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.

Potency estimation of vaccine batches remains unaffected by anesthesia for intracerebral injection.

Potency testing of rabies and whole-cell pertussis vaccine batches is still performed by an intracerebral (i.c.) challenge test, in conformity with international regulatory requirements. For the i.c. injection, the use of anesthesia is strongly recommended to alleviate the severe pain induced by the procedure. Today, anesthesia is not consistently mentioned in regulatory requirements, in contrast to the times when the potency tests were developed. The introduction of anesthesia is hampered, due to the lack of data on a hypothetical impact of anesthesia on potency estimation. Here, we show the comparative analysis of the extensive batch release data set of a rabies vaccine for human use that was tested in two laboratories of which only one applied anesthesia. In essence, we find that the mean batch test results were similar to each other, demonstrating that anesthesia for i.c. injection does not interfere with potency estimation. Consequently, we recommend the update of regulatory requirements and protocols and support the implementation of anesthesia for i.c. injection.

Report on the international workshop on alternative methods for human and veterinary rabies vaccine testing: state of the science and planning the way forward.

Potency testing of most human and veterinary rabies vaccines requires vaccination of mice followed by a challenge test using an intracerebral injection of live rabies virus. NICEATM, ICCVAM, and their international partners organized a workshop to review the availability and validation status of alternative methods that might reduce, refine, or replace the use of animals for rabies vaccine potency testing, and to identify research and development efforts to further advance alternative methods. Workshop participants agreed that general anesthesia should be used for intracerebral virus injections and that humane endpoints should be used routinely as the basis for euthanizing animals when conducting the mouse rabies challenge test. Workshop participants recommended as a near-term priority replacement of the mouse challenge with a test validated to ensure potency, such as the mouse antibody serum neutralization test for adjuvanted veterinary rabies vaccines for which an international collaborative study was recently completed. The workshop recommended that an in vitro antigen quantification test should be a high priority for product-specific validation of human and non-adjuvanted veterinary rabies vaccines. Finally, workshop participants recommended greater international cooperation to expedite development, validation, regulatory acceptance, and implementation of alternative test methods for rabies vaccine potency testing.

Vaccination against Feline Panleukopenia: implications from a field study in kittens.

BACKGROUND: Feline Panleukopenia (FPL) is a serious disease of cats that can be prevented by vaccination. Kittens are routinely vaccinated repeatedly during their first months of life. By this time maternally derived antibodies (MDA) can interfere with vaccination and inhibit the development of active immunity. The efficacy of primary vaccination under field conditions was questioned by frequent reports to the Paul-Ehrlich-Institut on outbreaks of FPL in vaccinated breeding catteries. We therefore initiated a field study to investigate the development of immunity in kittens during primary vaccination against FPL.64 kittens from 16 litters were vaccinated against FPL at the age of 8, 12 and 16 weeks using three commercial polyvalent vaccines. Blood samples were taken before each vaccination and at the age of 20 weeks. Sera were tested for antibodies against Feline Panleukopenia Virus (FPV) by hemagglutination inhibition test and serum neutralisation assay in two independent diagnostic laboratories. RESULTS: There was a good correlation between the results obtained in different laboratories and with different methods. Despite triple vaccination 36.7% of the kittens did not seroconvert. Even very low titres of MDA apparently inhibited the development of active immunity. The majority of kittens displayed significant titres of MDA at 8 and 12 weeks of age; in some animals MDA were still detected at 20 weeks of age. Interestingly, the vaccines tested differed significantly in their ability to overcome low levels of maternal immunity. CONCLUSIONS: In the given situation it is recommended to quantify antibodies against FPV in the serum of the queen or kittens before primary vaccination of kittens. The beginning of primary vaccination should be delayed until MDA titres have declined. Unprotected kittens that have been identified serologically should be revaccinated.

Potency testing of veterinary vaccines: the way from in vivo to in vitro.

Current quality control of inactivated animal vaccines still focuses on the potency of final products in a batch-wise manner. Animal welfare concerns as well as scientific considerations have led to the '3Rs-concept' that comprises the refinement of animal procedures, the reduction of animal numbers, and the replacement of animal models. Although the 3Rs-concept has been widely accepted as a fundamental principle, the number of approved alternatives for in vivo tests is still limited. To promote further progress, the international scientific workshop 'Potency Testing of Veterinary Vaccines: The Way from in vivo to in vitro' was held at the Paul-Ehrlich-Institut in Langen, Germany, on 01-03 December 2010. More than 130 participants from industry, academia and regulatory authorities discussed the current state of the 3Rs-concept, examples of its successful implementation as well as still existing hurdles. Special emphasis was laid on the 'consistency approach' that aims to ensure relevant quality attributes of vaccine batches by in vitro analyses during production rather than by in vivo potency tests on the final product. This report provides an overview of the insights gained, including the recommendations produced at the end of the workshop.

Vesicular stomatitis virus glycoprotein displaying retrovirus-like particles induce a type I IFN receptor-dependent switch to neutralizing IgG antibodies.

Vesicular stomatitis virus (VSV) infection rapidly induces IFN-alphabeta that confers initial survival, whereas long-term protection is mediated by neutralizing IgG responses. Because coadministration of IFN-alphabeta can enhance Ab responses against soluble Ags, we addressed whether virus-induced IFN-alphabeta also had an impact on the induction of neutralizing Ab responses. To this end, we generated apathogenic retrovirus-like particles (VLP) displaying the VSV gp (VLP-VSV). Reminiscent of live VSV, VLP-VSV induced VSV-neutralizing IgM responses that switched to IgG in a T help-dependent manner. In type I IFN receptor-deficient (IFNAR(-/-)) mice, VLP-VSV injection elicited neutralizing IgM, whereas the IgG switch was absent. The lack of subclass switch was associated with a reduced germinal center reaction. Conditional knockout mice with a lymphocyte-specific IFNAR ablation showed normal Ab responses against VLP-VSV, as well as against live VSV. Thus, IFNAR triggering critically promoted the T help-dependent subclass switch of virus-neutralizing Ab responses against VLP-VSV. Interestingly, in the context of VLP-VSV as well as VSV immunization, IFNAR triggering of B lymphocytes did not play a critical role.

Type I interferons directly regulate lymphocyte recirculation and cause transient blood lymphopenia.

Early viral infection is often associated with lymphopenia, a transient reduction of blood lymphocyte counts long before the onset of clinical symptoms. We have investigated lymphopenia in mice infected with vesicular stomatitis virus (VSV) or treated with the Toll-like receptor (TLR) agonists poly(I:C) and R-848. In all cases analyzed, lymphopenia was critically dependent on type I interferon receptor (IFNAR) signaling. With the use of bone marrow-chimeric mice, radioresistant cells, such as stroma and endothelium, could be excluded as type I interferon (IFN-alpha/beta) targets for the induction of lymphopenia. Instead, adoptive transfer experiments and studies in conditionally gene-targeted mice with a B- or T-cell-specific IFNAR deletion demonstrated that IFN-alpha/beta exerted a direct effect on lymphocytes that was necessary and largely sufficient to induce lymphopenia. Furthermore, after treatment with R-848, we found that other cytokines such as TNF-alpha also played a role in T-cell lymphopenia. Investigation of the molecular mechanism revealed that lymphopenia was mainly independent of G protein-coupled receptors (GPCRs) and chemokines. In an adhesion assay, B cells of poly(I:C)-treated mice showed moderately increased adhesion to ICAM-1 but not to VCAM-1. In conclusion, our data identify a new effect of direct IFN-alpha/beta stimulation of lymphocytes that profoundly affects lymphocyte redistribution.

Direct stimulation of T cells by type I IFN enhances the CD8+ T cell response during cross-priming.

Type I IFN (IFN-alphabeta), which is produced rapidly in response to infection, plays a key role in innate immunity and also acts as a stimulus for the adaptive immune response. We have investigated how IFN-alphabeta induces cross-priming, comparing CD8+ T cell responses generated against soluble protein Ags in the presence or absence of IFN-alphabeta. Injection of IFN-alpha was found to prolong the proliferation and expansion of Ag-specific CD8+ T cells, which was associated with marked up-regulation of IL-2 and IL-15 receptors on Ag-specific cells and expression of IL-15 in the draining lymph node. Surprisingly, neither IL-2 nor IL-15 was required for IFN-alpha-induced cross-priming. Conversely, expression of the IFN-alphabetaR by T cells was shown to be necessary for effective stimulation of the response by IFN-alpha. The finding that T cells represent direct targets of IFN-alphabeta-mediated stimulation reveals an additional mechanism by which the innate response to infection promotes adaptive immunity.

Cutting edge: enhancement of antibody responses through direct stimulation of B and T cells by type I IFN.

Type I IFN (IFN-alphabeta) is induced rapidly by infection and plays a key role in innate antiviral defense. IFN-alphabeta also exerts stimulatory effects on the adaptive immune system and has been shown to enhance Ab and T cell responses. We have investigated the importance of B and T cells as direct targets of IFN-alphabeta during IFN-alpha-mediated augmentation of the Ab response against a soluble protein Ag. Strikingly, the ability of IFN-alpha to stimulate the Ab response and induce isotype switching was markedly reduced in mice in which B cells were selectively deficient for the IFN-alphabetaR. Moreover, IFN-alpha-mediated enhancement of the Ab response was also greatly impaired in mice in which T cells were selectively IFN-alphabetaR-deficient. These results indicate that IFN-alphabetaR signaling in both B and T cells plays an important role in the stimulation of Ab responses by IFN-alphabeta.