Molecular and Cellular Impact of M1 and M2 Macrophages on Neuronal Action Potentials

Abstract

Neuroinflammation is an inflammatory response within the brain or spinal cord that may vary within the context of disease, injury or infection. Several factors can contribute to neuroinflammatory disorders such as cytokine and chemokines that are produced and released from peripherally derived immune cells or from locally activated cells such as microglia in the brain. The primary function of these cells is to clear inflammation, however, following inflammation, circulating monocytes are recruited and enter the CNS and contribute to neuroinflammation. Monocyte-derived macrophages, an important component of CNS inflammation, play a pivotal role in mediating neuroinflammatory responses. Macrophages are heterogeneous both in normal and in pathological conditions due to their plasticity and they are classified in two subsets, classically activated (M1) or alternatively activated (M2). There is accumulating evidence suggesting that extracellular vesicles (EVs) released from activated immune cells may play crucial roles in mediating neurotoxicity in the inflamed brain. EVs may act as antigen-presenting vesicles, carry and transfer cytokines and chemokines between cells, stimulate immune responses, and induce tolerogenic effects to suppress or induce inflammation. However, the possible role of EVs released by activated immune cells such as M1 and M2 macrophages in neurotoxicity seen in the inflamed brain is not known. In order to investigate the molecular and cellular impact of macrophages and EVs released from macrophage subtypes on neuronal functions, we established the conditions for the differentiation of monocytic cell lines into M2-like macrophages and characterize their phenotype in the presence of pro-anti-inflammatory cytokines including PMA, TNF alpha, IFN-gamma, LPS, DEX, and M-CSF. Furthermore, we also isolated and characterized EVs from M1 and M2 macrophages and observed no significant changes in their size and numbers. Furthermore, we treated primary neurons with M2 derived EVs and found a significant reduction in the action potential of neurons when exposed to those EVs. Collectively, these data suggested that M1 and M2 macrophages may possess differential neurotoxic effects mediated by EVs released by monocytic cells in the concept of neuroinflammation.