Editors' ChoiceImmunology

Switching Macrophage Responses

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Science Signaling  28 Jul 2009:
Vol. 2, Issue 81, pp. ec252
DOI: 10.1126/scisignal.281ec252

Macrophages exist in at least two functionally distinct states that are triggered in response to different stimuli: M1 macrophages are involved in proinflammatory responses [characterized by proinflammatory cytokine release, such as interleukin-1β (IL-1β), and production of reactive oxygen species (ROS)], and M2 macrophages are involved in resolution of inflammation (characterized by production of scavenger receptors and IL-1 receptor antagonist, and decreased production of proinflammatory cytokines, including IL-1β). To better understand these differences in macrophage functional states, Pelegrin and Surprenant developed an in vitro protocol that allowed them to define five different macrophage states from M1 through M2, with state 1 corresponding to M1 cells, state 5 corresponding to M2 cells, and state 3 corresponding to cells equally intermediate between M1 and M2 (M1/M2 cells). With these five-state cells, they showed that M1 cells responded to extracellular ATP with activation of the NALP3 inflammasome complex and caspase-1–mediated processing and release of mature IL-1β. In M1/M2 cells, ATP no longer stimulated IL-1β processing and release, despite ATP stimulating its receptor, the calcium ion channel P2X7R in M1, M1/M2, and M2 cells. In contrast, Escherichia coli stimulated IL-1β release regardless of cell state. The change in IL-1β production in response to ATP was not due to decreased abundance of unprocessed intracellular IL-1β, nor due to decreased abundance of P2X7Rs. ATP also inhibited the processing and release of IL-1β in M1/M2 cells in response to the toxins nigericin and maitotoxin (MTX), which activate the NALP3 inflammasome independently of P2X7Rs. ATP inhibited MTX-stimulated or E. coli–stimulated IL-1β processing and release when the P2X7R was blocked pharmacologically in M1 cells or in M1 cells genetically deficient in P2X7R. Thus, in the absence of P2X7R signaling, ATP fails to stimulate IL-1β processing and even has an inhibitory effect on IL-1β processing triggered by other mediators. The inhibitory response was replicated by application of pyrophosphate (PPi) or the nonhydrolyzable bisphosphonate clodronate, but not by adenosine, suggesting that ATP hydrolysis to produce PPi was mediating the inhibitory response. ATP triggered differential changes in the actin cytoskeleton in different state cells, and PPi mimicked the effects of ATP in M1/M2 and M2 cells, producing intense intracellular actin clustering and preventing actin polymerization in the cell periphery. In addition to different responses in IL-1β processing and release, M1 cells responded to ATP with an increase in ROS, but M2 cells exhibited a delayed and reduced response compared with M1 cells. In P2X7R-deficient M1 cells, but not in wild-type M1 cells, ATP, PPi, or clodronate blocked MTX stimulation of ROS in M1 cells. Thus, the authors propose that the P2X7R becomes uncoupled from the inflammasome during the transition from M1 to M2 and that ATP signals through PPi to influence actin organization and inhibit ROS production, thereby inhibiting IL-1β processing.

P. Pelegrin, A. Surprenant, Dynamics of macrophage polarization reveal new mechanism to inhibit IL-1β release through pyrophosphates. EMBO J. 28, 2114–2127 (2009).[PubMed]