HIV And Retroviruses | Varsity Tutors

SECTION 1

Historical Context & Discovery of HIV

The emergence of Human Immunodeficiency Virus (HIV) as a recognized pathogen represents one of the most consequential chapters in modern infectious disease history. In the early 1980s, clinicians in the United States began documenting clusters of unusual opportunistic infections—Pneumocystis jirovecii pneumonia and Kaposi sarcoma—among previously healthy young men. These observations catalyzed a global research effort that would transform virology, immunology, and public health policy. The identification of HIV as a retrovirus that specifically destroys CD4⁺ T lymphocytes provided the mechanistic foundation for understanding Acquired Immunodeficiency Syndrome (AIDS) and for developing targeted antiretroviral therapies.

1970

Discovery of Reverse Transcriptase

Howard Temin and David Baltimore independently discover reverse transcriptase in retroviruses, overturning the central dogma that genetic information flows only from DNA to RNA.

1981

First AIDS Cases Reported

The CDC publishes a report on five cases of Pneumocystis pneumonia in Los Angeles, marking the first official recognition of what would become the AIDS epidemic.

1983

HIV-1 Isolated

Luc Montagnier and Françoise Barré-Sinoussi at the Pasteur Institute isolate a novel retrovirus—later named HIV-1—from the lymph node of a patient with lymphadenopathy.

1987

AZT Approved

Zidovudine (AZT), the first antiretroviral drug, receives FDA approval. It functions as a nucleoside reverse transcriptase inhibitor (NRTI) and provides proof of concept for targeting the HIV replication machinery.

1996

HAART Era Begins

Introduction of highly active antiretroviral therapy (HAART)—combining protease inhibitors with NRTIs—dramatically reduces AIDS-related mortality and transforms HIV into a manageable chronic disease.

Understanding HIV requires an appreciation of retroviral biology, particularly the mechanisms by which the virus integrates its genome into host DNA, evades immune surveillance, and progressively depletes the CD4⁺ T-cell population. The central question that drives clinical immunology in this context is: how does a single retrovirus dismantle the most sophisticated adaptive immune system in nature?

SECTION 2

Core Principles of Retroviral Biology & HIV Structure

Retroviruses belong to the family Retroviridae and are distinguished by their unique replication strategy: they carry a diploid, positive-sense single-stranded RNA (+ssRNA) genome that is reverse-transcribed into double-stranded DNA (dsDNA) and permanently integrated into the host cell chromosome. HIV belongs to the genus Lentivirus, a subset of retroviruses characterized by long clinical latency periods and tropism for cells of the immune system. There are two clinically relevant types: HIV-1 (the predominant global pathogen) and HIV-2 (largely confined to West Africa, with lower virulence and transmissibility).

Reverse Transcription

HIV uses reverse transcriptase (RT) to convert its +ssRNA genome into dsDNA. RT is error-prone (no 3′→5′ exonuclease proofreading), generating approximately one mutation per replication cycle—fueling antigenic variation and drug resistance.

Proviral Integration

The viral enzyme integrase catalyzes insertion of proviral DNA into the host genome. Once integrated, the provirus is replicated with every host cell division, establishing a permanent latent reservoir that current therapies cannot eradicate.

CD4⁺ Tropism

HIV selectively binds the CD4 receptor via its envelope glycoprotein gp120, then engages a coreceptor—CCR5 (macrophage-tropic, R5 strains) or CXCR4 (T-cell-tropic, X4 strains)—to trigger membrane fusion mediated by gp41.

Immune Evasion

High mutation rates, heavy glycosylation of envelope proteins (the glycan shield), downregulation of MHC class I via the Nef protein, and establishment of latent reservoirs collectively allow HIV to evade both humoral and cell-mediated immunity.

Viral Structural Genes

The HIV genome encodes three structural polyproteins (Gag, Pol, Env) and six regulatory/accessory genes (Tat, Rev, Nef, Vif, Vpr, Vpu). Gag encodes capsid (p24), matrix (p17), and nucleocapsid. Pol encodes RT, integrase, and protease. Env encodes gp120 and gp41.

✦ KEY TAKEAWAY

Think of HIV as a sophisticated saboteur infiltrating a military command center. The virus specifically targets CD4⁺ T-helper cells—the generals coordinating the immune response. By integrating its genetic blueprint into the host DNA, HIV essentially plants a permanent sleeper agent that can reactivate at any time, making complete eradication impossible with current technology. Meanwhile, the virus's high mutation rate is analogous to constantly changing its disguise, preventing the immune system from mounting a lasting defense.

SECTION 3

HIV Virion Structure

The HIV-1 virion is an enveloped retrovirus. The outer lipid bilayer (host-derived) is studded with gp120/gp41 trimeric spikes essential for cell entry. Beneath lies the matrix (p17) and the conical capsid (p24) housing two copies of the +ssRNA genome along with reverse transcriptase, integrase, and protease.

The HIV virion is approximately 100–120 nm in diameter. The envelope glycoproteins exist as trimers of gp120-gp41 heterodimers; gp120 is the extracellular subunit responsible for CD4 binding, while gp41 is the transmembrane subunit that mediates membrane fusion. Importantly, the lipid bilayer is derived from the host cell membrane during budding, which means it also contains host proteins such as MHC molecules and ICAM-1. The p24 capsid antigen is clinically significant because it is the target of fourth-generation HIV screening assays that simultaneously detect anti-HIV antibodies and p24 antigen, enabling earlier diagnosis during the window period.

SECTION 4

HIV Replication Cycle

The HIV replication cycle can be dissected into discrete stages, each of which represents a potential therapeutic target. Understanding this cycle is essential for rationalizing the mechanism of action of each antiretroviral drug class and for predicting patterns of drug resistance.

Step-by-Step Replication

1. Attachment & Entry: gp120 binds CD4 on the target cell, inducing a conformational change that exposes the coreceptor binding site. Engagement of CCR5 (early, macrophage-tropic) or CXCR4 (late, T-cell-tropic) triggers gp41-mediated fusion of viral and host membranes. Drug targets: CCR5 antagonists (maraviroc), fusion inhibitors (enfuvirtide).
2. Uncoating: The viral capsid is released into the cytoplasm and partially disassembles, releasing the RNA genome and associated enzymes. Drug target: capsid inhibitors (lenacapavir).
3. Reverse Transcription: Reverse transcriptase synthesizes dsDNA from the RNA template using host tRNA^Lys3 as a primer. This enzyme has RNA-dependent DNA polymerase, DNA-dependent DNA polymerase, and RNase H activities. Drug targets: NRTIs (tenofovir, emtricitabine), NNRTIs (efavirenz, rilpivirine).
4. Integration: The pre-integration complex translocates to the nucleus. Integrase catalyzes 3′-processing and strand transfer to insert proviral DNA into the host chromosome. Drug targets: integrase strand transfer inhibitors (INSTIs: dolutegravir, bictegravir).
5. Transcription & Translation: Host RNA polymerase II transcribes proviral DNA. The viral Tat protein enhances transcriptional elongation by binding the TAR element. Rev protein facilitates nuclear export of unspliced and partially spliced viral mRNAs for translation of structural proteins.
6. Assembly, Budding & Maturation: Gag and Gag-Pol polyproteins assemble at the plasma membrane. Immature virions bud from the cell, acquiring the lipid envelope. Viral protease cleaves polyproteins into functional components, producing mature infectious virions. Drug targets: protease inhibitors (darunavir, atazanavir).

⚡ HIGH-YIELD

The homozygous CCR5-Δ32 mutation (approximately 1% of Northern European populations) confers near-complete resistance to R5-tropic HIV-1 infection. Heterozygous carriers have slower disease progression. This principle underlies the curative "Berlin patient" (Timothy Ray Brown) and "London patient" (Adam Castillejo) cases, who received stem cell transplants from CCR5-Δ32 homozygous donors.

SECTION 5

Immunopathogenesis & Disease Stages

The natural history of untreated HIV infection follows a characteristic pattern of CD4⁺ T-cell decline and viral load dynamics that can be divided into three phases. The interplay between viral replication, immune activation, and progressive immunodeficiency defines the clinical trajectory and provides the rationale for laboratory monitoring via CD4 counts and plasma HIV RNA levels.

During the acute phase (weeks 2–4), viral load peaks at 10⁶–10⁷ copies/mL while CD4 counts transiently drop. The immune response then partially controls viremia, establishing the viral set point—a strong predictor of disease progression. During clinical latency, CD4 counts decline at approximately 50–80 cells/µL per year. When CD4 falls below 200 cells/µL, the patient meets the definition for AIDS and becomes susceptible to opportunistic infections.

Three phases of untreated HIV infection

Phase	Duration	CD4⁺ Count	Viral Load	Clinical Features
Acute (Primary)	2–4 weeks	Transient sharp decline	Very high (10⁶–10⁷)	Mononucleosis-like syndrome: fever, lymphadenopathy, pharyngitis, rash, mucosal ulcers
Clinical Latency	2–10 years	Gradual decline (~50–80/yr)	At viral set point	Largely asymptomatic; may have persistent generalized lymphadenopathy
AIDS	Variable	< 200 cells/µL	Rising, often > 10⁵	Opportunistic infections (PJP, CMV, MAC, toxoplasmosis), malignancies (Kaposi sarcoma, primary CNS lymphoma)

📋 CD4 COUNT THRESHOLDS TO REMEMBER

CD4 < 500: Kaposi sarcoma, oral hairy leukoplakia, thrush. CD4 < 200: PJP, histoplasmosis, coccidioidomycosis (AIDS-defining). CD4 < 100: Toxoplasmosis, cryptococcosis, cryptosporidiosis. CD4 < 50: CMV retinitis, disseminated MAC, progressive multifocal leukoencephalopathy (PML).

SECTION 6

Clinical Vignette: Diagnosing & Managing HIV

Clinical reasoning around HIV requires integration of risk factor assessment, serologic testing, staging, and treatment initiation. The following worked example walks through a representative USMLE-style scenario.

Case: 28-Year-Old Male with Acute Febrile Illness

Step 1 — Identify Clinical PresentationA 28-year-old man presents with a 10-day history of fever, diffuse maculopapular rash, pharyngitis, cervical lymphadenopathy, and painful oral ulcers. He reports unprotected sexual contact with a new partner 3 weeks ago. This constellation of findings is consistent with acute retroviral syndrome (ARS), which occurs in 40–90% of patients during primary HIV infection.

Clinical suspicion: Acute HIV infection

Step 2 — Select Appropriate Diagnostic TestDuring the acute phase, anti-HIV antibodies may not yet be detectable (the window period). The recommended initial screening test is a 4th-generation combination assay that detects both HIV-1/2 antibodies and p24 antigen. If this is reactive, the next step is an HIV-1/HIV-2 antibody differentiation immunoassay. If antibody differentiation is negative or indeterminate (as expected in acute infection), proceed with HIV-1 RNA NAAT (nucleic acid amplification test) to confirm viremia.

Results: 4th-gen assay (+), Ab differentiation indeterminate, HIV-1 RNA = 850,000 copies/mL → Confirmed acute HIV-1

Step 3 — Stage the DiseaseBaseline labs: CD4⁺ count = 380 cells/µL. Since his CD4 count is > 200 and he has no AIDS-defining illnesses, this is classified as Stage 1 (CD4 ≥ 500) or Stage 2 (CD4 200–499) per CDC classification. With a CD4 of 380, this patient is Stage 2. However, note that the CD4 may be transiently depressed during acute infection and could recover partially.

CDC Stage 2 (CD4 200–499 cells/µL)

Step 4 — Initiate TreatmentCurrent guidelines recommend initiating ART in all HIV-positive patients regardless of CD4 count. A preferred first-line regimen is a two-NRTI backbone (e.g., tenofovir alafenamide/emtricitabine [TAF/FTC]) plus an INSTI (e.g., dolutegravir or bictegravir). Before starting, check HLA-B*5701 (if considering abacavir), hepatitis B serologies (TAF/TDF treat both HIV and HBV), and obtain a baseline genotype resistance test.

Rx: Bictegravir/TAF/emtricitabine (single-tablet regimen)

Step 5 — Assess for Opportunistic Infection ProphylaxisWith a CD4 of 380, this patient does not currently require PJP prophylaxis (threshold < 200) or MAC prophylaxis (threshold < 50). However, he should receive a tuberculin skin test or IGRA, toxoplasma IgG, and appropriate vaccinations. If CD4 drops below 200 despite ART, initiate TMP-SMX for PJP prophylaxis (which also covers toxoplasmosis).

No OI prophylaxis needed at CD4 380; monitor and reassess

SECTION 7

Antiretroviral Drug Classes & Targets

Antiretroviral therapy (ART) exploits the vulnerability of each step in the HIV replication cycle. Understanding the mechanism, key side effects, and resistance patterns of each drug class is a high-yield topic for USMLE Step 1. Modern ART employs combination regimens to minimize the emergence of drug-resistant mutants, since HIV's error-prone reverse transcriptase generates an estimated 10⁹–10¹⁰ virions per day, each potentially carrying unique mutations.

Major antiretroviral drug classes for USMLE Step 1

Drug Class	Target	Key Agents	High-Yield Side Effects
NRTIs	Reverse transcriptase (competitive, chain termination)	Tenofovir (TDF/TAF), emtricitabine, abacavir, zidovudine (AZT), lamivudine	TDF: nephrotoxicity, Fanconi syndrome. Abacavir: hypersensitivity (HLA-B*5701). AZT: bone marrow suppression (macrocytic anemia, neutropenia). All NRTIs: lactic acidosis (mitochondrial toxicity)
NNRTIs	Reverse transcriptase (non-competitive, allosteric binding)	Efavirenz, nevirapine, rilpivirine, etravirine	Efavirenz: vivid dreams, CNS symptoms, teratogenicity. Nevirapine: hepatotoxicity, Stevens-Johnson syndrome. Low genetic barrier to resistance (single mutation)
Protease Inhibitors (PIs)	HIV protease (prevents Gag/Gag-Pol polyprotein cleavage)	Darunavir, atazanavir, ritonavir/cobicistat (boosters)	Metabolic syndrome: dyslipidemia, lipodystrophy, insulin resistance. Ritonavir: CYP3A4 inhibitor (drug interactions). Atazanavir: indirect hyperbilirubinemia
INSTIs	Integrase (strand transfer)	Dolutegravir, bictegravir, raltegravir, elvitegravir	Generally well-tolerated; weight gain, insomnia, headache. Dolutegravir: high genetic barrier to resistance. Raltegravir: CK elevation, rhabdomyolysis (rare)
Entry Inhibitors	CCR5 coreceptor (maraviroc); gp41 fusion (enfuvirtide)	Maraviroc, enfuvirtide	Maraviroc: hepatotoxicity (with systemic allergic reaction). Enfuvirtide: injection site reactions (subcutaneous administration)

✦ KEY TAKEAWAY

Think of combination ART like a multi-layered security system in a building. A single lock (monotherapy) can be picked by a skilled intruder (drug-resistant mutant), but simultaneously defeating a deadbolt, an electronic keypad, and biometric scanner (three drug classes) is astronomically less probable. This is why at least two to three different drug classes are always used together—the probability of a single virion simultaneously harboring resistance mutations to all three classes is vanishingly small, on the order of 10⁻²¹.

SECTION 8

Latent Reservoirs, Immune Reconstitution & Cure Strategies

Even with suppressive ART achieving undetectable viral loads (< 20 copies/mL), HIV persists indefinitely in latent reservoirs—primarily resting memory CD4⁺ T cells harboring transcriptionally silent proviral DNA. The half-life of this reservoir is estimated at 44 months, meaning that complete eradication by ART alone would require over 70 years of continuous therapy. This biological reality has shifted research focus toward functional cures, sterilizing cures, and the phenomenon of immune reconstitution inflammatory syndrome (IRIS).

Current therapy vs. emerging approaches to HIV cure and complications

Concept	Current Standard (ART)	Emerging / Experimental
Viral Suppression	ART suppresses viremia to < 20 copies/mL; does not eliminate provirus	"Shock and kill" strategies use latency-reversing agents (LRAs) to reactivate latent provirus so it can be targeted by immune effectors
Cure Approaches	Lifelong daily ART required; treatment interruption → viral rebound within weeks	CCR5-Δ32 stem cell transplant (2 confirmed cures); CRISPR/Cas9 proviral excision (preclinical); broadly neutralizing antibodies (bnAbs)
Vaccine Development	No approved prophylactic vaccine despite > 30 years of effort	mRNA vaccine platforms, mosaic immunogens targeting conserved epitopes, bnAb elicitation strategies in clinical trials
IRIS	Paradoxical worsening when ART initiated in severely immunocompromised patients (CD4 < 50)	Occurs as reconstituted immune system mounts inflammatory response against pre-existing pathogens (e.g., TB-IRIS, cryptococcal IRIS). Management: continue ART + corticosteroids for severe cases

For USMLE purposes, remember that IRIS is most common in patients who start ART with very low CD4 counts and is especially associated with TB and cryptococcal meningitis. The key management principle is to continue ART (do not stop it) and treat the inflammatory reaction with corticosteroids if severe. Additionally, the concept of U = U (Undetectable = Untransmittable) is now well-established: persons living with HIV who maintain an undetectable viral load on ART have effectively zero risk of sexually transmitting the virus, as demonstrated by the PARTNER and HPTN 052 trials.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

A researcher isolates a virus that carries a single-stranded RNA genome and also possesses an enzyme that synthesizes DNA from an RNA template. The viral genome is then permanently integrated into the host chromosome. Which family does this virus most likely belong to, and what is the name of the enzyme responsible for DNA synthesis from the RNA template?

PROBLEM 2 — BASIC CALCULATION

A patient with untreated HIV has a baseline CD4⁺ count of 800 cells/µL. Assuming an average decline of 65 cells/µL per year and no treatment, approximately how many years would it take for this patient to develop AIDS (defined as CD4 < 200 cells/µL)?

PROBLEM 3 — INTERMEDIATE

A 35-year-old woman is diagnosed with HIV. Her CD4 count is 150 cells/µL and viral load is 250,000 copies/mL. In addition to initiating ART, what prophylactic medications should be started, and why? Address both PJP and MAC prophylaxis.

PROBLEM 4 — APPLIED

A physician is treating an HIV-positive patient who has been stable on a regimen containing abacavir. The patient develops fever, malaise, diffuse rash, and GI symptoms 10 days into therapy. HLA-B*5701 testing was not performed prior to initiation. What is the diagnosis, and what is the critical management step?

PROBLEM 5 — CRITICAL THINKING

Explain why the development of an effective prophylactic HIV vaccine has been exceptionally challenging compared to vaccines for other viral pathogens. Discuss at least three immunological barriers, referencing specific features of HIV biology.

SUMMARY

HIV & Retroviruses: Key Concepts Review

HIV is a lentivirus within the family Retroviridae, carrying a diploid +ssRNA genome that undergoes reverse transcription to dsDNA and permanent proviral integration via integrase. The virus enters host cells by binding the CD4 receptor via gp120 and a coreceptor (CCR5 or CXCR4), with fusion mediated by gp41. The natural history progresses through acute retroviral syndrome, clinical latency (with gradual CD4 decline), and ultimately AIDS (CD4 < 200 cells/µL) if untreated.

Diagnosis employs a 4th-generation combo assay (Ab + p24 Ag) followed by antibody differentiation and HIV-1 RNA NAAT. Treatment with combination ART (typically 2 NRTIs + 1 INSTI) is initiated for all patients regardless of CD4 count. Key drug classes target reverse transcriptase (NRTIs/NNRTIs), integrase (INSTIs), protease (PIs), and viral entry (CCR5 antagonists, fusion inhibitors). High-yield associations include HLA-B*5701 and abacavir hypersensitivity, CCR5-Δ32 and HIV resistance, CD4 count thresholds for opportunistic infections, and IRIS in patients starting ART at very low CD4 counts.