Human alpha-defensins inhibit BK virus infection by aggregating virions and blocking binding to host cells

BK virus (BKV) is a polyomavirus that establishes a lifelong persistence in most humans and is a major impediment to success of kidney grafts. The function of the innate immune system in BKV infection and pathology has not been investigated. Here we examine the role of antimicrobial defensins in BKV infection of Vero cells. Our data show that alpha-defensin human neutrophil protein 1 (HNP1) and human alpha-defensin 5 (HD5) inhibit BKV infection by targeting an early event in the viral lifecycle. HD5 treatment of BKV reduced viral attachment to cells, whereas cellular treatment with HD5 did not. Colocalization studies indicated that HD5 interacts directly with BKV. Ultrastructural analysis revealed HD5-induced aggregation of virions. HD5 also inhibited infection of cells by other related polyomaviruses. This is the first study to demonstrate polyomavirus sensitivity to defensins. We also show a novel mechanism whereby HD5 binds to BKV leading to aggregation of virion particles preventing normal virus binding to the cell surface and uptake into cells.

Identification of amino acid residues in BK virus VP1 that are critical for viability and growth

BK virus (BKV) is a ubiquitous pathogen that establishes a persistent infection in the urinary tract of 80% of the human population. Like other polyomaviruses, the major capsid protein of BKV, virion protein 1 (VP1), is critical for host cell receptor recognition and for proper virion assembly. BKV uses a carbohydrate complex containing alpha(2,3)-linked sialic acid attached to glycoprotein and glycolipid motifs as a cellular receptor. To determine the amino acids important for BKV binding to the sialic acid portion of the complex, we generated a series of 17 point mutations in VP1 and scored them for viral growth. The first set of mutants behaved identically to wild-type virus, suggesting that these amino acids were not critical for virus propagation. Another group of VP1 mutants rendered the virus nonviable. These mutations failed to protect viral DNA from DNase I digestion, indicating a role for these domains in capsid assembly and/or packaging of DNA. A third group of VP1 mutations packaged DNA similarly to the wild type but failed to propagate. The initial burst size of these mutations was similar to that of the wild type, indicating that there is no defect in the lytic release of the mutated virions. Binding experiments revealed that a subset of the BKV mutants were unable to attach to their host cells. These motifs are likely important for sialic acid recognition. We next mapped these mutations onto a model of BKV VP1 to provide atomic insight into the role of these sites in the binding of sialic acid to VP1.

JC virus minor capsid proteins Vp2 and Vp3 are essential for virus propagation

Virus-encoded capsid proteins play a major role in the life cycles of all viruses. The JC virus capsid is composed of 72 pentamers of the major capsid protein Vp1, with one of the minor coat proteins Vp2 or Vp3 in the center of each pentamer. Vp3 is identical to two-thirds of Vp2, and these proteins share a DNA binding domain, a nuclear localization signal, and a Vp1-interacting domain. We demonstrate here that both the minor proteins and the myristylation site on Vp2 are essential for the viral life cycle, including the proper packaging of its genome.