CD4 and infection: RORγt-expressing CD4+ T cells, which are thought to be Th17-type T cells, were only present in the lungs of mice infected with the reference strains, Mtb H37Ra and H37Rv, at 28, 56 and 112 days post-infection; this type of T cell was not induced in mice infected with Mtb K (Fig 4E, lower-left panel).