PMN secretion stimulates cytokine release and bacterial phagocytosis by monocytes. (A) Concentrations of IL-6 (top) and TNF (bottom) in air pouch fluid of mice with intact WBC count (ctrl), neutropenic mice (PMN depl), and neutropenic mice treated with PMN secretion (PMN depl + PMN-sec). (B) Leukocyte recruitment in the air pouch was induced by PAF and fluorescent IgG-opsonized S aureus bacteria were injected 1 hour before the cells were harvested. Upper panel shows the number of bacteria retrieved from the air pouch fluid expressed in percentage of total amount of injected bacteria. Bottom panel shows fluorescence intensity of monocytes as a measure of bacterial phagocytosis. (C) Recruitment of PMN (top, Gr1⁺, F4/80⁻), resident monocytes (middle, Gr1⁻, F4/80⁺), and inflammatory monocytes (bottom, Gr1⁺, F4/80⁺) in the air pouch after injection of L monocytogenes (10⁶). (D) Dissemination of L monocytogenes 24 hours after inoculation into the air pouch. Data show colony-forming units (CFU) in liver homogenates from mice with intact WBC, neutropenic mice, and neutropenic mice treated with PMN secretion. Data are expressed as means (± SD); n = 4-6 for each bar. * indicates significant difference compared with ctrl; ** indicates significant difference compared with both ctrl and PMN depl + PMN-sec.