Fig. 1.

Early IFN-γ production in virus-experienced CD4⁺ T cells mediates protection against Lpn. Control and LCMV-experienced memory mice were challenged with Lpn. (A) Experimental layout of the heterologous infection model. (B) Bacterial titers from bronchoalveolar lavage (BAL) fluid 3 d post infection (dpi) (n = 6 to 8). (C) Representative FACS plot of the IFN-γ response of CD4⁺ T cells. (D) Time course of IFN-γ response with or without Lpn restimulation (n = 5). (E) Bacterial titers 3 dpi in mice treated intranasally on d0 and d1 with αIFN-γ neutralizing or IgG control antibody (n = 6 to 10). (F) 5 × 10⁵ CD4⁺ T cells isolated from the spleen were transferred i.v. 1 d before Lpn infection into congenic hosts and the IFN-γ response was measured 60 h post infection (n = 4 to 5). (G) Mice received 5 × 10⁴ SMARTA cells i.v. 1 d prior to LCMV infection (memory) or 10⁶ naive SMARTA cells 1 d before Lpn challenge (control). IFN-γ response was measured on day 2 post-Lpn challenge (n = 3). (H) Bacterial titers 3 dpi of Rag2^−/−γc^−/− mice that received 2 × 10⁶ naive or memory CD4⁺ T cells 2 d prior to Lpn infection (n = 9 to 10). (I) LCMV memory mice were treated with αCD4 or αCD8 depleting antibodies (or IgG isotype control) 5 and 3 d prior to Lpn infection. Bacterial titers were determined on day 3 post Lpn challenge (n = 14 to 15). Mean ± SD, Mann–Whitney U test (B and H), two-way ANOVA (Šídák; D–F), Kruskal–Wallis test (G), or Brown–Forsythe and Welch ANOVA (I).