Neutrophil granule proteins : modulators of the immune response

Abstract

Polymorphonuclear leukocytes (PMN) are multifunctional cells of the innate immune system which represent the advance guard in inflammatory reactions. Via the release of granule proteins the PMN communicates with the endothelium, monocytes, dendritic cells, and lymphocytes and is thereby critically involved in the fine tuning of the host defense system. This thesis aimed at elucidating mechanisms by which the PMN and its granule proteins recruit, activate, and stimulate monocytes and macrophages. Moreover, PMN secretory products may contribute to tissue damage, which is another focus of this project.

Extravasation of PMN to the site of injury or infection precedes the recruitment of monocytes. In the first part of the project we addressed causal connections between these two events. There we demonstrate that PMN deposit heparin-binding protein (HBP) from rapidly mobilized secretory vesicles on the endothelial cell surface. In this location HBP interacts with monocytes rolling along the endothelium. Consequently, monocytes become activated and adhere to the vessel wall in increased numbers. In a second study we applied two in vivo models to elucidate the importance of the primary PMN extravasation for the subsequent recruitment of monocytes. We show that PMN extravasation is crucial for the early recruitment of inflammatory monocytes, a subset of monoytes that has recently been shown to be of high significance in inflammatory reactions. As principal mediators of this effect we identify the PMN granule components LL-37 and HBP which employ monocytic formyl peptide receptors to stimulate recruitment of inflammatory monocytes.

Macrophages are of crucial importance in bacterial clearance. In the second part of the project we investigated the impact of PMN secretion products on the macrophage s capacity to phagocytose IgG-opsonized bacteria. PMN secretion was found to strongly enhance the phagocytic capacity of macrophages and thorough investigation revealed HBP and human neutrophil peptides 1-3 (alpha-defensins) as principal mediators of this effect. Mechanistically, these two proteins induce the release of TNFalpha and IFNgamma from macrophages. These activate the macrophage in an autocrine fashion resulting in upregulation of the Fcgamma receptors CD64 and CD32.

Infection with Streptococcus pyogenes of the M1 serotype may lead to acute lung injury and in the third part of the project we addressed the involvement of PMN secretion products in this response. We demonstrate that depletion of PMN, but not of monocytes, completely abolishes the M1 protein-induced lung damage. The lung damage was restored by injection of PMN secretion into neutropenic mice indicating that secretion products of PMN are principle inducers of lung damage following infection with S. pyogens.