Assay Development: Vaccines
Vaccine development is a long and intensive process that begins with determining public health needs and priorities. Several organizations have compiled priorities for research and development, including the World Health Organization/United Nations Development Program for vaccine development. There are several stages of vaccine development and testing, starting with an exploratory stage to identify natural or synthetic antigens. Then, preclinical work begins and cell cultures and animal models are used to determine the immunogenicity and safety of a candidate vaccine.
Following approval of an Investigational New Drug Application with the United States Food and Drug Administration, clinical trials can begin. Specialized tests are developed in preclinical and early phase clinical trials, and validated in later clinical trial phases. Prior to batch release, the assays have well-characterized parameters and criteria, although the assay development process is not without challenges. The immune system is very complex; however, new technologies and techniques offer solutions. Eventually, a successful vaccine candidate will advance to phase III trials and be licensed. The entire rigorous process can take more than 15 years.
First, it is important to understand the underlying mechanism of action. “Vaccines generally aim to modulate or prime the immune response to a pathogen so that the body can quickly eliminate the pathogen and minimize disease,” says Claire Richards, PhD, senior scientist, infectious diseases, discovery services at Charles River Laboratories (Wilmington, MA).
Preclinical research on vaccines includes the development of assays, in part, for the quantification of immune response. Rafiqul Islam, MS, Executive Director of Bioanalytical Services at Celerion Inc., echoes the importance of immunogenicity testing: "Multiple tests are needed to measure immune responses to ensure that the vaccine candidate is at least capable of causing a robust immune response”.
Islam provides insight into the clinical development of vaccines, which “requires a specific set of specialized tests to demonstrate immunogenicity, efficacy, exposure and safety. The development and validation of these tests require the integration of therapeutic information, state-of-the-art technologies, customized reagents and robust quality management systems.
What tests are then used to assess the immune response? “A proper test to determine immune stimulation is usually needed with possible inhibition tests to show that the immune response can reduce the growth of the pathogen; [eg] antigen-specific ELISA, inhibition ELISA, neutralization test, [and] effective concentration to give 50% survival (EC50),” says Dr. Richards. Table 1 provides a summary of commonly used assays to support vaccine development.
The humoral, or antibody-mediated, immune response relies on the recognition of antigens by B lymphocytes and the subsequent release of antibodies. Ligand-binding techniques, such as ELISA (enzyme immunoassay), are often used to assess humoral response. Conversely, the cell-mediated immune response relies primarily on T cells and is often assessed by ELISpot (enzyme-linked immunospot) or flow cytometry.