Regardless of how it arose, is there a problem with the current M1/M2 model? For one, the concept is an in vitro construction that relies on stimulating macrophages in culture with a defined set of factors. One view posits that, although an oversimplification, this in vitro construction nevertheless provides a useful guide for thinking about in vivo biology. The problem with this perspective is that macrophages taken out of their native environments and placed in culture change dramatically: after a 7-day incubation period, cultured microglia and peritoneal macrophages completely lose their tissue-specific gene expression programs. 39 To conclude anything from these in vitro settings is to ignore the obvious: these are different cells. A second argument in favor of the M1/M2 paradigm acknowledges that it is an in vitro construction but insists the macrophage spectrum, with M1 and M2 as its polarized extremes, does exist in vivo. If this is true, then knowing something about M1 versus M2 activity in vitro would be useful in the same ways that in vitro experiments are useful: reductionism and standardization. Setting aside the obvious limitations of such use, relying on the M1/M2 spectrum model remains a perilous proposition. First, as noted above, macrophages placed into culture change dramatically and, thus, may no longer resemble anything that exists in vivo. Second, a spectrum is an array ordered according to the magnitudes of certain properties (consider, eg, a spectrum of light). A spectrum requires intermediates that bridge the 2 extremes. In macrophage biology, we have little evidence for an all-encompassing spectrum. We do have evidence for a stimulus-dependent activation macrophage network. Transcriptional profiling of human macrophages, for example, identified a broad transcriptional repertoire that challenges the M1/M2 paradigm. In vitro culture of human monocytes with macrophage colony-stimulating factor (M-CSF) or granulocyte macrophage colony-stimulating factor (GM-CSF), followed by activation with diverse stimuli, revealed considerable deviation from the M1/M2 axis, an insight that should be particularly relevant to investigators studying macrophage biology in cardiovascular disease because free fatty acids and high-density lipoprotein molecules were among such stimuli. 40 In other words, a macrophage encountering a stimulus relevant to cardiovascular disease produces mediators that lie outside the M1/M2 spectrum. What needs to be emphasized is that departure from the M1/M2 framework depends on the stimulus. One wonders how many other polarization states, beyond the nine that were identified, exist with additional stimuli or with macrophages isolated from specific organs (ie, beyond M-CSF–and GM-CSF–generated monocyte-derived macrophages).

Should we abandon the M1/M2 paradigm altogether? Beyond the reasons already mentioned, the reductive M1/M2 model arguably stifles, rather than enables discovery. A typical experiment might involve profiling macrophages isolated from the aortas of 2 different groups of mice with atherosclerosis. The investigators might measure a cassette of transcripts that