Role of brain-infiltrating macrophages in collagenase-induced intracerebral hemorrhage

Abstract

Intracerebral hemorrhage (ICH) is one of the major causes of stroke, constituting 10% of all stroke cases. Upon ICH, the primary mechanical insult by increasing volume of hematoma is followed by massive infiltration of macrophages in the perihematoma regions. Recently, it has been reported that the function of macrophages is distinct depending on their activation type

M1, or classical activation, leads to pro-inflammatory responses, while M2, or alternative activation, contributes to tissue repair or healing. Since the function of brain-infiltrating macrophages after ICH has not been elucidated, I characterized the phenotype of macrophages infiltrating brain parenchyma and investigated their role in ICH. Upon ICH injury, extravasated macrophage number increased in perihematomal region. To investigate the role of these brain-infiltrating macrophages, I depleted peripheral monocytes by i.p. clodronate liposome injection. In the monocytes-depleted mice, the ICH-induced neurological deficits were more severe compared to those of control mice, indicating a protective role of macrophages in ICH injury. The mRNA expression of Arginase-1 (an M2 marker) was upregulated in the ICH-injured brain, while iNOS (an M1 marker) expression was not significantly altered. In flow cytometry, mannose receptor (an M2 marker)-expressing macrophages increased at a delayed time point after ICH. The M2 polarization of the brain-infiltrating macrophages suggested that the brain microenvironment around macrophages may affect macrophage activation. To explore such possibility, bone marrow derived macrophages (BMDM) were co-cultured with mouse brain mixed glia cells (MBMG) and then tested for their activation phenotype. Upon co-culture with MBMG, the number of mannose receptor-positive BMDMs was significantly increased, which suggests that glia cells can induce macrophage M2 polarization. Furthermore, treatment with MBMG conditioned media increased the number of mannose receptor-expressing BMDM and the mRNA expression of Arg-1 and Ym1. Taken together, these data suggest that brain-infiltrating macrophages, after ICH, are differentiated to the M2 phenotype by brain glial cells, and thereby contribute to the recovery from ICH injury.