On macrophage diversity and inflammatory metabolic timers

Key advances Single-cell sequencing revealed unexpected macrophage diversity in all organs examined so far. Major differences were reported in gene expression between human and mouse macrophages in the setting of lung cancer, whereas gene expression in other immune cell populations closely correlated between species. New macrophage-specific Cre deleters have been developed and tested. An unexpected 'timing' mechanism mediated by lactate seems to control macrophage inflammatory transitions. In 2019, single-cell sequencing studies provided important insight into the diverse gene expression profiles of tissue macrophages, and new systems for specifically deleting macrophages were reported. A lactate 'timer' that controls inflammatory responses in macrophages was also described. Key papers were published in 2019 that solidify the notion that macrophage gene expression is remarkably diverse; single-cell sequencing combined with new macrophage-specific genetic tools has provided unexpected insights into the tissue-specific and inflammatory diversity of macrophages. In addition, a new mechanism of inflammatory control of macrophage diversity based on lactate-mediated histone modifications was discovered.