7.(b) Discuss the mechanism of action of cytotoxic-T cell.

Activation of Cytotoxic T Cells

The activation of naive CD8+ T cells is a multi-step process that typically occurs in secondary lymphoid organs, such as lymph nodes. It requires two main signals from antigen-presenting cells (APCs), primarily dendritic cells [3, 4, 18]:

• Signal 1: T-cell Receptor (TCR) Recognition: The T-cell receptor (TCR) on the surface of the CD8+ T cell recognizes specific foreign peptides (antigens) presented by Major Histocompatibility Complex class I (MHC-I) molecules on the APC. MHC-I molecules are found on almost all nucleated cells in the body, making them ideal for displaying intracellular threats [13, 14]. The CD8 co-receptor on the CTL binds to a non-polymorphic region of the MHC-I molecule, stabilizing the interaction [13].
• Signal 2: Co-stimulation: For full activation, the T cell requires co-stimulatory signals. The most well-known involves the binding of CD28 on the T cell to B7 molecules (CD80/CD86) on the APC. This signal ensures that the T cell only mounts a robust response when a genuine threat is present, preventing autoimmune reactions [4, 18].
• Signal 3: Cytokine Signaling (Differentiation): Cytokines secreted by APCs, especially interleukin-2 (IL-2) produced by activated helper T cells, drive the proliferation and differentiation of activated CD8+ T cells into effector CTLs and memory T cells [10, 12, 18].

Once activated, CTLs undergo clonal expansion, rapidly proliferating to generate a large army of effector cells capable of recognizing and destroying target cells throughout the body [10].

Mechanisms of Action of Cytotoxic T Cells

Activated CTLs employ two primary mechanisms to induce apoptosis (programmed cell death) in target cells: the perforin-granzyme pathway and the Fas-FasL pathway.

1. Perforin-Granzyme Pathway (Granule Exocytosis Pathway)

This is the predominant and fast-acting mechanism used by CTLs [2, 16, 21]. Upon encountering a target cell presenting the specific antigen on its MHC-I molecule, the CTL forms an "immunological synapse" with the target cell. Within this synapse, the CTL rapidly repositions its lytic granules towards the target cell membrane [9, 16].

• Perforin Release: The CTL releases perforin, a pore-forming protein, from its lytic granules [4, 16]. Perforin polymerizes in the target cell membrane, creating pores or channels [5, 16].
• Granzyme Entry: Through these perforin-formed pores, granzymes (serine proteases, e.g., Granzyme A and Granzyme B) enter the cytoplasm of the target cell [4, 5, 23].
• Apoptosis Induction:
  • Granzyme B: Primarily activates a cascade of caspases (cysteine-aspartic proteases), such as caspase-3, which dismantle cellular structures and DNA, leading to apoptosis [2, 4, 25]. It can also directly activate pro-apoptotic members of the BCL-2 family [2].
  • Granzyme A: Targets cellular DNA replication mechanisms, leading to single-stranded DNA nicks and eventually cell dysfunction [6]. It can also induce mitochondrial dysfunction [6].
• CTL Detachment: After inducing apoptosis, the CTL detaches from the dying target cell and can move on to destroy other infected or abnormal cells, acting as a serial killer [16].

2. Fas-Fas Ligand (Fas-FasL) Pathway

This pathway is a slower-acting but equally crucial mechanism, particularly important for eliminating self-reactive lymphocytes and for long-term immune regulation [8, 15, 21].

• FasL Expression: Activated CTLs express Fas ligand (FasL) on their cell surface [15, 20].
• Fas Receptor Engagement: Many target cells, especially those undergoing stress or viral infection, express the Fas receptor (CD95/APO-1) on their surface [15]. When FasL on the CTL binds to Fas on the target cell, it triggers a signaling cascade within the target cell [15, 20].
• Death-Inducing Signaling Complex (DISC) Formation: The binding of FasL to Fas leads to the recruitment of adapter proteins and pro-caspase-8, forming the Death-Inducing Signaling Complex (DISC) [21].
• Caspase Activation and Apoptosis: Pro-caspase-8 is activated within the DISC, leading to the activation of downstream effector caspases (like caspase-3), which execute apoptosis [15, 21, 25].

Both pathways ultimately lead to the controlled dismantling of the target cell, preventing inflammation and damage to surrounding healthy tissues, distinguishing it from necrosis [23].

Role of Cytokines

CTLs also release cytokines such as Interferon-gamma (IFN-γ) and Tumor Necrosis Factor-alpha (TNF-α). These cytokines:

• Enhance the immune response by activating macrophages and other immune cells [4, 7, 24].
• Have antiviral and anti-tumor effects, inhibiting viral replication and tumor cell growth [7, 14].

The table below summarizes the key differences between the two primary killing mechanisms of CTLs:

Feature	Perforin-Granzyme Pathway	Fas-FasL Pathway
Mechanism	Granule exocytosis of perforin and granzymes	Receptor-ligand interaction (FasL on CTL, Fas on target)
Speed of Action	Fast-acting, immediate cell death	Slower-acting, delayed cell death
Key Molecules	Perforin, Granzymes (A, B)	Fas Ligand (FasL), Fas receptor
Primary Role	Elimination of virus-infected and tumor cells	Apoptosis of self-reactive lymphocytes, immune regulation, some pathogen/tumor elimination
Caspase Activation	Indirect (Granzyme B activates caspases)	Direct (DISC formation activates Caspase-8)