(a) PD-1 expression by CD4+ T cells from Malian children before the malaria season (Before Malaria) and seven days after symptomatic P. falciparum infection (After Malaria). The non-parametric Mann-Whitney test was used to compare continuous variables between groups. (b) Resolution of P. yoelii (Py) blood-stage infection in rodents requires CD4+ T cells and antibody secreting B cells. Survival curves of Py pRBC infected wild-type C57BL/6 mice, wild-type mice depleted of CD4+ or CD8+ T cells on day 10, or C57BL/6 Aicda–/– μs–/– mice. Data represent 2 independent experiments with 5 mice/group. (c) Upregulation of CD49d and CD11a identifies Plasmodium-specific, infection-induced CD4+ T cells. Longitudinal analyses of PBL before (naïve) and 7 days following (Py pRBC) challenge. Data represent 4 independent experiments. (d) Prolonged Py blood-stage infection results in sustained CD4+ T cell proliferation. Histograms show the fraction of CD49dhiCD11ahi Plasmodium-specific (open) or CD49dloCD11alo naïve (filled) CD4+ T cells that have incorporated BrdU following a day 4-8 pulse (top panel) or day 27-30 pulse (bottom panel). Data represent 3 independent experiments with 3 mice/group. (e) Chronic virus (LCMV cl13) and prolonged Plasmodium blood-stage infection (Py pRBC), but not acute virus infection (LCMV Arm) induce T cell inhibitory receptors PD-1 and LAG-3 at day 31 on splenic, pathogen-specific (open) but not naïve (filled), CD4+ T cells. Data represent 3 independent experiments with 3-5 mice/group. (f) Pathogen-specific CD49dhiCD11ahi CD4+ T cells from Py infected mice exhibit dysfunctional IFN-γ, TNF and IL-2 production in response to PMA/ionomycin stimulation. Data (mean±s.d.) are from 4-5 mice/group and are representative of 3 independent experiments. Statistics in (f) were determined by two-tailed, unpaired student's t-test.