(A) FACS quantification of macrophages (Mac; CD45⁺CD11c⁺SiglecF⁺MHCII^int; solid line) and DCs (CD45⁺CD11c⁺SiglecF^–MHCII^hi; dashed line) in the lungs of uninfected or PR8-infected (7 dpi) WT, Gm-csf^–/–, and SPC-GM mice. (B) Mice were i.t. treated with clodronate liposomes (CL) to deplete alveolar macrophages or with empty liposomes (EL) 48 hours prior to PR8 infection, and AEC apoptosis and alveolar albumin leakage were determined at 7 dpi. (C) Treatment protocol. WT and SPC-GM mice were lethally irradiated and transplanted 1 × 10⁶ CD11c^+/DTR BM cells to generate CD11c^+/DTR→WT or CD11c^+/DTR→SPC-GM chimeric mice. 26 days later, when >90% of lung DCs were of CD11c^+/DTR donor phenotype (whereas alveolar macrophages were mainly of recipient CD11c^+/+ phenotype), chimeras were treated with clodronate or empty liposomes i.t. and with DTX or PBS i.p. to deplete lung macrophages and DCs, respectively, then infected with PR8 48 hours later. (D) Depletion efficacy in CD11c^+/DTR→WT chimeras at 28 dpi, determined by FACS. Fractions of alveolar (CD45⁺CD11c⁺MHCII^intSiglecF⁺; solid line) and lung tissue (CD45⁺CD11c⁺SiglecF⁺MHCII^int) macrophages and of alveolar DCs (CD45⁺CD11c⁺MHCII^hiSiglecF^–; dashed line) were determined from BALF and LH, respectively, and fractions of CD45⁺CD11c⁺MHCII^hiCD103⁺ (red gates) and CD45⁺CD11c⁺MHCII^hiCD11b⁺ (blue gates) DCs were determined from LH and MLNs. (E) Survival of clodronate liposome/DTX–treated CD11c^+/DTR→WT mice and empty liposome/PBS–, clodronate liposome/PBS–, or clodronate liposome/DTX–treated CD11c^+/DTR→SPC-GM mice after PR8 infection (n = 7–8). Data are mean ± SD. *P < 0.05, ***P < 0.005.