CD4 and cystic fibrosis: T cell activation was also assessed by analysing expression of HLA-DR on CD4 and CD8 T cells, revealing that CF children show lower proportions of HLA-DR+ CD4 and CD8 T cells relative to healthy control children [HLADR+ CD4 (median 0.24% vs 1.04%, p = 0.03), HLADR+ CD8 (median 0.01% vs 0.23%, p = 0.014)] (Figure 1D).