Broadly neutralizing human monoclonal antibodies against BK polyomavirus genotypes.

BK polyomavirus (BKV) causes polyomavirus-associated nephropathy (BKV-nephropathy) and polyomavirus-associated haemorrhagic cystitis following kidney transplantation and allogeneic haematopoietic stem cell transplantation. BKV strains consist of four distinct genotypes (BKV-I, -II, -III and -IV), with more than 80% of individuals seropositive for the BKV-I genotype and lower prevalences of infection or co-infection with the other genotypes. BKV-nephropathy occurs widely in immunosuppressed transplant recipients, with the recommended treatment including reduction of immunosuppressive drugs. High serum titres of BKV-neutralizing antibodies and treatment with BKV-neutralizing intravenous immunoglobulin are associated with reduced levels of BKV-DNAemia in kidney transplant recipients, suggesting anti-BKV antibodies can limit viral load. Thus, we set out to generate broadly neutralizing human mAbs against the viral protein 1 (VP1) major capsid protein of BKV genotypes I-IV using VelocImmune® transgenic mice that encode for human immunoglobulins. Hybridoma clones from VelocImmune® mice immunized with combinations of BKV I-IV VP1 and respective genotypes of BKV pseudoviruses (BK-PsVs) were screened using a high-throughput binding assay against BKV-I VP1 expressed in HEK-293 cells. The VP1-binding hybridoma clones were then assessed for neutralization with BK-PsV consisting of respective VP1 proteins from BKV-I, -II, -III or -IV on the virion surface. Overall, 36 broadly cross-neutralizing mAbs against BKV-I, -II, -III and -IV were identified. Importantly, 20 of the cross-reactive immunoglobulins were subjected to nucleotide sequencing, resulting in 6 clonotype families with 14 genetically distinct immunoglobulins. Several of the most effective mAbs were fully humanized with the IgG1 Fc domain with broad neutralization of BK-PsV-I, -II, -III and -IV genotypes with IC50s ranging from ~7 to 200 pM. Thus, these cross-neutralizing mAbs represent potential biologics to be developed into BKV therapeutics.