Neutrophil-specific deletion of Syk results in a defective host response to S aureus. Analysis of syk deletion by flow cytometry of Syk protein in (A) neutrophils (Ly6G⁺CD11b⁺) and (B) monocytes (Ly6G⁻CD11b⁺) from the bone marrow (BM), peripheral blood (PB), and air pouch lavage (AP). Syk^KO cells from syk^−/− chimeras or syk ^f/fVav1cre^Tg mice are negative controls (gray histograms) relative to syk^f/−LysMcre^Tg/+, syk^f/−Mrp8cre^Tg (dashed black lines), or syk^f/− (solid black line) mice. Histograms are representative of at least 4 animals. (C) CFU of S aureus and neutrophil infiltrate and (D) CFU of E coli and neutrophil infiltrate in the air pouch lavage of syk^f/+LysMcre^Tg/+ and syk^f/−LysMcre^Tg/+ mice 24 hours after the initiation of infection. (E) CFU of S aureus and neutrophil infiltrate and (F) CFU of E coli and neutrophil infiltrate in the air pouch lavage of syk^f/f, syk^f/−, and syk^f/−Mrp8cre^Tg mice 24 hours after the initiation of infection. Data represent at least 3 independent experiments, with at least 3 animals per group. The difference in S aureus counts in the air pouch is statistically significant; *P < .05 by Mann-Whitney U test. (G) Analysis of TNF-α and MIP-1α levels in S aureus–infected air pouches from syk^f/+LysMcre^Tg/+ and syk^f/−LysMcre^Tg/+ mice. *P < .05, unpaired t test.