Host – Dan Keller
Hello, and welcome to Episode Fifty-Three of Multiple Sclerosis Discovery, the podcast of the MS Discovery Forum. I’m your host, Dan Keller.
This week’s podcast features Dr. Jonathan Kipnis, who discusses his recent discovery of lymphatic vessels in the meninges. But first, here are some new items in the MS Discovery Forum.
According to our curated list of the latest scientific articles related to MS, 34 such articles were published between August 21st and 28th. To see these publications and the articles we selected as Editors Picks, go to msdiscovery.org and click on Papers.
Our Drug-Development Pipeline includes continually updated information on 44 investigational agents for MS. This week, we’ve added 6 pieces of information about alemtuzumab and fingolimod. To find information on all 44 compounds, visit msdiscovery.org and click first on Research Resources and then on Drug-Development Pipeline.
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And now to Part 1 of our interview with Dr. Jonathan Kipnis, Professor of Neuroscience and Director of the Center for Brain Immunology and Glia at the University of Virginia in Charlottesville. His group recently published in Nature their discovery and characterization of lymphatic vessels in the meninges.
Interviewer – Dan Keller
You've described in this paper about meningeal lymphatics, the novel but actually more conventional path for cerebrospinal fluid drainage from the CNS than I guess had been thought of before; it's sort of conventional as revolutionary. Can you tell me what you found and what led you to look?
Interviewee – Jonathan Kipnis
Yes, so we've been interested in the role of meningeal immune system for quite some time, and we've shown that changes in meningeal immunity could impact brain after a CNS injury, or also for normal brain function. So, for example, mice that have impaired meningeal immunity would show cognitive deficits and would show some little bit more prone to stress and other phenotypes.
So we've been very interested in understanding how meningeal immunity is being regulated. So the assumption was at some point that there is no immune cells in the brain, which is true, except for microglial which reside in the brain and compose 10% of the brain cells, but there is no peripheral immune cells within the brain. But in very nearby areas, which is the surroundings of the brain – the choroid plexus, the meninges, and the CSF – that's where actually there are immune cells, and there are all types of immune cells. And so we have been very interested to understand how the cells are getting in and getting out.
Through the use of parabiotic mice, we demonstrated last year, we showed that immune population of the meninges is not static; the cells are being repopulated, and about 50% of T cells, for example, is being exchanged within about 10 days, and major exchange between the CSF or the meninges with the deep cervical lymph nodes. So nothing was really new, we just sort of established things maybe more solid way. Those cells can get in while still nobody understands very well how they get in; we'll assume they get in through the meningeal vasculature, which is probably true.
But then how do they get out, or what happens with cells after they get to the CNS? Well, the assumptions were, well, they either die, magically disappear, or crawl under the nose through the cribriform plate and into the deep cervical lymph nodes through the nasal mucosa. They were okay explanations, but in our systems we did not find any of it to be sufficiently explaining what's going on in this really fast and pretty dramatic exchange of the immune system within the meningeal spaces.
So when we just looking at it a bit closer, and it is very, very well established that there is lymphatic drainage from the CNS, so this needs to be remembered. So people in many labs have shown that if you put stuff in the brain – which stuff I mean proteins – if you put proteins or antigens in the brain, whether it's in the parenchyma or in the meninges or in the CSF, you will find those proteins, and you will find immune response to these proteins in the deep cervical lymph nodes.
The question is, of course, how do they get there? And the path which was described just did not work in our hands, and so I was lucky to get a very, very talented postdoc, Antoine Louveau, at the lab. He realized that for us to understand how things get in and out, the only way to do it is to do live imaging and also to do a whole mount of the entire meninges. And I think that's when it was a breakthrough point. So Antoine laid out the entire meninges and was looking for location of the immune cells. And he said let's see where I see maximum accumulation of the immune cells, and then let's see how these places will change when we expose mouse to, for example, stress, learning, or EAE inflammation, viral infection, or whatever, let's see how these areas of dense immune population will change.
And so he realized that there is a lot of immune activity around the major sinuses in the meninges, and then he saw that there are immune cells which are in the vascular structures which were not blood vessels. And I think that was the turning point. And I said, okay, if the cells are within the vasculature which is not blood vasculature, what would it be? Well, so I went to colleagues here and said what do you label lymphatic vessels with? And they didn't understand why would you want to label for lymphatic vessels, because they don't work with the brain. And so we labeled a lymphatic marker and we saw the vessels, which were lining the major sinuses and going all the way along them. So that's a very long answer to your very simple question.
And Antoine Louveau's technique here that was the key to it was doing in situ fixation so he could get the meninges out intact?
To let's assume the meninges came out intact on the brain, and let's assume we had this beautiful staining. Let's say we did the coronal staining, and let's say we're labeling for lymphatic vessels. So you can imagine that what you'll see is at the border of the brain you will see a dot; you will see maybe three dots because there are three vessels going along the sinuses. And when you see a dot, you never take a dot seriously in immunohistochemistry. Now that we know that this dot represents the vessel, then we can actually go back and do those coronal sections and look at it. But back then only by seeing the whole meninges mounted as one on a slide, and by seeing those vessels there, I mean that's when we knew. And so to us it was obvious this is something that absolutely went under-noticed. And this technique of whole-mount meninges, I think, was absolutely crucial.
Did he find these vessels in all layers of the meninges, or any specific ones?
No, no. Major lymphatic vessels are following the superior sagittal and the transverse sinuses, which is in the dura. So all the blood from the brain is being – at least in mice. In humans it goes a little bit different, but also through the sinuses, although sinuses are located so not all the blood in the human brain goes through the parasagittal sinus, but in the mouse brain all the blood goes through the sinuses in the dura. So major sinuses through which all the blood is being collected from the brain, and then goes out there. And so along those sinuses we find the lymphatic vessels, so they are sitting in the dura.
And this system also has been found in humans?
Well, that's a good question. You know, it's very hard to obtain high-quality human samples from the dura, because nobody really cares about this area. So we were lucky in the triple operation of Bea Lopes, who's a really great neuropathologist here at UV; she was able to give us, I think, nine samples from patients of dura of the sinus; these were all fixed in formalin. So we looked at those, and as you can imagine, the sinus in the human is huge, so obviously compared to a mouse. So in two out of nine, we were able to identify vessels that looked like meningeal vessels, but I think it warrants much deeper and much farther investigation to be able to say, yes, here they are. But if you ask me personally, why wouldn't they be? Why would mice have them and humans won't have them. So I think it's a matter of identifying their location and the best markers to use for them, but I think they should be there again. In two out of the nine samples, we were able to demonstrate that this is something that looks very, very good.
And you did immunohistochemistry on these to show the lymphatic properties and not general blood circulatory vasculature properties, either in the mice or human?
Oh, yes. So in the mice we identified the characteristics of those vessels really, really well. You know, nothing is perfect, that every marker on mouse markers are expressed by different cells, so you need here to provide a series of markers and to demonstrate that this is also indeed the real lymphatic cells. So we stained for LYVE1 and we showed beautiful staining with LYVE1, also with macrophages. And those are vascular structures and they came out to be macrophages. But one of the major transcription factors that will define lymphatic and endothelial cells is a Prox1. So we demonstrated two ways of Prox1; one is transgenic mouse and the other is staining for Prox1. And we also did two other molecules. One is a Podoplanin which is expressed in tissue lymphatics, and these vessels are expressive.
And the other molecule, she is very interesting. It's a receptor for VEGF3, VEGF-C. And this receptor is first on the lymphatic and endothelial cells. In the periphery, lymphatic and endothelial cells will respond to recombinant VEGF-C and will expand. So what we did here, we also injected a recombinant VEGF-C and we showed these vessels expanding. So we know now that the receptor is actually functional in the vessels, but also we now can expand the vessels. Whether it will impact any neurological disease, we don't know, but at least we have the capability to do so.
And then we also identified them by flow cytometry. We took samples from skin and from diaphragm where lymphatics are very, very well defined, and using the exact same antibodies we did also flow cytometry on our meningeal samples. And we show that the cells look exactly like they look from the skin and from the diaphragm; of course, the numbers are much smaller. So I think in terms of their calculation in a mouse, we are very convinced.
Now for humans it's more difficult. Like I said, the sample was in formalin and it's very hard to work with those samples, and, again, the area is huge to go through. So we were able in humans to get two markers to work; one was LYVE1 and the other was Podoplanin. We could not make Prox1 to work, I think it's a problem with the antibody and not with the vessel or potentially with the tissue as well. And these vessels would not label for some other markers, which would be characteristic of, for example, macrophages. So we were able to attack on them two out of four markers that would potentially allow for him to see. But we are now trying to identify those vessels by other means in humans as well, and I think flow cytometry may be the way to go.
Now you've shown that these lymphatic vessels drain into the deep cervical lymph nodes, and it looks like you've also been able to rule out drainage through the cribriform plate back into the cervical lymph nodes. Is that true?
I'm glad you bring this up, this is very important. So if you think of CSF, CSF is composed of several things. So we have the liquid itself, we have the macromolecules within the CSF, and then we have the immune cells within the CSF. So I don't think there is anybody would argue against liquid being drained through the cribriform plate and through the granulation; this is funny to argue. And obviously we are not claiming anything until we're absolutely sure; there is beautiful works from many, many labs showing that. But for the macromolecules and for the immune cells, the path which was proposed through the cribriform plate most probably if it's not a wrong one, it's probably not the major one.
Thank you for listening to Episode Fifty-Three of Multiple Sclerosis Discovery. This podcast was produced by the MS Discovery Forum, MSDF, the premier source of independent news and information on MS research. Msdiscovery.org is part of the non-profit Accelerated Cure Project for Multiple Sclerosis. Robert McBurney is our President and CEO, and Hollie Schmidt is vice president of scientific operations.
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