Macrophages: plastic solutions to environmental heterogeneity

Macrophages are among the oldest cell types in the animal kingdom, and they have a long evolutionary history and experience various evolutionary pressures. It was clear from the earliest studies that variations exist in macrophage populations. Macrophages are known to adapt to their microenvironment. Although the paradigm for macrophage plasticity is their flexible program driven by environmental signals, the most common working hypothesis is that of a dichotomy between two major macrophage phenotypes, M1 and M2. A PubMed and Web of Science databases search was performed providing evidences that numerous authors have expanded the concept of plasticity and conducted experimental studies focusing on the complex program of phenotypes. This review evaluated a number of issues relating to macrophage plasticity, environmental heterogeneity and the potential for changes to be reversal or non reversal in an ecological context. The ecological principles of phenotypic plasticity which can assist in evaluating and interpreting macrophage experimental data are discussed as well.