As we know even after recovering from the HepB, some virus stays in liver cells as cccDNA and in extremely rare condition, may reactivate. This is also a hindrance in developing an effective cure of the same.
My query is, the HepB reactivation is still a possibility if one has Anti-Hbs titer and activated T-cells like in those recovered from infection (specially an acute infection)?
Further, that activated T-cells should be able to kill liver cells containing cccDNA whenever it tries to replicate and express proteins for multiplication ?
Does it remain dormant without doing anything for years?
Then how does it know when immunosuppression bring down immune system?
It reactivates following immunosuppression that indicates that it doesn’t remain dormant but replicates but copies are neutralized by antibodies.
Similarly, I guess activated T-cells should eventually kill all the liver cells over a period of time as cccDNA will try to multiply at some point giving signal to T-cells to attack ?
Great questions! Basically, after you have anti-HBs, your immune system acts to suppress any latent virus that may be around and stops it from spreading. This basically prevents any liver damage from happening over the long-term.
So you’re right in thinking “that activated T-cells should be able to kill liver cells containing cccDNA whenever it tries to replicate and express proteins for multiplication”
So, if you are immunosuppressed, your immune system will stop preventing new infections and hep B can spread.
This is what we think may happen, but we don’t really know yet and at what speed this happens.
Thomas is right, the immune system, including B cells that are best know for making antibodies but that also have other functions such as antigen presentation and cytokine secretion, keeps the residual HBV in check in the vast majority of folks who do not suffer immunosuppression, such as from HIV co-infection or organ transplant.
A bit more detail: Basically, we know that this reservoir of cccDNA exists in the body and some of the triggers that cause it to resurge. However, we know next to nothing beyond those 2 points for sure. We don’t even know for sure the reservoir is in the liver, although that is by far the most likely spot.
Persistence of the cccDNA actually explains a confusing thing about HBV immunity after an acute infection: Robust immune responses, including lots of anti-HBs antibodies in the blood, last for many decades after viral clearance without the “antigen re-stimulation” that is often needed to keep memory immune responses from gradually waning. This has led to the hypothesis that the residual low level of cccDNA induces HBV replication at extremely low levels (perhaps intermittently), and this replication is promptly squashed by the immune system. In this scenario, the HBV proteins made during the brief reactivations would restimulate the immune system and keep it robust. This is an unproven hypothesis, but is quite plausible and I suspect it is correct.
To specifically answer your questions:
Reactivation is extraordinarily unlikely if you are anti-HBs positive.
The T-cells probably do kill the infected cells, but T cells are hampered in the liver because it is an “immuno-privileged site” that dampens immune responses. The reason for that is because the antigens in the food we eat go through the liver first, and everyone would die of food allergies if the liver was not an immunosuppressive environment. That means the immune system essentially has to fight HBV with one hand tied behind its back.
We do not know if the residual HBV is completely dormant or just turned way down. We won’t be able to figure that out till we find where the cccDNA is hiding in the body. The studies that have been done can’t find any residual antigen in the liver, but the assays that need to be used in those studies (mostly immunohistochemistry) are not very sensitive and easily could miss very low levels of HBV proteins.
We suspect the residual HBV is constantly trying to replicate but the immune system is constantly suppressing it. If that is true, HBV senses immunosuppression by the absence of pressure against it.
Total killing of cccDNA-containing cells is what would be expected, but does not happen. Again we don’t know for sure why, but the immuosuppressive nature of the liver is likely to be one cause (but unlikely to be the only one).
Thank you Dr Thomas and Dr Tavis for your kind replies.
But Dr @john.tavis with all due respect, if liver was such an immuno-priviledged site, how do we explain clearance of infection by T-cells in more than 95% adults ?
Also, it is proven that cccDNA stays in every individual recovering from infection? Because @john.tavis as you said we don’t even know for sure where does it reside and various techniques have failed to locate it?
Maybe it’s not in every recovered person’s liver? Completely removed ?
Maybe that’s why reactivation so extremely rare?
Maybe their numbers (cccDNA containg cells) keep on reducing over many years if not completely killed at once? @ThomasTu
This also indicate the same. I mean replication squashed by the immune system promptly. Therefore, cccDNA cells should be all killed over a period of time.
Basically, we don’t know when someone is completely free of cccDNA because it’s very difficult to access someone’s liver and detect it.
This is also not known. In fact, if we did know how this worked, then this would be a major discovery in the immunology of hepatitis B because we would then have a way of activating the immune response against a chronic infection and get a functional cure.
So basically we don’t know so many things despite such an old disease. But we are sure that cccDNA stays somewhere in all recovered person’s body/ liver ?
Our best guess from animal models of HBV infection and some clinical studies is that cccDNA is still there after HBsAg-loss, but the potential risk of reactivation is extremely low outside of the conditions we mentioned earlier.
A followup on AJK’s “squashed by the immune system” comment:
It is unlikely that all suppressive activities of the immune system result in destruction of infected cells or in removal of the cccDNA without killing cells (this is known to occur, and is called cytokine-mediated clearance). Part of interferon alpha’s effect on HBV is by silencing the cccDNA (ie, turning it off rather than destroying it). We don’t know if that shutoff is permanent or could fail with time.
A theme here is that we don’t know a lot of really important things about how the immune system controls cccDNA and the fate of the cccDNA in the liver. That is because it is extraordinarily difficult to study these things in people’s livers. That would require closely-spaced biopsies exclusively for research purposes, and that is unethical for medical reasons.
Sorry for not directly answering your question from 14 Jan. We’ve been really busy pushing to get a big study out for publication.
Immune responses are not 100% or 0%–but a gradient. The immune system does work in the liver, just not as aggressively as in most other tissues. Also, there is a race between the virus and the immune system during the first ~6 months of infection, with the virus trying to suppress the immune system and the immune system trying to clear the virus. We don’t know most of the details of this race because it is extraordinarily hard to study, but some aspects are coming into view. For example, HBe helps suppress anti-HBc immune responses, HBs binds to anti-HBs antibodies keeping them from slowing expansion of the viral infection, HBx stops SMC5/6 from silencing the cccDNA, and somehow (I don’t understand but Jim Ou is studying this) the virus alters function of liver macrophages to make them less effective against the virus. In healthy adults infected with HBV the immune system wins this race ~90-95% of the time, whereas in newborns and infants the virus wins ~90-95% of the time.
Thank you Dr @john.tavis for explaining everything. I just wish, somehow, at some point of time, T cell could get rid of cccDNA containg liver cells. Maybe that happens that’s why reactivation is so rare?
I have another query:
Just wondering if one who has good Anti Hbs titer and takes a booster dose or third dose- will the Ag (HBsAg) neutralize all the available anti Hbs antibodies ? And may produce a window wherein one becomes susceptible to HepB infection as free antibodies are no more available or less than 10 IU/L ? Just a random question that came to my non stop thinking mind. @ThomasTu
T cells certainly can get rid of the cccDNA, just not all the time in everyone. That is why efforts to enhance T-cell activity in chronically infected people is being aggressively pursued as a new therapeutic approach. That would be the primary mechanism by which therapeutic vaccines would work.
The amount of antigen used during vaccination is much, much less than that amount of anti-HBs antibody in a person with a good antibody titer. There would be no window where a person is transiently susceptible during subsequent vaccinations.
That would certainly be “good”. There is an enormous amount of antibody in blood, and the vaccines carry only a very small amount of antigen due to the immune system’s remarkable sensitivity. The relative amounts are not even close.
AKJ some additional information on what we know about cccDNA will help explain the “cccDNA issue”.
cccDNA is fully chromatinized, which to say that it is coated with many of the proteins and “packed” in the same manner as all of the DNA in your cells. It is better to think of cccDNA as a “minichromosome”.
Like all chromatin in your cells, the cccDNA minichromosome exists in two forms. The active form is able to support the production of viral proteins so cells with “active cccDNA” also display these viral proteins on their surface and can be recognized by T-cells.
Unfortunately, there is also a “latent” form which is compacted and very difficult to target. This form cannot support the production of any viral proteins so cells with “latent cccDNA” do not display any viral proteins and are not recognized by any aspect of the immune system.
We know that “latent cccDNA” can persist for a very long time. In patients with a normally functioning immune system, we suspect that conversion of latent cccDNA to active cccDNA over time gets well controlled so that no infection or liver disease develops. However immunosuppressive drugs likely impair the control of reactivated cccDNA, leading to the development of reactivation of infection in patients on immunosuppresive therapy or chemotherapy.
So do we expect over time all latent cccDNA will eventually convert to activate cccDNA that will be targeted by the T cells- leading to complete eradication of cccDNA?
Unfortunately, since cccDNA acts like normal chromosomal DNA, it can transition from active to latent forms and back just like your chromosomal DNA does. We suspect that in its latent state, cccDNA is very long lived in the liver and also very difficult to target.
The good news is that with functional cure (recovery of immune function), reactivation of latent cccDNA is always effectively “quashed” by your immune system. This is a process which occurs for most people that get infected with HBV and these people go on to lead perfectly healthy lives without need for any treatment.
However, the long term persistence of latent cccDNA is why reactivation of HBV infection can happen in people who have had no evidence of active infection for many years when they take immunosuppressive therapy or chemotherapy: the reactivation of latent cccDNA can now occur and give rise to reactivation of infection.
So functional cure of HBV is better described as a remission of active infection that will persist as long as the healthy immune function is present.
But when we say -latent form transit to active and express proteins that are presented to T cells-, don’t you think that cell would be killed by T cells-? So it may not move back to silent form from active once liver cell containing cccDNA present expressed viral proteins to surface leading to recognition and lysis? Eventually, all latent cccDNA containing liver cells will disappear?
I mean to say once it get converted to active form and express proteins, it will along with liver cell be lysed/ killed?
Also once viral proteins are presented by liver cells to t-cells, doesn’t matter if it get converted to latent form as cell is going to be killed by t-cell anyway?