Macrophages (Mø) represent a multifunctional cell type in innate immunity that contributes to cytokine release, antigen presentation, and bacterial killing. In acute inflammation infiltrating neutrophils (PMN) release a wide array of stored proteins which functionally interfere with their environment. Here, we present a role for neutrophil-derived proteins in Mø bacterial phagocytosis.

Secretion from human PMN was obtained by antibody cross-linking of beta2-integrins. This treatment results in release of primary, secondary, and tertiary granules as well as secretory vesicles. The PMN secretion was then used to stimulate murine RAW264 cells or Mø differentiated from human monocytes. Cells were subsequently incubated with fluorescent Staphylococcus aureus and the number of phagocytosed bacteria per cell was quantified by fluorescence microscopy.

Treatment of human and murine Mø with PMN secretion caused a several fold increase in bacterial phagocytosis. Heat-inactivation or pepsin-treatment of the secreted material completely abrogated this effect. The PMN-derived secretion evoked a rapid activation of Mø as depicted by increase in cytosolic free Ca2+ and caused upregulation of maturation markers such as CD86, HLA II, CD40 and CD25. Increased expression of Fc? RI (CD64) and Fc? RII (CD32) in response to treatment with PMN secretion likely serves as an effectuating link to the enhanced phagocytosis.

Thus, secretion of PMN granule proteins activates Mø and stimulates enhanced phagocytosis of bacteria. This study provides a further piece of evidence for the multiple cross-talks among cells of the innate immune system.