Macrophage polarization in bacterial infections.

Converging studies have shown that M1 and M2 macrophages are functionally polarized in response to microorganisms and host mediators. Gene expression profiling of macrophages reveals that various Gram-negative and Gram-positive bacteria induce the transcriptional activity of a "common host response," which includes genes belonging to the M1 program. However, excessive or prolonged M1 polarization can lead to tissue injury and contribute to pathogenesis. The so-called M2 macrophages play a critical role in the resolution of inflammation by producing anti-inflammatory mediators. These M2 cells cover a continuum of cells with different phenotypic and functional properties. In addition, some bacterial pathogens induce specific M2 programs in macrophages. In this review, we discuss the relevance of macrophage polarization in three domains of infectious diseases: resistance to infection, infectious pathogenesis, and chronic evolution of infectious diseases.