FIG 2.

Influenza virus infection rapidly expands peptide-primed CD4 T cell memory. NP peptide- or IFA-immunized mice were challenged intranasally at 4 weeks postimmunization with 5 × 10⁴ EID₅₀ A/New Caledonia/20/99. On day 7 postinfection, CD4 T cells were isolated from MLN (A), PLN (B), and spleen (C) and tested in an IFN-γ ELISPOT assay for reactivity to the pool of NP peptides. Experimental groups included NP peptide-immunized and infected mice (Imm/Inf) and IFA-immunized and infected mice. Uninfected NP-peptide immunized mice (Imm/Uninf) were assayed as a control for memory at day 0 of infection (day 30 postimmunization). The data are presented as mean spots per million CD4 T cells from 3 to 5 mice per group and are representative of at least three experiments. ND, no detectable response for IFA-primed PLN. Error bars indicate standard deviations. *, P < 0.01; **, P < 0.005, Student's t test.