Neutralization Serotyping of BK Polyomavirus Infection in Kidney Transplant Recipients

Abstract

BK polyomavirus (BKV or BKPyV) associated nephropathy affects up to 10% of kidney transplant recipients (KTRs). BKV isolates are categorized into four genotypes. It is currently unclear whether the four genotypes are also serotypes. To address this issue, we developed high-throughput serological assays based on antibody-mediated neutralization of BKV genotype I and IV reporter vectors (pseudoviruses). Neutralization-based testing of sera from mice immunized with BKV-I or BKV-IV virus-like particles (VLPs) or sera from naturally infected human subjects revealed that BKV-I specific serum antibodies are poorly neutralizing against BKV-IV and vice versa. The fact that BKV-I and BKV-IV are distinct serotypes was less evident in traditional VLP-based ELISAs. BKV-I and BKV-IV neutralization assays were used to examine BKV type-specific neutralizing antibody responses in KTRs at various time points after transplantation. At study entry, sera from 5% and 49% of KTRs showed no detectable neutralizing activity for BKV-I or BKV-IV neutralization, respectively. By one year after transplantation, all KTRs were neutralization seropositive for BKV-I, and 43% of the initially BKV-IV seronegative subjects showed evidence of acute seroconversion for BKV-IV neutralization. The results suggest a model in which BKV-IV-specific seroconversion reflects a de novo BKV-IV infection in KTRs who initially lack protective antibody responses capable of neutralizing genotype IV BKVs. If this model is correct, it suggests that pre-vaccinating prospective KTRs with a multivalent VLP-based vaccine against all BKV serotypes, or administration of BKV-neutralizing antibodies, might offer protection against graft loss or dysfunction due to BKV associated nephropathy. Serological studies have shown that nearly all humans are chronically infected with BK polyomavirus (BKV). The infection isn't usually associated with noticeable symptoms. However, opportunistic replication of BKV in therapeutically immunosuppressed kidney transplant recipients (KTRs) can lead to dysfunction or loss of the engrafted kidney. BKV associated nephropathy can occur even in KTRs with high levels of anti-BKV antibodies that might be expected to neutralize the virus. In this report we provide a possible explanation: we show there are at least two BKV genotypes, which are distinct serotypes with respect to antibody-mediated neutralization. Using a novel neutralization-based approach, we found that about half of 108 KTRs did not have detectable levels of antibodies capable of neutralizing BKV genotype IV (BKV-IV) at the time of transplantation. Of these initially BKV-IV naïve KTRs, about half experienced acute BKV-IV specific seroconversion during the first year after transplantation. This likely reflects a de novo BKV-IV infection arising from the engrafted kidney. In a pilot study, we show that recombinant BKV-IV VLPs can induce high levels of BKV-IV-neutralizing antibodies in vaccinated animals. Our results suggest that administration of a BKV VLP-based vaccine to prospective KTRs might protect against the development of opportunistic BKV replication.