CD4 and parasitic infectious disease: In order to directly enumerate antigen-specific CD4 T cells in vivo, mice were initially vaccinated with a live vaccine strain of S. Typhimurium expressing the MHC Class II epitope 2W1S (BRD509::2W1S, strain SPN555) and subsequently infected with P. yoelii. Using this approach, the effect of parasite infection on the frequency and function of 2W1S-specific CD4 T cells was determined using a 2W1S MHC class-II tetramer [34].