Q.33 Which of the following is(are) involved in the activation of cytotoxic T cells?
(A) MHC I
(B) FcR
(C) T cell receptor
(D) CTLA 4

MHC class I (MHC I), the T cell receptor (TCR), and other elements like co-stimulation are essential for cytotoxic T cell activation, while CTLA-4 inhibits it and FcR plays no direct role. The correct answer is (A) and (C).​

Option Analysis

MHC I (A): MHC class I molecules present intracellular antigens (e.g., viral peptides) on infected cells to the TCR on naive CD8+ T cells, providing the primary signal for activation. Without MHC I-peptide complexes, cytotoxic T cells (CTLs) cannot recognize targets.​

FcR (B): Fc receptors are primarily on myeloid cells (e.g., macrophages, NK cells) for antibody-mediated functions like ADCC; T cells, including CTLs, do not express functional FcR for their activation. Transient FcR expression on activated T cells regulates subsets but does not drive CTL activation.​

T cell receptor (C): The TCR on CD8+ T cells binds specifically to peptide-MHC I complexes, initiating the signaling cascade (e.g., via CD3 ζ-chain) essential for CTL proliferation, differentiation, and cytotoxicity.​

CTLA-4 (D): CTLA-4 acts as an inhibitory checkpoint on activated T cells, competing with CD28 for B7 ligands on APCs to dampen TCR signaling and prevent overactivation.​

Cytotoxic T cells, or CD8+ T lymphocytes, are key effectors in cell-mediated immunity, targeting virus-infected or cancerous cells during activation of cytotoxic T cells. This process requires antigen-specific recognition and co-stimulation, crucial for CSIR NET Life Sciences preparation.​

Core Mechanisms

Naive CD8+ T cells activate when their TCR binds peptide antigens on MHC I from APCs or infected cells (signal 1). CD28-B7 interaction provides co-stimulation (signal 2), with cytokines like IL-2 amplifying proliferation into CTLs that release perforin and granzymes.​

Role of Each Option

• MHC I: Displays 8-11 amino acid peptides to TCR, enabling specificity; all nucleated cells express it.​

• FcR: Absent on resting CTLs; expressed transiently on some activated T cells for regulation, not activation.​

• TCR: Triggers intracellular signaling (ZAP-70, ERK) upon MHC I binding, forming immunological synapses.​

• CTLA-4: Inhibits via higher B7 affinity, trans-endocytosis, and phosphatase recruitment post-activation.​

Exam Relevance

For CSIR NET, focus on two-signal model: TCR-MHC I (signal 1) + CD28 (signal 2). Inhibitors like CTLA-4 highlight checkpoints, relevant to immunotherapy.​