A significant percentage of organ donors—7% in our series—had serological evidence of apparently resolved HBV infection, as manifested by absence of HBsAg and the presence of circulating anti-HBc and anti-HBs antibodies. Several reports on HBV reactivation in immunosuppressed individuals,2–6,14
as well as observations on the high risk of HBV transmission by liver allografts from anti-HBc positive donors,5–8
support the conclusion that occult HBV DNA in a donor liver may be a significant source of HBV infection. Our study confirms previous estimations of the infectious risk based on the donor or recipient's HBV serological status7
: naive recipients are highly susceptible to de novo infection regardless of the donor's anti-HBs status and patients with pretransplant vaccine immunity appear to be resistant. In this series, four HBV naive recipients developed infection while pretransplant vaccination appeared to be protective in two patients. This well recognised infectious risk suggests discarding or directing liver grafts from anti-HBc positive donors to selected recipients, such as HBsAg positive patients. However, due to organ shortage, HBsAg negative patients may occasionally receive these grafts because of a critical condition, rare blood group, or failure in the screening of the donor's anti-HBc status. The efficacy of long term high dose HBIG in lowering the rate of recurrence or the severity of the recurrent infection in HBsAg positive recipients10–12
and the apparent resistance to infection of patients with vaccine immunity prompted us to extend HBIG prophylaxis to HBV naive recipients of latently HBV infected liver grafts. The modified HBIG prophylaxis protocol was less stringent than that given to prevent HBV recurrence as we assumed that there were no (or less) circulating viral particles. Combination therapy with HBIG and lamivudine has been shown to prevent both HBV recurrence after transplantation in pretransplant HBV DNA negative HBsAg positive recipients15
and the emergence of HBsAg in anti-HBs negative recipients of liver grafts from anti-HBc positive donors.9
However, the risk of lamivudine induced mutations and the scarcity of alternative efficient therapies prompted us to adopt HBIG monotherapy. In this series, five naive patients underwent HBIG prophylaxis and anti-HBs titre surveillance. The development of de novo infection in one patient with low anti-HBs titres appears to be a surveillance failure and underlines the importance of achieving high levels of neutralising antibodies in naive patients. HBIG prophylaxis was also given to four patients with pretransplant natural immunity (anti-HBc positive and anti-HBs negative). These patients are at risk of HBV reactivation from donor liver but also from their own infection,5
although the rate of de novo HBV infection has been reported to be lower than that in naive patients.7
Indeed, none developed infection. Over the three year period of the study, three patients with pretransplant vaccine immunity were given HBIG at the perioperative period. The rationale for HBIG infusions was maintenance of high anti-HBs titres in a context where vaccine booster injections are poorly efficient. The decline of anti-HBs however was very slow and none developed infection.
An interesting finding was the marked fall in liver HBV DNA after transplantation. While liver HBV DNA was detected in eight of 12 grafts on day 0 of transplantation, it was no longer detectable in post-transplant specimens from the same grafts. Pre- and post-transplant PCR testing was performed on paraffin embedded liver biopsy sections, so the sample size and risk of DNA degradation were the same for both assays. In addition, each liver sample was checked by amplification of cellular DNA control. From an immunological viewpoint, this finding is paradoxical because the studied donors, who were presumably immunocompetent and had neutralising antibodies to HBsAg in most cases, had failed to clear liver HBV DNA. After recovery from acute infection, HBV may persist in liver tissue either integrated or in a replication competent form referred to as covalently closed circular DNA. “Clearance” of graft HBV DNA may be possible when HBV persists in a non-integrated form,16,17
and may be linked to regeneration following transplantation. Thus HBIG or anti-HBs antibodies acquired through vaccination may protect newly formed hepatocytes from HBV infection. From our data, anti-HBs neutralising immunity alone may be sufficient to control HBV replication as no antiviral therapy was given. However, maintenance of anti-HBc after seroconversion suggests persistent antigenic stimulation from core epitopes, probably expressed at low levels,17–19
and appears to be a marker of HBV latent infection in these liver recipients. Furthermore, in two patients who experienced de novo infection, liver HBV DNA was transiently undetectable before HBsAg seroconversion, showing that HBV replication may be reduced beyond detectable levels but not be turned off. These observations support the conclusion that anti-HBs prophylaxis must be given indefinitely.
Our study shows that the current anti-HBc testing is required to identify potential latent HBV carriers and that HBV molecular testing on serum or liver with the currently available techniques does not provide a definitive measure of the infectious risk of anti-HBc positive donors. As reported in previous studies on blood donors and in countries with a low seroprevalence of HBV infection,20
only one of 22 donor serum samples was positive for HBV DNA. Similarly, the persistence of HBV DNA in the liver of healthy donors with no evidence of liver dysfunction has been reported previously.5,16
In this series, HBV DNA was detected in 11/21 liver grafts (52%). It is of note that PCR was positive in 4/5 grafts of patients who developed de novo infection and in 7/16 grafts of patients who did not. This difference was not significant. Furthermore, the HBV detection rate may be underestimated due to low viral load, insufficient sensitivity of the PCR assay, or focal distribution of HBV infection. Additional studies are necessary to develop effective assays for the detection of latently infected donors.
In conclusion, circulating anti-HBs antibodies, without additional antiviral therapy, appear to control HBV replication in recipients of previously infected liver grafts and lead to the apparent clearance of liver HBV DNA. Further studies are needed to assess combined therapy with antiviral drugs, which raises concern about the lack of alternatives in cases of reactivation. Liver grafts from anti-HBc positive donors may be suitable for recipients with high anti-HBs titres before transplantation and for those who are given long term high dose HBIG to prevent HBV recurrence. The use of such livers in non-vaccinated HBV naive recipients should be carefully weighted. Indeed, HBIG are a human blood product and their administration is associated with increased costs, occasional adverse events, and time consuming protocols. On the other hand, the mortality rate among patients awaiting a liver graft is particularly high in those with rare blood groups or hepatocellular carcinoma. The risk of waiting longer for a suitable liver graft has to be taken into consideration. Finally, all HBV naive candidates for liver transplantation should be immunised against HBV, despite the low response rate of patients with end stage liver disease. When successful, such immunisation will provide an adequate anti-HBs response in situations where a liver from an HBsAg negative/anti-HBc positive donor is available.