CD4 and infection: Both CD4+ and CD8+ T cells started to infiltrate the lungs of WT mice as early as day 3 p.i., increasing rapidly to peak at day 8 p.i. (Fig. 3B), and declining thereafter, with levels at day 15 p.i. still higher than in mock-infected groups (data not shown), while the kinetics of recruitment of these cells to the MLN showed a peak at day 5 p.i. (Fig. 3C), with a progressive decrease at later time points of infection.