FIG. 4.

Depletion (depl.) of neutrophils (PMN) and AM results in increased lung virus titers (A), brain virus titers (B), and lethality (C). Neutrophils were depleted in BALB/c mice (20 mice/group) by repeated i.p. injections of MAb RB6-8C5 (▴) or control rat IgG (•) 5 h before a 1918 HA/NA:Tx/91 virus infection and on days 3, 5, and 7 p.i to maintain depletion. For AM depletion, i.n. administration of 50 μl of L-Cl₂MBP (□) on days −4 and −2 relative to the time of 1918 HA/NA:Tx/91 virus infection was performed. Control mice received 50-μl doses of L-PBS (○). An additional group of mice was depleted of both AMs and neutrophils (⧫). Mice were i.n. infected with sublethal doses of 0.2 LD₅₀ of 1918 HA/NA:Tx/91 virus. On the days indicated, mice (four per group) were sacrificed, and individual lung and brain tissues were homogenized in 1 ml of PBS. The titers of the lysates for virus infectivity in eggs were determined. The remaining eight mice were followed daily for survival. The key corresponds to panels A and B. Asterisks indicate that the control lung titers are significantly (P < 0.05) different from lung titers from depleted mice, as determined by ANOVA.