Immune responses to BK polyomavirus in healthy donors and renal transplant recipients

Abstract

abstract

Background: BKV is a human polyomavirus, a member of the Polyomaviridae family that includes JC virus and SV40. Within the general population the virus infects the host with no clear symptoms displayed by infected individuals. The virus possesses a high seroprevalence reaching 80% within the general population and with more than 19% of healthy subjects shedding BK virus in their urine. After primary infection, which is mainly sub-clinical, the virus persists in the urinary tissue and establishes latency. In immunocompetent individuals the virus persists in the urinary and renal epithelium in its established latent form. However, in individuals with impaired immune surveillance such as in patients receiving immune-suppressive therapies, as prescribed following organ transplant, the virus can reactivate. In kidney transplant recipients (KTR), 30-40% of patients develop BKV viremia while 1-10% progress to BK virus nephropathy. This project investigated host cellular and humoral immune responses to BK virus and to monitor viral genotypes in infected individuals and correlate these variant parameters with disease severity and progression. Methods: Urine, plasma, and PBMCs were collected from post kidney transplant recipients and healthy volunteers. Real-time PCR was used to identify subjects secreting BKV in their urine and sandwich ELISAs used to measure BK-specific immunoglobulin G antibody and soluble HLA-G levels in serum samples from healthy and post kidney transplant recipients. To detect cellular immune responses mounted against BKV, INF-γ secreting cells were quantified and HLA-G expression was measured using ELISPOT and surface staining assays, respectively. A nested PCR assay was developed and used to identify BKV genotypes among patients. Results: BKV specific cellular immune responses were compared between healthy subjects and those where BKV was identified in urine. Immunocompetent subjects with detected virus showed higher cellular responses than negative subjects, while in immunocompromised KTR individuals receiving immunosuppressive therapy it was identified that the presence of virus, as detected by viral DNA, was associated with the induction of anti BKV IgG responses but did not associate with the induction of BKV specific cellular immune responses. However, when immune-suppressive therapies were reduced cellular immune responses were found to be restored. Investigation of HLA-G cell surface protein expression on PBMCs of patients with BKV infection suggested that monocytes expressed more HLA-G than other leucocyte subsets. Additionally, patients with active BKV infection showed dramatic increases in soluble HLA-G expression levels in plasma in comparison to the control group, while no difference was observed in soluble HLA-G levels between the KTR groups tested. Pre-transplant anti BKV IgG levels were similar in patients with post-transplant BK viremia and those who never demonstrated BKV viremia. The level was stable in BKV negative patients during follow up, whilst in viremic patients a significant increase in BKV IgG levels associated with development of viremia in parallel with reduction in immunosuppressive therapy. Further, we observed that the presence of HLA-B44 and HLA-DR15 was significantly associated with increased risk of viremia among the recipients. Finally, by studying the influence of BKV genotypes on development of viremia, genotype lb-2 was found to be the most prevalent genotype among our patients, with no observed significant correlation between genotype and incidence of BKV viremia.

In conclusion: BKV cellular immune responses, without a detectable level of anti-BKV antibodies, could be associated with protection against BKV reactivation. Moreover, HLA-G expression on some immune cellular subsets increased after primary virus infection and may play a role in virus immun