Subcutaneous immunization with Streptococcus pneumoniae GAPDH confers effective protection in mice via TLR2 and TLR4

The surface localized Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) of Streptococcus pneumoniae is best known as housekeeping protein. Currently, GAPDH has been recognized as moonlighting protein and virulent factor. Therefore, we investigate whether GAPDH can act as a suitable vaccine candidate protein to prevent pneumococcal infection. In this study, mice received subcutaneous vaccination with recombinant GAPDH followed by challenge with D39 and 19F showing higher survival rate and lower bacterial loads in nasal washes and lung homogenates than control. Meanwhile, high titers of rGAPDH specific antibody and elevated titers of IgG subtype indicated that rGAPDH could elicit immune response in mice. Then, we investigated the mechanism that immunization with rGAPDH conferred protection against Streptococcus pneumoniae in host. In vitro experiments, rGAPDH induced phenotypic and functional maturation of BMDCs, because the high expression of CD40, CD86 and MHC II and the production of IL-12p70, IL-6 and TNF-α were observed after treatment with rGAPDH. However, the costimulatory molecules and cytokines declined significantly in TLR2^-/- and TLR4^-/- mice, indicating rGAPDH can be a potential ligand for both TLR2 and TLR4. Subsequent investigations suggested that rGAPDH could also activate the phosphorylation of MAPKs, PI3K-Akt and NF-κB. Meantime, upregulation of mir-146a and downregulation of mir-27a in BMDCs were observed. Taken together, our findings confirm that rGAPDH, a housekeeping protein, is also qualified as a vaccine candidate protein and rGAPDH activates BMDCs in a TLR2 and TLR4 dependent manner.