In contrast with proinflammatory M1 cells, alternatively activated macrophages express cytokines and receptors that are implicated in inhibiting inflammation and restoring homeostasis. This includes production of IL-10 to downregulate inflammatory cells, extracellular matrix protecting proteins like YM1, ornithine, and polyamines for wound repair, and higher levels of receptors associated with phagocytosis, such as scavenger receptors [18]. Just as the Th1 cytokine IFNγ has been associated with induction of proinflammatory M1 macrophages, the Th2 cytokine IL-4 has been associated with M2, or alternative, activation. In the periphery, M2 cells are not always associated with protective functions. In addition to parasite protection, wound repair, and debris clearance, these cells are also potential key players in asthma and allergy responses [7]. However, these types of harmful reaction may not be relevant in the CNS, demonstrating a divergence between peripheral and central cells. Interestingly, it appears that when there is a lack of M2 cell differentiation in the CNS, problems can arise (this is discussed in detail later).
