The immune cells known as macrophages are involved in debris cleanup and destruction of potentially harmful cells, among other tasks, but in recent years more attention has been drawn to the important role they play in the complex coordination of cellular activities relating to healing and tissue maintenance. It is even thought that a significant portion of the difference between limited human regeneration and proficient regeneration of the sort observed in salamanders might be explained by differences in macrophage behavior between these species.
Of more immediate practical use, macrophages involved in regenerative processes appear split into a few different classes with distinct behaviors and protein signatures. Although this is a case of arbitrary dividing lines drawn on a continuous spectrum rather than a case of clearly separate camps, it is a still a useful distinction to make. These types are known as polarizations, and the polarizations of interest in this discussion are M1 and M2. Both play important roles in the bigger picture, but M1 macrophages are generally less helpful in regeneration, spurring inflammation and fibrosis, while M2 macrophages are generally more helpful, suppressing inflammation and generating a more supportive environment for regrowth.
Researchers are finding that it is possible to enhance the outcome of regeneration by increasing the ratio of M2 macrophages to M1 macrophages. More M2 macrophages and fewer M1 macrophages produces more rapid, more effective regrowth of tissues, and in some cases induces regrowth that normally doesn’t occur with any reliability in mammals. This has been achieved in animal studies of nerve regeneration and bone healing, to pick a few examples. Interestingly, the cancer research community is interested in turning the dial in the opposite direction, generating more of the aggressive, inflammatory M1 macrophages that destroy cancer cells. As I said, both types have their part to play in the bigger picture.
Moving on to the topic at hand here today, the research below covers the impact of polarization on another part of the macrophage task list, that of debris clearance. Atherosclerosis is a condition in which oxidized, fatty metabolic waste enters the blood stream and sufficiently irritates a section of the blood vessel walls for the cells there to take action. Inflammatory signals draw macrophages that attempt to clean up the garbage, but macrophages are unfortunately unexpectedly frail in the face of this sort of fatty debris. Some ingest too much and either die or become senescent, dysfunctional foam cells, further aggravating the situation. Over time, a small irritated portion of a blood vessel wall swells into a self-perpetuating disaster zone of dead and dying macrophages. Eventually this happens somewhere critical, and driven by the hypertension of aging, a blood vessel wall or the fatty mass inside the vessel ruptures to cause a stroke or heart attack. It turns out that here, as elsewhere, it is the case that adjusting the natural balance towards more M2 and fewer M1 macrophages produces better outcomes, but just how useful this is for human medicine remains to be determined with any great certainty.
A certain immune reaction is the key, not to slowing atherosclerosis as cholesterol-lowering drugs do, but instead to reversing a disease that gradually blocks arteries to cause heart attacks and strokes. The study in mice focuses on reversing the effects of “bad cholesterol,” which is deposited into the walls lining blood vessels in levels influenced by both genetics and a person’s diet. By the fourth decade of life, and thanks to the chronic reaction to cholesterol, most people have inflamed “wounds” in their arteries, called plaques, which when severe enough can rupture to cause blood clots that block arteries. “Even the latest, most potent cholesterol-lowering drugs, PCSK9 inhibitors, let alone widely used statins, cannot fully reverse damage done to arteries over time. We need the next generation of drugs to go beyond cholesterol lowering to address the immune reaction to accumulated cholesterol, and to dismantle plaques as part of reversing or regressing mature disease.”
Once deposited into arteries, bad cholesterol – known to physicians as low density lipoprotein – triggers the body’s immune system, which is meant to destroy invading microbes but can drive inflammatory disease in the wrong context. Immune cells in the bloodstream called monocytes swarm to cholesterol deposits, and become either inflammatory or healing cell types based on signals there. In situations where disease is worsening in a plaque, past studies have shown that monocytes become M1 macrophages that amplify immune responses, increase inflammation, and secrete enzymes that gnaw at plaques until they rupture. The current study confirmed that monocytes arriving in plaques where disease is regressing instead become M2 “healing” macrophages, which dampen inflammation and prevent the ruptures that precede clotting.
When mice were engineered to lose the ability of monocytes to become M2 macrophages, they could no longer achieve normal disease regression. By surgically transplanting plaques from diseased mice into the arteries of healthy mice, the research team brought about dramatic drops in cholesterol levels. This drop has been shown to trigger a second benefit in mice, where monocytes automatically become M2 instead of M1 macrophages as plaques rapidly regress. It is not known whether cholesterol lowering alone triggers this M2 switch in humans, but new imaging techniques may soon be able to detect changes in the type and number of macrophages in plaques. In the meantime, if researchers learn how to boost the M2 switch, a number of clinical applications may become possible just as methods arrive that can measure their success. “A race is underway to develop treatments that enhance the decision of human monocytes to become M2 macrophages in cases where the disease has not yet caused clot formation, at which point it becomes irreversible.”
Using a number of mouse models of atherosclerosis regression, including the aortic arch transplant used in the present study, we have previously shown that aggressive lipid lowering promotes the resolution of plaque inflammation, which is characterized by a decreased content of macrophages and an increase in the level of markers of the M2 state. We now extend these findings to show that plaque regression and the attendant resolution of inflammation surprisingly require the recruitment of new monocytes, which assume the characteristics of M2 macrophages. Furthermore, contrary to the prevailing paradigm, the newly recruited monocytes are drawn from the Ly6Chi circulating subset, generally considered to be “inflammation-prone” precursors of M1 macrophages.
The characteristic rapid reversal of hyperlipidemia in mouse atherosclerosis regression models is likely to reduce the continuous stimulation of the plaque inflammatory response by atherogenic lipoproteins, but clearly is not sufficient for the resolution of inflammation. Based on our results, M2 enrichment must also occur, and how the change in lipoprotein environment causes this to happen also remains to be determined. Our finding that it depends on STAT6-dependent signaling in the newly recruited monocytes suggests that local factors in the regressing plaque stimulate this signaling pathway. STAT6 is activated by two key cytokines, IL-4 and IL-13. However, which of these cytokines is the main player, as well as their cellular source(s), in promoting plaque regression is unclear.
Though many questions remain, the present results provide insights into the dynamic nature of the inflammatory process and the role of Ly6Chi monocytes in plaques. These cells were previously thought to contribute only to plaque progression and inflammation, but are now shown here to be important in regression and inflammation resolution. One clinically relevant insight raised by our studies is that strategies that promote the accumulation of M2 macrophages in atherosclerotic lesions may be a promising approach toward promoting plaque regression, consistent with recent studies in mice in which treatment with IL-13- or IL-4-based therapy was protective against atherosclerosis progression.