Induction of the pneumococcal vncRS operon by lactoferrin is essential for pneumonia

Abstract

Streptococcus pneumoniae (pneumococcus), the major pathogen for pneumonia, commonly colonizes the lung, but the mechanism underlying the coordination of virulence factors during invasion via the host protein remains poorly understood. Bacterial lysis releases the components of the cell wall, and triggers innate immunity and the subsequent secretion of pro-inflammatory cytokines. Previously, the virulence of the pep27 mutant was shown to be attenuated as a feasible candidate for vaccine development. However, the role of pep27 gene, belonging to the vancomycin-resistance locus (vncRS operon), in virulence, is largely unknown. This study demonstrates that transferrin in the host serum reduces the survival of the host during S. pneumoniae infections in mice. The exposure of the pneumococcal D39 strain to lactoferrin induced the vncRS operon, lysis, and subsequent in vivo cytokine production, resulting in lung inflammation. However, these responses were significantly attenuated in pneumococci harboring a mutation in pep27. Mechanistically, the VncS ligand, identified as lactoferrin, induced the vncRS operon and increased the in vivo mortality rates. Thus, serum-induced activation of vncRS plays an essential role in inducing pneumonia.

Pneumococcal VncR Strain-Specifically Regulates Capsule Polysaccharide Synthesis

“…In contrast, EMSA results indicated that in presence of serum, the DBD of the VncR protein can bind to the cpsp of all these three serotypes, suggesting that bacterial transcription is dependent upon a coordinate regulation of various factors (Balleza et al, 2009), and that the DNA-binding protein primarily binds somewhere along the DNA non-specifically, followed by diffusion in search of its specific target (Li and Xie, 2011). In agreement with our previous report (Lee et al, 2018), this study proposes that when pneumococci get exposed to host serum during invasion, VncS senses the serum component LF, which results in the phosphorylation of VncS. Phosphorylated VncS then provides a phosphate group to VncR, and phosphorylated VncR binds to the cpsp , which permits the pneumococci to acclimatize to the blood environment.…”

“…In our previous study, mRNA levels of D39 (type 2) vncR were maximally induced by 5 min treatment of 10% human serum (Lee et al, 2018), however, 5 min exposure to 10% serum failed to activate vncR mRNA levels in BG7322 (type 6B) and WU2 (type 3) pneumococci (data not shown). Nonetheless, subsequent prolonged treatment with 10% serum revealed maximal induction of the vncR mRNA in types 3 and 6B in 2 h and 40 min, respectively (Figure 1).…”

“…Previously, supplementation of 30 mM LF caused the induction of type 2 pneumococcal vncRS , proposing LF as a VncS sensor ligand (Lee et al, 2018). Thus, the effect of LF supplementation was examined in D39, WU2, and BG7322 strains.…”

“…The S. pneumoniae D39 vncR deletion mutant was constructed in our previous study (Lee et al, 2018). Similarly, in this study, in WU2 and BG7322 pneumococcal strains, a donor DNA fragment in which the deleted vncR genes were substituted with an erythromycin resistance cassette ( ermB ), was created by overlapping PCR and incorporated into pneumococcal chromosomes by homologous recombination (Kim et al, 2012).…”

“…Previously, we showed that the VncS ligand, serum lactoferrin (LF), induced the type 2 pneumococcal vncRS operon and augmented in vivo mortality rates mediated by pep27 , a gene belonging to vncRS operon (Lee et al, 2018). Further, the expression of the cps2A gene, representing the extent of pneumococcal cps transcription, was upregulated in the presence of serum (Ogunniyi et al, 2002).…”