BK polyomavirus genotypes represent distinct serotypes with distinct entry tropism

Abstract

BK polyomavirus (BKV) causes significant urinary tract pathogenesis in immunosuppressed individuals, including kidney and bone marrow transplant recipients. It is currently unclear whether BKV-neutralizing antibodies can moderate or prevent BKV disease. We developed reporter pseudoviruses based on seven divergent BKV isolates and performed neutralization assays on sera from healthy human subjects. The results demonstrate that BKV genotypes I, II, III, and IV are fully distinct serotypes. While nearly all healthy subjects had BKV genotype I-neutralizing antibodies, a majority of subjects did not detectably neutralize genotype III or IV. Surprisingly, BKV subgenotypes Ib1 and Ib2 can behave as fully distinct serotypes. This difference is governed by as few as two residues adjacent to the cellular glycan receptor-binding site on the virion surface. Serological analysis of mice given virus-like particle (VLP)-based BKV vaccines confirmed these findings. Mice administered a multivalent VLP vaccine showed high-titer serum antibody responses that potently cross-neutralized all tested BKV genotypes. Interestingly, each of the neutralization serotypes bound a distinct spectrum of cell surface receptors, suggesting a possible connection between escape from recognition by neutralizing antibodies and cellular attachment mechanisms. The finding implies that different BKV genotypes have different cellular tropisms and pathogenic potentials in vivo. Individuals who are infected with one BKV serotype may remain humorally vulnerable to other BKV serotypes after implementation of T cell immunosuppression. Thus, prevaccinating organ transplant recipients with a multivalent BKV VLP vaccine might reduce the risk of developing posttransplant BKV disease.

Fig 1

Phylogenetic tree of BKV variants. The tree illustrates the relationship between the VP1 proteins of the seven tested BKV isolates (labeled branches) and other unique full-length BKV VP1 sequences found in GenBank (unlabeled branches). BKV genotypes Ia and Ib1 are indistinguishable on the basis of VP1 protein sequence.

Fig 2

BKV-neutralizing titers of human sera. The chart shows the neutralizing titer (displayed as log₁₀ values) of the specified BKV variant (columns) by serum samples from individual human subjects (rows). Log titers of 5 and above are shown in red, 4 to 4.9 in orange, 3 to 3.9 in yellow, 2 to 2.9 in green, and negative (undetectable to 1.9) in blue.

Fig 3

BKV-neutralizing responses in mice. The chart shows the neutralizing titer (log₁₀) of the specified BKV variant (column) by pooled plasma samples from mice (5 animals per group) immunized with VP1-only VLPs based on specified BKV variants (rows). (A) Samples taken 4 weeks after initial VLP priming. (B) Samples taken 4.5 weeks after administration of a booster dose of VLPs.

Fig 4

Identification of amino acids responsible for serotypes. (A) Amino acid variations in natural BKV variants and mutants created for this work. (B) Neutralizing titers (log₁₀) of six normal human sera against various VP1 mutant pseudoviruses. All sera on this panel showed >10-fold differences between BKV-Ib1 and Ib2 in initial testing (Fig. 1).

Fig 5

Hemagglutination by BKV variants. Bar graphs express the inverse of the last amount (in ng/ml) of VP1 VLPs that was able to hemagglutinate a 0.5% solution of red blood cells from the animal species indicated.

Fig 6

Transduction efficiency of BKV variants. BKV variants were normalized by VP1 content and tested for transduction of various human cell lines (top panel) or the ganglioside-deficient murine cell line, GM95 (bottom panel). GM95 cells were preloaded with the ganglioside GT1b or mock loaded. Error bars represent the standard deviations from quadruplicate testing. Similar results were observed in three independent experiments.