Maturation of IgG avidity to individual rubella virus structural proteins

Standardization of Assays That Detect Anti-Rubella Virus IgG Antibodies

Rubella virus usually causes a mild infection in humans but can cause congenital rubella syndrome (CRS). Vaccination programs have significantly decreased primary rubella virus infection and CRS; however, vaccinated individuals usually have lower levels of rubella virus IgG than those with natural infections. Rubella virus IgG is quantified with enzyme immunoassays that have been calibrated against the World Health Organization (WHO) international standard and report results in international units per milliliter. It is recognized that the results reported by these assays are not standardized. This investigation into the reasons for the lack of standardization found that the current WHO international standard (RUB-1-94) fails by three key metrological principles. The standard is not a pure analyte but is composed of pooled human immunoglobulin. It was not calibrated by certified reference methods; rather, superseded tests were used. Finally, no measurement uncertainty estimations have been provided. There is an analytical and clinical consequence to the lack of standardization of rubella virus IgG assays, which leads to misinterpretation of results. The current approach to standardization of rubella virus IgG assays has not achieved the desired results. A new approach is required.

Effects of Serial Skin Testing with Purified Protein Derivative on the Level and Quality of Antibodies to Complex and Defined Antigens in Mycobacterium bovis-Infected Cattle

Several serological tests designed to detect antibodies to immunodominant Mycobacterium bovis antigens have recently emerged as ancillary tests for the detection of bovine tuberculosis in cattle, particularly when used after the injection of purified protein derivative (PPD) for skin testing, which significantly boosts M. bovis-specific antibody responses. The present findings demonstrate the onset and duration of boosted antibody responses after the injection of M. bovis PPD for the caudal fold test (CFT) and Mycobacterium avium and M. bovis PPDs for the comparative cervical test (CCT), administered in series in cattle experimentally infected with M. bovis. While skin tests boosted the responses to certain antigens (i.e., MPB83 and MPB70), they did not affect the responses to other antigens (e.g., ESAT-6, CFP10, MPB59, and MPB64). Administration of the CCT 105 days after the CFT resulted in an even greater secondary boost in antibody responses to MPB83 and MPB70 and to a proteinase K-digested whole-cell sonicate (WCS-PK) of M. bovis. Both IgM and IgG contributed to the initial boost in the MPB83/MPB70-specific antibody response after the CFT. The secondary boost after the CCT was primarily due to increased IgG levels. Also, the avidity of antibodies to MPB83 and MPB70 increased after the CCT in M. bovis-infected cattle. The avidity of antibodies to the WCS-PK antigens increased in the interval between the CFT and the CCT but did not increase further after the CCT. Together, these findings demonstrate that the administration of PPDs for skin tests results in additive enhancement (i.e., when the CFT and CCT are performed in series), both qualitative and quantitative, of MPB83/MPB70-specific antibody responses.

Immunogenicity of recombinant human bocavirus-1,2 VP2 gene virus-like particles in mice

Human bocavirus (HBoV), a recently identified pathogen with a worldwide distribution is closely related to paediatric acute respiratory infection and gastroenteritis. The present study was performed to evaluate the immunogenicity of HBoV1 and HBoV2 virus-like particles (VLPs) as vaccine candidates in mice. Both HBoV1 and HBoV2 VLPs were expressed in the bacmid virus–SF9 cell system. Mice were inoculated three times at 3-week intervals with HBoV VLPs at one dose intramuscular (i.m.) or intradermal (i.d.) with or without the addition of the alum adjuvant. ELISA was used to detected antibody, and ELISPOT was used to test cellular immune responses. HBoV-specific IgG antibodies were induced and alum adjuvant improved the antibody titres and avidity, while the inoculation pathway had no influence. T helper type 1/ type 2 immune responses were balanced induced by HBoV1 VLPs but not HBoV2 VLPs. Serum IgG antibody cross-reactivity rates of the two subtypes were similar, but cross-reactions of HBoV1 immunization groups were higher. The single i.m. group had more interferon-γ-secreting splenocytes. These data indicate that HBoV VP2 VLPs have good immunogenicity with induction of strong humoral and cellular immune responses, and they may be potential candidate vaccines for HBoV infection.

Avidity Studies in Anisakis simplex-Associated Allergic Diseases

Gastroallergic anisakiasis (GAA) and Anisakis-sensitization-associated chronic urticaria (CU+) differ with respect to specific IgE levels. We hypothesised different immunoglobulin avidities in both entities as well as their dependence on TI and fish consumption. 16 patients with GAA and 17 patients with CU+ were included, and immunoglobulin levels were analysed by CAP (Phadia). IgE and IgG avidity indexes (AvIgE and AvIgG, resp.) were also determined. IgG avidity was higher in GAA than in CU+ (P = 0.035), whereas there was a tendency to lower IgE avidity in GAA (P = 0.095). When analysing all patients, AvIgG was positively correlated with specific IgE, IgG, and IgG₄ as well as total IgE (Rho between 0.66 and 0.71; P < 0.002), but AvIgE was negatively correlated with specific IgE (Rho −0.57; P < 0.001), specific IgG₄ (Rho −0.38; P < 0.05), and total IgE (Rho 0.66; P < 0.001). In GAA, weekly fish consumption was positively associated with AvIgE (Rho 0.51; P = 0.05). A multivariate regression showed that time interval was the main explaining factor for AvIgE in GAA. We could show a differential behaviour of immunoglobulin isotype avidities in both entities and their dependence on fish-eating habits as well as on the time elapsed to the last parasitic episode.

Quantitative and qualitative assay of rubella IgA antibody in breast milk

Breast milk contains immunological factors, such as IgA antibody, which help to prevent infectious diseases. A total of 197 paired samples of colostrum and breast milk was collected from postpartum mothers in Gunma City, Japan, and examined for anti-rubella IgA antibody by enzyme-linked immunosorbent assay (ELISA) and Western blotting (WB). The anti-rubella virus IgA ranged from 0.5 to 78.5 U/ml with a mean of 6.05 U/ml and a median of 3.6 U/ml in colostrum, and from 0.5 to 32.7 U/ml with a mean of 2.74 U/ml and a median of 2 U/ml in milk. The differences between the means of titers of total IgA and anti-rubella virus IgA in colostrum and in milk were significant statistically. The levels of anti-rubella virus IgA in both colostrum and breast milk correlated positively with the anti-rubella virus hemagglutination inhibition (HI) titers in the sera of mother, indicating that the levels of these different classes of antibodies correlated. Based on WB, anti-rubella virus IgA in both colostrum and breast milk reacted with the rubella viral protein E1 and C, but not with the E2 protein.

Evaluation of commercial rubella immunoglobulin G avidity assays

We compared the performances of five commercial rubella virus immunoglobulin G (IgG) avidity assays. The Adaltis (kappa = 0.28) and Diesse (kappa = 0.33) assays showed poor correlation, the Behring assay (kappa = 0.68) showed good correlation, and the Euroimmun (kappa = 0.95) and Radim (kappa = 0.94) assays showed excellent correlation with a well-established in-house rubella virus IgG avidity assay. The Euroimmun and Radim assays were statistically significantly better than the other commercial assays (P < 0.01).

Humoral immune response to primary rubella virus infection

An assay capable of distinguishing between the immune response generated by recent exposure to rubella virus and the immune response existing as a result of past exposure or immunization is required for the diagnosis of primary rubella virus infection, especially in pregnant women. Avidity assays, which are based on the premise that chaotropic agents can be used to selectively dissociate the low-avidity antibodies generated early in the course of infection, have become routinely used in an effort to accomplish this. We have thoroughly investigated the immunological basis of an avidity assay using a viral lysate-based assay and an enzyme-linked immunosorbent assay (ELISA) based on a peptide analogue of the putative immunodominant region of the E1 glycoprotein (E1208-239). The relative affinities of the antibodies directed against E1208-239 were measured by surface plasmon resonance and were found to correlate well with the avidity index calculated from the ELISA results. We found that the immune response generated during primary rubella virus infection consists of an initial low-affinity peak of immunoglobulin M (IgM) reactivity followed by transient peaks of low-avidity IgG3 and IgA reactivity. The predominant response is an IgG1 response which increases in concentration and affinity progressively over the course of infection. Incubation with the chaotropic agent used in the avidity assay abolished the detection of the early low-affinity peaks of IgM, IgA, and IgG3 reactivity while leaving the high-affinity IgG1 response relatively unaffected. The present study supported the premise that avidity assays based on appropriate antigens can be useful to confirm primary rubella virus infection.

Use of avidity enzyme-linked immunosorbent assay and avidity Western blot to discriminate between acute and chronic Neospora caninum infection in cattle

Avidity serological tests (avidity enzyme-linked immunosorbent assay [ELISA] and avidity Western blot) were developed and used to differentiate between acute (primary infection, reinfection, and recrudescence) and chronic Neospora caninum infection in cattle. In addition, the pattern of immunoglobulin G (IgG) avidity maturation against different specific antigens of N. caninum tachyzoites was studied. Sequential serum samples were collected from cattle naturally and experimentally infected with N. caninum. Four groups of experimentally infected cattle were included in the study and were representative of primary infection, reinfection, chronic infection, and noninfection. Serum samples were also collected from naturally infected cattle classified into nonaborting and aborting cows on the basis of clinical findings and serological profiles, and a third group composed of seronegative cows that seroconverted during the course of the experiment. All samples were tested by avidity ELISA and avidity Western blot. The IgG avidity ELISA allowed the discrimination between primary and chronic infection because all experimentally primary-infection cows showed low avidity indexes at week 4 postinfection (p.i.) compared with the high avidity values found at week 20 postinfection. However, this test did not allow the discrimination of reinfection or recrudescence from chronic infection. Regarding IgG avidity Western blot results, no antigenic markers correlating with acute (primary infection, recrudescence, and reinfection) or chronic infection were recognized. However, the 17-kD immunodominant antigen was mostly responsible for high avidity values obtained by avidity ELISA because it was intensively recognized by high-avidity antibodies in all chronically infected animals after urea treatment.

Significance of avidity and immunoblot analysis for rubella IgM-positive serum samples in pregnant women

A total of 512 IgM-positive serum samples from 449 pregnant women referred by microbiologists and medical laboratories for additional testing and final interpretation were collected over a 3-year period. Employment of an IgM capture enzyme immunoassay (EIA) confirmed only 31% of the initial EIA-IgM-positive samples. In order to discriminate acute rubella virus infections, which are associated with increased risk of fetal infection and embryopathy, from persistent or non-specific IgM, IgG avidity index and the presence of IgG with specificity to rubella virus E2 glycoprotein was determined by Western immunoblot. In only six patients (1.3%), a primary infection with rubella virus was diagnosed on the basis of IgM positivity, low avidity IgG, absence of E2-specific IgG in immunoblot concordant with clinical findings as well as consistent changes in follow-up samples. The serological results were not compatible with rubella re-infection. The infection status in 14 patients (3.1%) remained inconclusive even when both avidity assay and immunoblot were used, while restriction to either test did not allow a conclusive interpretation in 11.6% of patients. The use of both assays is clearly better, and therefore, recommended in IgM-positive samples.

Usefulness of specific IgG avidity for diagnosis of hepatitis A infection

AIM

Diagnosis of acute hepatitis A virus (HAV) infection is classically based on the detection of HAV-IgM. Nevertheless, HAV-IgM can be positive for patients with polyclonal stimulation of their immune system (i.e. immune reactivation). To improve the diagnostic yield, an avidity test for HAV-IgG antibodies was developed and tested.

METHODS

Avidity tests were performed in 128 sera: 11 selected samples from patients with past infection, 15 acute hepatitis A, 10 vaccinated subjects and 4 patients with immune reactivation as well as 84 HAV-IgM positive unselected sera, provided by routine laboratories.

RESULTS

Patients with past infection had avidities over 70%, whereas avidities in patients with acute hepatitis A were below 50% during the first month following the onset of symptoms. As expected, patients with immune reactivation had avidities over 70% consistent with past infection. The results obtained for the 84 unselected sera allowed reconsidering the diagnosis of acute hepatitis A for nearly a third of patients.

CONCLUSION

This test could improve the diagnosis of acute hepatitis A infection, particularly in elderly patients.

Diagnostic relevance of immunoglobulin G avidity for hepatitis A virus

Diagnosis of acute hepatitis A virus (HAV) infection is based on the detection of HAV immunoglobulin M (IgM). However, IgM could be detected due to nonspecific polyclonal activation of the immune system. An avidity test for anti-HAV IgG was developed to distinguish acute infection, where low-avidity antibodies are detected, from immune reactivation. The assay was tested on 104 samples, including 11 sera from patients with past infection, 15 sera from patients with acute infection and 4 collected after recovery, 10 sera from vaccinated subjects, 4 sera from patients with suspected immune reactivation, and 60 unselected HAV-IgM positive sera, collected over 1 year in a routine laboratory. The avidity index (AI) was expressed as percentage. The results were provided as the mean +/- one standard deviation. Patients with a history of prior infection had AIs of >70% (mean, 86% +/- 10), whereas the mean AI was 36% +/- 16 during acute HAV infection (P < 0.001). Within the first month after the onset of hepatitis, avidity was either noncalculable due to a very low IgG titer or <50%. In patients with immune reactivation, avidity was >70% (88% +/- 10%), a finding consistent with a prior infection. Among the 60 unselected sera, 35 (58%) had a noncalculable or <50% avidity, and most of them had a detectable HAV RNA, confirming HAV infection. In contrast, 16 (27%) had an avidity of >70%, and none was reverse transcription-PCR positive, suggesting immune reactivation. These 16 patients were significantly older than the others (50 +/- 16 years versus 26 +/- 14 years). The new anti-HAV IgG avidity assay we developed could improve HAV infection diagnosis, particularly in elderly patients.

Changes in avidity and level of immunoglobulin G antibodies to Mycobacterium tuberculosis in sera of patients undergoing treatment for pulmonary tuberculosis

Much is known about specific antibodies and their titers in patients with tuberculosis. However, little is known about the avidity of these antibodies or whether changes in avidity occur during the progression of the disease or during treatment. The aims of this study were to determine the avidity of antibodies to Mycobacterium tuberculosis in patients with pulmonary tuberculosis, to explore the value of avidity determination for the diagnosis of tuberculosis, and to study changes in levels of antibodies and their avidity during treatment. Antibody avidity was measured by an enzyme-linked immunosorbent assay with thiocyanate elution. Avidity indices and serum levels of immunoglobulin G to M. tuberculosis were determined for 22 patients with pulmonary tuberculosis before and during treatment and for 24 patients with other pulmonary diseases. Antibody levels and avidity were both significantly higher in untreated tuberculosis patients than in the controls. Avidity determination had more diagnostic potential than determination of the antibody levels. Tuberculosis patients with a long duration of symptoms had higher antibody avidity than those with a recent onset of symptoms, indicating affinity maturation of specific antibodies during active disease. In the early phase of treatment, a decrease in antibody avidity was observed for 73% of all tuberculosis patients, accompanied by an initial increase in antibody levels in 36% of these patients. These phenomena could be explained by an intense stimulation of the humoral response by antigens released from killed bacteria, reflecting early bactericidal activity of antituberculous drugs leading to the production of low-affinity antibodies against these released antigens.