CD4 and infection: The mesenteric CD4+ T cells showed a similar, but slightly slower, increase in PD-1 expression (average fold-increase of 2.9 in frequency, 1.8 in total number, and 1.2 in MFI) during the first five weeks post-infection, reaching a plateau at eight weeks post-infection (average fold-increase of about 6 in frequency, 5.6 in total number, and 1.7 in MFI) (Fig 2B and 2C and S1 Fig).