Lecture 12

Lecture Notes: Discovery of HIV and Evidence from the 1983 Paper

1. Background: Early AIDS Research

  • AIDS recognized in 1983, but the causative agent was unknown.

  • Early observations:

    • Patients had many opportunistic infections.

    • Indicates severe immune system suppression, especially T-cell deficiency.

  • Initially observed in:

    • Homosexual men

    • Later also seen in:

      • Blood transfusion recipients

      • IV drug users

  • Researchers suspected a virus that infects T cells.

Key Scientific Problem

Find the etiological agent (cause) of AIDS.

Researchers were testing whether the cause might be:

  • Herpesviruses

  • Retroviruses (especially HTLV – Human T-cell leukemia virus)

Koch’s Postulates (Framework for Proving Disease Causation)

  1. Microorganism must be found in diseased individuals but not healthy ones.

  2. Microorganism must be isolated and grown in pure culture.

  3. Cultured organism should cause disease in a healthy host.

  4. Organism must be re-isolated from the infected host.

Limitations

  • Impossible to ethically infect humans.

  • HIV is human-specific, making animal models difficult.

The paper attempts to partially satisfy these postulates.

Figure 1: Evidence for a Retrovirus

Question

Is the virus isolated from the patient a retrovirus?

Method

  1. Lymph node biopsy from an AIDS patient.

  2. Cells metabolically labeled with tritiated uridine (radioactive RNA label).

  3. Virus purified using a sucrose density gradient.

  4. Fractions tested for:

    • RNA (tritium signal)

    • Reverse transcriptase activity

Important Concepts

  • Retroviruses package reverse transcriptase inside virions.

  • Reverse transcriptase converts RNA → DNA.

Results

  • Peak of:

    • RNA labeling

    • Reverse transcriptase activity

  • Both occur in the same density fraction.

Interpretation

  • Virus particles contain:

    • RNA

    • Reverse transcriptase

➡ Therefore, the virus is likely a retrovirus.

Koch’s Postulates

Supports Postulate 2:

  • Isolation and biochemical characterization of the microorganism.

Figure 2: Electron Microscopy

Question

Do the isolated particles look like retroviruses?

Method

  • Electron microscopy of infected cell cultures.

Observations

Virus particles:

  • Budding from plasma membrane

  • Contain dense viral cores

Interpretation

These structures resemble retrovirus virions.

Also shows:

  • Virus can infect new T-cells in culture.

Koch’s Postulates

Supports:

  • Isolation

  • Propagation in culture

Table: Antibody Recognition

Question

Is this virus HTLV-1, or something new?

Method

Test whether infected cells are recognized by antibodies:

Antibodies used:

  • HTLV-1 antibodies (against proteins P19 and P24)

  • Patient sera

  • Control sera

Cell types tested:

  • Normal lymphocytes

  • HTLV-producing cells

  • Cells infected with the new virus

Results

  • HTLV antibodies do NOT recognize the new virus strongly.

  • Patient serum recognizes infected cells.

Interpretation

  • The virus is not HTLV-1.

  • Patients produce antibodies against a different viral antigen.

Figure 3: Viral Protein Identification

Question

What viral proteins are produced, and are they related to HTLV proteins?

Method

  1. Infected cells labeled with 35S-methionine
    → labels newly synthesized proteins

  2. Cells lysed to extract proteins.

  3. Immunoprecipitation

    • Antibodies bind specific proteins

    • Antibody–protein complexes pulled down using beads

  4. Proteins separated using SDS-PAGE.

Antibodies Used

  • Patient 1 serum

  • Patient 2 serum

  • Healthy serum

  • HTLV P24 antibody

  • Control goat serum

Results

Patients' sera detect a ~25 kDa protein (P25).

Key findings:

  • This protein not recognized by HTLV antibodies.

  • Appears only in patient samples.

Interpretation

  • Virus produces a capsid protein similar in size to HTLV.

  • But immunologically distinct.

➡ Suggests a new retrovirus.

Overall Conclusion of the Paper

Evidence suggests a new retrovirus infecting T cells.

Characteristics:

  • Infects T-lymphocytes

  • Contains reverse transcriptase

  • Produces distinct viral proteins

  • Different from HTLV-1

This virus would later be named:

Human Immunodeficiency Virus (HIV).

Scientific Context

The discovery occurred during competition between:

  • Luc Montagnier's French group

  • Robert Gallo's American group

Both were trying to identify the cause of AIDS.

Experimental Limitations

Problems with the study:

  • Small sample size

  • Poor figure labeling

  • Polyclonal patient sera complicates interpretation

  • Cannot fulfill Koch's Postulate #3 (infection experiment)

Despite this, the evidence strongly suggested a new retrovirus causing AIDS.

Graphical Abstract Assignment (Class Instructions)

Goal: Summarize the paper visually.

Recommended Structure

Panel 1 — Question

  • What is causing AIDS?

Panel 2 — Methods
Examples:

  • Patient lymph node biopsy

  • Retrovirus detection

  • Electron microscopy

  • Protein analysis

Panel 3 — Results

  • Retrovirus identified

  • New viral protein detected

  • Distinct from HTLV

Tips

  • Keep text minimal.

  • Use simple diagrams or stick figures.

  • Focus on main experiment → conclusion.

Modern Techniques (Compared to 1983 Methods)

Older methods used:

  • Radioactive labeling

  • Electron microscopy

  • Immunoprecipitation

Today researchers could use:

  • PCR

  • Next-generation sequencing

  • Fluorescent protein tagging

  • Proteomics

These would identify HIV much faster.