Thank you all for joining us today for the Pediatrics Year in Review for 2021. I've been asked to review the literature for basic science, which is, of course, an impossible task. So, I chose to focus on advancements in the field of trained immunity because I think this is a very quickly growing field and one that has broad implications for the care of critically ill children. There's growing recognition that the innate immune system is not static, but instead is adapting over time depending on prior exposures and stimuli. Here's a schematic of three different types of adaptations that have been described in the innate immune system. The first is priming, where after the first insult, there's not yet time to return to baseline before the second insult. And there, as a consequence, is an enhanced response to that second insult. This is in contrast to trained immunity, where there is a complete return to baseline after that first insult, but there's some innate immune memory such that with the second insult, there is augmented immune response. The opposite of this is tolerance, where after a return to baseline after the first insult, with the second insult, there is a dampened inflammatory response. Now, you can probably imagine how each of these adaptations might be beneficial or harmful depending on the circumstances. And so, it will be increasingly important that we understand how these might be playing out in our patients such that we can take advantage of tolerance when we need to decrease inflammation, for instance, or take advantage of trained immunity when we need to enhance it. The first manuscript that caught my eye last year was published by Jasmine Belkade's group in the journal Science and looked at trained immunity in the context of prenatal exposures. In this paper, they infected pregnant mice with a self-limited infection of Yersinia. This infection was mild, it did not harm the mice, and it did not reach the fetuses. They then went on to interrogate the offspring in young adulthood. When they looked at all of the barrier sites and all of the various immune cell populations, they found that there was a greatest increase in the Th17 population in the small and large intestine, and that this increase in Th17 was limited to the first litter that the mom had while she was infected and not a second litter where there was not an infection. They then took sera from the infected pregnant mice and transferred it to pregnant mice that had not been infected and found that they could recapitulate the Th17 response in the offspring with just the sera. When they investigated what cytokines were present in that sera, they found that there was a biggest increase in IL-6. They tested whether or not just giving IL-6 to the pregnant mice could have the same impact in the Th17 population in the offspring and found that yes, it could. Treatment with the anti-IL-6 antibody prevented the increase in Th17 cells in the population, further supporting that this skewing to the Th17 cells was mediated by IL-6. Then they went on to show that this was happening through the intestinal epithelial cells. They did this by knocking out the IL-6 receptor just in the intestinal epithelial cells and then measuring the Th17 response and found that without the IL-6 receptor, giving IL-6 prenatally did not increase the percentage or number of Th17 cells in the intestines. Finally, to test the functional consequences of this IL-6 training, they infected the offspring with Salmonella and found that there was improved survival in the offspring who had been prenatally exposed to IL-6. Take-homes from this paper for me were that prenatal exposures have the capability of impacting subsequent tissue susceptibility to infection and also auto-inflammatory disorders which I didn't have time to show all the data for. The next paper that caught my eye explored the impact of sequential viral infections on immune response and outcomes. This paper was by Alan Foxman's group and published in the Journal of Experimental Medicine. The first part of this paper explored how the antiviral interferon responses changes over time in response to SARS-CoV-2 infection in people, but I wanted to focus on the second half of the paper. Here they investigated how sequential infection with rhinovirus and then SARS-CoV-2 impacts outcomes. Here they used an epithelial organoid model where they first infected the epithelial cells with rhinovirus and three days later challenged them with SARS-CoV-2. They then assessed the viral load at various time points after infection and the expression of interferon-stimulated genes. First they found that after rhinovirus infection there was an up-regulation of various interferon-stimulated genes known to be important for the control of viral infection. In addition to those being up right before the infection with SARS-CoV-2, the pre-infection with rhinovirus enhanced the expression of these interferon-stimulated genes throughout SARS-CoV-2 infection. As a consequence, there was a dramatic decrease in the viral load of SARS-CoV-2 following rhinovirus infection than there was following mock infection. To demonstrate that the protective effect of rhinovirus infection prior to SARS-CoV-2 is mediated by the transcription factor IRF3 and the up-regulation of interferon-stimulated genes, they treated the cells with an IRF3 inhibitor prior to infection with rhinovirus. Inhibiting IRF3 successfully decreased the transcription of interferon-stimulated genes and it also inhibited the beneficial effects of rhinovirus such that the SARS-CoV-2 viral load was no longer decreased following rhinovirus infection. In addition, inhibiting IRF3 also increased the viral load of rhinovirus. So sequential rhinovirus than SARS-CoV-2 infection was only protective and improved viral clearance if the interferon response was intact. The authors to conclude that there's a period of time after a viral infection that we are protected from a subsequent viral infection, but only if the interferon response is intact. If the interferon response is not intact, then sequential viral infections are actually harmful. In this third paper, they did a deep exploration into the epigenomic and transcriptional consequences of influenza vaccination. This was published by Bali Palendran's group in Cell. In this study, volunteers were vaccinated with a seasonal influenza vaccine, TIV, an avian influenza vaccine, H5N1, or an avian influenza vaccine plus an adjuvant, HSO3. Following vaccination, their peripheral blood mononuclear cells were isolated at various time points and all these different measurements were obtained to fully assess the impact of vaccination on the PBMC. In these first set of experiments, they took the isolated peripheral blood mononuclear cells and exposed them to various bacterial or viral ligands and measured cytokine secretion. Here you can see that over many days following vaccination, there is a decrease in cytokine secretion in response to both bacterial and viral ligands and that this decrease in cytokine secretion is not fully reversed by day 180. This decrease in cytokine secretion was associated with a decrease in chromatin accessibility of these genes, which they went on to show was likely mediated by a decrease in the AP1 family of transcription factors. Moving on to the H5N1 vaccinations, they also showed that there was a decrease in cytokine secretion up to 42 days after the vaccination. Similar to the seasonal influenza vaccine, this decrease in cytokine secretion was associated with a decrease in chromatin accessibility of the AP1 family of transcription factors, including Fos and Jun here. Interestingly and unlike the seasonal influenza vaccine or the non-adjuvanted H5N1 vaccine, the adjuvanted H5N1 vaccine showed an increase in chromatin accessibility of the IRF family of transcription factors, which are responsible for the transcription of interferon-stimulated genes. When they looked at the interferon-stimulated gene transcription, they found that they were in fact increased in response to vaccination with the adjuvanted H5N1 and its booster, but not increased in the non-adjuvanted H5N1 vaccine. They then tested how these epigenomic modifications that they found after vaccination might impact the response to other viruses. They infected the PBMCs isolated from people who were vaccinated with the adjuvanted H5N1 vaccine with either dengue or Zika virus. They found that there was decreased viral load in the folks who had been vaccinated with the adjuvanted vaccine compared to those who had not. In addition, they found that the decrease in viral load was inversely proportional to the increase in IRF1, suggesting that an increase in IRF1 was improving the ability to combat these viruses. So vaccination can modify the inflammatory response to unrelated stimuli and careful choice of immune stimulants like adjuvants can tune the immune system to dampen inflammation while also increasing antiviral potential. Finally, I wanted to touch briefly on this article published in Nature by Kirsten Meyers group looking at the local and systemic responses to SARS-CoV-2 in both children and adults. In this study, they performed single cell RNA sequencing on samples from the nose and the blood from healthy children and adults and those infected with SARS-CoV-2. They found some interesting age-related differences shown here in the nose between children and adults with infants having a higher proportion of innate immune cells in the nose compared to older children and adults, which transitions to an increase in adaptive immune cells with time. So not surprising, but fun to see this switch from innate immunity, a dependence on innate immunity in the youngest children, to a more prominent adaptive immune response in the older kids. Intriguingly, when they compared healthy children to adults, they found an increase in baseline antiviral interference signaling in both the nose and the blood. So here shown in red, indicating that these processes were upregulated in children compared to adults at baseline. However, this increased signature only persisted in the immune cell population and not the epithelial cell population in response to SARS-CoV-2. So the consequences of this pre-existing antiviral interferon signature in children is unclear. I realize that was a whirlwind tour of some very complicated papers with broad implications for our understanding of innate immunity and its development over childhood. I didn't have time to go through all of the data in all of the papers, so I hope that you have the opportunity to go back and read them for yourself, because there is definitely some very interesting aspects that we didn't discuss today. But for me, the take-home points that I thought were most interesting from each paper are listed here. The first being that prenatal exposure might impact postnatal inflammatory responses to pathogens or environmental stimuli. That sequential viral infection might be protective if the interferon response is intact, but if it's not, they might be harmful. That immune cell activation by vaccination can modify the subsequent immune response to unrelated viral infections. We've known this for a while, but seeing how that subsequent response may be tuned depending on how the vaccination was given and with what adjuvants, I think it's really fascinating. And that children might have a preactivated interferon response compared to adults that could be protective in things like SARS-CoV-2 infection, but why it's not clearly protective in other viral infections like influenza and RSV, which we know impact children to a more significant degree than adults, is unclear. And I think all of these papers leave us with the question of how can we harness trained immunity to improve disease tolerance and outcomes, and how can we use this knowledge to both protect our patients from critical illness, to help them recover from critical illness more quickly, or to protect them from the side effects of prolonged critical illness like immunoparalysis and hospital-acquired infections. With that, I'd like to thank you for your attention, and I really hope that we have the opportunity to discuss these papers.

trained immunity

pediatrics

innate immune system

viral infections

influenza vaccination