Understanding Membranoproliferative Glomerulonephritis, Including C3 Glomerulopathy: Part 1

Hot Topic

Expires: July, 2025

Membranoproliferative glomerulonephritis (MPGN) is a pattern of glomerular injury. MPGN is characterized by mesangial expansion and hypercellularity, endocapillary hypercellularity, and double contour formation along the capillary walls. MPGN can result from 2 main causes: 1) deposition of Ig/immune-complexes (IC) and 2) deposition of complement factors. Based on the etiology of MPGN, Drs. Sethi and Fervenza proposed a new histologic classification of MPGN into Ig/IC-mediated MPGN and complement-mediated MPGN. In this recording, Dr. Sethi demonstrates the use of the new classification with case studies.

Continued in Part 2



Sanjeev Sethi, M.D., Ph.D.

Professor of Laboratory Medicine and Pathology
Consultant in the Division of Anatomic Pathology
Mayo Clinic in Rochester, Minnesota


Our speaker for this program is Dr. Sanjeev Sethi, Professor of Laboratory Medicine and Pathology, as well as a consultant in Anatomic Pathology at Mayo Clinic in Rochester, Minnesota. Dr. Sethi has recorded two Hot Topics on the topic of membranoproliferative glomerulonephritis or MPGN. In part 1, he provides a brief introduction to the diagnosis of MPGN, including C3 glomerulopathy, and presents 2 interesting cases that demonstrate the variable presentation of the disease. Thank you, Dr. Sethi, for presenting with us today.Welcome to Mayo Medical Laboratory’s Hot Topics. These presentations provide short discussion of current topics and may be helpful to you in your practice.

The Elephant that is MPGN

Thanks so much for the introduction. I’m going to be talking on membranoproliferative glomerulonephritis today, in short, MPGN. And MPGN, for all these years, has been somewhat of a poorly understood entity, primarily because different people look at it differently. So you have the nephrologist who sort of treats the MPGN; and then you have the lab people; nowadays the complement people, in particular, who do the testing for MPGN; then you have the pathologist who sort of makes the diagnosis of MPGN on the biopsy; and then you have the transplant folks who, eventually, most patients with MPGN do end up with end-stage kidney disease, and then they get a transplant. So, the transplant person is also involved. So different people are involved in MPGN in the diagnosis, treatment, management, and everybody has sort of a different view of MPGN.

What is MPGN?

So, what is MPGN? MPGN, basically, is a pattern of injury; it’s not a disease entity itself. So, what happens in MPGN, you get injury in the kidney or, in particular, in the glomerulus, where you get deposition of immunoglobulins, you get deposition of complement, sometimes even fibrin, all that deposition leads to a very active phase, what’s called the proliferative phase, or that’s where the term “GN” glomerulonephritis or inflammation of the glomeruli comes from. So you’re dealing with inflammation, and that’s what glomerulonephritis is. But eventually, like everything else, things do heal up. There is healing, there is resolving, and when the healing and resolving happens, it’s called the membranous phase or the membrano-phase. The term together then is called membranoproliferative glomerulonephritis, indicating a phase of healing and proliferation together.

Pathology of MPGN

The glomerulus, which is the unit in the kidney, is inflamed. And when it’s inflamed, it’s usually enlarged. There are lots of cells in there; that’s why we use the word “hypercellular glomeruli.” What kind of hypercellularity? So you have hypercellularity in the mesangium; that’s one area of the glomerulus, or you can have hypercellularity within the capillary loops, that’s called endocapillary. This, is then the proliferative phase. Eventually, healing takes place, and that results, actually, in thickening of the capillary walls, and that’s the membrano-phase. The immunofluorescent studies will show you the immunoglobulins, will show you the complement, and will show you the fibrin. And then the EM, if you do electron microscopy, you will see these immunoglobulins, and you will see the C3–that’s the complement or the fibrin in terms of deposits, they're called electron-dense deposits.

Here is what MPGN looks like. You look at the top 2 slides, those are pictures of glomeruli that show the MPGN pattern. So, if you look at the bottom slide, you can see a normal glomerulus; that is the glomeruli have open loops. You can see that the glomerulus and mesangium and you can see 1 to 2 or 3 cells. The capillary lumen is kind of open, and you can see the Bowman space is also open.

When you go to the MPGN, you can see it’s a very busy glomerulus; that means there’s lots of cells in there. So this is a clearly already proliferative or the inflammatory phase, the “GN,” the glomerulonephritis phase, of MPGN. You can see a lot of cells. See, these cells that are sitting in the mesangium, this is the mesangial hypercellularity. When the cells sit in the capillary lumen, it’s the endocapillary proliferation. So both mesangial and endocapillary hypercellularity is taking place. It tends to make these capillary tufts that you see in the normal, sort of just loose, lying all over the place, makes it look very lobular, and you can see this lobular accentuation of these tufts. This is very classic for MPGN.

Let me show you another higher power. You can clearly see these sort of deposits within the capillary lumen. This is what’s causing the injury. And then, this leads to this proliferation. And if you look at these black arrows over here, these are pointing to the double contours, or the healing phase, that I was talking about. So this is what’s called the healing phase or the membranous phase. And this whole entity, in itself, is called MPGN or membranoproliferative glomerulonephritis.

Now, all cases of kidney biopsies, we stain them for various immunoglobulins: IgG, IgA, IgM, κ and λ light chains. We also do some complement staining: C3, C1Q. And the reason I don’t have a normal for you is because most normal glomeruli would not stain for any immunoglobulins or any complement factors.


Okay, on the other hand, I can show you a normal EM. This is what a normal electron microscopy looks like. And you can see over here, this is a capillary loop, where my white arrow is going to. They are supported in the middle down at the bottom by the mesangium. There are 3 main complements of each capillary loop. One is the podocytes, that’s marked as the Ep or the epithelial cell, and you can see these nice fenestrations; that’s a normal podocyte. Next one, the basement membrane, and that’s the basement membrane. And then underneath, you have this endothelial cell that sort of sends out the cytoplasmic fenestrations, and you can see that lining the capillary lumen. Anything that’s going on over there is called the subepithelial region. And anything that goes on over here is called the subendothelial. And that stuff that happens within the glomerular basement membrane is called intramembranous, so 3 areas where a lot of action takes place. You need to recognize there are 3 regions to the capillary wall—the subepithelial, the subendothelial, and then the intramembranous. And this is the mesangium that sort of supports these capillary tufts.

Active Phase-Endocapillary Hypercellularity

Now what happens in MPGN? So this is when the inflammation is taking place. This is what normal looks like, and this is what abnormal or MPGN looks like. You can see it’s very busy. The entire lumen is stuffed. So if you look over here, that’s the basement membrane that I just showed you. Now everything that’s happening over here in this particular slide is all subendothelial. That means it’s below the endothelium. And, as a pathologist, I can recognize that these big white arrows are pointing to this dark black stuff; that’s all subendothelial deposits. So you get these deposits that are probably made of immunoglobulin or complement factors. Second is you can see a lot of cells in here, so a lot of cells are coming in to clean up this mess, and some of them are mononuclear cells; some of this might be an endothelial cell itself. Occasionally, you’ll get neutrophils. At the same time, the other thing that’s happening is a lot of this fuzzy stuff that you see here, the second black arrow, this is material, maybe just matrix material, being made by the endothelial cells, maybe made by some mesangial elements, or maybe even made by these infiltrating cells. We don’t know. But, bottom line is, a lot of this matrix stuff is being made and this is sort of a reaction to this, all this material that’s deposited over here.

Chronic Phase-Double Contour

Eventually, when the healing is complete, this is what it looks like. So here is a healed glomerulus now, and you can see that now what’s happened is the lumen is now opened up again. Yet, if you compare it with that first one, it’s not perfect. This was that perfect capillary lumen and now look at it, it’s still sort of distorted. Here is the old basement membrane, the original one, and then during the healing phase, you get this entrapment of these cellular elements. Sometimes, you get some small deposits that are entrapped in there. Sometimes, an entire cell gets entrapped like over here. And this is what’s called the double contour or the tram-track appearance. And this is sort of pathognomonic of an MPGN in the diagnosis that from the time this was described, it was said, you’ve got to see the tram-track appearance. Remember tram-track meaning there are 2 tracks; so there is the old basement membrane, and here’s the new basement membrane. So you get this railroad or tram-track appearance or double contour. Basically, all that it indicates is healing is taking place. Now there is some resolving compared to this phase. When it’s very active, you don’t see that double contour so clearly; but when it’s healed, you can see the double contours.

The take-home message, is when you get all these deposits in the subendothelial region, whether it’s complement or whether it is immunoglobulins or fibrin, it’s not a good thing because it drives inflammation. It drives inflammation the more inflammatory cells that come in make the capillary loop even worse. So what the glomerulus does is it sort of heals itself by making new basement membrane. It sort of entraps these elements in there and, maybe tries to fool the neutrophils and the monocytes in not seeing these injurious elements, and they’re sort of entrapped within this thickened glomerular basement membrane or entrapped within this so-called double contour. The idea being, let’s avoid these immune deposits to be seen by the neutrophils, because once the neutrophil sees them, it releases proteases and it releases certain cytokines that can damage the capillary wall. So it’s one way of sort of hiding these immune deposits by making these double contours. The blue arrow points to the old basement membrane, and the yellow arrow points to the new basement membrane. So you have this double contour that’s formed.

MPGN- "Old" Classification, Based on Location of Deposits on EM

So based on these double contours and based on these deposits, MPGN, for at least 20+ years, was classified into type I, type II, and type III. And it was purely a description based on where the deposits were situated. So most often, the deposits were subendothelial. For example, over here, this is where the deposits showed up. There was a lot of inflammation. Then you got the double contour being formed, and this was called MPGN type I. Regardless of the underlying etiology, it was called MPGN type I. And then, you had some cases where the deposits were intramembranous; that means they were within the membrane. And this particular entity, is still today what’s called MPGN type II or dense-deposit disease. The “dense” coming from the deposits were extremely dense, and the word “transformation” of the glomerular basement was used.

On the other hand, if you saw deposits that were not just subendothelial but also subepithelial, so this is on the other side of the basement membranes; you can clearly see these are subepithelial. So you have subepithelial deposits and you have subendothelial deposits; so this was called MPGN type III. So we had 3 types of MPGN, I, II, and III, based on where the deposits were on electron microscopy, regardless of the underlying etiology. It was purely a morphologic description on the presence of the location of the deposits on electron microscopy. And this held good in the 70s, and 80s, and even 1990s. Subsequently, we realized that there were some forms of MPGN that had an underlying etiology, and that was when hepatitis C was quite common; probably is still a common cause of MPGN. Then we called it primary and secondary. Every time a patient with MPGN had hepatitis C, they said this was probably a secondary MPGN to hepatitis C. Everything else was labeled as primary.

Now, let’s go into 3 or 4 cases. So in every case of MPGN, you can figure out the underlying etiology. Classification of MPGN type I, type II, type III does not hold good anymore because when you say I, II, and III, that tells you that you don’t know the underlying etiology. But today, in every case of MPGN, we should be able to figure out the underlying cause and etiology and then treat the patient accordingly.

Patient #1

So let’s look at this first case, this is a 47-year-old man. This is a classic presentation of MPGN. They have proteinuria. They have hematuria, because they have damaged the glomeruli, so the protein is leaking out into the urine, red blood cells are leaking out into the urine. The red blood cells are clogging up the tubules, so the patient gets renal failure. And you can see that basic tests like serum creatinine shows high, 4.7; clearance is down to 14; he’s got urinary protein 2 gm/24 hours; a really active sediment (when we say active sediment, we mean red blood cells and red blood cell casts). So he’s got an active disease, what’s also called nephritic syndrome.

If you look at this particular patient’s biopsy, it shows you all the classic MPGN findings. So it has a globular accentuation, has the mesangial proliferation, has the endocapillary proliferation, has the extremely thickened capillary walls and double contours. So it has the MPGN look on light microscopy.

You do the IF, now I can show you a positive IF in this case, because it lights up for IgM, that’s the predominant immunoglobulin. In this case, it lights up for C3, it lights up for κ light chains, and λ light chains.

And if you look at the deposits, in this case, these are all subendothelial deposits, and you can see that classic double contour of MPGN. So this is a classic MPGN with IgM, C3, κ, and λ, and subendothelial deposits.

Patient #2

Let’s look at case 2. This is a 66-year-old man with a past medical history of multiple urinary abnormalities. Again, he’s got proteinuria; he’s got hematuria. These are classic manifestations of MPGN, and most patients with kidney disease or glomerulonephritis, also hypertension, so he’s got that as well.

Biopsy done in this case shows you the same story again. It shows you a very proliferative-looking picture. You can see the glomeruli are hypercellular. And if you look at a high power and look carefully, you can start appreciating the double contours. So again, he’s got that MPGN.

When you do the immunofluorescence in this case, remember the previous one I showed you showed IgM, C3, κ, and λ. This one shows you the IgM, just like the previous one, shows you the C3 like the previous one; but in this particular setting, the lambda is completely negative and the κ is nicely positive. So basically, he’s got a protein that stains for IgM and κ and is negative for λ light chains. You do the EM in the case, this is again a busy slide because it’s a really active phase when the patient was biopsied. The double contours have not yet formed very nicely; but again, the white arrows that’s the old basement membrane, and here’s some new basement membrane being formed. A lot of action in the capillary lumen; these black arrows, points you to all these deposits, so again, subendothelial deposits. Keep that in mind, patients 1 and 2, it’s subendothelial deposits.

Patient #3

This is patient 3. This is an 18-year-old young woman. She was just found to have hematuria and a little bit of proteinuria initially. But then a repeat urinalysis showed a fair amount of protein, 7 gm of protein in the urine.

And when the biopsy was done, you can clearly see this is a lobular sort of accentuation, has very thick double capillary walls. You can see the double contour right over there. So, again, it has that MPGN look to it.

In this particular case, when we did the immunofluorescence, it didn’t show anything. It just showed C3. Everything else was negative, including κ, λ, IgA, IgG, IgM, C1Q. And when we did EM, we found these huge, dense intramembranous deposits. So that’s case 3.

Patient #4

And this is the last case. This is gentleman, 61-year-old man, with a monoclonal gammopathy. That means he’s got a small M spike in his serum; it’s an IgG κ. He has no complement levels. He’s got the picture of MPGN: he’s got hematuria, he’s got red blood cell casts,

And I looked at this biopsy and, again, multiple pictures here showing you the proliferative look of MPGN. You can see the lobular accentuation of the capillary tufts. You can see the mesangial hypercellularity. If you look carefully here, you can even on light microscopy, you can see the tram-track appearance of the double contours. So again, classic MPGN-like picture.

IF, again just like case 3, shows you strong C3, yet nothing for an IgA, nothing for an IgG, nothing for an IgM, nothing for κ or λ light chains, nothing for C1Q. EM shows you these large amounts of deposits. These are mesangial deposits.

Let me show you a few more pictures. These are, again, mesangial deposits. Now let’s look at the capillary wall. In this particular cast setting, you can see the subendothelial deposit here. You can also see a big huge subepithelial deposit. That means it’s on the other side of the basement membrane.

Now, here are the 4 cases. One, 2, 3, and 4, all have an MPGN based on the light microscopy.

And, if you went by the EM, the first patient has subendothelial deposits. Second patient has subendothelial deposits. Patient 3 has the intramembranous deposits, and patient 4 has both subendothelial and subepithelial deposits.

Membranoproliferative Glomerulonephritis

So the classification of these 4 cases would really be MPGN type I for case 1 and 2, MPGN type II for case 3, and MPGN type III for case 4, regardless of the underlying etiology. Most cases of MPGN, in the past, were treated with immunosuppressives. So all 4 patients would receive some form of immunosuppression.

Now if you go back to the IF on these cases, remember patients 1 and 2 showed immune complexes; the first one was IgM, C3, κ and λ; the second one was an IgM κ alone; and patients 3 and 4 had complement alone. So really, by just looking at the immunofluorescence, you can tell that patients 1 and 2 have immune deposits, and patients 3 and 4 don’t have immune deposits.

Just based on that, I can tell you that the underlying etiology is different. So, for example, patient 1 had an MPGN from immune deposits. When you see a lot of IgM, it usually indicates a viral infection. And indeed, this particular patient was found to have hepatitis C. So MPGN in patient 1 was due to hepatitis C. Patient 2, if you remember, it lit up for IgM κ. There was no λ. So I could tell by looking at this particular MPGN, this was most probably due to a monoclonal protein, an IgM κ, and this was secondary to this particular paraprotein or dysproteinemia. And indeed, the workup did show the IgM κ. Patients 3 and 4 didn’t have any immune complexes. It showed only complement and, initially, we would call this complement-mediated MPGN. Now the term, C3 glomerulopathy is used. That means this is a purely complement-driven disease entity. Patient 3, we called it dense-deposit disease because of the dense deposits within the glomerular basement member. In patient 4, we called it a C3 glomerulonephritis now, and we’ll get into this a little bit in the second lecture. So, clearly, you can see that MPGN I, II, and III, even though they have different underlying etiologies, and you can clearly look at this particular patient. Patient number 1 would need to be treated for his hepatitis C. Patient number 2 would need to be treated for the dysproteinemia. Patients number 3 and number 4 have very specific abnormalities in the alternative pathway, and they need to be treated for that. Patient number 3 actually has a mutation, that young girl has a mutation in a complement-regulating protein, and patient number 4 actually has factor H antibodies.

MPGN – A Simple Classification

So based these particular findings, we reclassify the MPGN. We said MPGN should really be classified into 2 major disease entities: 1) When you see immunoglobulins; if you see immunoglobulins in the biopsy material, that would be one type of MPGN. 2) The other one would be where the immunoglobulins are essentially negative, but there is a lot of complement factor. When you have immunoglobulins in the immunofluorescence, we call it immunoglobulin or immune complex-mediated MPGN. Typically, immune complexes can come from 1 of 3 entities. So, if you were at the Mayo Clinic, where you’re dealing with mostly a geriatric or an older population, most likely, the MPGN is coming from serum monoclonal protein. If you are living in a third-world country where there’s a lot of infections, in all likelihood, you might have an infection-driven immune complex MPGN. And then of course, there are autoimmune diseases like lupus, and rheumatoid arthritis, and Sjögren syndrome, and so on. All of these entities can lead to an immune complex-like MPGN. What you can clearly see from this particular figure is that the treatment for dysproteinemia, treatment for autoimmune disease, treatment of infections is obviously very different, and you can’t just lump MPGN into I, II, and III with total disregard to the underlying etiology. And, on the other hand, when you have just complement being deposited, in the past you call this MPGN complement-mediated MPGN; the term C3 glomerulopathy is now used. This is typically due to dysregulation of the alternative pathway of complement. And, again, this is further subdivided into dense-deposit disease and C3 glomerulonephritis, and we’ll get into this a little bit more in the second talk.


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