Extrinsic and intrinsic control of macrophage inflammatory responses.

Macrophages make major contributions to inflammatory immunopathology. In this work, we examine three disease scenarios, in which M1s play a major role early in the disease but eventually transitions into a population of cells with immunoregulatory activity. We propose that the transition from an inflammatory to a regulatory phenotype is a natural progression that regularly occurs in stimulated macrophages and that the timing of this transition is critical to maintaining homeostasis. In the first section of this review, we discuss the exogenous microenvironmental cues that may induce macrophages to enter a regulatory state. In the second half of this review, we discuss a novel mechanism, whereby TLR-stimulated macrophages can intrinsically induce their own regulatory activation state. They do so by secreting and synthesizing endogenous "reprogramming" signals that work in an autocrine fashion to promote a regulatory phenotype. We propose that these endogenous regulatory mechanisms exist to prevent macrophage-mediated immunopathology. Thus, macrophages can respond to endogenous and exogenous cues to regulate their activation state, and without these controlled regulatory responses, M1 would persist to the detriment of the host.