Since R5 viral entry rates were relatively low (approximately 5% of non-activated CD4+ cells compared to 60% with X4 virus; Figure 1B), these results suggest that only a few HIV+ CD4+ T cells are required to induce a CD8+ T cell response in this assay, irrespective of co-receptor usage, since we observed only a 2-fold higher CD8+ T cell response with X4 than R5 HIV infection despite a 12-fold higher HIV entry. Here, CD4 is linked to HIV infectious disease.