CD4 and infection: To further understand the underlying mechanism of the contrasting pathogenicity of SIVcpz- versus HIV-1-infected hu-BLT mice, CD4+ T cells were isolated from splenic tissues of SIVcpz-infected (n = 3 for SIVcpzMB897 infection, n = 3 for BF1167 infection) and HIV-1-infected (n = 3) animals at 16 wpi using negative magnetic cell sorting.