Q.43 Which one of the following statements is true with regard to processing and presentation of
protein antigens?
(A) In the class II MHC pathway, protein antigens in the cytosol are processed by
proteasomes
(B) In the class I MHC pathway, extracellular protein antigens are endocytosed into vesicles
and processed
(C) In the class I MHC pathway, transporter associated antigen processing (TAP) protein is
required for translocating processed peptides generated in the cytosol
(D) Invariant chain in endoplasmic reticulum is involved in transport of peptides and loading
of class I MHC

MHC Antigen Processing: Correct Answer and Detailed Explanation

The correct answer is (C) In the class I MHC pathway, transporter associated antigen processing (TAP) protein is required for translocating processed peptides generated in the cytosol. This statement accurately describes the classical MHC class I pathway, where cytosolic proteins are degraded by proteasomes into peptides that TAP transports into the endoplasmic reticulum for loading onto MHC I molecules.​

Class I MHC Pathway Overview

MHC class I molecules present endogenous antigens, such as viral or self-proteins from the cytosol, to CD8+ T cells. Proteins in the cytosol undergo proteasomal degradation, generating peptides that enter the ER via TAP, a heterodimeric transporter. This process enables immune surveillance against intracellular pathogens.​

Why Option (C) is Correct

TAP specifically translocates 8-10 amino acid peptides from the cytosol into the ER lumen, where they bind nascent MHC class I molecules stabilized by chaperones like calnexin and tapasin. Defects in TAP impair MHC I presentation, as seen in bare lymphocyte syndrome, confirming its essential role.​

Analysis of Incorrect Options

• Option (A): False. Class II MHC processes exogenous antigens endocytosed into vesicles, degraded by lysosomal proteases like cathepsins, not cytosolic proteasomes.​

• Option (B): False. Class I MHC handles cytosolic antigens; extracellular proteins follow the class II pathway via endocytosis, not class I.​

• Option (D): False. Invariant chain (Ii) associates with MHC class II in the ER to block peptide binding and direct it to endosomes for exogenous peptide loading; it does not affect class I.​

Key Concepts in Antigen Presentation

• Endogenous vs. Exogenous Pathways: Cytosolic (class I) vs. vesicular (class II) distinguishes antigen sources.​

• Cross-Presentation Exception: Dendritic cells can load exogenous antigens onto MHC I via TAP-dependent routes, but this does not alter classical pathways.​

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