CASP4

A team has discovered a protein that plays critical roles in alerting the body to a bacterial infection and initiating an effective immune response to fight back against the invasion. A team led by researchers at Massachusetts General Hospital (MGH) has discovered a protein that plays critical roles in alerting the body to a bacterial infection and initiating an effective immune response to fight back against the invasion. They found that when certain bacteria are ingested by human immune cells, the protein, called NLRP11, can recognize these bacteria through a portion of their outer coat, filling a previous gap in this recognition pathway. NLRP11 is present in humans but not in mice, which are the most common laboratory model for human infection. The group's discovery, which is published in Science Immunology, may therefore help investigators develop improved "humanized" mouse models of infections and diseases involving the immune system. For the study, the researchers focused on macrophages, which are immune cells that ingest and degrade microorganisms (such as bacteria) and stimulate the action of other immune cells. A genetic screen revealed that macrophages require the NLRP11 gene to become fully activated after ingesting certain bacteria. Mechanistically, the team found that the NLRP11 protein that's produced from the gene enables a macrophage to sense the presence of bacterial lipopolysaccharide molecules within its interior. Upon sensing bacterial lipopolysaccharide molecules, NLRP11 triggers an inflammasome, an intracellular multimeric protein complex that turns on a pathway that activates inflammatory responses to control infection. In this case, NLRP11 leads to activation of an enzyme called caspase-4 that is a component of this inflammasome. These actions by NLRP11 result in an effective defense by the immune system that kills invading bacterial cells and prevents widespread infection in the body. "NLRP11 is present in humans and other primates but absent in mice, likely explaining why it has been missed previously, as most screens looking for innate immune signaling molecules have been carried out in mice," says senior author Marcia B. Goldberg, MD, an investigator in the Division of Infectious Diseases at MGH and a professor in the Departments of Medicine and Microbiology at Harvard Medical School. "Our results demonstrate that NLRP11 is a previously missing link and a component of the human caspase-4 inflammasome activation pathway." Goldberg and her colleagues are conducting even more thorough studies of NLRP11's functions in human macrophages, and they are comparing the differences that result from the presence of NLRP11 in human macrophages and its absence in mouse macrophages. "This may enable the development of mouse macrophages (and mice) that are more similar to human macrophages (and humans) for bacterial infection experiments," she says. Additional co-authors include Maricarmen Rojas-Lopez, María Luisa Gil-Marqués, Vritti Kharbanda, Amanda S. Zajac, Kelly A. Miller, Thomas E. Wood, Austin C. Hachey, and Keith T. Egger. This work was supported by Real Colegio Complutense Fellowship, Real Colegio Complutense at Harvard (MLGM), National Institutes of Health grant R01 AI081724 (MBG), National Institutes of Health grant R01 AI173030 (MBG), National Institutes of Health grant F32 AI145128 (ASZ), National Institutes of Health grant F32 AI126765 (KAM), and National Institutes of Health grant T32 AI007061 (ASZ).

Blocking an immune response-related enzyme holds promise in preventing or treating severe COVID-19 symptoms by reducing inflammation, tissue injury and blood clots in the lungs, new research in mice suggests. Scientists who have long studied this molecules functions in bacterial infections traced development of extensive lung damage in infected mice to heightened levels of the enzyme triggered by the invading SARS-CoV-2 virus. Versions of this enzyme exist and have similar functions in both mice and humans - they're called caspase 11 and caspase 4, respectively. After finding that the molecule is an attractive therapeutic target, researchers are exploring compounds that could safely and effectively block its activation. "The whole idea is if this molecule is not there, the mouse will do better, which means if you target this molecule, then humans should do better," said co-senior study author Amal Amer, professor of microbial infection and immunity in The Ohio State University College of Medicine. The research was published online recently in Proceedings of the National Academy of Sciences. Amer teamed with Ohio State flu and COVID virologist Jacob Yount to look into caspase 11s role in coronavirus infection. Their labs ran a number of experiments comparing COVID infection outcomes in normal mice and mice genetically engineered so they dont produce the enzyme. "From the first experiment, we saw caspase 11 knockout mice had less severe infections and started to recover after only a couple of days," said Yount, associate professor of microbial infection and immunity and co-senior author of the study. Previous research has shown that caspase 11 in mice has many of the same immune-response functions as caspase 4 in humans. In both species, the enzyme is produced upon the onset of an infection. This study supports the notion that what was seen in the mice has relevance to humans: The researchers analyzed nationally available COVID-19 patient data and found that caspase 4 was highly expressed in people hospitalized in the ICU - linking its presence to severe disease. Lung tissue samples from COVID patients also showed high activation of the molecule. The sickest COVID-19 patients develop acute respiratory distress syndrome resulting from the combination of high levels of pro-inflammatory proteins called cytokines, fluid accumulation in air sacs that seeps into lung tissue and blood clots, or thrombosis, caused by damage to cells lining vessel walls. In a series of experiments, the research team found that inhibiting caspase 11 reduced the intensity of multiple effects. They used a version of the SARS-CoV-2 virus that other scientists have engineered specifically to cause disease in mice. (The human coronavirus does not make mice sick.) Among the most striking findings: lower recruitment and inflammatory priming of first-responder cells called neutrophils, white blood cells whose job is to heal wounds and clear away infection - they are important, but have a tendency to perpetuate inflammation that damages tissue and contributes to blood clot formation. A technique used to image the tiniest capillaries in mouse lungs also showed that while the blood vessels in the lungs of normal mice infected with the virus were spotted with clots, the capillaries of mice lacking caspase 11 remained free of thrombosis. "What happens in the lung with COVID can be worse than with other infections. Its amazing that caspase 11 is controlling many of those unique aspects of COVID-19 pathology," Yount said. Amer said this research has opened up new ways of thinking about the enzymes possible role in a host of diseases. Its role in exacerbating lung damage in COVID-19 was an unexpected finding - the activation of caspase 11 and caspase 4 in bacterial infections has a protective function, setting up immune cells to kill bacterial pathogens. Caspase 11 is known to need the help of a specific protein called gasdermin-D to ward off bacterial infections, but this work showed the enzyme intensified lung damage in COVID-19 infection without making use of gasdermin-D. The link to blood clotting also suggests caspase 11s effects in the presence of infection probably dont stop with the lungs, and may affect disease conditions in the heart, brain and elsewhere in the body. "We discovered caspase 11 has other pathways, and we are looking at the function of caspase 11 in all of the types of cells that cause thrombosis," she said. In the meantime, Amer's lab is already testing a caspase 11 blocker that she believes has potential to become a human drug candidate. "This molecule has been found to inhibit thrombosis, inflammation and secretion of cytokines, and it also inhibits caspase 11," she said. "No one had put together that inhibition of caspase 11 has an effect on these downstream problems. This caspase inhibitor may save the day." Eltobgy MM, Zani A, Kenney AD, Estfanous S, Kim E, Badr A, Carafice C, Daily K, Whitham O, Pietrzak M, Webb A, Kawahara J, Eddy AC, Denz P, Lu M, Kc M, Peeples ME, Li J, Zhu J, Que J, Robinson R, Rosas Mejia O, Rayner RE, Hall-Stoodley L, Seveau S, Gavrilin MA, Zhang X, Thomas J, Kohlmeier JE, Suthar MS, Oltz E, Tedeschi A, Robledo-Avila FH, Partida-Sanchez S, Hemann EA, Abdelrazik E, Forero A, Nimjee SM, Boyaka PN, Cormet-Boyaka E, Yount JS, Amer AO. Caspase-4/11 exacerbates disease severity in SARS-CoV-2 infection by promoting inflammation and immunothrombosis. Proc Natl Acad Sci U S A. 2022 May 24;119(21):e2202012119. doi: 10.1073/pnas.2202012119