Opsonophagocytic assay (OPA)

The opsonophagocytic assay (OPA) is a functional immunological assay used to measure the ability of serum antibodies to mediate opsonization and phagocytic killing of bacteria in the presence of complement and effector immune cells. The assay evaluates the capacity of antigen‑specific antibodies to bind bacterial surface antigens, promote complement deposition, and facilitate uptake and intracellular killing by phagocytes, such as differentiated HL‑60 cells or primary polymorphonuclear leukocytes. Serial dilutions of heat‑inactivated serum samples are incubated with defined bacterial target strains, an exogenous source of active complement, and phagocytic effector cells, followed by enumeration of surviving bacteria by colony forming unit (CFU) counting. Opsonophagocytic antibody titers are expressed as the reciprocal of the highest serum dilution achieving a predefined level of bacterial killing, typically ≥50%.

The OPA is widely applied in vaccine development and immunogenicity assessment and is recognized in EMA and WHO‑endorsed frameworks as a functional correlate or supportive surrogate of protection for several bacterial pathogens, particularly those for which opsonization and phagocytosis represent key immune clearance mechanisms. Pathogens commonly evaluated in VisMederi using OPA include Streptococcus pneumoniae and Neisseria gonorrhoeae, with capability to implement the assay for other bacterial targets such as Group B Streptococcus, Staphylococcus aureus, Haemophilus influenzae, Neisseria meningitidis, Klebsiella pneumoniae, and other encapsulated or extracellular bacteria. When appropriately validated and qualified, the OPA is considered suitable for use in accordance with EMA expectations for functional antibody assays supporting the clinical development and evaluation of bacterial vaccines.