A kind of white blood cell generally associated with immunological reactions to foreign particles may have the ability to remove liver cells that have undergone apoptosis, a regulated cell death process.
The research, which was published in eLife as a Reviewed Preprint, provides what the editors call “persuasive evidence” that neutrophils destroy liver cells going through apoptosis by engulfing and devouring them, a process they refer to as “perforocytosis.” The results may have ramifications for brand-new therapeutic strategies to address human autoimmune liver illness (AIL), which may be brought on by a lack of neutrophils.
A class of immune cells known as phagocytes removes billions of apoptotic cells per day from adults. Around 50–70% of the white blood cells in humans are neutrophils, which are a subtype of phagocyte. They were commonly believed to be prohibited from apoptotic cells, unlike other phagocytes, since they cause inflammation that can harm neighbouring healthy cells and tissues. The most recent discoveries now refute that presumption.
According to co-lead author Luyang Cao, Associate Investigator in the Department of Neurosurgery, State Key Laboratory of Biotherapy and Cancer Center, West China Hospital, Sichuan University, China, “Apoptotic cells are well characterised but they are not often found within human samples, possibly because they are removed so efficiently by phagocytes.” This implies that the precise phagocytes in charge of eliminating apoptotic cells are yet unknown, and it is uncertain if they are exclusive to particular human tissues.
Cells from the liver tissue of individuals with tumours brought on by hepatocellular carcinoma or hepatic hemangioma were acquired by the research team in order to identify the phagocytes responsible for eliminating apoptotic cells in the liver. For the purpose of determining whether cells in the sample were apoptotic, they used two distinct staining methods.
The researchers found that up to 22 neutrophils were present in each of the 281 apoptotic liver cells from the livers of 32 individuals. The removal of apoptotic liver cells has historically been attributed to a class of phagocytes known as Kupffer cells, however the researchers discovered that there were relatively few Kupffer cells in the samples. They consequently proposed that the procedure they dubbed perforocytosis—the elimination of deceased liver cells—was carried out mostly by neutrophils. This differs from the typical method used by the majority of other phagocytes, which is to swallow apoptotic cells.
The scientists used intravital microscopy, a live imaging technology, to see the process in mouse livers in order to validate the mechanism by which neutrophils remove apoptotic liver cells. Intravital microscopy enables the live observation of biological processes inside living animals. They used an anti-Ly6G antibody to mark neutrophils and a protein called Annexin V to label liver cells. The scientists saw that neutrophils penetrated and eliminated dead liver cells in the mice, which is consistent with their results in human samples. The procedure was quick and harsh; it took four to seven minutes for the dead cells to be entirely digested.
According to co-corresponding author Hexige Saiyin, Assistant Professor in the State Key Laboratory of Genetic Engineering, School of Life Sciences, Fudan University, China, “Our discovery of neutrophils burrowing into and clearing out apoptotic liver cells helps to solve some of the mysteries surrounding the apoptotic clearance process.”
The researchers then aimed to determine if decreasing the number of neutrophils in mice had an effect on the removal of liver cells that have undergone apoptosis. The proportion of apoptotic cells was much greater in a sample of liver cells from neutrophil-depleted animals than in normal mice (0.92% vs. 0.2%), indicating that neutrophil depletion hinders the removal of apoptotic cells. Moreover, they observed the existence of additional phagocytes in the neutrophil-depleted animals, suggesting that these phagocytes served a compensating function in the absence of neutrophils.
Autoimmune disorders, such as AIL, are often associated with the improper clearance of apoptotic cells. The scientists observed an increase in autoantibodies in the neutrophil-depleted animals, which are immune cells that wrongly target the body’s own healthy cells rather than outside invaders like viruses or bacteria. Antibiotic treatments had little effect on this rise, which only appeared in animals without neutrophils and not in mice lacking other phagocytes. According to this, neutrophil depletion is linked to defective liver cell apoptosis clearance and, therefore, to the production of autoantibodies that may cause AIL illness. By analysing biopsy samples from people who had the AIL condition, the team was able to confirm this conclusion. Once again, they discovered that the neutrophil-mediated clearance of apoptotic cells was compromised in each patient.
The method and importance of perforocytosis, as well as if perforocytosis happens in organs other than the liver, are all things that need more study, according to the scientists. How to use this recently discovered apoptotic clearance mechanism in the therapeutic management of AIL is a crucial next step.
Further understanding of the crucial role that neutrophils play in apoptotic clearance may have significant implications for the treatment of inflammatory and autoimmune illnesses, since the inability to eliminate dead cells is associated to these conditions. Recently, we screened and discovered a number of drugs that significantly increased neutrophil perforocytosis and showed excellent therapeutic benefits to treat AIL in animal models. Jingsong Xu, senior author and former principal investigator at the Department of Neurosurgery, State Key Laboratory of Biotherapy and Cancer Center, West China Hospital, Sichuan University, comes to a conclusion (current address: Department of Pharmacology, Center for Lung and Vascular Biology, University of Illinois, Chicago).