the latent HIV reservoir because the latent reservoir of
infected memory cells decays very slowly

27. CD4 T Lymphocytes, Memory Cells, &
Latency
 It is unlikely that an HIV infection can be cured
 If there were a million infected memory cells in the body, it would
take about 70 years for them to decay
 When exposed to antigen or when drug therapy is
discontinued, the memory cells become activated and
release infectious virus
 It is possible that other drug-insensitive reservoirs may
also exist among macrophages, hematopoietic stem
cells, or brain cells

28. Monocytes & Macrophages
 Monocytes and macrophages play a major role in the
dissemination and pathogenesis of HIV infection
 Certain subsets of monocytes express the CD4 surface
antigen and therefore bind to the envelope of HIV
 The HIV coreceptor on monocytes and macrophages is the
CCR5 chemokine receptor
 In the brain, the major cell types infected with HIV appear to
be the monocytes and macrophages, and this may have
important consequences for the development of
neuropsychiatric manifestations associated with HIV
infection
 Infected pulmonary alveolar macrophages may play a role
in the interstitial pneumonitis seen in certain patients with
AIDS

29. Monocytes & Macrophages
 Macrophage-tropic strains of HIV predominate
early after infection, and these strains are
responsible for initial infections even when the
transmitting source contains both M-tropic and T-
tropic viruses
 It is believed that monocytes and macrophages
serve as major reservoirs for HIV in the body
 These cells also aid dissemination of the virus in to
various organs of the body

30. Lymphoid Organs
 Lymphoid organs play a central role in HIV infection
 Lymphocytes in the peripheral blood represent only about 2% of the total
lymphocyte pool, the remainder being located chiefly in lymphoid organs
 It is in the lymphoid organs that specific immune responses are generated
 The network of follicular dendritic cells in the germinal centers of lymph
nodes traps antigens and stimulates an immune response
 Throughout the course of untreated infection—even during the stage of
clinical latency—HIV is actively replicating in lymphoid tissues
 The microenvironment of the lymph node is ideal for the establishment
and spread of HIV infection
 Cytokines are released, activating a large pool of CD4 T cells that are highly
susceptible to HIV infection
 As the late stages of HIV disease progress, the architecture of the lymph
nodes becomes disrupted

31. Neural Cells
 Neurologic abnormalities are common in late stages of infection and are an AIDS-
defining condition
 Central nervous system disease occurs to varying degrees in 40–90% of patients.
These include:
 HIV encephalopathy
 Peripheral neuropathies, and
 Most serious—AIDS dementia complex
 Both direct and indirect pathogenic mechanisms might explain the
neuropsychiatric manifestations of HIV infection
 The predominant cell types in the brain that are infected with HIV are monocytes
and macrophages
 Virus may enter the brain through infected monocytes and release cytokines that
are toxic to neurons as well as chemotactic factors that lead to infiltration of the
brain with inflammatory cells
 HIV is present rarely, if at all, in neurons, oligodendrocytes, and astrocytes

32.  Lecture tbc…….
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33. THANK YOU