Maggot secretions skew monocyte-macrophage differentiation away from a pro-inflammatory to a pro-angiogenic type

Abstract

Background: Maggots of the blowfly Lucilia sericata are used for the treatment of chronic wounds. Earlier we reported maggot secretions to inhibit pro-inflammatory responses of human monocytes. The aim of this study was to investigate the effect of maggot secretions on the differentiation of monocytes into pro-inflammatory (MØ-1) and anti-inflammatory/pro-angiogenic macrophages (MØ-2) as these cells play a central role in wound healing.

Methodology/principal findings: Freshly isolated monocytes were incubated with secretions and GM-CSF or M-CSF for 6 days and then stimulated with LPS or LTA for 18 h. The expression of cell surface molecules and the levels of cytokines, chemokines and growth factors in supernatants were measured. Our results showed secretions to affect monocyte-macrophage differentiation leading to MØ-1 with a partial MØ-2-like morphology but lacking CD163, which is characteristic for MØ-2. In response to LPS or LTA, secretions-differentiated MØ-1 produced less pro-inflammatory cytokines (TNF-alpha, IL-12p40 and MIF) than control cells. Similar results were observed for MØ-2 when stimulated with low concentrations of LPS. Furthermore, secretions dose-dependently led to MØ-1 and MØ-2 characterized by an altered chemokine production. Secretions led to MØ-2, but not MØ-1, producing enhanced levels of the growth factors bFGF and VEGF, as compared to control cells. The expression of cell-surface receptors involved in LPS/LTA was enhanced by secretions, that of CD86 and HLA-DR down-regulated, while receptors involved in phagocytosis remained largely unaffected.

Conclusions: Maggot secretions skew the differentiation of monocytes into macrophages away from a pro-inflammatory to a pro-angiogenic type.

Figure 1. Light microscopy analysis of the effect secretions on the differentiation of monocytes to macrophages.

Monocytes were differentiated to MØ-1 in the presence of GM-CSF (A) or in the presence of GM-CSF and 35 µg of secretions/ml (B) and their morphology evaluated. Similarly, monocytes were differentiated to MØ-2 in the presence of M-CSF (C) and in the presence of M-CSF and secretions (D). Results indicated in days are from a representative experiment.

Figure 2. Cytokine production in response to LPS.

We measured the production of IL-12p40 (A,B) and IL-10 (C,D) by control and secretions-differentiated MØ-1 and MØ-2 in response to a range of LPS. The results, expressed in ng/ml, are means±SEM of 9–10 experiments. Open bars: control macrophages; filled bars: secretions differentiated macrophages. *p<0.05 for differences from control macrophages.

Figure 3. Cytokine production in response to LTA.

We measured the production of IL-12p40 (A,B), TNF-α (C,D), IL-6 (E,F) and IL-10 (G,H) by control and secretions-differentiated MØ-1 and MØ-2 induced by a range of LTA. The results, expressed in ng/ml, are means±SEM of 12 experiments. Open bars: control macrophages; filled bars: secretions differentiated macrophages. *p<0.05 for differences from control macrophages.