Jonathan Barratt, PhD, FRCP: Sanjeev, you're going to have to take us very slowly through the kidney biopsy evaluation for someone who has C3G. I know there are multiple potential diagnoses and it's a pattern that could lead on very many different routes, diagnostic routes, and investigation routes.

Sanjeev Sethi, MD: Let me start off by saying although we understand C3G better now, this is not a new disease entity. We're calling it C3 glomerulonephritis now or C3G now. This entity has been around, it's just that we gave it a different terminology because we didn't understand it. In the past it was called membranoproliferative glomerulonephritis one, two, three. And we know very well that membranoproliferative glomerulonephritis or MPGN for short, is just a pattern of injury, and that can result from various etiologies including infections or autoimmune disease, monoclonal gammopathies, as well as complement abnormalities. When you have dysregulation of the complement pathways, the complement material or complement fragments are activated, the complement fragments get deposited in the kidney and then they result in inflammation and scarring just like in IgA. In IgA you have IgA that gets deposited, in this, in C3 glomerulonephritis, or what we used to call MPGN in the past, complement products get deposited. That's just the basic background but on the biopsy so what do you see? Again, this is not a very extremely difficult diagnosis to make. Like I told you, the diagnosis in a lot of the diseases is made on immunofluorescence microscopy so the first thing most pathologists do is look at immunofluorescence before we look at light microscopy. For IgA, every biopsy gets stained for IgA, IgG, IGM, C1Q, C3, albumen, fibrinogen and then Kappa Lamda. I'm sounding repetitive but once you look at the IF and a glomerulus lights up for C3, that means brightly positive for C3 and everything else is negative. There's negative IgA, there's negative IgG, there's negative IGM, negative C1Q, no Kappa Lamda, so you know you're dealing with probably a C3 glomerulopathy. The diagnosis is really based on immunofluorescence where you see this bright C3 staining. Sometimes you can see a little bit of IgG and IGM, typically there's an area that is scarred. They're not typically seen in the exact same location as C3 but we do understand that small amounts of IgG or IGM are OK. Typically, it's dominated by this bright C3 and everything else being active. Before I go to the light microscopy, when you see this bright C3 the differential diagnosis of course for me the first thing is this is C3 glomerulopathy. The other question is could this be anything else? Two other things that always come up to my mind; is am I looking at glomeruli that are extremely sclerosed and scarred? If you have a biopsy that is scarred from other diseases including IgA or maybe diabetes or hypertensive kidney disease that has led to a lot of scarring, when you see a lot of scarred glomeruli, they will also show a lot of C3. You must be careful that this C3 that you're seeing is the real deal, in other words this is general IgG along the capillary walls and the mesangium and you're not dealing with on a biopsy you've got three glomeruli that are all sclerosed, they all have C3 and then that's not C3 glomerulopathy. That's one thing to keep in mind that people forget that scarred glomeruli will entrap C3. The same goes the reverse way too. You have a C3 glomerulopathy which shows you the bright C3 but you have five glomeruli that are sclerosed, out of 10 let's say. Those five glomeruli that are sclerosed will also entrap some immunoglobulins so there'll be a little bit of IgG and lots of IGM sitting there. You must be careful now not calling these immune complex GN and a lot of studies get sort of messed there. They call them immune complex disease. Truly, these were just sclerosed glomeruli. The reverse also holds good; be careful not to over call C3 glomerulopathy when in fact it is a disease entity with scarred up glomeruli. This must be carefully looked at. The diagnosis again is truly made on immunofluorescence, bright C3 in the absence of everything else keeping in mind that you don't want to be dealing with sclerosed glomeruli because it can lead you down the wrong pathway. On light microscopy, this disease entity results from chronic deposition of complement products along the capillary walls as well in the mesangium. It's not predominantly a mesangial disease unlike IgA which is dominated by mesangial proliferative lesions. This is more of a capillary wall deposit over time. Most often, the pattern that's most common is what we call MPGN and that's what was called membranoproliferative glomerulonephritis. In other words, unlike microscopy what you see is extremely thickened capillary walls. That's where the deposits have been sitting, the complementary deposits are sitting. Over time, these complement deposits also elicit sort of a remodeling to say because this is not a one day shot. This occurs over weeks, months, if not years. The glomerulis is good at remodeling or sort of entrapping these deposits so you get extremely thickened glomerulis based membranes. There are also some mesangial deposits. There's also mesangial expansion, because these deposits are deposited in the capillary wall or sub-endothelial there's also influx of leukocytes. This whole picture of deposits along the capillary walls, the leukocytes trying to clean these deposits, the glomerulis trying to re-model at the same time leads to the sort of lobular look of the glomeruli capillary toughs that's called MPGN. We used to call this a disease in the past itself but now we know very well that this is just a pattern. This is a common pattern of C3 glomerulonephritis. On immunofluorescence you get bright C3, on light microscopy typically an MPGN pattern. You can sometimes get mesangial proliferative, sometimes you can also get extremely severe crescentic pattern, but MPGN is the most common pattern. Now, the diagnosis of C3G is already made over here on IF and on light microscopy so why did we need electron microscopy? We do electron microscopy in all these cases. The deposits in C3G can be quite heterogeneous so you can have two types of deposits. One is the deposit of are extremely dense so they are dark, we use the world in pathology, osmophilic, they sort of lie the capillary walls. Very often they look intramembranous and this entity when you get these dark, dense complement deposits we call it dense deposit disease. On the other hand, when the deposits are not dark, dense, saucered shape or snakelike almost along the capillary walls and they're just patchy, they can be large, but they are not that dense, and they are not lining the base membranes then we call it C3 glomerulonephritis. Both are very often associated with corresponding deposits in the mesangium as well. Sometimes these deposits have a certain look to them that most pathologists can tell this is dense deposit disease versus C3GN but these deposits on immunofluorescence also have- they have a way of forming what we call mesangial rings. That means you have complemented in a circle almost and you can see that on electron microscopy. You can see the deposits in a circle, and we call these mesangial rings of C3, very typical of C3 glomerulopathy. IF bright C3, light microscopy, MPGN light pattern, on electron microscopy there's a little dichotomy, dense deposit disease when you get these dense intramembranous deposits. On the other hand, when these deposits are chunky and sort of haphazard, all over the place, we call it C3 glomerulonephritis. The prognosis of C3 glomerulopathy is made by all three; IF, LM and EM.

Transcript Edited for Clarity