Macrophage: SHIP of Immunity

Immunology. Why does it exist? Two words. Cure disease. People get diseases. “Test tubes” do not. People fund immunologists for solutions to their health problems. But, immunologists often study leukocytes in test tubes – the laboratory – away from diseases. Why? Because much can be learned from analyzing cellular biochemistry and behaviors in vitro that cannot be ascertained when leukocytes are in animals. At the same time, isolated leukocyte reactions often do not reflect how the immune system operates as a unit. So, it is critical to verify in vitro observations in vivo. Among leukocytes, macrophages are the central initiating and directing element in immune systems, and serve this role through four basic “SHIP” functions in vivo: Sample; Heal; Inhibit; and Present (antigen) (1–4). The polar-opposite functions of Heal (M2-type) and Inhibit (M1-type) can have profoundly different effects on host survival, and require unique and major changes in macrophage metabolism and physiology. In turn, macrophage populations are necessarily heterogeneous as they adapt to protect hosts in different ways: they exhibit “plasticity.” Some have focused on measuring ever-expanding lists of cell surface or various other “markers” (mostly in vitro) to try and sub-type macrophages. But, the “heterogeneity” created by such studies can be “illusory” because there are many more markers than there are functions (e.g., M1/inhibit and M2/heal). Thus, it is important to focus on classifying macrophages by functions, such as SHIP, to navigate through a “sea of plasticity.” And, thereby realize the enormous potential of macrophages/innate immunity for improving health.