<p>A subpopulation of effector CD4⁺ T cells is primarily responsible for reactivity to immunogenic <i>Francisella</i> epitopes.</p>

<p>C57Bl/6 mice were injected subcutaneously with either 3 × 10⁴ CFU of <i>F. tularensis</i> LVS (immune) or sterile PBS (control), and lymphocytes were isolated 3 weeks later and pooled from groups of 4 mice. Cocultures of 3 × 10⁵ splenic CD4⁺ T cells and 2 × 10⁵ naïve B cells were stimulated for 24 h with a mixture of 5 selected immunogenic peptides (PP-4) (Table B in <a href="plospathogens.org/article/info:doi/10.1371/journal.ppat.1014597#ppat.1014597.s002" target="_blank">S1 Appendix</a>) or left without peptides (unstim) and the IFN-γ response was determined.

A concentration of 5 µM was used for each peptide. (A) The numbers of IFN-γ producing cells (SFCs) were determined using ELISpot. Furthermore, the % of intracellular IFN-γ-positive cells among live (B) total CD4⁺ T cells or (C) subpopulation of CD4⁺CD62L^-CD44⁺ effector T cells was determined by FACS analysis.