CD4 and infection: To establish the identity of the target cell population that supported initial infection at the site of inoculation, rectum and distal colon sections were tested with a combined RNAScope and immunofluorescence assay specific for SIVsab vRNA, and they were costained for CD4+ T cells and myeloid lineage cells (CD68+ CD163+ HAM56+) in all AGMs (Figure 6).