Transglutaminase 2 modulates efferocytosis in human monocyte-derived macrophages

TRANGLUTAMINASE 2 MODULATES EFFEROCYTOSIS IN HUMAN MONOCYTE-DERIVED MACROPHAGES S. Eligini1, S. Fiorelli1, M. Brioschi1, C. Banfi1, E. Tremoli1,2, S. Colli2 1Centro Cardiologico Monzino I.R.C.C.S, Milan, Italy;2Dept. of Pharmacological and Biomolecular Science, Università degli Studi di Milano, Milan, Italy ABSTRACT Background Macrophage are heterogeneous both in morphology and function. M1 and M2 phenotypes represent the two extreme of a wide spectrum of activation described in health and in pathological conditions, as atherosclerosis. M1 are pro-inflammatory and have a central role in host defense, while M2 drive non-inflammatory responses and tissue remodeling. We previously reported that human monocyte-derived macrophages (MDMs) spontaneously differentiated in vitro show two dominant morphotypes, spindle and round, that co-exist in the same culture and that display some features typical of M1 and M2 profile. Recently, transglutaminase (TG)2, a multifunctional enzyme expressed in various cell types, has emerged as a novel marker of M2 phenotype, being involved in several M2 functions as phagocytosis of apoptotic cell (efferocytosis). Aim of this study was to establish the occurrence of TG2 in the two different MDM phenotypes and to address its role in some macrophage functions. Materials and methods Mononuclear cells were isolated from venous blood of healthy donors and cultured in medium 199 supplemented with 10% autologous serum for 7 days in the presence or in the absence of cystamine, a competitive inhibitor of TG2 activity. TG2 was determined by immunofluorescence. The uptake of apoptotic Jurkat T cells was detected by flow cytometry whille neutral lipid accumulation was quantified after MDM staining with Fat Red 7B. Results Round MDMs showed significantly higher TG2 labeling and lipid content. TG2 inhibition markedly influenced morphology, efferocytosis, and lipid accumulation: MDMs exposed to cystamine were characterized by a marked predominance of the spindle morphotype, low efferocytotic potential and poor lipid content. Among the potential mechanisms involved, we highlighted the reduced expression of CD14, responsible for the recognition of apoptotic cells, and that of the scavenger receptor SRA that drives lipid uptake. Conclusions TG2 plays a role in efferocytosis and lipid accumulation by MDMs.