Shane Crotty: A Landmark Study on Upper Airway Mucosal Immunity

A video snippet of our conversation. Full videos of all Ground Truths podcasts can be seen on YouTube here. The audios are also available on Apple and Spotify.

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Shane Crotty: A Landmark Study on Upper Airway Mucosal Immunity

Transcript

This is the first time a Ground Truths podcast is being posted simultaneous with a new publication, this one in Nature, by Professor Shane Crotty and his colleagues at La Jolla Institute for Immunology. Shane is one of the leading immunologists and virologists in the country; he and his group published in 2020 the first detailed analysis for how our immune system responds to SARS-CoV-2. Shane also, among many other notable contributions during COVID, illuminated the role of hybrid immunity vs COVID, the differences between and additivity of vaccination and infection.

Today’s paper in Nature is indeed a landmark contribution doing something that hasn’t been done before—to understand the underpinnings of mucosal immunity of the upper airway. 100 participants had monthly nasal and nasopharyngeal swabs throughout the pandemic. With a median of >100,000 cells per swab recovered, they undertook single-cell sequencing and full characterization of the cells (tissue-resident memory B cells, CD4+ and CD8+ T cells, germinal center follicular helper T cells and B cells, etc.) to determine optimal immune protection of the upper airway, the effect of infections by different variants, breakthrough infections, vaccination, and age.

Here is the transcript of our conversation about the new report with links to the audio:

Eric Topol (00:06):

Hello, it's Eric Topol with Ground Truths, and with me today is Professor Shane Crotty from the La Jolla Institute of Immunology (LJI), not too far away from where I work at Scripps. And Shane has been a go-to immunologist colleague here in the Mesa, and he and his colleagues were the ones that really first published the response to SARS-CoV-2 as far as the immunologic response. And today we're doing something very unique. We're going to go over for the first time in the two year plus history of Ground Truths, going to have a publication with at least simultaneous or near simultaneous podcast. Shane, welcome and congratulations on this really important paper in Nature.

Shane Crotty (00:57):

Thanks, Eric. Thanks for having me. Yeah, somebody asked if I was going to go over to Scripps for the podcast and I was like, yeah, we could.

Eric Topol (01:06):

You could. You could. But no, it's good. And it's nice having the logo of this great institute you work at right in the right corner. And you've done so many contributions with your colleagues at La Jolla Institute. It's really a privilege to have a chance to learn from you and particularly about what we're going to talk about today, which is mucosal immunity to upper airway infections, which is especially germane to COVID. And we're actually in the middle of a significant wave of COVID right now. And I guess it would maybe be fair to say, Shane, that we've never truly understood the underpinnings, the real details of upper airway mucosal immunity. Is that a fair statement?

Shane Crotty (01:53):

Yeah, it is a fair statement.

Eric Topol (01:56):

Okay. So today we're going to crack the case. This paper from you and your colleagues, of course, you're the senior author and first author, Sydney Ramirez did a remarkable study. I mean, just extraordinary. This is why we're doing a special podcast about it. Maybe you could just kind of give us the overview of the design because you were doing things that haven't been done before.

Shane Crotty (02:24):

Sure. And, I would say the genesis even of it goes back to what you were introducing. I mean, during the pandemic, we like a lot of scientists spent a lot of time and energy trying to help understanding immune responses to this virus, and immune memory to this virus, and what was involved in protective immunity. And we're certainly proud of the work that we did. And it was hard work. And after a while we were exhausted and we stopped.

Shane Crotty (02:59):

And then we came back to it after a while and said, well, the virus is still here. And so many people have contributed so much to better understanding the virus and creating vaccines. But there are clearly still things we don't understand. What are those biggest knowledge gaps and where might we be able to contribute? And really to me the biggest one was location, location, location. This is a virus that infects your nose, infects your upper airway—your nose, and throat, and oral cavity. And then obviously if you get severe disease, the severe disease and death are from the lungs. And it's just been a big knowledge gap in terms of understanding what actually occurs in those tissues immunologically and what is associated with protective immunity or what could be associated with protective immunity. And sort of looking forward what might be helpful for mucosal vaccine development from things that we could learn.

Shane Crotty (04:12):

So we started from what we would call the basics, and what does immune memory look like in the upper airways in normal people? And that hasn't been available really even in, and we started this two years ago, even in the biggest atlases published of the human body. There was no upper airway tissue representation at all. And that's because technically it's just tough to access and difficult to reproducibly get at. And so, we recruited people to a group of 20 to 30 people to come to LJI once a month, and just started testing out, published and unpublished sampling techniques to see were there ways where we could reproducibly sample immune cells in the upper airways from people. And once we got things, so the keys for us were you got to have enough cells that you can collect to learn something from. And luckily with modern techniques of flow cytometry and single cell sequencing, you don't need that many cells. And so, we could get a hundred thousand cells on a swab and that's enough to do a lot with. And second, how reproducible was it? So we showed, we had people come in every month for a year and we could reproducibly find the same things in their swab; same cell types in their swabs. And the third thing was that people would come back.

Shane Crotty (06:05):

We found that if you have good nurses doing the techniques, we could find ways that this would be a sampling approach that was tolerable and people would come back for repeat measures, which is really valuable to see what's happening in people over time. So that was what we started from in the study and built from.

Eric Topol (06:27):

And if I am correct, you sampled two places with the swabs, one in the nose and one of the throat. Or, I think one which you have in the paper as the MT for something about the median nasal turbinate and the other adenoid in the back of the throat. Is that right?

Shane Crotty (06:50):

So all the sampling is a swab into your nose. And when we were doing that, we were really excited to see the diversity of immune cells, particularly T cells and B cells, memory T cells and B cells that we isolated. They're like, wow, there's actually a lot of interesting immune memory up in there. And the lab said, oh, by the way, we're seeing T follicular helper cells (TFH). Now that happens to be my favorite cell type.

Eric Topol (07:22):

Why is that, Shane? Of all the cells, why do you say that's your favorite? I know you publish a lot on it.

Shane Crotty (07:31):

Because those are the T cells that are required for basically all neutralizing antibody responses. All high-quality antibody responses depend on—almost all high-quality antibody responses depend on—T cell help. That T cell help comes from T follicular helper cells. Antibody evolution is certainly one of the coolest processes of the immune system. And all of that depends on T follicular helper cells. So the fact that for example, you could get Omicron neutralizing antibodies even after only being vaccinated with ancestral vaccine, that's the immune system making guesses of what variants would look like. And those guesses come about through this antibody evolution that's driven by T follicular helper cells. So, it's really one of the most brilliant things the immune system does, and that's a cell type that's really key, but those processes happen in lymphoid tissue. That's what happens in lymph nodes and spleen. And here we were sampling epithelium, your nasal epithelium, so the cells didn't really belong there.

Shane Crotty (08:37):

And so, that's what turned the study in another direction. And we said, okay, let's figure out why is it that these cells are present in these swabs? And we had a couple of possibilities. One possibility was that the swab was going all the way back to the posterior wall of your nasopharynx, your top of your throat and sampling adenoid tissue. So adenoid tonsils and adenoids are a true lymphoid tissue and they're a mucosal lymphoid tissue. And so, we came up with multiple ways to validate that that's what we were testing. And in fact, it was the Sydney Ramirez, a clinician, and the ENTs involved who said, well, let's just look. And so, they actually did endoscopies with the swab to actually see where the swab went. We've got videos of the swabs going into the adenoid crypt in the back, and then we've got measurements of here are the cells that you find on those swabs.

Shane Crotty (09:58):

And what's cool about it is that, yes, so we did studies with two sets. We then shifted to doing studies with two sets of swabs. One where we essentially went “halfway back” where we were detecting that epithelium of your nasal passages and then one where it was all the way back and detecting the adenoid lymphoid tissue. So here we've got two different sites in your upper airways that are about an inch apart, and we're detecting essentially completely different cells of the immune system at those two places. And we tend to think of the cells present in that epithelial tissue as probably the sentinels, the cells that are sitting there that can potentially immediately react and try and protect you against a viral or bacterial infection. Whereas the lymphoid tissue, the adenoids, is really about generating the immune responses in the first place and priming immune responses. And that's where these germinal centers can occur, which are where the TFH are where you can get antibody evolution. And so, we found in the course of the study that with this non-invasive technique that we can.

Eric Topol (11:14):

By the way, I don't want to be signing up for the one way up there because I mean just a mid-nose enough for me. So wow, I got to give credit to your study participants for coming back every month for a year to have that. Some people call it a brain biopsy.

Video of swab of nasopharyngeal tissue

Shane Crotty (11:33):

Right. So I will tell you, it is a different experience than the COVID nasopharyngeal swab might've gotten through your car window. If you're actually sitting down in a comfortable space and there's a nurse doing it with these particular goals. We really found, we had a hundred people in the study and a total of 300 swabs, and the vast majority of people came back if we asked them to.

Eric Topol (12:06):

That's great.

Shane Crotty (12:07):

And we're certainly very thankful for the volunteers. Obviously they were volunteering in the first place to participate. So I'm a little hesitant about the video because I've told people to not show it to potential volunteers because it definitely doesn't encourage you to volunteer. You're like, wait, that's what's happening? But actually, I've had it done on me.

Video of the swab to the nasopharynx for adenoid (lymphoid tissue) access.

Eric Topol (12:37):

Not that bad.

Shane Crotty (12:39):

It's really pretty compelling. And by doing these repeated samples, we actually now have the capacity to look at ongoing immune responses like after an infection or vaccination in people and see how that results in the immune system changing and what might be the source of the protective immunity that comes up. So we've actually got data in the paper looking at this antibody evolution in real time. So we've got affinity maturation of B cells occurring in just normal healthy adults of mucosal B cells against COVID. And so, that's really helping us learn what's possible, basically to figure out, okay, if you're going to try and make a vaccine, what types of immune cells are even possible to generate in this tissue? And where might you try and generate them? Or if you're trying to study some disease state, what are types of cells that might be problematic?

Eric Topol (13:45):

Yeah, I mean, I think the idea that so many of us have been pushing for a nasal vaccine to induce mucosal immunity because, as you know very well, the current shots are not very good at any durable or substantial protection from upper airway infections of COVID or SARS-CoV-2 and other infections. So I think one of the most important parts of this report is that it lends itself well to helping towards artificially, if you will, make a vaccine to get the protective features that you were able to identify. Maybe you could just [speculate], if you had the ideal nasal airway, what would the cellular profile look like?

Shane Crotty (14:44):

Ah, I see. Yeah, great question. So, first of all, antibodies are great. So most of my career has been dedicated to most licensed vaccines. The correlate of protection is antibodies. Antibodies clearly can be protective, and if you can get them that’s excellent, so certainly I would want, in terms of the non-cellular component, I would want antibodies present, neutralizing antibodies present in it.

Eric Topol (15:26):

Are these IgA or IgG?

Shane Crotty (15:31):

Yeah, in an ideal situation, what would I want? I'd want a mix of both, basically. The IgAs look like they have a little more protective efficacy, but the IgGs, just at a molecular level have a longer half-life, stick around a little. So yeah, I'd want both. And then really the premise for most of what we do is saying, in situations where antibody isn't enough or the antibodies don't stay around long enough, or you've got a variant that now obviates the protective efficacy of that particular antibody, are there other types of protective immunity you can have? And the immune system has other stuff besides antibodies for a reason. Of the lymphocytes in your blood, most of them aren't antibody producing cells. Most of them are other things. And so, well sticking with adjacent to antibodies, those antibodies in the mucosa, I'd want them to be made by cells that were literally right there. So plasma cells living in that site so that you've got basically the highest concentration of antibodies you can get because they're not having to diffuse through the whole body. They're just already at their highest concentration right there. Now antibodies come from B cells, that's what encodes the antibodies.

Shane Crotty (17:03):

And so, the B cells can make neutralizing antibodies if it turns out that you haven't made enough neutralizing antibodies, or if there's a variant that escapes those, maybe there are other B cells that could make, once you get infected, more B cells that could make more antibody rapidly infection, or B cells that recognize this variant that is mismatched to the current antibodies you have. But memory B cells are basically a library of different antibody specificities representing different guesses about what viral variants or structures might look like. And so, I would want memory B cells in that upper airway tissue that could reactivate quickly. There are memory B cells in your blood and we don't know how long it takes. And that's one of the reasons we're hoping we and others build upon this study. But it might take, let's say five days for memory B cells to go from your blood into your upper airway.

Eric Topol (18:06):

Oh, right.

Shane Crotty (18:08):

That's right, you were already quite sick by that point. Instead, if memory B cells are right there, as soon as virus showed up, they got activated. Now maybe after (we’re not sure yet), but maybe after 48 hours those cells are now activated and doing something useful. That would be optimal. So then we can pivot to the T cell side. So there’s a fantastic recognition that T cells being physically present in tissues, tissue resident memory cells, as they’re most often called, can really have fantastic protective capacities. From a lot of mouse model systems where you can see T cells are in the skin or the liver, or whatever [tissue] are already there, they’re more protective than if the cells are in the blood. So if you could also have T cells essentially permanently parked in the epithelium of your nasal passages and in the adenoid, hopefully those could essentially be sentinels for protective immunity, and as soon as you get infected, those T cells would reactivate and start killing off infected cells. ’That’s the mix that I would want to see. And I think there’s at least some reasonable evidence in the context of COVID that people who have T cells in their upper airways maybe manage to control the virus so quickly that it’s a subclinical infection; they never notice when they get infected. And so, building on those types of observations, that’s what I would want.

Eric Topol (19:56):

That sounds good. I like that. I’d like to have that in my nasal airway. Now, just to make sure I’ve got this, what you found, of course, the memory B cells, the T cell memory, CD8+, that is the cell-killing T cells that you mentioned, the resident T cells. One clarification on that, they are not really going to do much until there’s been some cells that have been infected with the virus, right? Then they come alive and kill those cells. So they’re not immediate, but they can work pretty quickly still though, right? If they’re resident T cells?

Shane Crotty (20:45):

Yeah, in theory it might take as little as 12 hours for a virus to infect a cell, and then you get some antigen presentation on that cell that could activate the T cell.

Eric Topol (20:58):

And that’s all happening perhaps within the incubation phase of the virus, right?

Shane Crotty (21:07):

Correct. That’s a tough thing to study, but conceptually that’s the way people tend to sketch it out.

Eric Topol (21:13):

Right. Now the other part of the story is, and you alluded to it earlier, is the lymphoid tissue up there, higher up where there are these germinal centers; is there anything different you want in these germinal centers? Do they contribute to mucosal immunity that you haven’t already mentioned?

Shane Crotty (21:36):

So they really contribute in this forward looking sense or really in the classroom kind of sense. The germinal centers are where you’re basically teaching the B cells in advance of seeing the infection either with your vaccine or with your previous infection, evolving better B cells and better antibodies and hopefully instructing them where to go reside to then be ready for the next infection. If you get really great protection that next time, hopefully then you don’t need to start.

Eric Topol (22:14):

Right. So it’s like the training grounds for this coordinated response, I guess. Now you also noted this, I mean this is a rich paper, which is we’re illuminating something that’s never been done before in human beings. I mean it’s pretty damn important and impressive. But you also found that you had an age relationship. Can you tell us about that?

Shane Crotty (22:39):

Sure. This is one of our favorite parts of the study. I’d say in particular for several of the clinicians who were involved, because the general conversations people have about upper airway lymphoid tissue, like your tonsils and including your adenoids, is that adults don’t really have functional lymphoid tissue in the upper airway that your tonsils atrophy by the time maybe you’re 20 or something. So, immunologically, functionally, what that means is if you have let’s say an intranasal vaccine or you get infected with a new [virus] like SARS-CoV-2, if those would normally be the sites that start your immune response, where does it now happen? And instead what we saw was, we had such a diverse group of people in our studies—we realized we had people from age 18 to 68—and so we could directly ask, in normal healthy individuals across a large age span of adulthood is there functional mucosal lymphoid tissue? And the answer was yes, it was there. But it definitely declines over time, and it's declining on a log scale. Our simplest statement was that 75% of everybody we sampled still had functional tissue, but the younger the people were, the more functional it was, and the more germinal centers actually we saw; again these training grounds.

Eric Topol (24:35):

So this is really important because we know for COVID and obviously for influenza and other respiratory infections that people of advanced age are much more susceptible. And here you are finding something that supports that ,and it's almost like, the thymus, it involutes. After that, what age 20, and our lymphoid tissue [involutes]. We're just set up to fail. Old codgers, like me we're defenseless, I guess, right?

Shane Crotty (25:12):

So what I've liked about that in a positive sense is that it's not that all of these things go to zero. Like for example, naive T cells are definitely less abundant in people over the age of 60 than under, but they're not zero. And the mucosal lymphoid tissue is definitely less abundant in people over the age of 60, but in most people it still wasn't zero. And I always think about these things from a vaccine immunology perspective, and fundamentally the difference between getting vaccinated and infected frequently is that the whole point of the vaccine is you get to generate the immune response on your own time. And so, even if you're starting with five times fewer T cells or five times fewer germinal centers, if you're getting to do all that training ground in advance, you can end up with just as many bispecific T cells as a 20-year-old or just as many memory B cells as a 20-year-old because these things occur on an exponential scale because of the cell divisions. And so, it might take you three extra days, for example, to get to the same level, which again, if you're racing a virus, can be the difference between life and death. But if it's not a race and if you're doing it in the context of a vaccine, it's a much smaller factor. And that's some of what we've been trying to learn.

Eric Topol (26:42):

Now we only have started to scratch the surface of your findings. One of the things that drives me nutty in reading papers, especially from great immunologists like you, is that in each figure there's like 20 different panels. We get to one of the figures, figure three is all the way to panel W. I mean that starts with A. That gives you a little impression of the data. It's rich, another one goes to N or R. I mean we're talking about a lot of data. So I've only started to really deconvolute what you've done here, which is just an amazing study. But what are some other things that we should touch on before wrapping up?

Shane Crotty (27:35):

A lot of the goal in this study was to establish baselines of what is normal in humans in the upper airways. And that's one reason why in this case there actually are a lot of figure panels because we could work out a bunch of individual parts of the immune system that really hadn't been characterized in this way before. And something we really cared about was durability of immune memory. It's often talked about, well, mucosal responses are inherently short-lived. And we're like, well, what does that mean? Does that mean there's just no memory? Is it different kinds of memory? And so, this is the first measurement of memory B cells in this tissue in an antigen specific way. And we were doing it in people who had had recent COVID breakthrough infections. And we saw really the mucosal memory was stable for six months. And so, to me that's quite encouraging that it's not one month and it's gone, at least with an infection, it's at least six months and it looks like it'll project out for substantially longer.

Shane Crotty (28:53):

Amongst those cells, many of them are IgA. IgA is this antibody isotype that's particularly mucosal associated. And only 5% of the memory B cells circulating in blood were IgA. Whereas many of the memory B cells in the local tissue were IgA, which we think is also telling us that there's a lot of immune memory and the immune system in this tissue that we're probably not sampling in the blood. And so, sampling blood's great, right? It's accessible and we can learn a lot from it, but it does look like there is some tissue compartmentalization.

Eric Topol (29:37):

Oh, not a question. And the findings you had of the resident T cell is so indicative of that. And what's really striking, of course Shane, is that as we assess the immune system in people at large, we look at a lymphocyte neutrophil ratio [in the blood], we get almost nothing. And then in the course of the pandemic, you and your colleagues there provided such granular data on B and T cells, CD4 and CD8 T cells, and that you illuminated things that are not done ever clinically. These are research, high tier research labs like yours. The only question I have on before I just wrap up with the nasal vaccine story, interferon wasn't really part of this. As we know SARS-CoV-2 can shut down the interferon response, it's considered a frontline part of the defense. Where does that fit into the mucosal immunity of the upper airway?

Shane Crotty (30:46):

Yeah, it's really important. And that's in this basic divide we do in the immune system, the innate immune system and the adaptive immune system. So everything I was talking about is the B cells, the T cells, and antibodies. That's all the adaptive immune system. That's all virus specific. And then the innate immune system is the generalists, and really sort of the fire alarm, just sensing some danger. And definitely in COVID interferon is very important. I'm quite intrigued to see if using these techniques. I'm curious to see if some of these other aspects of the immune system can compensate somewhat for the fact that this virus. To me, if this virus has one superpower, it's its incredible ability to evade triggering interferon for as long as it does. And that has this massive cascading effect to almost everything about the pandemic essentially. And so, I'm intrigued by whether in people who have immunity are there ways that these other cells of the immune system or even antibodies can do things when a viral infection occurs, that helps trigger the overall immune system to recognize that something's there, even in the absence of type 1 interferons. That's where I think for now it fits in.

Eric Topol (32:14):

Well. I think you've so aptly described, not surprisingly, the superpower of SARS-CoV-2, which I think a lot of people haven't realized that it's so good at shutting down that defense system. Now on the basis of you having really gotten this understanding of the mucosal immunity in the upper airway, does this make you think that the nasal vaccine that we aspire to have is more of a reality? Do you kind of know what the ideal profile might look like to keep people healthy and resist infections? Do you think this is achievable in any durable sense at high level success with a nasal spray vaccine?

Shane Crotty (33:04):

I'm optimistic for several reasons. One is we really saw a lot of different immune memory cell types that were present, that was encouraging and seeing the B cell memory durability for at least six months—pretty flat line for that six months—was encouraging. It looks like the immune system knows how to keep these cells around if it wants to for a significant period of time. We'll have to do more in follow up. But again, it was encouraging. Third, we had some people who were vaccinated only and some people who had breakthrough infections. And really in the vaccinated only, we didn't see T cell memory in the upper airways. And I actually consider that encouraging because it suggests local exposure does give you the memory and exposure in your arm really doesn't. So I think there is something to improve upon. It can be improved upon. And lastly, I get asked all the time, I'm sure you get asked all the time: Why aren't there more intranasal vaccines or inhaled vaccines, more mucosal vaccines in some way?

Shane Crotty (34:25):

And I think there's more than one reason, but I tend to be very practical, and I think one practical reason is there's very little to measure, to guide you in your vaccine development. If you have six ideas or six constructs that you think might work in humans as a nasal vaccine, you basically just have to pick one, try something, and hoping there's not much you can measure it clinical trials for what might be the type of response even. So for example, the FluMist vaccine, it's the only licensed inhaled vaccine, intranasal vaccine. In adults it doesn't have a clear correlate of protection. If you get vaccinated with that, your circulating antibody responses don't increase, but also increases in nasal antibody didn't correlate with protection well. So, what does that mean? That probably means there's other things going on up there that could be indicative of protection but weren't being measured before. So I'm hopeful with these types of approaches. Now, if you're an intranasal vaccine developer, you maybe have 4, 5, 6, 7, 8 ideas or constructs. If you can try those in a few people and make these different measurements and you've got your favorite immune profile that you might, now you have something to, it's more of an engineering problem. It's not a throwing a dart problem. You're like, yeah, this has given me the type of response that I like and I'm going to try and push this into clinical trials. So those are the things that I'm optimistic about moving forward.

Eric Topol (36:04):

Well, I love it because we really need it. And if anybody's optimistic that means a lot; it's yours. What you've done here has been quite extraordinary because you defined for the first time really the underpinnings of the mucosal immune response, the upper airway, you did it by age, you did it by variant, you did it by vaccine and infection. And most importantly, perhaps for longer term is you established what are the desirable features to have, which didn't exist before. It seemed like whatever I read for nasal vaccines, they were measuring some IgA or IgG, and they didn't get down to the memory B cells and the tissue resident T cells, memory cells, and all these other things that you found. You did all this single cell sequencing and flow cytometry. The work is just really fantastic. So Shane, just in closing, I just want to congratulate you.

Eric Topol (37:05):

You made seminal findings along the pandemic. You were the one that really illuminated hybrid immunity, the advantage of if you don't want to have an infection of COVID, but if you did have that and a vaccine, you kind of had some extra synergy, if you will. But here you've done something, you and your team. Unique. Congratulations on that. No surprise that it's in Nature this week. I'm sure a lot of people will share your optimism that we will have something beyond just shots in the future because COVID isn't going away. There's other respiratory pathogens. And finally, somebody did the right study, who knows immunology inside and out. So Shane, thanks very much.

Shane Crotty (37:52):

Thanks Eric. Very much appreciated particularly coming from you.

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