Antibodies Reactive to Commensal Streptococcus mitis Show Cross-Reactivity With Virulent Streptococcus pneumoniae Serotypes

Abstract

Current vaccines against Streptococcus pneumoniae, a bacterial species that afflicts people by causing a wide spectrum of diseases, do not protect against all pneumococcal serotypes. Thus, alternative vaccines to fight pneumococcal infections that target common proteins are under investigation. One promising strategy is to take advantage of immune cross-reactivity between commensal and pathogenic microbes for cross-protection. In this study, we examined the antibody-mediated cross-reactivity between S. pneumoniae and Streptococcus mitis, a commensal species closely related to S. pneumoniae. Western blot analysis showed that rabbit antisera raised against S. mitis reacted with multiple proteins of virulent S. pneumoniae strains (6B, TIGR4, and D39). Rabbit anti-S. pneumoniae IgG antibodies also showed binding to S. mitis antigens. Incubation of rabbit antisera raised against S. mitis with heterologous or homologous bacterial lysates resulted in marked inhibition of the developments of bands in the Western blots. Furthermore, plasma IgG antibodies from adult human volunteers intranasally inoculated with S. pneumoniae 6B revealed enhanced S. mitis-specific IgG titers compared with the pre-inoculation samples. Using an on-chip protein microarray representing a number of selected membrane and extracellular S. pneumoniae proteins, we identified choline-binding protein D (CbpD), cell division protein (FtsH), and manganese ABC transporter or manganese-binding adhesion lipoprotein (PsaA) as common targets of the rabbit IgG antibodies raised against S. mitis or S. pneumoniae. Cumulatively, these findings provide evidence on the antibody-mediated cross-reactivity of proteins from S. mitis and S. pneumoniae, which may have implications for development of effective and wide-range pneumococcal vaccines.

Mouse IgG2a antibodies specific for the commensalStreptococcus mitisshow stronger cross-reactivity withStreptococcus pneumoniaethan IgG1 antibodies

“…5 Emerging evidence reveals that rabbits immunized with S. mitis generate IgG antibodies that are reactive with both S. mitis and S. pneumoniae. 4 In line with this, we have recently shown that mouse IgG antibodies specific for S. mitis exhibited cross-reactivity toward S. pneumoniae strains D39 (serotype 2) and TIGR4 (serotype 4). 3 Murine IgG antibodies are heterogeneous consisting of four subclasses (IgG1, IgG2a, IgG2b and IgG3).…”

“…All strains were cultured, harvested, and stored as described previously. 4 To lyse the bacterial cells, a Precellys Lysing Kit and Homogenizer (Precellys 24, Bertin Instruments) was used as per the manufacturer's instructions. After homogenization, the bacterial cell lysate was centrifuged at 1,000 g for 5 min at 4°C and the supernatant was collected and stored at −80°C for further use.…”

“…To determine antibody levels in mouse sera samples, a whole cell ELISA was used as described previously. 4 In brief, each well of a 96-well plate (Maxisorb, Nunc, Thermo Scientific) was coated overnight with 100 µl of bacterial suspension (OD600 0.5), which was washed and then fixed with 10% formalin. The plate was washed and blocked with a blocking buffer (PBS + 0.05% Tween + 1% BSA) and incubation for 1 h at 37°C.…”

“…In recent years, Streptococcus mitis, a commensal that shares phylogenetic, antigenic, and ecological characteristics with S. pneumoniae, has appeared as an interesting vaccine candidate that holds the potential to confer optimal immunity to pneumococcal infections. 3,4 Although recent studies have indicated a crucial role for IgG antibodies in defense against pneumococcal infections, it is unclear as to how S. mitis stimulates the host's immune system to generate humoral responses. 5 Emerging evidence reveals that rabbits immunized with S. mitis generate IgG antibodies that are reactive with both S. mitis and S. pneumoniae.…”

Phosphatidylcholine biosynthesis in Mitis group streptococci via host metabolite scavenging

“…They also include the major human pathogen, S. pneumoniae . S. pneumoniae has >99% 16S rRNA sequence identity with S. mitis and S. oralis (8–10), and with which they exchange capsule biosynthesis and antibiotic resistance genes (11, 12) and show antibody cross-reactivity (13).…”