platelet factor 4
Posted on by Dr. Francis Collins
Spiders spin webs to catch insects for dinner. It turns out certain human immune cells, called neutrophils, do something similar to trap bacteria in people who develop sepsis, an uncontrolled, systemic infection that poses a major challenge in hospitals.
When activated to catch sepsis-causing bacteria or other pathogens, neutrophils rupture and spew sticky, spider-like webs made of DNA and antibacterial proteins. Here in red you see one of these so-called neutrophil extracellular traps (NETs) that’s ensnared Staphylococcus aureus (green), a type of bacteria known for causing a range of illnesses from skin infections to pneumonia.
Yet this image, which comes from Kandace Gollomp and Mortimer Poncz at The Children’s Hospital of Philadelphia, is much more than a fascinating picture. It demonstrates a potentially promising new way to treat sepsis.
The researchers’ strategy involves adding a protein called platelet factor 4 (PF4), which is released by clot-forming blood platelets, to the NETs. PF4 readily binds to NETs and enhances their capture of bacteria. A modified antibody (white), which is a little hard to see, coats the PF4-bound NET above. This antibody makes the NETs even better at catching and holding onto bacteria. Other immune cells then come in to engulf and clean up the mess.
Until recently, most discussions about NETs assumed they were causing trouble, and therefore revolved around how to prevent or get rid of them while treating sepsis. But such strategies faced a major obstacle. By the time most people are diagnosed with sepsis, large swaths of these NETs have already been spun. In fact, destroying them might do more harm than good by releasing entrapped bacteria and other toxins into the bloodstream.
In a recent study published in the journal Blood, Gollomp’s team proposed flipping the script . Rather than prevent or destroy NETs, why not modify them to work even better to fight sepsis? Their idea: Make NETs even stickier to catch more bacteria. This would lower the number of bacteria and help people recover from sepsis.
Gollomp recalled something lab member Anna Kowalska had noted earlier in unrelated mouse studies. She’d observed that high levels of PF4 were protective in mice with sepsis. Gollomp and her colleagues wondered if the PF4 might also be used to reinforce NETs. Sure enough, Gollomp’s studies showed that PF4 will bind to NETs, causing them to condense and resist break down.
Subsequent studies in mice and with human NETs cast in a synthetic blood vessel suggest that this approach might work. Treatment with PF4 greatly increased the number of bacteria captured by NETs. It also kept NETs intact and holding tightly onto their toxic contents. As a result, mice with sepsis fared better.
Of course, mice are not humans. More study is needed to see if the same strategy can help people with sepsis. For example, it will be important to determine if modified NETs are difficult for the human body to clear. Also, Gollomp thinks this approach might be explored for treating other types of bacterial infections.
Still, the group’s initial findings come as encouraging news for hospital staff and administrators. If all goes well, a future treatment based on this intriguing strategy may one day help to reduce the 270,000 sepsis-related deaths in the U.S. and its estimated more than $24 billion annual price tag for our nation’s hospitals [2, 3].
 Fc-modified HIT-like monoclonal antibody as a novel treatment for sepsis. Gollomp K, Sarkar A, Harikumar S, Seeholzer SH, Arepally GM, Hudock K, Rauova L, Kowalska MA, Poncz M. Blood. 2020 Mar 5;135(10):743-754.
 Sepsis, Data & Reports, Centers for Disease Control and Prevention, Feb. 14, 2020.
 National inpatient hospital costs: The most expensive conditions by payer, 2013: Statistical Brief #204. Torio CM, Moore BJ. Healthcare Cost and Utilization Project (HCUP) Statistical Briefs. Agency for Healthcare Research and Quality (US); 2016 May.
Sepsis (National Institute of General Medical Sciences/NIH)
Kandace Gollomp (The Children’s Hospital of Philadelphia, PA)
Mortimer Poncz (The Children’s Hospital of Philadelphia, PA)
NIH Support: National Heart, Lung, and Blood Institute