Question

Which statements are true for NKT cells ?
A. NKT cells can act as both helper cells and cytotoxic cells.
B. NKT cells do form memory cells.
C. NKT cells killing appears to depend predominantly on FasL-Fas interactions.
D. NKT cell include both CD4\(^+\) and CD4\(^-\) subpopulations, which may also differ by cytokine production.
E. NKT cells do not express a number of markers characteristic of T lymphocytes.
Choose the correct answer from the options given below :

• A. A and B only
• B. A, B and C only
• C. A, B, C and D only
• D. A, C, D and E only

Hint:

Unlike conventional T cells that recognize peptide antigens presented by MHC molecules, NKT cells specifically recognize lipid and glycolipid antigens presented by the non-polymorphic CD1d molecule.

Answer 1

The Correct Option is C

Step 1: Concept:
The question tests knowledge regarding the unique characteristics, cellular subsets, and effector mechanisms of Natural Killer T (NKT) cells in the immune system.

Step 2: Step-by-step Explanation:

• Statement A: NKT cells function uniquely at the intersection of innate and adaptive immunity. Upon activation, they rapidly secrete huge amounts of cytokines (like IFN-\(\gamma\) and IL-4), acting as helper cells. They can also directly lyse target cells, acting as cytotoxic cells. Thus, A is true.

• Statement B: Traditionally, innate-like cells were thought to lack memory. However, recent immunological research has identified that specific subsets of NKT cells can undergo clonal expansion and form long-lived memory populations that mount an enhanced response upon re-challenge. Thus, B is generally considered true in modern contexts.

• Statement C: The cytotoxic killing mechanism of NKT cells primarily relies on the extrinsic apoptotic pathway, predominantly mediated by the interaction between Fas Ligand (FasL) on the NKT cell and the Fas receptor on the target cell. Thus, C is true.

• Statement D: NKT cells are phenotypically heterogeneous. In humans and mice, they can be categorized into distinct subpopulations, such as CD4\(^+\) and CD4\(^-\) (which are mostly CD4\(^-\)CD8\(^-\) double negative). These subsets distinctly differ in their cytokine secretion profiles. Thus, D is true.

• Statement E: By definition, NKT cells express a semi-invariant T-cell receptor (TCR), which is a definitive hallmark of T lymphocytes. They heavily express CD3 and other standard T-cell markers alongside NK cell markers (like NK1.1). Therefore, stating they "do not express" T-cell markers is false.

• Since Statements A, B, C, and D are true, and E is false, the correct grouping corresponds to Option (C).

Step 3: Final Answer:
The combination of true statements is A, B, C, and D only.