Hello, and welcome to Episode Thirty-Four of Multiple Sclerosis Discovery, the podcast of the MS Discovery Forum. I’m your host, Dan Keller.
This week’s podcast features a follow-up interview from last week’s episode with Dr. Pierre-Antoine Gourraud. This week, we interview Dr. Jill Hollenbach about killer immunoglobulin-like receptors – or KIR – and their relationship with human leukocyte antigen and MS. But to begin, here’s a brief summary of some of the latest developments on the MS Discovery Forum at msdiscovery.org.
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Now to the interview. Dr. Jill Hollenbach is an assistant professor in the department of neurology at the University of California, San Francisco. She met with science journalist Cynthia McKelvey, to talk about KIR in MS.
Interviewer – Cynthia McKelvey
Why don’t we start by introducing what KIR is, how it’s different from the potassium channel and what its relationship is to HLA.
Interviewee – Jill Hollenbach
Well, KIR is an acronym, it stands for killer immunoglobulin-like receptor. These are receptors on the surface on the surface of natural killer cells. They use generally, not in every case, but use HLA as their ligand and they have either an activating or inhibitory effect on natural killer cells.
You mentioned in your talk earlier today at UCSF that they are difficult to study. Why is that?
Just in terms of their genetic architecture. KIR occupy a complex on chromosome 19, it’s a multigene complex. And so on any individual haplotype that’s one chromosome, an individual can have between 4 and 14 KIR genes. These genes are really recently evolved, and so they’ve kind of arisen in humans as a result of repeated events of recombination and gene duplication. So what that means is that one KIR gene often at the nucleotide level looks an awful lot like another KIR gene. And so we’ve had a lot of issues. A lot of the methodologies that are available right now in terms of sequencing, part of this has to do with a lack of human genome reference alignments, but there has been a lot of difficulty in examining these genes because they look so much like one another.
How does that relate to why they haven’t really seen them on genome-wide association studies?
There’s a couple of reasons why we don’t see them on genome-wide association studies. One is that, as I mentioned, there haven’t been a lot of good reference alignments, so as a result we don’t actually see a lot of SNP markers on most of the common platforms that are used for genome-wide association studies. And then the markers that are there are often lost to quality control, because we have a lot of gene content variation, which is kind of like a copy number variant. And so if we only see a result for one chromosome, for example, for a given SNP, that is not going to pass general quality control thresholds. And, of course, you have to recognize that when you’re doing these GWAS studies, you’re looking at a hundred thousand, five hundred thousand, a million markers, two and a half million markers, so you’re not going one-by-one and saying, well this KIR-1, we expect to only see one copy or that sort of thing. So it just gets thrown out in the mix with things that don’t pass QC.
How does KIR relate to multiple sclerosis specifically?
Well, we’re trying to figure that out. So there’s been a small number of studies examining KIR in multiple sclerosis, and what seems clear is that variation in the KIR does play a role in susceptibility to multiple sclerosis. It may play a role in progression; we’re just not sure. There’s not been enough work done to say definitely what’s going on, but there’s enough evidence to say that something is going on. And some of the work that I talked about in my talk today, an analysis of KIR variation along with HLA in an African-American MS cohort, a very large study population, it seems clear that there is some association of KIR variation with susceptibility or protection for multiple sclerosis.
Why study the African-American cohort? What does that tell us about MS in general?
We want to study them because they’re different from one another; so an African-American population is going to look very different genetically with respect to KIR and HLA from a European-American population. So we want to know two things. We want to know is there something different going on with these genes with respect to disease in these different populations, OR at the same time we want to know is something the same going on? And so we can learn something both from these commonalities and differences, and both can be really important in genetics. So if there’s something that is important that’s specific in the African-American population, we want to know that, and we can only find out by looking at a number of different ethnic groups.
Let’s talk a little bit more about interaction between KIR and the HLA ligand, and how that plays in with Bw4. And if we can define all of those things, too, that would be great.
Okay. Well, so KIR molecules, as I mentioned before, need to see something, they need to have a ligand on their target cell. We have both inhibitory receptors and activating receptors. The job of the NK cell is to perform immune surveillance. So NK cells kind of wander around the body, and what they’re looking for are cells that don’t look healthy. So what does that look like and what is an unhealthy cell? It’s a cell that is virally infected, it’s a tumor cell. Those are the two main things that NK cells are looking for. And it’s a really important function because they’re part of what we refer to as the innate immune system; it’s the first line of defense against these kind of unhealthy events.
And so what does an unhealthy cell look like? Well, one of the things that happens in both viral infection and tumors is downregulation of MHC class I. That’s what the KIR are looking for. So when an NK cell encounters a healthy cell, it will see HLA class I, it immediately recognizes this is self, this is healthy cell at least in terms of what I’m able to see as an NK cell, and it will move on and it won’t cause any damage to the cell after making contact between the KIR and that ligand.
On the other hand, if the KIR doesn’t see this HLA ligand, the inhibitory KIR, an activating KIR – and we’re still not completely sure what the activating KIR ligands are – but the activating KIR is also bound to something on the surface of this cell. If the activating KIR is bound but the inhibitory KIR is not, what happens is the NK cell is going to lyse that cell which is presumably unhealthy in some ways – tumor or viral infected.
Now HLA class I – actually all HLA molecules – have another primary really important role which is antigen presentation to T cells. Class I molecules present antigen to the CD8-positive T cells, and so that’s how these T cells perform their role in terms of the active immune response. KIRs see a different part of the HLA molecule than the T cell receptors, and so they see kind of this piece of the HLA class I molecule that’s kind of on the side of where the T cell receptors sit. And the variation that they see on that HLA class I molecule can kind of be defined by these broad categories based on the particular amino acid residues. And it’s generally just from two to four amino acid residues that determine whether or not a given KIR can see a given HLA class I molecule.
So one of these epitopes, as we call them – and if they were originally defined on a serological basis because specific antibodies could recognize them – so one of these epitopes is referred to as Bw4; these are epitopes that we mainly see on HLA-B molecules – not all – so depending on the population, human population, may be from 40 to 60 or 70% of HLA-B molecules will have this Bw4 epitope. Some HLA-A molecules also bear the Bw4 epitope. So that’s what some KIR molecules, specifically KIR3DL1, is seeing is Bw4.
The results of the study that I talked about today and what we saw is in this African-American multiple sclerosis cohort, individuals that have both 3DL1 and HLA alleles with the Bw4 epitope appear to be protected from multiple sclerosis. And so we see higher frequencies of this combination in our control population relative to patients. So that suggests a protective effect of that combination, 3DL1 plus Bw4.
Where do you see the research going from here?
Right now the data that we’re looking at is strictly in terms of carrying frequencies for these particular genes. So these genes are actually highly variable at the allele level, so any given gene like KIR3DL1 has many, many variants that are already known, and likely many variants that we haven’t identified yet because the technology has not been there. The technology is just about now caught up to the point where we are able to examine at the sequence level the variation within these specific KIR genes, and so I think that that’s really the next step. And we’re actually taking steps to start examining this cohort and others in terms of this fine-grained variation in the KIR genes.
Very good. Thanks.
Thank you for listening to Episode Thirty-Four 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. MSDF’s executive editor is Robert Finn. 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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