Fig. 4. Lung PD1^HiFR4^Hi CD4⁺ T cells are tissue resident.

(A) WT mice were infected with PR8. The expression of CD69, CXCR6, and Bhlhe40 in lung CD4⁺ PD-1^HiFR4^Hi, CD4⁺ PD-1^LowFR4^Low NP_311–325 T cells, or splenic T_FH cells at 28 d.p.i. (B to E) CD45.1⁺ (host) or CD45.1⁺ CD45.2⁺ (partner) WT mice were infected with PR8. Parabiosis surgery was performed at 21 d.p.i. Mice were euthanized 14 days later for analysis. (B) Schematic of parabiosis experiments. (C) Composition of Host-derived or Partner-derived CD4⁺ T cells in the spleen of the parabionts. (D) Frequencies of Host-derived or Partner-derived cells in the lung PD-1^HiFR4^Hi or PD-1^LowFR4^Low total CD4⁺ T cell compartment. (E) Frequencies of Host-derived or Partner-derived cells in influenza-specific lung CD4⁺ PD-1^HiFR4^Hi or CD4⁺ PD-1^LowFR4^Low NP_311–325 T cell compartment. In (A), the representative histograms were from at least two independent experiments (three to four mice per group). In (C) to (E), parabiosis data were pooled from two different experiments. P values were calculated by one-way ANOVA in (D) and (E). Data are means ± SEM. **P < 0.01, ***P < 0.001, and ****P < 0.0001.