Transcriptional regulation, occurrence and putative role of the Pht family of Streptococcus pneumoniae

Abstract

Restricted to the genus Streptococcus, the Pht protein family comprises four members: PhtA, PhtB, PhtD and PhtE. This family has the potential to provide a protein candidate for incorporation in pneumococcal vaccines. Based on sequence analysis and on RT-PCR experiments, we show here that the pht genes are organized in tandem but that their expression, except that of phtD, is monocistronic. PhtD, PhtE, PhtB and PhtA are present in 100, 97, 81 and 62 % of the strains, respectively, and, by analysing its sequence conservation across 107 pneumococcal strains, we showed that PhtD displays very little variability. To analyse the physiological function of these proteins, several mutants were constructed. The quadruple Pht-deficient mutant was not able to grow in a poor culture medium, but the addition of Zn 2+ or Mn 2+ restored its growth capacity.Moreover, the phtD mRNA expression level increased when the culture medium was depleted in zinc. Therefore, we suggest that these proteins are zinc and manganese scavengers, and are able to store these metals and to release them when the bacterium faces an ion-restricted environment. The data also showed that this protein family, and more particularly PhtD, is a promising candidate to be incorporated into pneumococcal vaccines.

Role of Pht Proteins in Attachment of Streptococcus pneumoniae to Respiratory Epithelial Cells

“…They share extensive sequence similarity, with PhtB and PhtD having the highest sequence homology (87%) (8,9). Pht proteins have been shown to be strongly immunogenic and to induce protective humoral immunity in the host, therefore establishing Pht proteins as strong vaccine candidates.…”

“…Binding sites for transcriptional repressor AdcR have been found upstream of pht genes (22,23). AdcR binding has been shown to be induced under conditions of high Zn 2ϩ , which results in inhibition of transcription of pht genes under its dependence (9,23). The concentrations of ions essential to pathogen survival are carefully controlled at mucosal surfaces; the zinc concentration in bronchoalveolar lavage fluid is 5-to 10-fold lower than that in the human plasma (24,25).…”

“…Immunization of mice with Pht proteins has been shown to induce protection against nasopharyngeal colonization (10,11), sepsis, and pneumonia (8,10,12,13). Of all the Pht proteins, immunization with PhtD is likely to elicit the broadest protection against pneumococcal infections as PhtD is present among all pneumococcal strains (8,9). Immunization of rhesus macaques with a protein-based vaccine containing PhtD was shown to protect the animals from pneumococcal pneumonia (14).…”

“…The biological function of Pht proteins in pneumococcal viru-lence is still poorly understood, but roles in metal scavenging (9), complement inhibition (27,28), and in adherence to respiratory epithelium (29) have been suggested. Our aim in this study was to further assess the possible role of Pht proteins in pneumococcal colonization by comparing the ability of wild-type and mutant pneumococcal strains lacking one or more of the Pht proteins to bind to respiratory epithelial cell lines and also to assess the possible inhibitory function of Pht antibodies in the adherence.…”

Protection against Streptococcus pneumoniae Invasive Pathogenesis by a Protein-Based Vaccine Is Achieved by Suppression of Nasopharyngeal Bacterial Density during Influenza A Virus Coinfection

“…We then sought to determine if prior vaccination could effectively prevent the necessary increase in bacterial density permissive to invasive infection. As a vaccine, we used S. pneumoniae histidine triad protein D (PhtD), a highly conserved, surface-exposed adhesin protein that facilitates attachment to the NP and lung epithelium cells (10)(11)(12). PhtD as a vaccine component has been shown to be protective in a number of mouse S. pneumoniae infection models (11,13,14) and is included in vaccines currently in human trials (15)(16)(17).…”