Infiltration of M2-polarized macrophages in infected lymphatic malformations: possible role in disease progression

Abstract

Background: Lymphatic malformations (LMs), slow-flow vascular anomalies resulting from abnormal development of lymphatic channels, often progress rapidly after trauma or infection. Objectives: To explore the possible mechanism by which local infection promotes the progression of LMs. Methods: Immunohistochemistry in serial sections and immunofluorescence were performed to label polarized macrophages. Tertiary lymphoid organs (TLOs) in LMs were identified using antibodies against CD3 (a T-cell marker), CD20 (a B-cell marker) and PNAd (a high endothelial venule marker). Pearson's correlation and cluster analysis were carried out to delineate the relationship between macrophage infiltration and TLO formation. Rat models of LM were established to examine the role of lipopolysaccharide in LM development. Results: Compared with normal skin tissues, both M1- and M2-polarized macrophages were prevalent in LMs. Moreover, M2-polarized macrophages were significantly increased in infected LMs with an elevated density of TLOs. M2-polarized macrophages were observed in the centre of TLOs accompanied by intensive staining of macrophage colony-stimulating factor, a strong chemotactic factor for monocytes/macrophages, suggesting that macrophages might be recruited through TLOs. Cluster analysis and Pearson's correlation suggested a close relationship between macrophage infiltration and TLO formation. Furthermore, the expression of CD68 was also correlated with that of vascular endothelial growth factor (VEGF)-C and Ki67. Importantly, in an established LM rat model, lipopolysaccharide promoted the progression of the malformations with increased macrophage infiltration and TLO formation. Conclusions: M2-polarized macrophages that may be recruited through TLOs in infected LMs may contribute to the progression of the disease by secreting VEGF-C, and therefore accelerating the proliferation of lymphatic endothelial cells.