Strategic Approaches for Developing Alternative Tests for Safety and Potency of Vaccines

Abstract

Safety control tests for vaccines need to be capable of quantifying in vivo toxicity of vaccines. Endotoxin contamination is a threat for vaccine safety and has traditionally been controlled using either an in vivo rabbit pyrogen test or the in vitro Limulus amebocyte lysate (LAL) assay. An in vitro pyrogen test was developed using prostaglandin E2 induction in rabbit peripheral blood. Responsiveness of LAL and rabbits to various endotoxins, however, differ from that of humans. A monocyte activation test was developed using human monocytes but is not applicable where the use of donated blood is limited to therapeutic purposes. Therefore, a clinically relevant in vitro pyrogen test was developed using a human monocytic cell line responsive to various pyrogens in a manner consistent with that of human peripheral blood. Residual pertussis toxin (PT) activity of acellular pertussis vaccines (aPs) is quantified by the mouse histamine sensitization (HS) test. The Chinese Hamster Ovary Cell clustering test can sensitively detect PT activity but is not adequate for testing aPs due to hampered interaction of aggregated PT molecules with cell surface receptors. An in vitro test system has been developed to estimate HS activity using the combined assay results of carbohydrate binding ELISA (B-subunit) and enzymatic-HPLC (A-subunit). Potency tests need to evaluate protective immunogenicity of vaccines. Protective potency of whole cell pertussis vaccine can be measured by the method whose results were shown to correlate with clinical protection. Identifying and quantifying the structures of antigen molecules relevant to the induction of protective immunity may be the ultimate goal.