Precursor T-cell proliferation in the thymus is, at least in part, dependent upon IL-7. We thus quantified plasma IL-7 concentration in all groups of patients. At study entry, HCV-and HIV/ HCV-infected patients presented with elevated plasma IL-7 (median = 10.3 pg/mL, range (6.7-12.9) in acutely HCV-infected patients; 8.3 pg/mL (6.3-10.5) in chronic HCV-infected patients and 7.15 pg/mL (4.3-13.5) in co-infected subjects), as compared to that observed in healthy control individuals (p,0.001 for any patients' group; Figure 4A) . Surprisingly, while lymphocytopenia established, IL-7 plasma concentrations significantly decreased in both groups of HCV-infected patients (30, 54, 18 and 29% decrease at M1 to M4 in acute infection, p,0.05; 25, 46, 26 and 16% decrease at M1 to M4 in chronic infection, p,0.05; Figure 4B left and central panels). In contrast, IL-7 plasma levels did not significantly evolve in co-infected individuals during the first month of IFNa therapy ( Figure 4B right panel) . Only patients with the highest IL-7 plasma levels showed a reduction in the concentration of this cytokine.