MHC class II molecules expressed on the surface of thymic cortical epithelial cells normally have a wide repertoire of different peptides bound to them. By engineering a construct that fuses the MHC class II protein to a single peptide sequence, and expressing this construct in thymic cortical epithelial cells that have their endogenous MHC class II genes knocked out, it is possible to generate a mouse line where all MHC class II proteins expressed on all thymic cortical epithelial cells are bound to the same peptide. These mice are often referred to as ‘single-peptide’ mice. Examination of the T cell developing in these single peptide mice would likely show:
A. A significant reduction in the numbers of mature CD4 T cells
B. No change in the numbers of mature CD4 T cells
C. A block in T cell development at the CD4+CD8+double-positive stage
D. A repertoire of T-cell receptors on mature CD4 T cells restricted to a single Vbeta
E. A block in T cell development at the CD4-CD8-double-negative stage