Cytotoxic T cells (CTL) are capable of recognizing:

E. All of the above.

DISCUSSION: It was long thought that CTL recognize antigen expressed on the surface of the target cell destined to be killed, similar to the recognition of an antigen by an antibody; however, it was found that the mechanism of CTL recognition of antigens was fundamentally different from the mechanism of antibody recognition. CTL can detect antigens derived from cell surface–associated proteins, but in addition can recognize proteins that are normally in the cytoplasm or in the nucleus. In fact, the normal location of the protein can be anywhere within the cell.

To understand how a CTL can distinguish an antigen from a protein that is normally located within the cytoplasm or nucleus of the cell, investigators had to determine exactly what was being recognized by the CTL. T cells were found to recognize short linear fragments of processed or even denatured protein. It was found that proteins that were synthesized endogenously were degraded within the cytoplasm into 9– or 10–amino acid long peptides. These peptides were then transported to the endocytoplasmic reticulum and associated with newly synthesized MHC class I molecules. Certain peptides could fit within the MHC class I molecule and were then transported to the cell surface as a complex. It was this complex, consisting of a 9– to 10–amino acid peptide within a MHC class I molecule that was presented to, and recognized by, CD8+ CTL.