A 23-year-old man who was recently infected by the HIV virus volunteered for investigative studies of his immune response to the virus.Investigators identified cytolytic T lymphocytes (CTLs)in the patient's blood that recognized a particular peptide derived from a protein encoded by the HIV gag gene.Six months later,viral isolates from the patient showed point mutations in the gag gene sequence encoding that peptide.Which of the following statements about the HIV gag mutations is most likely to be correct?
A)The patient's CTL response to the virus provided selective pressure for the emergence of virus carrying the mutation.
B)The CTL that recognized the original gag-encoded peptide will still be able to recognize the mutated Gag peptide.
C)The CTLs specific for the original Gag peptide were incapable of killing HIV-infected cells.
D)The gag mutation will enhance the ability of natural killer cells to recognize and kill HIV infected cells.
E)The gag mutations have more relevance to the viral evasion of the antibody response rather than the CTL responsE.

Question

Quizplus · Accepted Answer

Cytolytic T lymphocytes (CTLs)are highly specific for short peptide epitopes of viral proteins bound to self-MHC molecules.Random mutations in the viral genome that alter these peptide sequences will lead to escape from CTL detection.Therefore,CTLs do provide a selective pressure for such mutations.Given these selective pressures,it is most likely that the mutations that are found in the virus that has survived in the patient are no longer recognized by the CTL.Because only one or two amino acid residues of a viral peptide will contact a particular TCR,point mutations that ablate T cell recognition are likely to occur.The emergence of escape variants with mutated peptides implies that the CTLs that recognized the original peptide were capable of killing the infected cells,but mutations arose before all infected cells were killed.Natural killer cells recognize alterations in self class I MHC expression but do not recognize specific viral peptide sequences.Although antibodies may be produced specific for distinct epitopes of the same proteins that elicit CTL responses,the information given about changes in a T cell epitope implicate the relevance of the CTL response.

A 23-Year-Old Man Who Was Recently Infected by the HIV