Intracellular Infections and Lymphocyte Targeting

Intracellular Infections: Lymphocyte Targeting and Apoptosis

a. Lymphocyte Type for Targeting Infected Cells

• Cytotoxic T lymphocytes (CTLs), also known as CD8+ T cells, are the lymphocytes that specifically target infected cells for apoptosis during intracellular infections.

b. Diagram of CTL Activation and Apoptosis Induction

• (No actual diagram can be rendered in this text-based format, but the elements will be described below and would be annotated on a physical diagram)

Key Events for Diagram (c. and d.)

• 1. Antigen Presentation:

  • Infected cells process viral antigens into peptides.

  • These peptides are presented on the cell surface via MHC class I molecules.

• 2. CTL Activation:

  • A T cell receptor (TCR) on a naive CD8+ T cell binds to the MHC class I-peptide complex on the infected cell.

  • Co-stimulatory molecules (e.g., B7 on the infected cell binding to CD28 on the T cell) provide a second signal, crucial for T cell activation.

  • The activated CD8+ T cell proliferates and differentiates into cytotoxic T lymphocytes (CTLs).

• 3. Target Cell Recognition:

  • The CTL uses its TCR to scan other cells, looking for the same MHC class I-peptide complex.

  • If the complex is found on another infected cell, the CTL binds to it.

• 4. Apoptosis Induction:

  • CTL releases granules containing perforin and granzymes.

  • Perforin creates pores in the target cell membrane.

  • Granzymes enter the target cell through these pores and activate caspases, initiating apoptosis (programmed cell death).

  • Alternatively, the CTL can express Fas ligand (FasL) which binds to Fas receptor on the target cell, also triggering apoptosis.

• 5. Target Cell Death:

  • The infected cell undergoes apoptosis, fragmenting its DNA and shrinking.

  • This process prevents the virus from replicating and spreading.

e. Adaptive Immune Response and Antibody Role in Viral Infections

• Adaptive Immune Response Targeting of Virally Infected Cells:

  • The adaptive immune response targets virally infected cells primarily through the action of CTLs, as described above.

  • The presentation of viral peptides on MHC class I molecules signals to the immune system that the cell is infected and needs to be eliminated.

  • This mechanism is highly specific, as the TCR on the CTL must recognize the specific viral peptide presented by the MHC molecule.

• Role of Antibody in Viral Infections:

  • Antibodies, produced by B cells, play several critical roles in combating viral infections:

    • Neutralization: Antibodies can bind to free virus particles, preventing them from infecting new cells. This is particularly important during the extracellular phase of viral infection. If $V$ represents the virus and $Ab$ represents the antibody, we can represent this interaction as: $V + Ab \rightarrow V:Ab$ (neutralized virus).

    • Opsonization: Antibodies coat the surface of virus particles or infected cells, making them more easily recognized and phagocytosed by macrophages and neutrophils. If $P$ represent phagocytes, the process can be simplified as: $V:Ab + P \rightarrow Phagocytosis$

    • Complement Activation: Antibodies can activate the complement system, leading to the lysis of infected cells or further enhancement of phagocytosis and inflammation. Antibody binding to a virally infected cell can initiate the classical complement pathway: $V_{cell} + Ab + C1 \rightarrow Classical pathway$

    • Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC): Antibodies can bind to infected cells, marking them for destruction by natural killer (NK) cells. The NK cells bind to the antibody via their Fc receptors and release cytotoxic granules: $V<em>{cell} + Ab + NK</em>{cell} \rightarrow Lysis$

  • Antibodies are especially effective against viruses with an extracellular phase in their life cycle, where the virus circulates in the bloodstream or other bodily fluids before infecting new cells.

In summary, while CTLs directly target and eliminate infected cells, antibodies prevent the spread of infection by neutralizing viruses, enhancing phagocytosis, and activating the complement system.

Here is a mini-essay based on the provided notes:

The adaptive immune response employs multiple strategies to combat viral infections, with cytotoxic T lymphocytes (CTLs) and antibodies playing central roles. CTLs, or CD8+ T cells, are crucial for targeting and eliminating cells infected with viruses. This process begins with antigen presentation, where infected cells display viral peptides on MHC class I molecules, signaling their infected status to the immune system. Upon recognition of the MHC class I-peptide complex by a CTL's T cell receptor (TCR), and with the necessary co-stimulatory signals, the CTL becomes activated. The activated CTL then scans other cells for the same MHC class I-peptide complex. Upon binding to an infected cell, the CTL induces apoptosis through the release of perforin and granzymes or via Fas ligand (FasL) binding to the Fas receptor on the target cell. This targeted killing is essential for preventing viral replication and spread.

Antibodies, produced by B cells, complement the action of CTLs through various mechanisms. They neutralize free virus particles, preventing them from infecting new cells, and facilitate the clearance of viruses through opsonization and complement activation. Antibody-dependent cell-mediated cytotoxicity (ADCC) also allows natural killer (NK) cells to target and destroy infected cells. Each of these mechanisms ensures comprehensive protection against viral threats.