Detection of BK virus antibodies measured by enzyme-linked immunosorbent assay (ELISA) and two haemagglutination inhibition methods: a comparative study

A quantitative PCR assay for SV40 neutralization adaptable for high-throughput applications

A neutralization assay incorporating a quantitative SYBR Green PCR endpoint has been developed for SV40. The present study demonstrates that crude virus samples can serve as suitable amplification templates for quantitative PCR without the need for nucleic acid extraction. The denaturation temperature of thermocycling appears to be sufficient to release the encapsidated viral genome and allow its availability as a PCR template. Issues arising from inhibitors of PCR present in crude virus samples can be circumvented easily by a 100-fold dilution step. Using a streamlined procedure that eliminates sample nucleic acid extraction (a hitherto rate-limiting step that diminishes throughput substantially), quantitative PCR was applied in order to assess: (1) the replication kinetics of SV40 and (2) the inhibition of SV40 productive infection by neutralizing antibodies. A similar high-throughput approach might be feasible for related polyomaviruses (e.g., BKV and JCV) as well as for other families of viruses.

Discovery and epidemiology of the human polyomaviruses BK virus (BKV) and JC virus (JCV)

Although discovered over thirty years ago, many aspects of the epidemiology of BKV and JCV in the general population, such as the source of infectious virus and the mode of transmission, are still unknown. Primary infection with both BKV and JCV is usually asymptomatic, and so age seroprevalence studies have been used to indicate infection. BKV commonly infects young children in all parts of the world, with the exception of a few very isolated communities, adult seroprevalence rates of 65-90% being reached by the age of ten years. In contrast, the pattern of JCV infection appears to vary between populations; in some anti-JCV antibody is acquired early as for BKV, but in others anti-JCV antibody prevalence continues to rise throughout life. This indicates that the two viruses are probably transmitted independently and by different routes. Whilst BKV DNA is found infrequently in the urine of healthy adults, JCV viruria occurs universally, increasing with age, with adult prevalence rates often between 20% and 60%. Four antigenic subtypes have been described for BKV and eight genotypes are currently recognized for JCV. The latter have been used to trace population movements and to reconstruct the population history in various communities.

Serum antibodies to viral pathogens and Toxoplasma gondii in HIV-infected individuals

Sera from 38 HIV-infected individuals were examined longitudinally for antibodies to viruses that may increase morbidity in HIV infection, as well as commensal viruses and Toxoplasma gondii. HTLV infection was seen in Norway for the first time as four patients had antibodies to HTLV-II and one had antibodies to HTLV-I. Antibodies to hepatitis B virus (HBV) were found in 47.2%, while 21.6% of the patients had antibodies to hepatitis C virus (HCV). There was no evidence of acquisition of HBV or HVC during the mean observation period of 2 years. A titre increase in CMV antibody with time was observed for 7 out of 21 patients and a decrease for 2 patients. For Epstein-Barr virus, herpes simplex, varicella-zoster, rubella and measles viruses, human polyomavirus BK as well as for Toxoplasma gondii, antibody prevalences and titres were within the range seen in normal populations. Also, no longitudinal changes were observed in titres of these antibodies, indicating that humoral immunity remained intact during the study period. The high prevalences of HTLV-I/II, HBV and HCV antibodies in HIV-infected patients reflect common modes of virus transmission, and the fluctuations in CMV antibody titre are indicative of reactivations. Such coinfections may influence disease progression.

Antibodies to dsDNA are produced during primary BK virus infection in man, indicating that anti-dsDNA antibodies may be related to virus replication in vivo

Experimental immunizations with both the Polyomavirus BK and with the isolated viral genomic dsDNA regularly induce antibodies with a relative affinity for BK virus dsDNA. In the present study we demonstrate that the anti-dsDNA responses to BK virus in experimental animals also appear during natural BK virus infection in man. Fifty-nine children were examined over time for serological signs of primary BK virus infection. Of eight children found to undergo primary infection with BK virus, anti-BK dsDNA antibodies appeared in all. In 4 of the 8 patients the antibodies cross-reacted significantly with mammalian dsDNA, and weak cross-reactions were also noted in at least three other patients. The antibodies resembled those induced in the experimental model with regard to their relative affinity for BK dsDNA. In contrast, most, but not all, anti-dsDNA antibodies from 10 SLE patients cross-reacted extensively with dsDNA from viral and mammalian origin. Thus, a dsDNA virus like BK virus may provoke immunological intolerance to dsDNA, but, with qualities different from those produced during SLE. The present observations demonstrate that induction of anti-dsDNA antibodies is not restricted to experimental immunization of animals, but does also take place in humans during naturally acquired BK virus infection. The relevance of this model for the spontaneous production of anti-dsDNA antibodies is discussed.

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.

Prevalence of anti BK virus antibody in Portugal and Norway

Specific IgG antibodies against BK virus measured by ELISA were used as a marker of previous infection. Results with sera from healthy people of different counties in Portugal were compared with previous findings in Norwegian sera. No significant difference between the prevalence and level of BKV IgG could be found between Portugal and Norway, and when comparing the different counties of Portugal. Thus, the way of transmission seemed to follow the same routes both in rural and urban counties in Portugal, and in Norway.

Inoculation with BK virus may break immunological tolerance to histone and DNA antigens

BK virus particles contain histones of host cell origin that combine with viral DNA to form minichromosomes. Data from earlier immunoblotting experiments demonstrated that rabbits inoculated i.v. with purified infectious BK virus produced antibodies not only to the viral structural protein VP1 but also to migrating polypeptides of a molecular mass ranging from 14-16 kDa. These proteins were believed to represent certain histone classes. To examine this hypothesis, sera from five rabbits inoculated with BK virus were analyzed by ELISA for antibodies against polynucleosomes, the individual histone classes, and double-stranded DNA, as well as against antigens carried by the structural viral proteins. Antibodies against polynucleosomes and also against histones H1 and H3 were found in sera from two of five inoculated rabbits. The same sera contained antibodies reacting with double-stranded DNA, whereas no antibodies against H2A, H2B, and H4 were detected. BK virus inoculation may thus lead to a break of tolerance, resulting in autoantibody production against highly conserved antigens that, in this context, may be regarded as "self-antigens", irrespective of the species in which they originate.

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

We have monitored BK virus (BKV) antigen expression and multiplication in human monocytes and in a human macrophage (M luminal diameter)-like cell line (U937) in the presence or absence of dilution series of human or rabbit anti-BKV antisera. After infection with BKV alone, restricted expression (structural antigens and T-antigen) and multiplication was recorded in monocytes from some donors, while in U937 cells and monocytes from other donors, no signs of viral activity were detected. Monocyte cultures established from the same donor at different times demonstrated antigen expression/multiplication on two occasions but not on the third. A pronounced enhancement of BKV expression/multiplication in human monocytes and multiplication in U937 cells was seen with some dilutions of all antisera (human and rabbit) used. The pattern of enhancement and the dilution resulting in maximum viral activity was constant and seemed to be determined by the serum, but the exact level of enhancement for a given serum differed considerably in monocytes from different donors and seemed to be determined by the cells. In the latter respect, monocytes taken from the same donor some weeks apart showed variations at the same level, as did cells from different donors. PMA (phorbol-12-myristate-13-acetate) stimulation of monocytes and U937 cells resulted in stronger antibody enhancement in terms of infectivity, without affecting the number of monocytes showing antigen expression. No expression/multiplication of BKV was detected in the murine M luminal diameter-like cell line P338 DI.

Characterization of BK virus-specific antibodies in human sera by Western immunoblotting: use of a zwitterionic detergent for restoring the antibody-binding capacity of electroblotted proteins

After initial problems related to denaturation of antigenic epitopes, we developed a Western immunoblotting method for the characterization of antibodies reacting with BK virus (BKV) structural polypeptides. When a zwitterionic detergent, Empigen BB, was added to the running buffer during electroblotting, the antibody-binding capacity of electrophoretically separated BKV polypeptides was partially restored. Antibodies reacting with different BKV antigens were detected and visualized by biotinylated anti-species-specific antibodies, peroxidase-conjugated streptavidine, and diaminobenzidine staining. Human sera containing anti-BKV antibodies reacted with VP1, but a serum containing antinuclear antibodies also reacted with VP4, -5 and -6 (histones). Serum from a rabbit inoculated with purified BKV reacted with VP1, and also with VP4, indicating that BKV inoculation may imply production of antibodies against histones.

BK virus infection in patients with AIDS

Antibodies to the human papovavirus BK (BKV) were determined in a group of 25 homo- and bisexual males with AIDS, 24 men with AIDS-related complex (ARC) and 18 healthy male homosexual controls from Copenhagen. The AIDS patients had a significantly lower prevalence and level of anti-BKV antibodies tested by IgG-ELISA, hemagglutination inhibition and neutralization tests than the ARC patients. About half of the anti-BKV antibody positive AIDS patients demonstrated primary infections or reactivations but without specific IgM production. The titers were low compared to primary infections in children. At least 2 of the patients lost their serological markers in the late phase of the disease. It is therefore possible that the low prevalence of BKV infection in AIDS patients is caused by loss of serological markers even if the level of total IgG is normal or increased.

BK virus infection in children with cancer: serological response studied by haemagglutination inhibition, neutralization, and IgG- and IgM-class specific ELISA tests

We recently developed enzyme-linked immunosorbent assays (ELISAs) for the detection of anti-BK virus IgG- and IgM-antibodies, and also a convenient and rapid serum neutralization test. Complemented by a traditional haemagglutination inhibition test (HAI) these methods were used to investigate the longitudinal response to BKV infection in sequentially taken sera from 29 children under treatment for cancer. In separate experiments it was shown that the results were not at any extent influenced by antibodies against other polyomaviruses. At the time of diagnosis the prevalence of specific IgG- and IgM-antibodies and the geometric mean IgG levels were not significantly different for the patients compared with a group of healthy children. The primary infections seemed to occur at the same age for the two groups of children. Seven of the patients had a primary infection with BKV. The results indicate that the host response in moderately immunosuppressed children during primary infection is the same as expected for healthy individuals with the development of specific IgG, HAI, and NT antibodies, and, usually, production of BKV-IgM for several months. The results indicated that whether specific IgM was demonstrated in the first sample or appeared later during a reactivation episode, this parameter was correlated with profound immunosuppression. Significant titre changes, detectable IgM antibodies and/or seroconversions were demonstrated in 69% (20/29) of the cancer patients. Such indications of recent viral activity was recorded in 42% (8/19) of children with meningococcal infections. The observation periods for the two groups of patients are, however, not directly comparable.

Neutralization test for BK virus: plaque reduction detected by immunoperoxidase staining

We developed an immunoperoxidase staining test to detect structural antigens of BK virus (BKV) in Vero cell cultures. This test was used to examine the neutralizing activity of human and immunized animal sera. It was shown that sera positive for BKV antibodies measured by hemagglutination inhibition test and enzyme-linked immunosorbent assay (ELISA) were able to prevent expression of BKV structural antigens in cell cultures. The correlation between titers in the hemagglutination inhibition test, levels of BKV IgG measured by ELISA, and the titers assayed by the immunoperoxidase neutralization test was high. We suggest that this type of test may be used instead of conventional neutralization tests for other viruses with slowly developing cytopathogenic effects.

Age-dependent prevalence of BK virus IgG and IgM antibodies measured by enzyme-linked immunosorbent assays (ELISA)

Enzyme immunoassays (ELISA) have been developed for the detection of BK virus IgG- and IgM-antibodies. Specific IgG is detected by an antigen-coated solid phase test; IgM by an antibody capture method. These methods have been used to study the age-distribution of BK virus antibodies in Tromsø county in Northern Norway. The serum panels tested were: 60 sera from paediatric patients aged 0-1 year; 220 sera from healthy persons aged 1-82 years; 74 sera from healthy blood donors; 107 sera from healthy pregnant women. The age-distribution of BKV-IgG antibodies showed that primary infections took place predominantly between the ages of 1 and 6 years, and that there were no sex differences, either in the age-specific prevalence or in the level of BKV-IgG. We found no significant differences in the prevalence of BKV-IgM antibodies in healthy children and adults and pregnant women. BKV-IgM was detected in 26 of the 461 sera tested (5.6%).