A child who suffers from a persistent viral infection is found to have a deficiency in lymphocyte production and very few T and B cells. Other bone marrow-derived cells are produced in normal numbers, and MHC molecule expression on cells appears normal

Question

Transfusion of mature T cells from an unrelated donor who had recovered from a previous infection by the same virus would not be expected to help the child clear his infection. Which one of the following is a reasonable explanation for why this therapeutic approach would fail?
A. Viral infections are cleared by antibodies, not T cells.
B. The patient's own immune system would destroy the transfused T cells before they could respond to the viral infection.
C. T cells recognize peptides, not viral particles.
D. Donor T cell viral antigen recognition is restricted by MHC molecules not expressed in the patient.
E. In responding to the previous infection, the donor would have used up all his T cells specific for that virus.

Answer 1

ANS: D. T cells are "self MHC restricted," meaning they specifically recognize infected cells that display microbial peptides displayed by self MHC molecules. There may be no MHC molecules shared by donor and patient, and therefore the transfused T cells would not recognize virus-infected cells in the patient. Because the patient has very few B cells and T cells, his immune system is unlikely to be able to recognize and destroy (i.e., "reject") the transfused T cells. T cells do not recognize structures on intact viral particles but rather peptides derived from viral proteins bound to MHC molecules. Prior viral infection in the donor would be expected to generate memory T cells specific for the virus.