Objectives: Hepatitis B virus reactivation, particularly when occurring under immunosuppressive therapy, poses a risk for patients with chronic or resolved hepatitis B virus infections. This study aimed to evaluate the incidence and clinical outcomes of hepatitis B virus reactivation in kidney transplant recipients with resolved hepatitis B virus infection.
Materials and Methods: We retrospectively screened patients who underwent kidney transplant between 2012 and 2023 seen at our center (Izmir, Türkiye). We included transplant recipients with resolved hepatitis B virus infection. We analyzed risk factors associated with reactivation and the clinical course of patients who developed hepatitis B virus reactivation.
Results: Our analyses included 95 kidney transplant recipients; mean follow-up period was 70 ± 31 months. Baseline serology revealed that 9.4% of patients had isolated anti-HBc positivity, whereas 90.6% were positive for both anti-HBc and anti-HBs. None of the patients received antiviral prophylaxis at baseline; however, 14 patients received prophylaxis during the follow-up period. Among the 81 transplant recipients who did not receive prophylaxis, hepatitis B virus reactivation occurred in 3 patients (3.7%). No serious adverse outcomes, such as liver failure, liver transplant, or death due to hepatitis B virus reactivation were observed.
Conclusions: Although hepatitis B virus reactivation is uncommon in hepatitis B surface antigen-negative/anti-HBc-positive patients after kidney transplant, reactivation remains a potential risk. In patients not scheduled for long-term prophylaxis, careful assessment regarding the necessity, efficacy, and safety of antiviral intervention is critical for optimal management. Key words: Hepatic failure, Hepatitis B reactivation, Immunosuppressive treatment, Renal disease, Viral hepatitis

Introduction
Hepatitis B virus (HBV) infection is more prevalent in patients with end-stage renal disease compared with the general population. Prevalence of HBV infection among kidney transplant recipients (KTRs) ranges from 2.2% to 20.9%, and infection is associated with increased mortality, graft loss, and the progression of liver disease.¹ Although antibodies that develop after infection generally confer long-term immunity, the protective role of anti-HBs titers against reactivation remains poorly defined. Loss of anti-HBs may serve as a predictor for reactivation. Consequently, HBV reactivation (HBVr) can occur in both chronically infected patients and those with resolved infection, particularly under immunosuppression.² For patients who are hepatitis B surface antigen (HBsAg) negative/anti-HBc positive, guidelines from the American Association for the Study of Liver Diseases suggest either prophylactic antiviral therapy or preemptive monitoring; however, the optimal strategy remains debated. For patients who are managed without prophylaxis, measuring HBV DNA levels every 1 to 3 months is recommended.³ Similarly, guidelines from the American Gastroenterological Association, Asian-Pacific Association for the Study of the Liver, and European Association for the Study of the Liver do not recommend routine antiviral prophylaxis for patients at low risk of HBVr who are receiving immunosuppressive therapy.^4-6 In non-liver solid-organ transplant (SOT) recipients, the American Association for the Study of Liver Diseases recommends prophylaxis during the first 6 to 12 months of intensive immunosuppression, followed by monitoring of alanine aminotransferase (ALT) for 6 to 12 months after prophylaxis cessation, with HBV DNA testing if ALT levels rise.³ Conversely, guidelines of the Kidney Disease: Improving Global Outcomes do not recommend routine monitoring or antiviral prophylaxis for KTRs with resolved HBV infection due to the relatively low risk.⁷ Previous studies have reported that the risk in this population ranges from 2% to 9.6%.⁸ Nevertheless, immunosuppressive therapies significantly increase the risk of HBVr, a preventable cause of acute liver failure in SOT recipients with prior HBV exposure.^9-11 Therefore, determining the appropriate strategy (that is, antiviral prophylaxis versus preemptive monitoring) is critical. In this study, we aimed to elucidate the incidence and clinical outcomes of HBVr in KTRs with resolved HBV infection receiving immunosuppressive therapy. We also investigated the effects of anti-HBs levels, rituximab use, anti-thymocyte globulin (ATG) induction, and history of acute rejection on the risk of HBVr.

Materials and Methods
The institutional review board and research and ethics committee of the University of Health Sciences, Izmir Bozyaka Training and Research Hospital approved this study (12.06.2023; protocol no. 2023/92). The study was conducted in accordance with the ethical guidelines of the 1975 Declaration of Helsinki. We screened patients who underwent kidney transplant between 2012 and 2023 at our center (Izmir, Türkiye). We collected patient demographics, etiology of kidney disease, donor type, and dialysis duration. We excluded patients with missing HBV serology, HBsAg positivity, HBsAg-negative/anti-HBc-negative status, follow-up duration of less than 1 year, and hepatitis C virus-HIV coinfection. Our final study group had 95 KTRs with resolved HBV infection (positive anti-HBc but undetectable HBV-DNA and negative HBsAg). We analyzed the following risk factors for association with HBVr: age, sex, donor type, pretransplant hepatitis viral markers of recipients and donors, induction and immunosuppressive regimens, and rituximab use. The primary outcome was HBVr, defined as the appearance of HBV DNA in serum (which was negative before exposure to immunosuppressive treatments) or reverse seroconversion (reappearance of HBsAg or loss of anti-HBs).^12,13 Secondary outcomes included hepatitis flare, liver failure, and mortality attributable to HBVr. Hepatitis flare was defined as an abrupt elevation in ALT levels >3 times the upper limit of normal. Hepatitis B virus-associated liver failure was defined as severe liver injury accompanied by at least 1 of the following: encephalopathy, ascites, impaired synthetic function (international normalized ratio ≥1.5 or total bilirubin >3 mg/dL), or death.

Statistical analyses
We used SPSS Statistics for Windows version 29.0 (IBM Corp) for statistical analyses. We presented categorical variables as frequencies and percentages and continuous variables as means ± SD. We conducted group comparisons using the independent sample t test for continuous variables and the Pearson χ2 test for categorical variables. P < .⁰⁵ was considered statistically significant.

Results
Our study included 95 anti-HBc-positive KTRs, representing 13.7% of the transplant population at our center, seen from January 2012 through October 2023. Mean follow-up period was 70 ± 31 months (range, 20-123 mo). Mean age of KTRs was 54 ± 11 years (range, 29-75 y), 66% were men, 56% received a kidney from a deceased donor, and 14% underwent a second transplant (Table 1). Delayed graft function was observed in 34% of the patients. All patients received ATG as induction therapy. Maintenance immunosuppression consisted of tacrolimus-based regimens in 84 patients, cyclosporine-based regimens in 13 patients, and mammalian target of rapamycin inhibitor-based regimens in 2 patients. Regarding hepatitis B serology, 9.4% of patients had isolated anti-HBc positivity (anti-HBc positive/anti-HBs negative), and 90.6% of patients were positive for both anti-HBc and anti-HBs. Mean anti-HBs titer was 307 ± 344 mIU/mL (range, 0-1000 mIU/mL). Anti-HBs titers exceeded 100 mIU/mL in 67% of patients (40% had >300 mIU/mL, and 13% had >1000 mIU/mL). During the follow-up period, antiviral prophylaxis was initiated in 14 patients. Indications for prophylaxis included chemotherapy for malignancy in 1 patient and intensive immunosuppression for rejection and/or glomerular disease recurrence in 13 patients. No HBVr developed in the 14 patients who received prophylaxis. Among the 81 recipients managed without antiviral prophylaxis, the incidence of HBVr was 3.7% (n = 3/81). All 3 patients with HBVr experienced a hepatitis flare. Reactivation occurred at a median of 70 months (range, 32-103 mo) posttransplant. In these 3 patients, mean pretransplant anti-HBs level was 91 ± 47 mIU/mL (range, 38-129 mIU/mL). During posttransplant follow-up, anti-HBs titers remained >10 mIU/mL in all 3 cases, with 1 case exceeding 300 mIU/mL. Although anti-HBs loss was not observed in all of these cases, HBsAg changed from negative to positive in all of them (Table 2). Comparison of patients with and without HBVr revealed no significant differences regarding ATG cumulative doses, rituximab usage, transplant type, donor characteristics, or maintenance immunosuppression regimens (Table 1 and Table 2). No serious adverse outcomes, such as liver failure, liver transplant, or death attributable to HBVr, were observed.

Discussion
Hepatitis B virus reactivation occurs predominantly in HBsAg-positive patients undergoing immunosuppressive therapy. However, the risk of HBVr in HBsAg-negative/anti-HBc-positive patients remains poorly defined.^14,15 Comorbidities and immunosuppressive regimens may further potentiate this risk. Consequently, determining the optimal strategy, that is, prophylactic antiviral therapy versus preemptive monitoring, is imperative. This study aimed to elucidate this clinical dilemma. In our cohort, the HBVr rate was 3.7% among HBsAg-negative/anti-HBc-positive recipients managed without prophylaxis. This finding is consistent with Kim and colleagues who reported a 2% HBVr rate over a median follow-up of 6.7 years in a cohort of 449 similar KTRs.¹⁶ Similarly, Querido and colleagues observed HBVr in only 2 of 70 patients (despite 15.7% receiving rituximab) over a median of 151 months.¹⁷ In contrast, Chiu and colleagues reported a significantly higher incidence of HBVr in non-liver SOT recipients with resolved infection (12% overall, 16% in KTRs).¹⁸ These discrepancies may be attributable to differences in baseline recipient/donor characteristics, geographic HBV prevalence, immunosuppressive protocols, and varying approaches to antiviral prophylaxis. With regard to timing of reactivation, HBVr in our study occurred between 32 and 103 months posttransplant, suggesting that the risk persists throughout the duration of immunosuppression and emphasizing the necessity of long-term monitoring. A systematic review and meta-analysis of 16 retrospective studies (n = 2913) similarly reported that the time to HBVr varied widely, ranging from 5 months to 15 years, with a pooled incidence of 2.5%.¹⁹ Interestingly, in all 3 of our cases with HBVr, anti-HBs titers were >10 mIU/mL during both the pretransplant and posttransplant periods; 1 patient maintained a titer >300 mIU/mL. Our findings indicated that the presence of anti-HBs did not preclude reactivation. This contrasts with Jeon and colleagues, who suggested that anti-HBs positivity confers protection and that monitoring might not be necessary for anti-HBs-positive patients.²⁰ However, a systematic review of non-liver SOT recipients found that, although anti-HBs negativity was associated with a higher risk of HBVr, no significant difference in incidence occurred between recipients with titers >100 IU/L and those with titers between 10 and 100 IU/L.¹⁹ In our study group, specific precipitating factors were identified in 2 of the 3 patients with incidence of HBVr: 1 case was attributed to increased pulse steroid and maintenance therapy for suspected chronic rejection and the other case to prolonged, intensive immunosuppression necessitated by a second transplant and delayed graft function (29 days). No identifiable risk factor was detected in the third case. Notably, no HBVr occurred in the 14 patients who received limited-term prophylaxis during periods of intensified immunosuppression (eg, for malignancy or rejection). This suggests that targeted prophylaxis during high-risk periods may be an effective strategy for KTRs with resolved infection. We found no significant differences in ATG dosage, rituximab use, transplant type, or donor characteristics between patients with and without HBVr. This lack of significance may stem from the limited sample size or the use of single-dose rituximab for induction. Although our statistical analysis did not isolate specific risk factors, we advocate for antiviral prophylaxis in patients undergoing treatment for acute rejection, particularly when T-cell-depleting agents (ATG) or B-cell-depleting agents (rituximab) are utilized. Kim and colleagues noted that, although rituximab, anti-HBs status, or ATG alone might not pose a significant risk, the simultaneous presence of multiple factors increases HBVr susceptibility.¹⁶ Yin and colleagues also identified ABO-incompatible transplant, ATG use, rituximab use, and history of acute rejection as potential risk factors, recommending routine monitoring or prophylaxis when these risks are present.¹⁹ Current guidelines strongly recommend antiviral prophylaxis during rituximab therapy and for 12 months after discontinuation. Regarding dosing, Lee and colleagues observed a higher (although not statistically significant) rate of HBVr in patients receiving standard dose rituximab (375 mg/m²) compared with reduced doses.²¹ Fortunately, the clinical course of HBVr in our study was benign; no serious adverse outcomes, such as liver failure, liver transplant, or death, were observed, aligning with Chiu and colleagues, who also reported no HBV-related liver failure or death among 14 recipients with HBVr.¹⁸ In contrast, Lee and colleagues reported 1 HBV-related death in a rituximab-treated group of patients with resolved infection.²²

Conclusions
Although HBVr is infrequent shown in KTRs with resolved HBV infection, it represents a tangible risk. Consequently, the importance of rigorous follow-up cannot be overstated. For KTRs not scheduled for long-term prophylaxis, future studies should focus on defining the necessity, efficacy, and safety of targeted antiviral prophylaxis, particularly during the first 6 to 12 months of intensive immunosuppression or during treatment for allograft rejection.