Diagnosis and Treatment of Immune-Mediated Kidney Diseases - Episode 17
Sanjeev Sethi, MD, discusses clues in the pathology of kidney biopsies that could lead to a C3G diagnosis.
Jonathan Barratt, PhD, FRCP: Sanjeev, because we've mentioned this several times with monoclonal gammopathy being important, just run us through the things that you would see that would help us as a nephrologist that would point us to that on a kidney biopsy that makes us want to look for that diagnosis.
Sanjeev Sethi, MD: As you can see, we’ve devoted a fair amount of time on complements. On the biopsy, like I said, the C3G is a relatively, I would say, easy diagnosis to me because it lights up for C3 and if you can get away, if you've been doing this for a long time, get away with the nonspecific IgG and IgM, it's not that difficult. The question is, when you have a monoclonal gammopathy in circulation, what's that doing over there? Can you make more of it? What do we do? There are 2 things you can do. Is the monoclonal gammopathy entrapped in there or not? Sometimes the monoclonal immunoglobulins, when they are sitting in the glomerulus, don't light up for one reason or the other. When we do immunofluorescence, again, IgA, IgG, kappa, lambda the whole deal, typically monoclonal proteins will stain for a heavy chain. You get an IgG usually, and then a light chain. It's either kappa or lambda, typically kappa, it'll be IgG-kappa. We know that's driving the inflammation. This is a monoclonal story. We are straightforward. Of course, there's complement in there as well, but that's a different disease entity. It's really a monoclonal immunoglobulin that is deposited in the kidney, and that's what is driving the inflammation. Some people like to use the word PG and MID proliferative glomerulonephritis for monoclonal immunoglobulin deposits. Really long entity. I used to call it just GN from monoclonal immunoglobulins. Whatever terminology, it’s monoclonal immunoglobulins driving the glomerular disease and you can do the immunofluorescence and you see it. That's straightforward. You don't have to worry about it. The patients who have C3G and have monoclonal gammopathy, the monoclonal gammopathy is probably activating the pathway in circulation. The monoclonal immunoglobulin itself is not deposited there. That's what makes this sort of a trickier diagnosis. In some cases, in my experience, is less than 5%. The monoclonal immunoglobulin is sitting there yet our immunofluorescence studies don't pick it up. We use the word masked immunoglobulins just masked meaning that these immunoglobulins are there, but your IF is not sensitive enough, or you need to use a salvage technique to show that truly those monoclonal immunoglobulins are there. What we do is we use something called protease. We treat the kidney biopsy tissue prior to the immunofluorescence. That means if there are small amounts of immunoglobulin sitting there that have the epitopes that are recognized by that antibody that you use for staining, now they will open. We treat these kidney biopsies by an enzyme called pronase. People use other enzymes and we treat it, and then we do the immunofluorescence microscopy in a small percentage of C3G patients. It varies by center. My own experience is less than 5%. You will unearth, if I may use that word, you will unearth a monoclonal immunoglobulin that's sitting there that you would've called C3G in the past. Some people say that this is issue of fixation so depending upon the fixatives we use, you're more likely to have a hidden immunoglobulin that you need to use a protease to sort of unearth. But other places they never see a case. Particularly in Europe, it's extremely infrequent because they do not use this fixative. At large centers like Columbia and the Mayo, our specimens are received from centers from Texas, California, Bermuda, Hawaii, everywhere. They must send it to us in a particular fixative. If you just send the tissue in saline, by the time it comes to us, it's all gone. They say, this could be a fixative related issue, that you have amassed immunoglobulin, because those are - the fixative somehow injured that, so we have to use techniques to sort of show it up. Whether it's the real deal or not, we don't know.
Transcript Edited for Clarity