Hi everyone. My name is Nader Yeya and in this COVID session we're going to talk about how can we apply animal and human studies to children. This title was assigned to me and I do want to use it to call attention to the fact that it implies that children are neither animal nor humans. Once we get past that then there's actually a fair amount of cool stuff to discuss. These are my disclosures. So pediatric COVID from the get-go we know has better outcomes. It's not clear whether it's less infectious. It's not clear whether it's equally infectious but just less severe. It actually appears to be both and we'll talk a little bit about that. It does have this weird post-infectious multi-system inflammatory syndrome in children or MIS-C which has also been called the pediatric inflammatory multi-system syndrome temporarily associated with severe acute respiratory syndrome coronavirus 2 or PIMMS-TS in Europe. We're going to stick with MIS-C for obvious reasons. When reviewing the literature there is some decent translational data. This is a paper in science which suggested that there's pre-existing and de novo humoral immunity to SARS-CoV-2 in humans which is partly a function of pre-existing cross-reacting coronavirus antibodies which seem to peak in childhood. So the six to seventeen-year-olds seem to have these higher levels of non-COVID, non-SARS-CoV-2 antibodies which are already floating around in their system which are conferring some degree of immunity which is helpful suggesting that it is in fact less infectious in this population. We've also found in studies of subjects who underwent tonsillectomy and have tissue available in a tonsil biobank. Let your mind marinate on that for a while. People have identified resident memory CD8 positive T cells with functional specificity for SARS-CoV-2 and these are unexposed oropharyngeal tissue. This is this is stuff which was collected a good year or two, the biobank ended a good year or two before the first cases of coronavirus were reported. When you look at the graph on the left, okay, the heat map on the left, you see that the percentage of memory CD8s are present for the nuclear capsid and several of the open reading frame proteins, much more so in children than they were in adults. When you look at the figures on the right, the pediatric CD8s appear to be more poly-functional. They appear to have the functions that were included in this were interferon, IL-2, TNF-alpha and CD107 and the pediatrics were more likely to have greater than two functions. The adults were primarily mono-functional and when you look at specific functions and secretions, there were increases in the particular increase in CD107 or degranulation activated CD8s expressing CD107 in children relative to adults. All of this is to say that this pre-existing CD8, which can cross-react with SARS-CoV-2, could be in part why children appear less susceptible to this. There is a mouse model that people had developed and in this paper from Cell, when you compare the same infectivity of young versus old, you actually need a higher dose in younger animals to affect the same level of weight loss or get any sort of significant phenotype. When you look at weight loss, then there's a day four weight loss, which is essentially returned to normal by day seven in the younger mice. In older mice, there's just persistent sustained weight loss over the course of the first week of experimentation. This parallels a much improved survival without a significant difference in mortality in the young mice infected with SARS-CoV-2, whereas there's a clear mortality difference in the older mice. When you look at the lungs specifically, there is much more heterogeneity of whether or not the lungs have any significant pulmonary edema in the pediatric model, whereas the adult model, the older mice have a consistent, consistent increase in lung edema, which parallels what we see in humans. This seems like a pretty reasonable mouse model to suggest that there's some age-related differences in immune function that are just intrinsic to why the pediatric population would be somewhat resistant to COVID. With respect to vaccination, the news is also better, although not necessarily reproduced or translated to what we are actually seeing in adults in humans, where the younger mice actually mounted a better antibody response in response to vaccination. This could also be a function of the different doses of vaccines, which are approved for adults versus children, so it's hard to make too much of this and the translatability of this right now without further data. In addition to being less infectious because of pre-existing humeral and cell-based immunity, children consistently also have a milder phenotype for the same viral load. This study looked at the actual innate immune response to COVID in children, and they found that it resolved faster than adults. However, they found that children were able to resolve their COVID response without actually sacrificing any of their immune function, so it's not like they had a less robust immune response. On the left, on this spider web, you see that children generally have lower symptomatology in every domain relative to adults. Okay, that being said, their symptoms also resolved a little bit faster, although this is really just the sample size here isn't big enough to like really make too much statement about this, but the viral load and the clearance of the viral load doesn't actually change, differ too much between adults and children from what we can tell. When you look at their innate immune response, on the left columns you have adults on top, children on the bottom, and adults have a more robust in different domains on this star plot of different domains of the immune response. Okay, like the number of domains are as the red lines extend, the red and the blue lines extend outward, that's how many more domains are involved and the robustness of the response. So on day zero to five, adults and children, adults on top, children on the bottom, have similar, although adults clearly have a somewhat higher response, but they both have pretty robust immune responses. By day six to 14, you can see that the pediatric response is massively attenuated and the adult response is somewhat attenuated, but if you pay particular attention to the interferon signal, which is the red segment in the pie chart of the star graph, then you see that the interferon signal specifically is quite diminished in children and it's still fairly strong in adults. When people have looked at specifically the humeral response and across different classes of immunoglobulins, then adults also have a much more robust humeral response to COVID than pediatric, than children do. This is a series of star plots looking at children on the left, severe adults in the middle, and mild adults on the right and pediatrics appears to have a humeral response more consistent with that of a mild adult, which is also consistent with what we see clinically. This particular paper focused very much on the IGA contribution, that was the novel contribution that they thought could be different. IGA makes a lot of sense for a respiratory virus as it interacts with a lot of the mucosa lining the respiratory system first, and so it would make sense that an IGA-driven response could in fact be the difference. We'd already talked about the fact that children may have pre-existing humeral immunity as well as cell immunity, but there also may be differences in actually how children versus adults mount a response to COVID, and here we see that levels of IGG are different, but levels of IGA are way different between severely affected adults and the entire pediatric cohort. This group also performed an antibody depletion study, and they suggested that the attenuation of the neutrophil phagocytosis activity was equal whether you depleted IGG, but it was much more noticeable that IGA depletion attenuated antibody-dependent neutrophil phagocytosis pretty much only in severe adults. When we shift over to thinking about MIS-C there's a lot of evidence that we're looking at an overactive th1 response and so this was a paper which looked at the immunopathologic signature of MIS-C and compared MIS-C children to children with pediatric COVID which we've already decided is a milder disease on par with mild adult COVID. And if you look at the the T cell population there seems to be a much higher T cell response in the MIS-C patients and a lower monocyte response in the classic as well in classical as well as non-classical monocyte responses in MIS-C but a very exaggerated T cell response. It should be noted that these patients are almost at this point have invariably gotten steroid by time of blood draw. The other thing that this paper noticed and really delved into is they looked at the T cell receptor repertoire and the expansion. They found a consistent expansion of TRBV11-2 which was only seen early. This also gradually decreased over the time in the patients that they had repeat measurements in but again I remind you that like by this point people the children have been exposed to steroids IBAG and a bunch of immunomodulation so that you may only see this sort of signal early. This TRBV11-2 expansion has also been reported by other groups. And there's also this sense that the risk the T cell response may actually be somewhat targeting the endothelium. So in this study which looked at the performing deep immune profiling in MIS-C they found a transient immune activation which they compared to pediatric COVID as well as to adult COVID. On the left are grades two to five of adult COVID and the darker gets in the adults then the more severe the COVID becomes and then the blue is pediatric COVID and the green triangles are MIS-C. So MIS-C has for both CD4 and CD8 a higher CD38 response of activated T cells and when you look at the co-expression of CX3CR1 or a endothelial targeted fractal kind then you actually see a higher level of those driving the CD38 positive response suggesting that this could potentially explain this targeting of the endothelium could explain the vascular phenotype and the shocky phenotype of MIS-C relative to the pulmonary phenotype of a primary COVID infection. Overall we conclude children may or may not be animals and or human. This is likely a case by case scenario depending on your particular child. It's clear that COVID is less infectious in younger subjects and some of the mechanisms that have been demonstrated are prior coronavirus exposures conferring humoral immunity because of cross-reactivity with SARS-CoV-2. We've also seen that these cross-reactive tonsil CD8s are polyfunctional. They have strong CD107 and interferon gamma co-expression stronger response to SARS-CoV-2 antigens including the nuclear capsid and the and several of the open reading frame proteins and it's important that it's at the tonsil. This was a tonsil or biobank because that's a likely portal of entry for respiratory virus like COVID. There are clearly age-related differences in immune function. Children appear to have a more efficient and thus shorter innate immune response but it seems more than adequate to clear the virus. The robust adult IGA response could be postulated to drive excessive inflammation that could explain the more severe ARDS and lung phenotypes AC and adult COVID. MIS-C appears to be a post-infectious phenomenon characterized by persistent dysregulated T cell response. This could be a super antigen type response to the initial SARS-CoV-2 antigens which were presented. It seems to be predominantly Th1 type inflammation. The TRBV11-2 clonal expansions is a consistent signal that multiple groups have reported and the endothelial targeting could explain the vascular phenotype of MIS-C relative to the pulmonary phenotype of COVID. All of the MIS-C data unfortunately is limited by small sample size and concurrent immunomodulation.

COVID-19 in children

animal studies

human studies

immunity to SARS-CoV-2

age-related differences in immune function