Neutrophils are the white blood cells protecting us against infections. They are commonly considered to be the “soldiers” going from blood to tissues, in a disciplined way, and they die on the battlefield to protect us from microbes. Current understanding of neutrophils, and in particular their migration, is the underlying knowledge behind the widespread use of the absolute neutrophil count as a clinical laboratory test. However, enabled by new technologies for more precise measurements of neutrophil migration, a picture much more complex and nuanced is emerging during various disease processes. Precise characterizing of neutrophil functions could help appreciate the limits of neutrophil counting and the contribution of neutrophils to health and disease processes. Using new microfluidic tools, MGH investigators are learning more about how neutrophils break the rules that they would follow under normal healthy conditions.
Read MoreRule #1: Neutrophils reaching chemoattractant targets stay with the target
One myth about neutrophils is that once they entered a tissue, they do their job and then die, and are removed by macrophages. However, recent reports in zebra fish and mice showed that neutrophils can return into circulation after traveling to targets inside tissues. To test if human neutrophils are capable of the same behavior, MGH investigators probed the migration of human neutrophils through U-shaped channels with chemoattractant at the tip of the U. ► View the unique motility pattern of human neutrophils. To their great surprise, the investigators found that close to 100% of human neutrophils, after chemotaxing towards a source of chemoattractant, can reverse direction of migration and move away from the chemoattractant (a process now called retrotax). With these devices, MGH investigators are the first to show that human neutrophils could retrotax for distances longer than one millimeter (200 times the body length of a neutrophil). The microfluidic tools developed at MGH are the first in vitro tools to systematically study this novel neutrophil phenotype and identify conditions that modulate retrotaxis. These tools could be useful for studying the details of acute and chronic inflammatory processes, and developing new interventions to control these processes in health and disease.
Rule #2: Neutrophils only start moving when the green light of chemokine gradient is ON
Neutrophils are usually very disciplined. The cells flow with the blood and do not enter tissue unless a chemokine signal is present. A chemical gradient is the essential condition for neutrophil migration. Any persistent migration of neutrophils over long distances (hundreds of microns) and for extended periods in one direction (tens of minutes) is commonly associated with the presence of chemoattractant gradients. However, MGH investigators have recently shown that neutrophils can migrate spontaneously in the absence of any chemoattractants. ► View the spontaneous migration of neutrophils. The investigators recently discovered that the spontaneous migration of human neutrophils (in the absence of a chemoattractant) can be useful for diagnosing and monitoring sepsis in patients with burn injuries.
Rule #3: Neutrophils are always in top shape and rarely break down
Neutrophil migration is a robust function, and genetic, permanent defects of neutrophil migration are very rare. Less than 20 people in the U.S. live with defective neutrophils (for example, chronic granulomatous disease). In fact, the ubiquitous use of the absolute neutrophil count in clinics around the world is based on the implicit assumption that the majority of neutrophils are fully functional most of the time. MGH investigators have designed microfluidic tools that are capable of making the most precise measurements of neutrophil migration to date. By studying the speed, persistence, and directionality of neutrophil migration, investigators found that specific migration signatures exist in response to various chemokines.
Investigators compared neutrophils from healthy donors (► view) with neutrophils from patients after major burns (► view) and found that the migration of neutrophils is impaired for weeks after the injury. It is likely that the behavior of neutrophils after burn injury represents just the tip of the iceberg and changes in neutrophil migration speed, directionality, and persistence likely exist in other diseases that have a significant inflammatory component. To address the logistic challenges in the handling and processing of blood samples for neutrophil measurements, the MGH investigators have recently designed devices that can isolate neutrophils from only one droplet of blood. ► See how this device works.
Rule #4. Neutrophils obey the speed limit
The average speed of neutrophils is about 10 microns per minute for neutrophils responding to chemoattractant gradients, with a broad range of values reported between 0.1 and 40. Using microfluidic devices that confine the migration of neutrophils to channels smaller than the size of the cells, MGH investigators measured the average migration speed of neutrophils from healthy individuals at a consistent 20 microns per minute. To better understand the determinants of human neutrophil migration speed and uncover new ways for restoring the defective migration of neutrophils in patients with major burn injuries, MGH investigators in 2014 organized the first World Cell Race. Relying on model cells that are common in the labs studying the molecular mechanisms of cells migration, the investigators asked scientists from around the world to compete in the race, using any genetic manipulations, doping, or selection tools at their disposal. To compete in the race, the cells had to navigate a maze towards a source of chemoattractant. The winner cells were not the fastest of all nor the smartest, but these were definitely the most efficient overall at navigating the maze along the shortest path, avoiding dead ends, and finding ways around traffic jams behind slower cells. ► Watch the winner cells of the 2014 Word Cell Race.
Relevant publications
Boneschansker L, Yan J,Wong E, Briscoe DM, Irimia D. Microfluidic platform for the quantitative analysis of leukocyte migration signatures. Nat Commun. 2014 Sep 3;5:4787. PubMed PMID: 25183261; PubMed Central PMC4155519
Hamza B, Wong E, Patel S, Cho H, Martel J, Irimia D. Retrotaxis of Human Neutrophils during Mechanical Confinement inside Microfluidic Channels. Integr Biol (Camb). 2014 Feb;6(2):175-83. PubMed PMID: 24419464; PubMed Central PMC3928968
Hoang AN, Jones CN, Dimisko L, Hamza B, Martel J, Kojic N, Irimia D. Measuring neutrophil speed and directionality during chemotaxis, directly from a droplet of whole blood. Technology (Singap World Sci). 2013 Oct 2;1(1):49.
PubMed PMID: 24809064; PubMed Central PMC4010229
Kurihara T, Jones CN, Yu YM, Fischman AJ, Watada S, Tompkins RG, Fagan S, Irimia D. Resolvin D2 Restores Neutrophil Directionality and Improves Survival after Burns. FASEB J. 2013 Jun;27(6):2270-81. PubMed PMID: 23430978; PubMed Central PMC3659356
Jones CN, Dalli J, Dimisko L, Wong E, Serhan CN, Irimia D. Microfluidic chambers for monitoring leukocyte trafficking and humanized nano-proresolving medicines interactions. Proc Natl Acad Sci U S A. 2012 Dec 11;109(50):20560-5. PubMed PMID: 23185003; PubMed Central PMC3528552
Butler KL, Ambravaneswaran V, Agrawal N, Bilodeau M, Toner M, Tompkins RG, Fagan S, Irimia D. Burn injury reduces neutrophil directional migration speed in microfluidic devices. PLoS One. 2010 Jul 30;5(7):e11921. PubMed PMID: 20689600; PubMed Central PMC2912851
Ambravaneswaran V, Wong IY, Aranyosi AJ, Toner M, Irimia D. Directional Decisions during Neutrophil Chemotaxis inside Bifurcating Channels. Integr Biol (Camb). 2010 Nov;2(11-12):639-47. PubMed PMID: 20676444; PubMed Central PMC3001269
Contact
Daniel Irimia, M.D., Ph.D.
Massachusetts General Hospital
114 16th Street, Building 114
Charlestown, MA 02129