Infestation with pathogen-free Dermacentor andersoni nymphs programs adoptively transferred HA-specific TCR transgenic CD4+ T cells to express IL-4. Panel A shows the percentage of divided cells in representative CFSE-dilution histograms for uninfested controls, mice receiving a primary (1°) infestation or a repeated infestation (2°) four weeks after an initial exposure to tick feeding. Panel B represents the mean percent ± standard error of CFSE labeled TCR transgenic CD4 T cells. Panel C depicts representative intracellular staining for IFN-γ vs IL-4 expression. Panel D presents quantitative analysis of cytokine expression. For panels B and D, asterisks indicate P < 0.05 compared to uninfested controls. N = 4 mice per group.