Which statement about MHC class I molecules is true?

Which statement about MHC class I molecules is true?

• A. They present peptides from cytosolic proteins to CD8+ T cells.
• B. They present extracellular antigens to CD4+ T cells.
• C. They are encoded by HLA-DR genes.
• D. They are not involved in antigen presentation.

Answer: (A)

MHC class I molecules are responsible for presenting peptides derived from proteins made inside the cell to CD8+ cytotoxic T cells. This endogenous peptide presentation pathway involves proteasomal degradation of cytosolic proteins, transport of the resulting peptides into the endoplasmic reticulum by TAP, and loading onto MHC class I with the help of chaperones before the peptide-MHC complex travels to the cell surface for recognition by CD8+ T cells. This is distinct from how extracellular antigens are handled (that pathway uses MHC class II and presents to CD4+ T cells) and from the fact that HLA-DR genes encode MHC class II molecules, not class I. Since MHC class I is actively involved in antigen presentation on nearly all nucleated cells, the statement describing this endogenous peptide presentation to CD8+ T cells is true.

MHC class I molecules are responsible for presenting peptides derived from proteins made inside the cell to CD8+ cytotoxic T cells. This endogenous peptide presentation pathway involves proteasomal degradation of cytosolic proteins, transport of the resulting peptides into the endoplasmic reticulum by TAP, and loading onto MHC class I with the help of chaperones before the peptide-MHC complex travels to the cell surface for recognition by CD8+ T cells. This is distinct from how extracellular antigens are handled (that pathway uses MHC class II and presents to CD4+ T cells) and from the fact that HLA-DR genes encode MHC class II molecules, not class I. Since MHC class I is actively involved in antigen presentation on nearly all nucleated cells, the statement describing this endogenous peptide presentation to CD8+ T cells is true.