Terrestrial Paradise: Multiplex Immunoassay for Specific Pneumococcal Polysaccharide Antibodies

Streptococcus pneumoniae is an encapsulated Grampositive, facultative anaerobic bacterium. It is a cause for major mucosal as well as invasive diseases, including otitis media, pneumonia, bacteremia, meningitis [1]. The WHO estimates that annually half a million children under five years of age die due to pneumococcal infections [2]. In elderly, a similar high burden of invasive pneumococcal disease is found [3]. A variety of diagnostic tests exists to identify the causative pneumococcal serotype in case of a suspected infection [4]. An indirect, but specific, method to investigate the involvement of S. pneumoniae in a given infectious disease is analysis of the serological response, i.e. the increase in serotype specific antibodies during the course of disease [5]. Streptococcus pneumoniae comes in 93 different serotypes. This is a challenge for the immune system to defend against, as antibodies which have been produced during an infection with a given serotype will not protect against an infection with a different serotype. This also presents a challenge for vaccine development, to select the most prevalent serotypes to be included into a vaccine and a challenge to medical immunologists, to measure the antibodies against all these different serotypes. The latter aspect is the topic of this Commentary. Before multiplex immunoassays were available, the only way to determine serotype specific pneumococcal antibodies was by Enzyme-Linked Immunosorbent Assay (ELISA) [6,7]. Ideally, 93 different ELISAs would be required to detect specific antibodies to all serotypes. In practice, the best that could be achieved was to test for antibodies to eight of the most common serotypes within the IgG, IgA, IgG1 and IgG2 (sub)class. This meant that every blood sample had to be tested on 32 ELISA plates, which was extremely time-consuming and also required quite a lot of material. Furthermore, because of the restricted dynamic range of ELISA, each sample had to be tested by serial dilution, limiting the number of samples to 8 per ELISA plate.