A semi-quantitative serological method to assess the potency of inactivated rabies vaccine for veterinary use

Pseudotyped Viruses for Lyssavirus

Lyssaviruses, which belong to the family Rhabdoviridae, are enveloped and bullet-shaped ssRNA viruses with genetic diversity. All members of Lyssavirus genus are known to infect warm-blooded animals and cause the fatal disease rabies. The rabies virus (RABV) in lyssavirus is the major pathogen to cause fatal rabies. The pseudotyped RABV is constructed to study the biological functions of G protein and evaluation of anti-RABV products including vaccine-induced antisera, rabies immunoglobulins (RIG), neutralizing mAbs, and other antiviral inhibitors. In this chapter, we focus on RABV as a representative and describe the construction of RABV G protein bearing pseudotyped virus and its applications. Other non-RABV lyssaviruses are also included.

Immunogenicity of an inactivated rabies vaccine for animals derived from the recombinant ERAGS strain

Purpose

The aims of the present study were to evaluate the immunogenicity of an inactivated rabies vaccine based on the ERAGS strain

Materials and Methods

The ERAGS virus propagated in Vero cells was inactivated with 3 mM binary ethylenimine for 8 hours. Three types of inactivated rabies vaccines were prepared to determine the minimum vaccine virus titers. Four further types of inactivated rabies vaccines were prepared by blending inactivated ERAGS with four different adjuvants; each vaccine was injected into mice, guinea pigs, and dogs to identify the optimal adjuvant. The immunogenicity of a Montanide (IMS) gel-adjuvanted vaccine was evaluated in cats, dogs, and cattle. Humoral immune responses were measured via a fluorescent antibody virus neutralization method and a blocking enzyme-linked immunosorbent assay.

Results

The minimum virus titer of the inactivated rabies vaccine was over 10^7.0 50% tissue culture infectious doses (TCID₅₀ values)/mL. Of the four kinds of adjuvants, the IMS gel-adjuvanted vaccine induced the highest mean viral neutralizing antibody (VNA) titers of 6.24 and 2.36 IU/mL in guinea pigs and dogs, respectively, and was thus selected as the vaccine for the target animals. Cats, dogs, and cattle inoculated with the IMS gel-adjuvanted vaccine developed protective VNA titers ranging from 3.5 to 1.2 IU/mL at 4 weeks post-inoculation (WPI).

Conclusion

Our data indicate that cats, dogs, and cattle inoculated with an inactivated rabies vaccine derived from the ERAGS strain developed protective immune responses that were maintained to 12 WPI.

Predicting the presence and titre of rabies virus-neutralizing antibodies from low-volume serum samples in low-containment facilities

Serology is a core component of the surveillance and management of viral zoonoses. Virus neutralization tests are a gold standard serological diagnostic, but requirements for large volumes of serum and high biosafety containment can limit widespread use. Here, focusing on Rabies lyssavirus, a globally important zoonosis, we developed a pseudotype micro-neutralization rapid fluorescent focus inhibition test (pmRFFIT) that overcomes these limitations. Specifically, we adapted an existing micro-neutralization test to use a green fluorescent protein-tagged murine leukaemia virus pseudotype in lieu of pathogenic rabies virus, reducing the need for specialized reagents for antigen detection and enabling use in low-containment laboratories. We further used statistical models to generate rapid, quantitative predictions of the probability and titre of rabies virus-neutralizing antibodies from microscopic imaging of neutralization outcomes. Using 47 serum samples from domestic dogs with neutralizing antibody titres estimated using the fluorescent antibody virus neutralization test (FAVN), pmRFFIT showed moderate sensitivity (78.79%) and high specificity (84.62%). Despite small conflicts, titre predictions were correlated across tests repeated on different dates both for dog samples (r = 0.93) and in a second data set of sera from wild common vampire bats (r = 0.72, N = 41), indicating repeatability. Our test uses a starting volume of 3.5 µl of serum, estimates titres from a single dilution of serum rather than requiring multiple dilutions and end point titration, and may be adapted to target neutralizing antibodies against alternative lyssavirus species. The pmRFFIT enables high-throughput detection of rabies virus-neutralizing antibodies in low-biocontainment settings and is suited to studies in wild or captive animals where large serum volumes cannot be obtained.

A rapid one-step immunochromatographic test strip for rabies detection using canine serum samples

We developed an immunochromatographic test strip using colloidal gold-coated staphylococcal protein A (SPA) for the detection of rabies antibody in canine serum samples. The recombinantly expressed rabies virus phosphoprotein (RV-P) and the anti-staphylococcal protein A (anti-SPA) polyclonal antibody were coated on the test (T) and control (C) lines on a nitrocellulose membrane, respectively. This layout is designed such that the polyclonal antibody in canine serum is captured by the colloidal gold-SPA conjugates, before the rabies antibody complex is specifically selected by the RV-P deposited on the T line, forming a 'sandwich' pattern. Unbound excess colloidal SPA then proceeds to the control line where SPA specifically interacts with the anti-SPA antibody, producing a red precipitation at the C line, indicating the validity of the strip. We tested 165 canine serum samples with the strips, and the results were compared with those obtained using ELISA. The specificity and sensitivity of ICTS were found to be 93·1 and 92·2%, respectively. As a rapid technique, not demanding expensive instrumentation, the strip offers potential in disease monitoring, especially in rabies-endemic developing countries.

SIGNIFICANCE AND IMPACT OF THE STUDY

Simple and cheap techniques to detect rabies virus or monitor immunity against it are central in maintaining epidemiological control over the disease, particularly in endemic developing countries. While many techniques meet this requirement, they are confined to this usage as they are time-consuming and demand expensive instrumentation. Our immunochromatographic test strip can detect rabies antibody with high specificity and sensitivity; the output can be measured with naked eye. It allows safe and quick detection that will be of value in the surveillance of the immunization status of potential targets in rabies-endemic regions and will aid disease control.

A multi-dose serological assay suitable to quantify the potency of inactivated rabies vaccines for veterinary use

The mouse vaccination-challenge test, which is the most widely used method for determining the potency of inactivated rabies vaccines, is imprecise, time-consuming, and causes severe distress to the test animals. An alternative single-dose serological method has been implemented in the European Pharmacopoeia Monograph 0451 to replace the mouse challenge test for batch release. This single-dose limit method provides semi-quantitative results, but is not suitable for quantifying potency. We have now extended this serological method to a multi-dose format which allows a quantification of vaccine potency. In studies including all rabies vaccine strains relevant for Europe, we found dose-dependency for all vaccines and standard preparations. We have demonstrated that the multi-dose serological approach provides reliable quantitative potency results and is more precise than the mouse vaccination-challenge test. We have shown that adjuvanted vaccines can be calibrated against non-adjuvanted material, and that reference material can be calibrated against the International Standard. The method is therefore capable of assigning potency with the additional advantage of requiring fewer animals and reducing distress. Once the applicability of the method has been further verified in a collaborative study, it can complement the single-dose assay and eventually eliminate the need for the mouse challenge test.