Thank you very much for that introduction, and thank you to the organizers for inviting me to speak, as well as all of you who are here on the last day of the conference to learn a little bit about immunology. So like you heard, my name's John Riley. I'm an assistant professor at the University of Pennsylvania, and the topic that I was assigned to talk to you about today is a storm brewing in the lungs, the impact of mechanical ventilation and hypoxia on immunology. Now I'll be honest, when I got assigned this topic, it's a big topic. And I could talk about it for hours. So instead of doing that, I'm going to have about 10 to 15 minutes to talk to you about a high-level view of how what we do with the mechanical ventilator, as well as hypoxia, a really common sign in our ICUs, impacts immunology. The only disclosures I have are that I have research funding to study ARDS from the National Institutes of Health and the Department of Defense. I have no other disclosures. So my learning objectives for this talk are to discuss the impact of mechanical ventilation on innate immunity, to describe the concept of biotrauma as a form of ventilator-induced lung injury and how lung-protective mechanical ventilation can reduce biotrauma, and then to identify the impacts of tissue hypoxia on immunology and how by understanding how tissue hypoxia affects immunology, we might identify targets for future therapeutics. So I'll start by talking about mechanical ventilation. So the advent of mechanical ventilation was one of the best advances in our field of critical care medicine in the late 1800s. And the technology compared to the old iron lung of negative pressure ventilation has advanced remarkably. In the 1950s, during the polio epidemics, we transitioned from negative pressure ventilation to positive pressure ventilation. And then this technology advanced further to what we see today, highly adept computers that are able to do significant different modes and measure different pressures and volumes and really led us through the most serious ill patients in the COVID-19 pandemic. However, even from those early days of the iron lung, there was recognition that the mechanical ventilator can cause harm. And there's this concept of ventilator-induced lung injury that I think everyone in this room has heard of before that was really studied aggressively in the 1980s and 90s before we developed the open lung concept and lung-protected mechanical ventilation. And people break out the types of ventilator-induced lung injury into four different categories, although there's significant overlap, volume trauma, adalectotrauma, barotrauma. And then there's this more nebulous concept of biotrauma, where there's this idea that the mechanical ventilator itself through repetitive stretch of the alveolar capillary membrane actually induces an inflammatory response that impacts lung injury and multisystem organ failure. So the thought is this biotrauma causes excess lung inflammation, and that in its own right can cause acute lung injury, or in patients with ARDS, it can worsen their lung injury. But because the right side of the heart pumps the entire volume of blood through the lungs, all of that excess lung inflammation can then spill over into the systemic system to systemic inflammation, resulting in multisystem organ failure. So why do we think that there's this concept of biotrauma? Well, there are really excellent studies that were conducted in the 1990s, first in preclinical models and then in human models. This is some data from old studies of rat models of ARDS. So these rats had intertracheal administration of LPS, and then they were mechanically ventilated for several hours using one of four different strategies. The lungs were then lung lavage cytokine measurements were then made in the different ventilator strategies and compared across them. And you can see here the control strategy is low tidal volume, low PEEP. Then there's a moderate tidal volume, high PEEP strategy, moderate tidal volume, ZEEP, or zero PEEP strategy, and then high tidal volume in ZEEP. And there's an increase in these inflammatory mediators and cytokines as you go to more lung injurious mechanical ventilation modes with higher tidal volumes and lower PEEP, suggesting that the actual mode that you use in the ventilator influences the innate immunity. Now after all these studies in the 90s, we all had the ARMA study. And I think most people in the room know about the ARDS network ARMA study. But this was a randomized controlled study that compared 6 cc's per kilo of ideal body weight tidal volumes to 12 cc's per kilo of ideal body weight and identified an approximately 9% reduction in mortality. Now in a post hoc analysis of this trial, there was blood that was collected both at baseline and day three for enrollment in the trial. And they looked at levels of three cytokines, IL-6, IL-8, and IL-10, based on whether the patient survived or not. And not surprisingly, I think people know in this room that these cytokines are strongly associated with survival in patients with ARDS and were in the ARMA trial. But much more interestingly is if you look at the cytokines by ventilator strategy. So remember, this is a randomized controlled trial. So confounders should be balanced. And if you look at the baseline levels of IL-6, IL-8, and IL-10 in the two arms, 6 cc's and 12 cc's, they were approximately equal at baseline. But then if you look at day three, the low tidal volume ventilation group had a more substantial reduction in these inflammatory mediators, suggesting just by changing the ventilator mode and avoiding ventilator-induced lung injury, we can quell the inflammatory cascade. So the thought is that biochemical and biophysical injury, like shear, overdescension, and cyclical stretch, and I'm very much so oversimplifying this, leads to cytokine production. But there's also a whole host of research that it increases permeability of the alveolar capillary membrane, results in reactive oxygen species generation, complement activation, and damage-associated molecular patterns releasing, which causes injury. And these are all pathways that are being studied in acute hypoxic respiratory failure as potential therapeutic targets. So lung protective low stretch strategy appears to reduce biotrauma. I haven't shown you data, but there's also data that there's further evidence that proning also reduces biotrauma by reducing ventilator-induced lung injury. Now future strategies that we're developing to hone our ventilator protection strategies by limiting ventosynchrony, improving lung protection by different PEEP and driving pressure approaches, should closely evaluate the effects on immunity. Because this is probably one of the mechanisms by which we propagate lung injury. I'll now switch gears from something we do to patients, the therapy we do to patients, to something the syndromes we take care of, a sign that the syndromes we take care of causes on patients. And that's hypoxia. So hypoxia, as all of you know, is a defining feature of the acute respiratory distress syndrome. So it's important. We think it's important. ARDS is acute onset diffuse pulmonary edema, severe hypoxemia that's unexplained by cardiac dysfunction. And I think we can all agree that ARDS is an extremely inflammatory process, whether it's secondary to sepsis, pancreatitis, trauma. Inflammation is a cornerstone of the pathology of ARDS. And that's for a lot of different reasons. It's pretty complicated. But actually, we're starting to understand more and more how hypoxia is one of the drivers of the propagation of the immune system in ARDS and acute hypoxic respiratory failure. So how do we know this? Well, we do have simple experimental studies looking at hypoxia through high-altitude exposure, where as the oxygen gradient goes down in high-altitude exposure, this causes stress on the healthy body that results in the release of DAMS into the circulating system, as well as cytokines and other inflammatory mediators. And if you repeatedly expose someone to high-altitude exposure, they actually eventually dampen down this response and have immune sensitization. Now there are many pathways that hypoxia induces. And I can't cover all of those pathways in this talk. But probably the best-studied pathway is hypoxia-inducible factor signaling, or HIF signaling. So this is a family of transcription factors that are induced by hypoxia. So what happens, if you can see here on the slide, is in the setting of hypoxia, the HIF1 alpha subunit is released and allowed to go into the nucleus and dimerized with a HIF1 beta subunit. This then binds to hypoxia response elements on DNA and results in the transcription of target genes. And this transcription leads to erythropoiesis, increased metabolism and angiogenesis, and can enhance cytokine activation, VEGF expression, and neutrophil recruitment. There's been a lot of focus on HIF signaling in the oncology literature, where as a tumor grows, the center of the tumor develops hypoxia and uses this mechanism to enhance angiogenesis and enhance growth of the tumor. And there are therapeutics that are being developed to block this growth specifically for cancer patients. But in our patients, they don't have localized hypoxia. They have global hypoxia. And there is increasing evidence that HIF signaling can propagate this inflammatory injury. And I think of it as a cycle of activation, or the title of my talk, A Storm in the Lungs. So you start with hypoxia, say, a patient with pneumonia. The lung endothelial cells and the alveolar epithelial cells, as well as leukocytes, are exposed to this hypoxia. And they activate the HIF pathway from oxygen deficiency, stretch. And actually, certain bacteria can also activate the HIF pathway. This then leads to an inflammatory response in cytokines, which causes more injury to the lung, chemical and mechanical, which causes more hypoxia that then leads to more HIF activation. And you can imagine this cycle just causing worse and worse lung injury. And this is the type of cycle that seems like an inhibitor might be able to break this cycle and storm in the lungs. So in conclusion, tissue hypoxia alters the immune response, harming the patient. And that interventions to correct this aberrant response represent promising future therapeutics. I thank you for your time. And I'd be happy to take questions at the end of the session.

mechanical ventilation

biotrauma

lung-protective strategies

hypoxia

acute respiratory distress syndrome