Enhancement of hemagglutination inhibition by complement

A monoclonal antibody-based immunoassay for measuring the potency of 2009 pandemic influenza H1N1 vaccines

Background

The potency of inactivated influenza vaccines is determined using a single radial immunodiffusion (SRID) assay. This assay is relatively easy to standardize, it is not technically demanding, and it is capable of measuring the potency of several vaccine strain subtypes in a multivalent vaccine. Nevertheless, alternative methods that retain the major advantages of the SRID, but with a greater dynamic range of measurement and with reduced reagent requirements, are needed.

Objectives

The feasibility of an ELISA-based assay format was explored as an alternative potency assay for inactivated influenza vaccines.

Methods

Several murine monoclonal antibodies (mAbs), specific for the 2009 pandemic H1N1 influenza virus hemagglutinin (HA), were evaluated for their potential to capture and quantify HA antigen. Vaccine samples, obtained from four licensed influenza vaccine manufacturers, included monovalent bulk vaccine, monovalent vaccine, and trivalent vaccine. Traditional SRID potency assays were run in parallel with the mAb–ELISA potency assay using the reference antigen standard appropriate for the vaccine samples being tested.

Results

The results indicated that the ELISA potency assay can quantify HA over a wide range of concentrations, including vaccine at subpotent doses, and the ELISA and SRID potency values correlated well for most vaccine samples. Importantly, the assay was capable of quantifying A/California HA in a trivalent formulation.

Conclusions

This study demonstrates the general feasibility of the mAb approach and strongly suggests that such ELISAs have potential for continued development as an alternative method to assay the potency of inactivated influenza vaccines.

Neutralizing and protective epitopes of the 2009 pandemic influenza H1N1 hemagglutinin

AIMS AND METHODS

To facilitate antigenic characterization of the influenza A 2009 pandemic H1N1 [A(H1N1)pdm09] hemagglutinin (HA), we generated a panel of murine monoclonal antibodies (mAbs) using as the immunogen mammalian-derived virus-like particles containing the HA of the A/California/04/2009 virus. The antibodies were specific for the A/California/04/2009 HA, and individual mAbs suitable for use in several practical applications including ELISA, immunofluorescence, and Western blot analysis were identified.

RESULTS AND CONCLUSIONS

As the panel of mAbs included antibodies with hemagglutination inhibition (HI) and virus neutralizing activities, this allowed identification and characterization of potentially important antigenic and neutralizing epitopes of the A/California/04/2009 HA and comparison of those epitopes with the HAs of other influenza viruses including seasonal H1N1 viruses as well as the A/South Carolina/1918 and A/New Jersey/1976 H1N1 viruses. Three mAbs with the highest HI and neutralizing titers were able to provide passive protection against virus challenge. Two other mAbs without HI or neutralizing activities were able to provide partial protection against challenge. HA epitopes recognized by the strongest neutralizing mAbs in the panel were identified by isolation and selection of virus escape mutants in the presence of individual mAbs. Cloned viruses resistant to HI and antibody neutralization were sequenced to identify mutations, and two unique mutations (D127E and G155E) were identified, both near the antigenic site Sa. Using human post-vaccination sera, however, there were no differences in HI titer between A/California/04/2009 and either escape mutant, suggesting that these single mutations were not sufficient to abrogate a protective antibody response to the vaccine.

Appearance of slow-reacting, complement-requiring hemagglutination-inhibiting antibody in swine infected with pseudorabies virus

Slow-reacting, complement-requiring hemagglutination-inhibiting (HI) antibody was detected in sera from pigs infected with pseudorabies virus; approximately 16 hemolytic units of complement were necessary for the detection of such antibody. Higher HI antibody titers were obtained when antigen and serum were allowed to incubate before addition of complement than when all three components were incubated at the same time. A HI test was developed in which antigen-serum mixtures were incubated at 4 degrees C for 48 h and then with complement at 37 degrees C for 1 h; this gave an improved sensitivity over the previous incubation without complement.

Microneutralization test for influenza A and B and parainfluenza 1 and 2 viruses that uses continuous cell lines and fresh serum enhancement

A modified microneutralization test for influenza A and B and parainfluenza 1 and 2 viruses was developed. Use of continuous cell lines (Madin-Darby canine kidney and LLC-MK2), hemagglutination for virus detection, and transfer plates and other microtiter equipment resulted in a highly mechanized technique suitable for titrating large numbers of sera easily and relatively inexpensively. Titers of heat-inactivated human sera were enhanced 0.5 to 3.5 log2 by addition of fresh hamster or human serum to the test. Seroconversion rates and identification of seronegative persons were not changed by fresh serum enhancement, but the magnitude of seroconversion was often increased and the distribution of positive titers was broadened. For influenza A and B, seroconversion rates in the microneutralization test were equivalent to those obtained in rhesus monkey kidney tube neutralization tests. For influenza B, seroconversion rates by micro-neutralization were almost double those obtained with the hemagglutination inhibition test, but the rates were similar for influenza A/USSR (H1N1). Preexisting microneutralization titers correlated well with resistance of infection with influenza B. Limited experience with parainfluenza 1 and 2 was similar to previously reported findings with the tube neutralization test.

Effect of heat-labile factors on the neutralization of vaccinia virus by human

The effect of unheated guinea pig and human prevaccination serum on the neutralization of vaccinia virus was studied. Enhancement of neutralization was found in all sera containing antibody and was more marked (20- to 150-fold) in sera obtained in the first weeks after primary vaccination than in sera from immune adults or cord sera (4- to 25-fold). The enhancing factor was thought to be complement because it was destroyed by heating and ethylenediaminetetraacetic acid treatment. The fresh serum-enhanced neutralization test represents a highly sensitive and specific test which can be applied to measure low levels of vaccinia antibody not detectable by other means.