In a Clinical Trial of a New Antiviral Vaccine Composed

In a clinical trial of a new antiviral vaccine composed of a recombinant viral peptide and adjuvant,4% of the healthy recipients did not show evidence of response to the immunization.Further investigation revealed that all the nonresponders expressed the same,single allelic variant of HLA-DR but all the responders were heterozygous for HLA-DR alleles.Which of the following is the most likely explanation for this finding?

A) Response to the vaccine requires T cell recognition of complexes of the viral peptide with HLA-DR,but the peptide cannot bind to the allelic variant of HLA-DR found in the nonresponders.
B) The nonresponders could not express class II MHC proteins.
C) The viral peptide is not an immunodominant epitope.
D) The nonresponders underwent determinant selection of another viral epitope.
E) Because of technical errors,the nonresponders had not received adequate doses of the vaccinE.