Are “Resting” Microglia More “M2”?

Over the last decade, the concept and nomenclature of microglial phenotype polarization has been carried over from the peripheral macrophage literature. However, it is not entirely correct to view these two cell types as overlapping. Microglia, although related to macrophages, have several differences and their own unique repertoire of features. In particular, microglia arise from a distinct early yolk sac progenitor population and therefore have different developmental origins than macrophages (1). Furthermore, once in the central nervous system (CNS), microglia are maintained through local self-renewal (2). Under normal conditions, there is no infiltration of peripheral macrophages (3). Microglia also maintain expression profiles distinct from peripheral macrophages (4). This demonstrates that microglia are not simply macrophages that have migrated into the brain; rather, they are a distinct cell type. These ideas have led to a reassessment of microglia activity, leading many researchers to shift their thinking on glial biology in general. One of the more outdated concepts carried over from macrophages is the idea that microglia in the healthy brain exist in a “resting” state. Through a variety of stimuli these “resting” cells can rapidly be “activated,” yielding microglia that are cytotoxic (5). These “activated” microglia were described in and were thought to play a major driving role in many neurodegenerative diseases (6). Thus, microglia “activation” took on a largely negative connotation. Taking cues from ongoing work in peripheral macrophages, this initial concept of microglia “activation” being generally harmful gave way to more specific ideas suggesting there was not just one kind of “activation.” Under the influence of either proor anti-inflammatory cytokines, microglia could be polarized into an inflammatory or anti-inflammatory phenotype, designated classically and alternatively activated microglia, respectively (7). Others have given different designations not solely based on inflammation but from inhibiting vs. healing functions, and label them M1 and M2, respectively (8). It is now becoming clear that the responses of microglia, like macrophages, are heterogeneous: these responses can include proand anti-inflammatory signatures within single cells and small, nearby populations and are driven by the local environment that can supply M1 and M2 polarizing cues simultaneously (9). Therefore, microglial responses are much richer than the dichotomous nomenclature suggests. It is common to represent microglia polarization as a spectrum with each respective phenotype occupying the extremes of the scale (10). However, it is unclear whether the diverse functional responses observed are a product of many different kinds of microglia subsets or simply varying ratios of M1 and M2 microglia. While there is much work that details this spectrum of activation, and much remains to be investigated, an interesting but overlooked area is the phenotype of the “resting” microglial cell. Classically, those who study microglial function in a healthy, normal brain are not the same groups that study microglia in an “activated” or disease setting. However, it appears that the microglia in each of these settings may be more similar than originally thought. The notion that microglia are truly “resting” has long been cast aside. The advent of in vivo techniques, in particular 2-photon microscopy, has revealed the constant surveillance and activity of microglia, even in the absence of traditional activation signals (11). Therefore, it might be incorrect to view microglia in the normal, healthy CNS as a distinct population that can become “activated.” Rather, it seems likely that even at baseline, microglia are already on the activation spectrum. So the question arises, where on the spectrum are they? Inflammation in the brain is typically associated with harmful outcomes. Even acute, low level inflammation can impair synaptic function, leading to cognitive dysfunction and behavioral abnormalities (12). Moreover, neuroinflammation has been recognized as a pathological hallmark in most chronic neurodegenerative diseases (13). This demonstrates the sensitivity of the brain to inflammation and the importance of protecting the CNS from insult. This protective role has generally been assigned to M2 microglia. Many investigators have observed protective effects of M2 cells; such as elevated neuronal survival and process extension after treatment with M2 conditioned media (14), or as reported in numerous papers that detail the beneficial effect of direct treatment with M2 inducing agents (15). However, all of these take place during pathology and do not consider the normal protective function of microglia. Thus, given the critical role of basal microglia in maintaining homeostasis, an attractive hypothesis is that under normal conditions, microglia are skewed toward a protective, anti-inflammatory phenotype. In fact many of the normal functions of microglia are reminiscent of M2 cells, although they are not as prominent as fully polarized cells. Even without stimulation, microglia are vital sources of important, neurosupportive cytokines