Antibody-mediated enhancement of BK virus infection in human monocytes and a human macrophage-like cell line

Immunohistochemical detection of KI polyomavirus in lung and spleen

Little is known about the tissue tropism of KI polyomavirus (KIPyV), and there are no studies to date describing any specific cell types it infects. The limited knowledge of KIPyV tropism has hindered study of this virus and understanding of its potential pathogenesis in humans. We describe tissues from two immunocompromised patients that stained positive for KIPyV antigen using a newly developed immunohistochemical assay targeting the KIPyV VP1 (KVP1) capsid protein. In the first patient, a pediatric bone marrow transplant recipient, KVP1 was detected in lung tissue. Double immunohistochemical staining demonstrated that approximately 50% of the KVP1-positive cells were CD68-positive cells of the macrophage/monocyte lineage. In the second case, an HIV-positive patient, KVP1 was detected in spleen and lung tissues. These results provide the first identification of a specific cell type in which KVP1 can be detected and expand our understanding of basic properties and in vivo tropism of KIPyV.

The pathobiology of polyomavirus infection in man

This article traces the discovery of polyomaviruses and outlines investigations, which shed light on potential modes of transmission of this increasingly important group of human pathogens. The pathobiology of the virus is summarized with particular reference to interactions with host cell receptors, cell entry, cytoplasmic trafficking, and targeting of the viral genome to the nucleus. This is followed by a discussion of sites of viral latency and factors leading to viral reactivation. Finally, we present biochemical mechanisms that could potentially explain several key elements of tissue pathology characteristic of BKV mediated damage to human kidney.

Antibody-dependent enhancement of viral infection: molecular mechanisms and in vivo implications

Besides the common receptor/coreceptor-dependent mechanism of cellular attachment, some viruses rely on antiviral antibodies for their efficient entry into target cells. This mechanism, known as antibody-dependent enhancement (ADE) of viral infection, depends on the cross-linking of complexes of virus-antibody or virus-activated complement components through interaction with cellular molecules such as Fc receptors or complement receptors, leading to enhanced infection of susceptible cells. Recent studies have suggested that additional mechanisms underlie ADE: involvement of complement component C1q and its receptor (Ebola virus), antibody-mediated modulation of the interaction between viral protein and its coreceptor (human immunodeficiency virus) and suppression of cellular antiviral genes by the replication of viruses entering cells via ADE (Ross River virus). Since ADE is exploited by a variety of viruses and has been associated with disease exacerbation, it may have broad relevance to the pathogenesis of viral infection and antiviral strategies.

Polyomavirus BK

Polyomavirus hominis 1, better known as BK virus (BKV), infects up to 90% of the general population. However, significant clinical manifestations are rare and limited to individuals with impaired immune functions. BKV has been associated with diverse entities such as haemorrhagic cystitis, ureteric stenosis, vasculopathy, pneumonitis, encephalitis, retinitis, and even multi-organ failure. In addition, BKV has been implicated in autoimmune disease and possibly cancer. Due to high prevalence and frequent reactivation, the role of BKV in some of these pathologies has been difficult to define. Development of BKV diseases is likely to require complementing determinants in the host, the target organ, and possibly the virus, that are subject to modulators such as immunosuppression. These complex aspects are highlighted in polyomavirus-associated nephropathy (PAN), an emerging disease in renal allograft recipients that may jeopardise the progress in renal transplantation accomplished in the past 10 years. Intervention is difficult due to the lack of specific antivirals and relies mostly on improving immune control. Diagnostic strategies using urine cytology and BKV load measurements in plasma have led to earlier diagnosis of PAN, which increased the success rate of intervention. Case series suggest that cidofovir might be effective, especially when combined with reduced immunosuppression.

Identification of a new control region in the genome of the DDP strain of BK virus isolated from PBMC

The various strains of human polyomavirus BK (BKV) show a marked heterogeneity in the non-coding control region (NCCR), which includes the origin of replication and the regulatory region for early and late transcription. A new BKV strain (DDP, U91605) was identified by direct detection and sequencing of PCR products of BKV-NCCR DNA obtained from PBMC samples of HIV-positive or -negative subjects. The DDP strain NCCR sequence showed an organisation not described previously in vivo with the maximum homology with the archetypal strain (WW) (M34048), as compared with those collected in GenBank. Structurally, P68, Q39, and S68 boxes were perfectly conserved, whereas the R63 box was completely deleted. This deletion involves the loss of sequences able to bind cellular factors essential for the DNA transcription, such as NF1 binding sites, normally present twice in the R box and the modification of SP1. It is possible that these rearrangements represent a cause of the loss of the VP1 region observed in 9/22 PBMC samples and never observed in urine isolates, which are similar to the WW strain.

Noncoding control region of naturally occurring BK virus variants: sequence comparison and functional analysis

The human polyomavirus BK (BKV) has a proven oncogenic potential, but its contribution to tumorigenesis under natural conditions remains undetermined. As for other primate polyomaviruses, the approximately 5.2 kbp double-stranded circular genome of BKV has three functional regions: the coding regions for the two early (T, t antigens) and four late (agno, capsid proteins; VP1-3) genes separated by a noncoding control region (NCCR). The NCCR contains the origin of replication as well as a promoter/enhancer with a mosaic of cis-acting elements involved in the regulation of both early and late transcription. Since the original isolation of BKV in 1971, a number of other strains have been identified. Most strains reveal a strong sequence conservation in the protein coding regions of the genome, while the NCCR exhibits considerable variation between different BKV isolates. This variation is due to deletions, duplications, and rearrangements of a basic set of sequence blocks. Comparative studies have proven that the anatomy of the NCCR may determine the transcriptional activities governed by the promoter/enhancer, the host cell tropism and permissivity, as well as the oncogenic potential of a given BKV strain. In most cases, however, the NCCR sequence of new isolates was determined after the virus had been passaged several times in more or less arbitrarily chosen cell cultures, a process known to predispose for NCCR rearrangements. Following the development of the polymerase chain reaction (PCR), it has become feasible to obtain naturally occurring BKV NCCRs, and their sequences, in samples taken directly from infected human individuals. Hence, the biological significance of BKV NCCR variation may be studied without prior propagation of the virus in cell culture. Such variation has general interest, because the BKV NCCRs represent typical mammalian promoter/enhancers, with a large number of binding motifs for cellular transacting factors, which can be conveniently handled for experimental purposes. This communication reviews the naturally occurring BKV NCCR variants, isolated and sequenced directly from human samples, that have been reported so far. The sequences of the different NCCRs are compared and analyzed for the presence of proven and putative cellular transcription factor binding sites. Differences in biological properties between BKV variants are discussed in light of their aberrant NCCR anatomies and the potentially modifying influence of transacting factors.

Infection enhancement of influenza A NWS virus in primary murine macrophages by anti-hemagglutinin monoclonal antibody

Antibody-dependent enhancement (ADE) of influenza A NWS virus infection was investigated in primary murine macrophages (M phi) using anti-hemagglutinin(HA) monoclonal antibody (mAB). Contrary to previous reports of abortive influenza virus infection in primary M phi, this study demonstrated that the NWS virus replicated productively in both resident peritoneal M phi and thioglycolate-elicited peritoneal M phi providing cleavage of the HA was achieved by trypsin; 5 micrograms/ml of trypsin was the optimum concentration for the induction of infectivity. Under multiple-cycle growth conditions in the presence of mAB at various concentrations in trypsin-containing media, ADE was demonstrated in both M phi in the presence of subneutralizing concentrations of mAB. Flow cytometric analysis showed that the mechanism of virus entry into M phi could be through HA to specific virus receptors, or HA plus antibody to Fc receptors. These results indicate that ADE of the NWS virus infection actually occurs on Fc receptor-bearing primary murine M phi depending on the concentration of antibody in the presence of the appropriate protease for cleavage of viral HA.

Detection of specific IgA antibodies against BK virus by ELISA

The development and evaluation of an ELISA for analysis of anti-BKV specific IgA antibodies in human sera are described. All children with cancer with a primary BKV infection developed specific IgA antibodies, without any specific symptoms during the infection. Specific IgA was found in 61% of sera from healthy persons containing BKV IgG antibodies, using the chosen cut-off value, and BKV IgM in 4%. These results indicate that IgA production is more persistent than IgM. The high frequency of specific IgA antibodies could either be explained by frequent reactivations or long-lasting persistence of antibodies.

Antibodies against BK virus in renal transplant recipient sera: results with five different methods indicate frequent reactivations

Sera from 10 renal transplant recipients were investigated longitudinally for antibodies to human polyomavirus BK (BKV) using specific IgG-, IgA- and IgM-ELISA, hemagglutination inhibition (HAI) and neutralization tests (NT). Nine patients had anti-BKV serum antibodies before transplantation. The serological findings in 5 patients were compatible with reactivation of BKV infections. The results obtained with different tests were generally in good agreement. However, according to the definition of significant titer increase. 1-3 patients would have been undetected by employing HAI alone. High levels of antibodies detected by the other tests generally corresponded well with NT titers, i.e. the ability to neutralize infectivity. All the 5 patients with reactivation also showed an increase in specific IgA antibodies.

Infection enhancement of influenza A H1 subtype viruses in macrophage-like P388D1 cells by cross-reactive antibodies

The contribution of cross-reactive hemagglutination inhibition (HI) antibodies to infection enhancement of influenza A H1 subtype NWS virus and two antigenic drift strains was investigated in a macrophage-like cell line P388D1. When P388D1 cells, previously treated with neuraminidase (NA) to remove the viral receptors, were infected with NWS virus exposed to rabbit antiviral immunoglobulin (IgG) showing various levels of cross-HI titers, virus yields were enhanced in the presence of a subneutralizing antibody, depending on their cross-HI titers. By flow cytometric analysis using a fluorescein isothiocyanate (FITC)-labeled NWS virus, the efficiency of attachment of virus-rabbit IgG complexes to Fc receptors on NA-treated cells showed close correlation with its cross-HI titer. These data suggest that cross-reactive HI antibodies could contribute to infection enhancement through the formation of potent infectious immune complexes with drift strains to mediate virus infection via Fc receptor uptake. Two monoclonal antibodies (mAB) in mouse IgG subclasses IgG1 and IgG2a showing strain-specific or cross-reactive HI activity were tested for their infection enhancement characteristics. A strain-specific mAB enhanced infection of homologous NWS virus, but not that of two other drift strains in either antibody dilution. In contrast, a cross-reactive mAB caused infection enhancement of all three virus strains in the presence of the subneutralizing antibody. This indicates that cross-reactivity, but not the IgG subclass, acts as an enhancing factor to this phenomenon. The antibody, with the same specificity as cross-reactive mAB, was detected semiquantitatively by competitive enzyme-linked immunosorbent assay (ELISA) with results almost consistent with cross-HI titers of polyclonal rabbit antiviral IgGs. These data suggest that the antibody detected by this assay might be one of the potent antibodies governing cross-HI activity as a whole antibody and causing infection enhancement of drift strains.

Infection of monocytes by human immunodeficiency virus type 1 blocked by inhibitors of CD4-gp120 binding, even in the presence of enhancing antibodies

Infection of monocyte/macrophages (M/M) by a variety of viruses (including HIV-1) has been shown to be enhanced in the presence of low concentrations of antiviral antibodies. This process has been hypothesized as occurring through binding of the virus-antibody complex to Fc or complement receptors followed by endocytosis. In the current study, we explored whether such a mechanism might provide a CD4-independent route of infection by HIV-1 for any of several populations of M/M. In the absence of anti-HIV antibodies, replication of HIV-1 in M/M was blocked by viral binding inhibitors such as soluble CD4 or OKT4A mAb. Furthermore, while infection of the M/M populations by a low multiplicity of infection of HIV-1 was found to be somewhat enhanced by the presence of very low concentrations of anti-HIV antibodies, this process was also consistently inhibited by recombinant soluble CD4 and by OKT4A antibody. These results suggest that under the variety of conditions studied, CD4 binding was an essential step in the infection of M/M by HIV. Moreover, they are consistent with the notion that "enhancing" antibodies may serve to concentrate HIV onto CD4 receptors or, alternately, may act at other steps in the process of viral entry and replication.