CX3CL1/fractalkine is released from apoptotic lymphocytes to stimulate macrophage chemotaxis

Abstract

Cells undergoing apoptosis are efficiently located and engulfed by phagocytes. The mechanisms by which macrophages, the professional scavenging phagocytes of apoptotic cells, are attracted to sites of apoptosis are poorly defined. Here we show that CX3CL1/fractalkine, a chemokine and intercellular adhesion molecule, is released rapidly from apoptotic lymphocytes, via caspase-and Bcl-2-regulated mechanisms, to attract macrophages. Effective chemotaxis of macrophages to apoptotic lymphocytes is dependent on macrophage fractalkine receptor, CX3CR1. CX3CR1 deficiency caused diminished recruitment of macrophages to germinal centers of lymphoid follicles, sites of high-rate B-cell apoptosis. These results provide the first demonstration of chemokine/chemokine-receptor activity in the navigation of macrophages toward apoptotic cells and identify a mechanism by which macrophage infiltration of tissues containing apoptotic lymphocytes is achieved. (Blood. 2008;112:5026-5036) IntroductionWhen apoptosis occurs at high rates in mammalian tissues, apoptotic cells are almost invariably encountered in situ in association with macrophages. 1 These professional scavengers are attracted to the dying cells and engage in their safe, nonphlogistic disposal by phagocytosis. Examples of this innate immune response to dying cells are readily apparent during normal organogenesis, in normal adult tissues, such as the germinal centers of lymphoid follicles, in inflammatory responses, and in pathologic conditions including tumors. The efficient clearance of apoptotic cells by phagocytes is a homeostatic mechanism that militates against histotoxic, proinflammatory, or immunogenic effects that may result from persistence of apoptotic cells. [1][2][3] In recent years, much progress has been made in improving our understanding of the molecular mechanisms underlying the interactions between apoptotic cells and macrophages and the immunologic implications of those interactions. [1][2][3][4][5][6] Before the tethering/ engulfment phases of macrophage-mediated apoptotic-cell clearance, phagocytes are required to navigate effectively to sites of apoptosis. Active release of chemoattractant ("find-me") signals from apoptotic cells at an early stage after engagement of the cell-death program would be predicted to underpin this process, but knowledge of the molecules involved is currently limited. Lysophosphatidylcholine (LPC) is released from apoptotic cells and functions in soluble form as a chemoattractant for mononuclear phagocytes. 7 Significantly, no chemokine family members have previously been implicated in this chemotactic process. Here we show that the chemokine and adhesion molecule CX3CL1, 8,9 also known as neurotactin or fractalkine (FKN), together with its cognate receptor CX3CR1, 10,11 plays an active role in the chemotaxis of macrophages to apoptotic cells. FKN is a type I transmembrane protein, the extracellular portion of which comprises the chemokine domain attached to a mucin stalk. Well known for its roles in inflammatory processe...

Do not let death do us part: ‘find-me’ signals in communication between dying cells and the phagocytes

“…The deficiency of CX3CR1 has also been associated with decreased numbers of macrophages in B-cell germinal centers. 36 Although macrophage recruitment was affected, there was no increase in apoptotic B cells, indicating that the reduced number of macrophages was still capable of efficient clearance. This suggests that several find-me signals may be released by one cell and loss of one may be compensated sufficiently enough by others.…”

“…It has been observed that apoptotic cells can undergo membrane blebbing and release small vesicles during the death process. Truman et al 36 showed that these blebs can participate in monocyte chemotaxis toward apoptotic germinal center B cells. 35 Later, it was shown that Fractalkine/CX3CL1 (FKN) associated with apoptotic microparticles was a chemotactic factor.…”

“…35 Later, it was shown that Fractalkine/CX3CL1 (FKN) associated with apoptotic microparticles was a chemotactic factor. 36 This 90-kDa membraneassociated chemokine gets processed to a 60-kDa form during the early stages of apoptosis (presumed to occur via extracellular proteases) and released in PtdSer exposing microparticles. Although the precise mechanism of release for the microparticle-associated FKN is not known, phagocyte migration was dependent on the chemokine receptor CX3CR1, as CX3CR1-deficient macrophages failed to migrate toward apoptotic B cells.…”

“…36,37 Why the 90-kDa fragment does not confer similar migratory effects during cell death remains to be determined. It is possible that PtdSer found on microparticles could further enhance chemotaxis.…”

Microglia and Monocyte-Derived Macrophages in Stroke

“…Dying cells release signals such as ATP, uridine triphosphate, and CX 3 CL1. CX 3 CR1-expressing mononuclear phagocytes migrate toward the dying neurons that release CX 3 CL1 [143][144][145]. M2 macrophages that highly express CX 3 CR1 are considered to be a major subset that engages in phagocytosis as shown in in vitro neuronmicroglia co-culture and organotypic slice culture [146,147].…”