Medical Microbiology & Immunology
Lecture 24 Virology Retroviruses (HTLVs, HIVs, and AIDS Molecular)
1) To understand the term retroviruses. Retroviruses are enveloped, single stranded
(+), RNA viruses, which contain an enzyme known as reverse transcriptase, which
converts the RNA genome to DNA genome after infecting the host cells. The virus
DNA integrates into the host genome. Transmissible only by intimate contact.
2) To recognize the difference between oncoviruses and lentiviruses.
Oncoviruses transform cells to produce new virus indefinitely and can transduce
growth promoting genes called oncogenes, which are partly responsible for
malignancies. Human T-limpotrophic virus type1 (HTLV-I) and type 2 (HTLV-II)
are oncogenes that infect T-lymphocytes.
Lentiviruses are slow disease causing viruses including: HIV-1 and HIV-2, the
viruses that cause AIDS, which infect and specifically kill T4 lymphocytes. They
also infect monocytes-macrophages, dendritic cells, Langerhans cells, brain cells, etc.
3) To describe the structure and replication of retroviruses.
Structure: Retroviruses are remarkably similar with a virion size of about 100 nm,
two copies of (+) RNA genomes (diploid) coated with nucleocapsid proteins and
RNA protein complex enclosed in capsid protein, matrix covers the capsid, and
nuclear-derived membrane with surface and transmembrane glycoproteins.
Replication: viral entry - HIV-1 virions interact with the cellular membrane through
surface glycoprotein gp120 and CD4+ on T lymphocytes or macrophages, the
transmembrane glycoprotein gp41 aids membrane fusion, CXCR4 and CCR5 are
coreceptors required for entry (interact with gp120) → Uncoating → Reverse
transcription RNA converted to dsDNA (forms pre-integration complex) →
Migration to the nucleus → Integration into host genome → Transcription of viral
genes by host RNA polymerase II, including envelope glycoproteins and viral
regulatory proteins → Translation and processed by viral encoded protease → Viral
assembly and release by budding through the plasma membrane.
4) To explain the mechanisms by which retroviruses transform to an oncogenic
state. Three mechanisms are known to exist:
1. The acute transforming viruses acquire the cellular gene (oncogene) that results in
loss of normal growth control upon expression. Cannot be cytocidal. No known
human correlates.
2. Insertional mutagenesis occurs when integration of a retrovirus causes
inappropriate expression of cellular genes resulting in uncontrolled growth. The
cellular oncogenes are called protooncogenes. No known human correlates.
3. HTLV-I, the causative agent of adult T cell leukemia, integrates a provirus in
leukemia cells at a unique location resulting in the continual expression of the
viral Tax gene that activates viral transcription and expression of cellular genes
(possibly protooncogenes) resulting in malignant transformation
5) To explain the genomic complexity of retroviruses (HTLV-I, HTLV-II, and
HIV).
HTLV-I
HTLV-II
Structural genes gag-pol-env are flanked by long
terminal repeats (LTR), which act as a promoter,
also encodes regulatory genes Tax and Rex
HIV
Also has gag-pol-env flanked by LTRs, but encodes
regulatory proteins Tat and Rev, and accessory
proteins Vif, Vpr, Vpu, and Nef. HIV-2 encodes all
accept Vpu, it encodes Vpx instead.
6) To understand the roles of viral structure and regulatory proteins.
Proteins
gag
pol
env
tat
rev
nef
vif
vpr
vpu (HIV-1)
vpx (HIV-2)
Function
The group specific antigen gene encodes structural proteins and in some cases the protease.
The polymerase gene encodes reverse transcriptase, integrates, and protease in some cases.
The envelope gene encodes the two glycoproteins found in the envelope, gp120 and gp41 by
processing gp160. Exhibits extensive variation resulting in considerable polymorphism.
Transcriptional post-transcriptional activator.
Post-transcriptional activator, transport structural proteins RNAs to the cytoplasm, involving
RNA splicing and stability.
Negative factor, down regulates CD4+ expression, required for viral pathogenesis.
Infectivity
Up regulates virus expression, prevents cell proliferation, arrest cells in G2/Mphase
Virus release and/or assembly
Virus assembly (?)
7) To know the complex interaction of HIV and immune system and the role of host
factors in HIV infection.
Immune system interaction: destroys CD4+ cells reducing the CD4+/CD8+ ratio from
2.0 to 1.0-0.2 in infected patients. Active virus production results in cell death.
CD4+ cells expressing gp120 and gp41 on their membrane can be killed by antibodyplus-complement lysis. MHC I expression with viral peptides can be killed by CTL
response. Number of infected T cells = 0.0 1-1%. Lymph nodes contained 10-100
times more virus than circulating T cells. Uninfected CD4+ T cells can be depleted
by syncytial formation, destruction is mediated by soluble gp120 which interferes
with T cell maturation resulting in apoptosis.
Humoral and cell mediated immune response: neutralizing antibodies primarily target
gp120 and gp41, but antibodies against most HIV-1 proteins can be found in infected
patients. CD4+ T cells present viral peptides on MHC II causing the release of
antiviral cytokines (INF-g, TNF). CD8+ T cells present viral peptides on MHC I
resulting in killing of virus-infected cells and release of antiviral substances. Cell-tocell spread avoids virus recognition by antibodies. The immune system is unable to
keep pace with the rapidly mutating virus. T cell and B cell function is impaired,
suppressing MHC I and II and interfering with cytokine function.
8) To discuss genetic variation in HIV. HIV-1 possesses the most error-prone reverse
transcriptase resulting in a myriad of variants. The variability is mainly in the
envelope region of the genome but also observed in other regions, and is stimulated
by immunologic pressure for change, alteration in cell tropism, and replication
efficiency. This genetic variation causes problems in vaccine development.
9) To describe pathogenesis and immunopathologies of HIV-I infection.
Pathogenesis: complex and poorly understood. HIV-1 targets CD4+ surface marker
on key lymphocytes, monocytes, and macrophages (probably the first to be infected).
R5 HIV-1 is commonly transmitted from person to person or mother to child. Buyers
can also infect other tissues expressing CD4 including: enterocytes, renal epithelium,
and brain astrocytes.
Latency: there is no viral latency in HIV-1 infection. Clinical latency observed in
HIV-1 infection is a long symptomatic symptomatic asymptomatic period following
infection. Factors that terminate clinical latency include mutations, altered cell
tropism, activation of infected T cells by mitogens and DNA viruses.
Immunopathology: production of CD4+ T lymphocytes due to direct killing of the
target cells. Leads to generalized failure of cell mediated immune responses resulting
in opportunistic infection and malignancy.