Secondary necrosis of apoptotic neutrophils induced by the human cathelicidin LL-37 is not proinflammatory to phagocytosing macrophages

Abstract

Cathelicidins are CHDP with essential roles in innate host defense but also more recently associated with the pathogenesis of certain chronic diseases. These peptides have microbicidal potential and the capacity to modulate innate immunity and inflammatory processes. PMN are key innate immune effector cells with pivotal roles in defense against infection. The appropriate regulation of PMN function, death, and clearance is critical to innate immunity, and dysregulation is implicated in disease pathogenesis. The efferocytosis of apoptotic PMN, in contrast to necrotic cells, is proposed to promote the resolution of inflammation. We demonstrate that the human cathelicidin LL-37 induced rapid secondary necrosis of apoptotic human PMN and identify an essential minimal region of LL-37 required for this activity. Using these LL-37-induced secondary necrotic PMN, we characterize the consequence for macrophage inflammatory responses. LL-37-induced secondary necrosis did not inhibit PMN ingestion by monocyte-derived macrophages and in contrast to expectation, was not proinflammatory. Furthermore, the anti-inflammatory effects of apoptotic PMN on activated macrophages were retained and even potentiated after LL-37-induced secondary necrosis. However, this process of secondary necrosis did induce the release of potentially harmful PMN granule contents. Thus, we suggest that LL-37 can be a potent inducer of PMN secondary necrosis during inflammation without promoting macrophage inflammation but may mediate host damage through PMN granule content release un-der chronic or dysregulated conditions.

LL-37 Directs Macrophage Differentiation toward Macrophages with a Proinflammatory Signature

“…Preliminary experiments revealed that these C-terminal peptides are more effective than LL-37 in killing of Staphylococcus aureus as detected by radial diffusion assays (T. Vos, A. van der Does, B. Ravensbergen, H. Beekhuizen, and P. Nibbering, unpublished results). In agreement, is has been reported that the LL-37-derived peptides covering residues 13-35 are at least as effective as LL-37 with respect to LPS neutralization (31), modulation of TLR-mediated responses (34), and inducing secondary necrosis of apoptotic neutrophils (35). These structure-function studies can be helpful in the design of new candidate peptides for the treatment of infections.…”

Neutrophil Apoptosis: Relevance to the Innate Immune Response and Inflammatory Disease

“…Neutrophil necrosis occurs by direct cytotoxicity induced by noxious stimuli, by apoptotic neutrophils undergoing secondary necrosis resulting from failed clearance or by pore-forming toxins such as PVL. The cathelicidin LL-37 has also been shown to induce rapid secondary necrosis of neutrophils that can potentiate the anti-inflammatory effect of macrophages without compromising clearance mechanisms [66] . Although these studies did not show any adverse effects on macrophage function with suggestion of promotion of a less inflammatory macrophage response, this must be weighed against the presence of toxic granule contents in the inflammatory milieu that previously had always been thought to contribute to host tissue damage.…”

Dangerous attraction: phagocyte recruitment and danger signals of apoptotic and necrotic cells

“…the death of intestinal epithelial cells) [64][65][66][67]. In contrast to apoptosis necrosis generally is considered to be associated with inflammation, although there are scattered reports describing conflicting observations-particularly when the dying cells are [68,69]. Potentially cytotoxic and autoantigenic intracellular components get exposed and act as endogenous danger signals or damage associated molecular patterns (DAMPs) that 'inform' the immune system about the tissue damage [70,71].…”