Heparin-based ELISA reduces background reactivity in virus-like particle-based papillomavirus serology

Clinicopathologic Implications of Complement Genetic Variants in Kidney Transplantation

Genetic testing has uncovered rare variants in complement proteins associated with thrombotic microangiopathy (TMA) and C3 glomerulopathy (C3G). Approximately 50% are classified as variants of uncertain significance (VUS). Clinical risk assessment of patients carrying a VUS remains challenging primarily due to a lack of functional information, especially in the context of multiple confounding factors in the setting of kidney transplantation. Our objective was to evaluate the clinicopathologic significance of genetic variants in TMA and C3G in a kidney transplant cohort. We used whole exome next-generation sequencing to analyze complement genes in 76 patients, comprising 60 patients with a TMA and 16 with C3G. Ten variants in complement factor H (CFH) were identified; of these, four were known to be pathogenic, one was likely benign and five were classified as a VUS (I372V, I453L, G918E, T956M, L1207I). Each VUS was subjected to a structural analysis and was recombinantly produced; if expressed, its function was then characterized relative to the wild-type (WT) protein. Our data indicate that I372V, I453L, and G918E were deleterious while T956M and L1207I demonstrated normal functional activity. Four common polymorphisms in CFH (E936D, N1050Y, I1059T, Q1143E) were also characterized. We also assessed a family with a pathogenic variant in membrane cofactor protein (MCP) in addition to CFH with a unique clinical presentation featuring valvular dysfunction. Our analyses helped to determine disease etiology and defined the recurrence risk after kidney transplant, thereby facilitating clinical decision making for our patients. This work further illustrates the limitations of the prediction models and highlights the importance of conducting functional analysis of genetic variants particularly in a complex clinicopathologic scenario such as kidney transplantation.

Comparisons of VLP-Based ELISA, Neutralization Assays with Native HPV, and Neutralization Assays with PsV in Detecting HPV Antibody Responses in HIV-Infected Women

OBJECTIVE

Human papillomavirus (HPV)-associated cancers are important public health problems in HIV-infected people. Assays based on HPV virus-like particles (VLP) and pseudoviruses (PsV) are commonly used to examine HPV antibody responses in HIV-infected people, but neutralization assays with native HPV have not been utilized and a comparison of these three assays is lacking. We evaluated the agreement of assays using VLP, native HPV and PsV in detecting HPV16 and 18 antibodies in HIV-infected women.

METHODS

The VLP-based ELISA (VLP-ELISA) was used to detect antibody responses to HPV16 and 18 and cottontail rabbit papillomavirus (CRPV) VLP antigens. Neutralization assays with native HPV (NA-HPV) and with PsV (NA-PsV) were conducted to examine HPV16 or 18 neutralizing antibodies. Intra class correlation coefficients (ICC) and kappa coefficients were used to assess the agreements of seropositivity between the assays.

RESULTS

The seroprevalence detected by the VLP-ELISA, NA-HPV and NA-PsV in 94 HIV-infected women was 35%, 51% and 27% for HPV16 and 14%, 44% and 21% for HPV18. Cross-reactivity between HPV16 and HPV18 was 0.35, 0.04 and 0.33 (kappa coefficients) for the VLP-ELISA, NA-HPV and NA-PsV. The agreements of seropositivity between the three assays were low. Six women who were HPV16 DNA positive were seropositive by the NA-HPV but only two were HPV16 seropositive by the VLP-ELISA or NA-PsV. One HPV18 DNA positive woman was seropositive by all three assays. Repeated tests indicated excellent reproducibility of the NA-HPV.

CONCLUSION

HPV serology results vary across different assays. The NA-HPV appears to be a sensitive and reliable approach in detecting natural HPV antibodies in HIV-infected women. The NA-HPV can be applied in both HPV natural history studies and vaccine studies in HIV-infected people.

Validation of multiplexed human papillomavirus serology using pseudovirions bound to heparin-coated beads

This study developed and validated a high-throughput human papillomavirus (HPV) serology method based on Luminex technology, using pseudovirions (PsVs) of eight mucosal HPV types (HPV-6, -11, -16, -18, -31, -45, -52 and -58) and two cutaneous HPV types (HPV-5 and -38) bound to heparin-coated beads. Analysis with neutralizing type-specific monoclonal antibodies against the included HPV types indicated the type specificity of the assay. Analysis of negative-control serum samples from 63 children and 71 middle-aged women with up to one lifetime sexual partner indicated high specificity. Positive-control serum samples from subjects with known HPV DNA status or clinical diagnosis found expected sensitivities for most of the HPV types in 219 European serum samples, but lower than expected in 124 samples from Africa. HPV-45 and -52 did not react as expected with the human serum samples. The PsV-Luminex method was used to determine the HPV-seropositivity-associated relative risk for future cervical cancer using 208 serum samples from a prospective study of 18 814 women followed for 23 years, analysed previously with standard HPV-16 ELISA. The PsV-Luminex method gave similar results to ELISA (kappa=0.77). As expected, HPV seropositivities assayed using the PsV-Luminex method found an increased risk of cervical cancer for HPV-16 [odds ratio (OR)=7.7, 95 % confidence interval (CI)=2.6-23] and HPV-31 (OR=4.1, 95 % CI=1.6-10.8), non-significant tendencies for increased risk for other mucosal HPV types and no risk for the cutaneous HPV types. In summary, multiplexed HPV serology using mammalian-derived PsVs selected for native conformation by binding to heparin-coated beads was validated as a high-throughput HPV serological method for most of the analysed HPV types.

Differential binding patterns to host cells associated with particles of several human alphapapillomavirus types

The focus of this research was to compare the binding profiles of human papillomavirus (HPV) 11, 16, 18 and 45 virus-like particles (VLPs) to HaCaT cells and to the extracellular matrix (ECM) secreted by these cells. All four HPV types tested bind to a component(s) of the ECM. HPV11 VLP binding is blocked when the ECM is pretreated with an anti-laminin 5 (LN5) polyclonal antibody. A series of treatments utilizing heparins and heparinase revealed that HPV18 VLPs are dependent on heparan sulfates (HS) for binding to cells and ECM. HPV16 and HPV45 VLPs are dependent on HS for binding to HaCaT cells and dependent on both HS and LN5 for binding to ECM. These studies emphasize the need to study the binding characteristics of different HPV types before applying universal binding principles to all papillomaviruses.

One-step chromatographic purification of human papillomavirus type 16 L1 protein from Saccharomyces cerevisiae

The prophylactic human papillomavirus vaccine is based on recombinant L1 protein produced in yeast or insect cells. L1 is a major capsid protein that self-assembles into virus-like particles (VLP). Conventionally, several chromatography steps are required to purify it; the steps are time consuming, and they result in losses of the target protein. Ultracentrifugation using a sucrose cushions or cesium chloride density gradients, and size-exclusion chromatography, has also been routinely used for small scale purification of L1 protein. However, these methods require a great deal of time and labor, and are not suitable for industrial-scale purification. To resolve these problems, we have developed two simple one-step chromatography methods for purifying recombinant HPV16 L1 protein produced in Saccharomyces cerevisiae. Eighty percent of the contaminating protein was removed by ammonium sulfate precipitation and by precipitating contaminants prior to the chromatography step. One method uses heparin chromatography and the other, cation-exchange chromatography, and recoveries by the two methods were both about 60%, the highest recoveries of L1 protein achieved so far. We confirmed that HPV16 L1 protein purified by either method self-assembles into VLP. We anticipate that these one-step chromatography methods will reduce the time, cost and labor needed for purification of L1 protein, and facilitate the study of prophylactic HPV vaccines.

ELISAs using human bocavirus VP2 virus-like particles for detection of antibodies against HBoV

Human bocavirus (HBoV) has been identified worldwide in children with lower respiratory tract infections with an incidence of approximately 2–11%. The role of HBoV in pathogenesis, however, is largely unknown, and little is known about the epidemiology of the virus. To study the seroepidemiology of HBoV infection, the capsid protein was expressed in insect cells. Expression of the putative major capsid protein VP2 in insect cells led to the formation of virus-like particles exhibiting the typical icosahedral appearance of parvoviruses with a diameter of approximately 20 nm. The expressed particles were used to establish an enzyme-linked immunosorbent assay (ELISA) method, and serum samples from groups of children of various ages in China were tested for IgG antibodies against HBoV. HBoV antibodies were detected in as high as 36% of healthy children under 9 years. Of children hospitalized with lower respiratory tract infections, 31% were seropositive, and all age groups of these children showed a significantly higher level of HBoV IgG antibody than their healthy counterparts. When divided into age cohorts, results showed that more than 48% of healthy children had seroconverted by age of 4. Thus, HBoV appears to be a common infection in children. The potential pathogenesis of this virus, especially its role in lower respiratory tract infections in children warrants further investigation.

Development of a non-denaturing electrophoresis system for characterization of neutralizing epitopes on HPV virus-like particles

The precise structure of the HPV16 major neutralizing epitope recognized by H16.V5 monoclonal antibody is unknown. This paper describes a novel polyacrylamide gel electrophoresis (PAGE) for separation of HPV virus-like particles (VLPs) using cetyltrimethylammonium chloride (CTAC) as a solubilizing agent. CTAC PAGE employs KOH/CH3CO2H (pH 4-5.4) as a buffer system, K+ as the leading ion and 3-aminopropionic acid as a trailing ion. The unique characteristics of a cationic electrophoresis system allow separation of VLPs without heat denaturation. HPV VLP gel migration patterns were dependent on pre-treatment conditions: (1) thiol-agent reduction alone resulted in a 174 kDa band (interpreted as a L1 trimer), a 53 kDa band (size of the L1 monomer), as well as higher Mr aggregates consistent with a pentamer size; (2) both heat denaturation and thiol-agent reduction resulted in a 53 kDa band. Western blot analysis showed that the 174 kDa L1 trimer was strongly immunoreactive with H16.V5 and HPV16 VLP ELISA positive human sera, whereas no reactivity was seen with the monomeric L1 unit. These data suggest that a structure consistent with the migration pattern of a L1 trimer contains the major neutralizing epitope recognized by the H16.V5 MAb and human sera.

Optimization and validation of a multiplexed luminex assay to quantify antibodies to neutralizing epitopes on human papillomaviruses 6, 11, 16, and 18

A human papillomavirus (HPV) multiplexed competitive Luminex immunoassay first described by Opalka et al. (D. Opalka, C. E. Lachman, S. A. MacMullen, K. U. Jansen, J. F. Smith, N. Chirmule, and M. T. Esser, Clin. Diagn. Lab. Immunol. 10:108--15, 2003) was optimized and validated for use in epidemiology studies and vaccine clinical trials. Optimization increased both the analytical sensitivity and the clinical specificity of the assay to more effectively discriminate the low-titer antibody response of HPV-infected persons from noninfected individuals. The characteristics of the assay that were optimized included monoclonal antibody (MAb) specificity, scaling up the conjugation of virus-like particles (VLPs) to microspheres, VLP concentration, MAb concentration, sample matrix, sample dilution, incubation time, heat inactivation of sample sera, and detergent effects on assay buffer. The assay was automated by use of a TECAN Genesis Workstation, thus improving assay throughput, reproducibility, and operator safety. Following optimization, the assay was validated using several distinct serum panels from individuals determined to be at low and high risk for HPV infection. The validated assay was then used to determine the clinical serostatus cutoff. This high-throughput assay has proven useful for performing epidemiology studies and evaluating the efficacy of prophylactic HPV vaccines.