Retroviridae: HIV

Classification and Subtypes

Classification

• Family Retroviridae: Viruses use reverse transcriptase enzymes to produce deoxyribonucleic acid (DNA) from their ribonucleic acid (RNA) genome (reverse of the usual pattern = “retro”).
• Genus Lentivirus:
  • Cause chronic deadly diseases 
  • Long incubation periods 
  • Integrate viral cDNA into host DNA, infecting nondividing cells

Two subtypes of human immunodeficiency virus (HIV)

• HIV-1 (discovered first, more virulent and more infectious than HIV-2):
  • Group M: stands for “major”; more than 90% of HIV/AIDS cases, several subtypes or recombinant forms
  • Group N: stands for “non-M, non-O”
  • Group O: stands for “outlier”; isolated to West-Central Africa
  • Group P: stands for “pending the identification of further human cases”; discovered in August 2009, isolated to Cameroon
• HIV-2:
  •  Lower infectivity, less virulent
  •  Largely confined to West Africa
  •  8 known subgroups (A to H)

Structure and Genome

HIV virion

• Roughly spherical 
• Approximately 110 nm in diameter
• Structure:
  • Virion envelope contains 2 membrane glycoproteins:
    • Gp41 (transmembrane)
    • Gp120 (surface/docking protein)
  • Matrix protein p17: surrounds the virus core
  • Virus core: 
    • Capsid protein p24
    • 2 single-stranded, enveloped positive-sense ribonucleic acid (RNA) coated with nucleocapsid protein
    • Enzymes (reverse transcriptase, integrase, protease)

HIV genome (9 genes that encode 15 viral proteins)

• Env gene encodes gp160, which is cleaved to form envelope glycoproteins:
  • gp120: attachment to host CD4+ T cell
  • gp41: fusion and entry to host cell
• Gag gene encodes p24 and p17, capsid and matrix proteins, respectively.
• Pol gene encodes the enzymes reverse transcriptase, integrase, and protease.
• Regulatory elements: tat, rev, vpr, vif, nef, and vpu

Genetic variability (very high)

• Recombination between 2 RNA copies in every particle
• Fast replication
• High mutation rate

Life Cycle

Tropism

• HIV can infect CD4+ T cells, macrophages, and microglial cells.
• HIV surface glycoprotein gp160 (composed of gp120 and gp41): key for viral attachment to the CD4 surface molecules and chemokine co-receptors
• CD4: expressed on the surface of T helper cells, regulatory T cells, monocytes, macrophages, and dendritic cells
• CCR5: expressed on T cells (memory and activated CD4 lymphocytes), gut-associated lymphoid tissues (GALTs), macrophages, dendritic cells, and microglia
• CXCR4: expressed on T cells (naïve and resting CD4 lymphocytes as well as CD8 cells), B cells, neutrophils, and eosinophils
• HIV strains: can express CCR5 (R5 or M) tropism, CXCR4 (X4 or T) tropism, or dual tropism (X4R5)

HIV replication cycle

• Target cells: CD4+ T cells, macrophages, and dendritic cells
• HIV cell entry: 
  • The virion first enters via a break in the mucous membrane. 
  • The virion crosses the mucosal barrier and seeks target cells.
• HIV replication:
  1. Membrane fusion and binding of receptors (entry): The virion (carrying viral RNA, reverse transcriptase, integrase, and other proteins) initiates entry into the host cell. 
     • Virion binds the CD4 receptor and a chemokine receptor (CCR5 on macrophages, CXCR4 on T cells). 
       • Macrophage tropic viruses: R5 strains
       • T cell tropic viruses: X4 strains
     • Binding of gp120 with CD4 and the chemokine receptors leads to a conformational change, exposing the fusion domain at gp41.
     • The conformational change pulls the viral and cellular membranes together, fusing them. 
  2. The capsid protein shell (surrounding the viral RNA and proteins) is uncoated as the virion traverses the cytoplasm.
  3. Reverse transcription: Reverse transcriptase-mediated synthesis of proviral DNA (from the viral RNA) occurs.
  4. Integration: Viral DNA is transported across the nucleus and is integrated into the host DNA, facilitated by integrase. 
  5. Replication: Viral DNA is transcribed; multiple copies of new HIV RNA are formed and transported to the cytoplasm. 
     • New HIV RNA becomes the genome of a new virus. 
     • Other copies of the RNA are used to make new HIV proteins.
  6. Assembly: New viral RNA + proteins + enzymes move to the cell surface and form a noninfectious particle.
  7. Budding and maturation: 
     • Particles (viral RNA + proteins) eventually bud out of the host cell with the immature HIV.
     • Viral protein protease then cleaves the newly synthesized polyproteins, producing mature HIV.

HIV Infection

Transmission

• Sexual:
  • > 80% worldwide through heterosexual contact
  • Risk increases:
    • In the presence of other sexually transmitted diseases
    • With repeated intercourse with an infected partner
    • In the case of high viral load
  • Antiretroviral therapy decreases transmission risk.
• Parenteral:
  • Intravenous drug users
  • Blood transfusion
  • Needle sticks in medical professionals
• Vertical (mother to child):
  • During pregnancy
  • During birth
  • Through breastfeeding
  • Risks decreased by antiretroviral therapy

Pathophysiology

• Initial entrance is most commonly through the anogenital mucosa.
• Interstitial dendritic cells may serve as an initial target.
• New infection is more commonly spread by macrophage-tropic viruses.
• The infected cells then fuse with CD4+ T cells and the infection spreads.
• Initial rapid increase in viral RNA corresponds with seroconversion (acute phase).
• Viral RNA levels then drop in response to the emergence of HIV-specific CD8+ T cells.
• Eventually, RNA level reaches a “set point” (over 6 months).
• Progressive slow decline in CD4+ cells:
  • Direct cytopathic effects of the virus
  • gp120-mediated cytotoxicity
  • Activation-induced apoptotic cell death
  • CD8+ cell killing of infected CD4+ cells
• Acquired immunodeficiency syndrome (AIDS) develops when CD4+ T cells < 200 cells/μL.

Clinical presentation

• Early/acute phase:
  • 60% are asymptomatic.
  • If symptomatic, mononucleosis-like illness:
    • Fever
    • Sore throat
    • Headache, rash, diarrhea
    • Myalgia/arthralgia
  • If symptoms last > 14 days, more rapid progression to AIDS
• Chronic infection (without AIDS):
  • Generally lasts 8–10 years
  • Mostly asymptomatic
  • Accompanied by fatigue, sweats, and weight loss
  • Generalized lymphadenopathy
  • Can have recurrent candidiasis, folliculitis, and severe presentations of herpes, papilloma, or shingles
• AIDS: defined by CD4+ count < 200 cells/μL and presence of AIDS-defining conditions (opportunistic illnesses secondary to immunosuppression)

Identification

• HIV antibody (enzyme-linked immunosorbent assay (ELISA))
• HIV antibody and antigen
• HIV antigen
• HIV RNA