When red blood cells break down, they release heme, a component of hemoglobin, that can contribute to inflammation and damage tissues and blood vessels. Researchers at Lund University have now identified a mechanism by which heme activates an inflammatory reaction in the blood. They were also able to block this pathway in experimental models. The findings could have implications for diseases and conditions involving extensive breakdown of red blood cells, including hemolytic uremic syndrome and malaria.
Heme is an essential component of hemoglobin, the protein in red blood cells that transports oxygen. When red blood cells breakdown, a process known as hemolysis, heme can be released into the bloodstream. Once outside the red blood cell heme is harmful and can contribute to inflammation. Hemolysis occurs in conditions such as sickle cell anemia, malaria, trauma and burns, but also when blood is exposed to mechanical stress, such as during cardiopulmonary bypass or through contact with mechanical heart valves.
In the new study, the researchers show how heme can trigger a chain reaction in the blood that activates inflammation making blood vessels more permeable. They found that heme binds to and activates the protein factor XII, which activates the kallikrein-kinin system, an inflammatory protein system in the blood. This leads to the release of bradykinin, a molecule that causes blood vessels to dilate and become more permeable. As a result, fluid leaks into surrounding tissues and contributes to swelling, inflammation and a drop in blood pressure.
"This explains how fragmented red blood cells can activate severe inflammation. Free heme can trigger an inflammatory response regardless of what caused the red blood cells to fragment in the first place," says Diana Karpman, Professor of Pediatrics at Lund University.
The researchers studied blood samples from 34 children with hemolytic uremic syndrome, HUS, as well as samples from healthy controls. HUS is a serious complication that can occur following EHEC infection and is characterized by low platelet levels, acute kidney injury and the breakdown of red blood cells. In some of the children, the researchers found evidence of kallikrein-kinin system activation. Depending on the marker used, evidence of activation was observed in 10 of 34 or 17 of 32 children, respectively. Activation was associated with the extent of red blood cell fragmentation (degree of hemolysis) and kidney involvement. The findings were confirmed in experimental models in which the researchers prevented activation using C1-inhibitor, a drug used for another indication, that inhibits the kallikrein-kinin system.
"The fact that we were able to block the reaction experimentally makes the finding particularly interesting. However, we have not yet shown that such a treatment works in patients. This now needs to be investigated further," says Alexandra Gerogianni, who conducted the study during her postdoctoral research at Lund University.
The findings provide a novel mechanistic explanation for how the breakdown of red blood cells can contribute to inflammation. Although the researchers chose to study HUS, hemolysis occurs in many other diseases and clinical situations. Globally, many people are affected by diseases characterized by extensive hemolysis occurring during the disease process, most notably malaria.
"We chose to study HUS because it is an area we work with, but the implications could be much broader. Free heme is present in many conditions involving hemolysis. An important question going forward is how much this mechanism contributes to inflammation and disease," says Diana Karpman.
A chain reaction in the blood
When red blood cells break down heme is released. Heme binds to and activates factor XII, which in turn activates the kallikrein-kinin system. This leads to the release of bradykinin, which makes blood vessels more permeable and contributes to inflammation. In experiments, the researchers were able to inhibit the reaction using the drug C1-inhibitor.
