How immune cells form swarms at injuries and infections [with video]

Which molecules direct neutrophil swarm formation at sites of tissue damage and infection? We employed intravital microscopy of skin and lymph nodes to analyze neutrophil swarming upon sterile tissue injury and infection in mice, provide an initial molecular map of the factors that contribute to the coordinated neutrophil movement and aggregation, and show that “swarm”-like intercellular communication exists between neutrophils in the living tissue.

HFSP Long-Term Fellow Tim Lämmermann and colleagues
authored on Fri, 07 June 2013

White blood cells called neutrophils are key components of the immune system’s defense against infection. These cells also help in wound healing after tissue damage.  How these cells leave the bloodstream to reach sites of infection or damage has been well characterized, but much less is known about the dynamics of neutrophil behavior within tissues. Once outside the bloodstream, individual neutrophils often show strikingly coordinated chemotaxis and cluster formation, reminiscent of the swarming behavior of insects. While a series of sequential phases had been described before, the molecular players that direct each step of the swarming response on the single cell and population level were unknown.

Here we used an advanced microscope to directly look at neutrophil swarms around small burns in the skin or in bacteria-infected lymph nodes.  Our systematic study identifies a lipid called leukotriene B4 (LTB4) that allows neutrophils to communicate with each other in the complexity of an inflamed tissue. LTB4 secreted from neutrophils acutely amplifies local cell death events, attracts other neutrophils from distant tissue regions and thus enhances the radius of highly directed cell recruitment (see Figure). We further show that LTB4, together with other chemoattractants, maintains neutrophil aggregates at the wound site. In these cell clusters, adhesive integrin receptors enable neutrophils to stick to each other and form a tight cellular seal that isolates the wound or infectious site from surrounding viable tissue

Figure: Scheme of neutrophil swarming in inflamed tissues. (1) Upon local tissue injury, neutrophils (green) close to the damage site sense short-range attractant factors (yellow). (2) Cell death (red) occurring at the injury site significantly correlates with amplified neutrophil recruitment from distant tissue regions. (3) Hereby, single neutrophils release the lipid leukotriene B4 (LTB4, blue), which amplifes local cell death signals. LTB4 acts as a signal relay molecule between neutrophils, which (4) enhances the radius of neutrophil tissue recruitment and thus coordinates the neutrophil population response. (5) Accumulating neutrophils form large cell aggregates that displace the surrounding collagen fibers (grey) of the connective tissue. These neutrophil clusters require LTB4 (and other attractants) in combination with adhesive forces (mediated by integrin receptors) to form a tight wound seal, whose borders cease in kinetic concert with the late recruitment of CX3CR1-positive macrophages and monocytes (orange) (6). Adapted from Lämmermann et al., Nature (2013).

Together, we provide the first detailed molecular map of the multi-step process of neutrophil swarming in the extravascular tissue space, along with an experimental strategy for both further dissection of the process and the testing of drug candidates meant to promote or interfere with this process. Our data further reveal how local events can be propagated over large distances in physiologically complex tissues, and how auto-signaling amongst neutrophils produces co-ordinated, self-organized swarming behavior.

Reference

Neutrophil swarms require LTB4 and integrins at sites of cell death in vivo. Lämmermann T, Afonso PV, Angermann BR, Wang JM, Kastenmüller W, Parent CA, Germain RN. Nature. 2013 May 26. doi:10.1038/nature12175.

Pubmed link

Nature link