IgG4 Drives M2 Macrophages to Cortisol, Lcn-2 and IL-10 Release: Implications in Maintenance of Tolerance and Allergen Immunotherapy

Rationale: M2 macrophages play a role in resolving inflammatory responses: macrophages are a prominent source of i) cortisol, and ii) of human lipocalin-2 (LCN-2) having a glucocorticoid-responsive element in its promoter. We addressed whether macrophages are a source of IL-10 and whether IgG antibodies have an impact in regulating them, for understanding allergen immunotherapy (AIT).

Methods: Primary macrophages from healthy PBMCs or monocytic cell line THP-1 were differentiated into M2 macrophages by M-CSF and LPS, and for further sub-differentiation with IL-4/IL-13 (M2a), or with IgG immunoglobulins (M2b). The supernatants were analyzed in radioimmunoassay for cortisol, or by ELISA for LCN-2 and IL-10. Alternatively, Bos d 5 was co-incubated with these supernatants, either loaded or emptied from its ligand by dialysis against deferoxamine, as controlled by Prussian Blue staining. 

Results: Prussian Blue staining detected iron in M2b > and M2a, but not in M2c macrophages.  Only IgG4, but not IgG1 immune complexes rendered M2b macrophages capable of secreting significant levels of cortisol, LCN-2 and IL-10. When ligand-emptied Bos d 5 was incubated to the M2b supernatants, it decreased the free levels of cortisol and LCN2. 

Conclusions: Alternatively activated macrophages, are differentially regulated by IgG classes: Only IgG4 is leading to cortisol, LCN-2 and IL-10 secretion. Moreover, exogenous unloaded lipocalin allergens may lower the levels of bioavailable cortisol, and LCN-2 and IL-10 release. Our data unravel a novel mechanism of how IgG4, being a hallmark in AIT, is able to regulate M2 macrophages towards a tolerogenic phenotype.