Finally, it should be noted that while the in vitro-derived M1 and M2 nomenclature is widely used to describe macrophage activation states, these classifications are imperfect and reflect only a subset of states existing on a continuum of macrophage activation (Mosser and Edwards, 2008). Thus, characterizing M1 as ‘bad’ and M2 as ‘good’ macrophages is overly simplistic since both types have important functions and it is likely that an imbalance in their ratios causes pathology, especially if the imbalance is prolonged. Chronic inflammation involving both M1 and M2 macrophages is documented in many CNS diseases and injuries such as Alzheimer's disease, MS, SCI, TBI, and stroke (Colton et al., 2006; Kigerl et al., 2009; Mikita et al., 2011; Hu et al., 2012; Kumar et al., 2013). Therapeutically targeting PPARs may help to ‘re-balance’ these two phenotypes in the injured CNS and promote neuroprotection.
