Open Access

Streptococcus pneumoniae has evolved versatile strategies to colonize the nasopharynx of humans. Colonization is facilitated by direct interactions with host cell receptors or via binding to components of the extracellular matrix. In addition, pneumococci hijack host-derived extracellular proteases such as the serine protease plasmin(ogen) for ECM and mucus degradation as well as colonization. S. pneumoniae expresses strain-dependent up to four serine proteases. In this study, we assessed the role of secreted or cell-bound serine proteases HtrA, PrtA, SFP, and CbpG, in adherence assays and in a mouse colonization model. We hypothesized that the redundancy of serine proteases compensates for the deficiency of a single enzyme. Therefore, double and triple mutants were generated in serotype 19F strain EF3030 and serotype 4 strain TIGR4. Strain EF3030 produces only three serine proteases and lacks the SFP encoding gene. In adherence studies using Detroit-562 epithelial cells, we demonstrated that both TIGR4Δcps and 19F mutants without serine proteases or expressing only CbpG, HtrA, or PrtA have a reduced ability to adhere to Detroit-562 cells. Consistent with these results, we show that the mutants of strain 19F, which preferentially colonizes mice, abrogate nasopharyngeal colonization in CD-1 mice after intranasal infection. The bacterial load in the nasopharynx was monitored for 14 days. Importantly, mutants showed significantly lower bacterial numbers in the nasopharynx two days after infection. Similarly, we detected a significantly reduced pneumococcal colonization on days 3, 7, and 14 post-inoculations. To assess the impact of pneumococcal serine proteases on acute infection, we infected mice intranasally with bioluminescent and invasive TIGR4 or isogenic triple mutants expressing only CbpG, HtrA, PrtA, or SFP. We imaged the acute lung infection in real-time and determined the survival of the mice. The TIGR4lux mutant expressing only PrtA showed a significant attenuation and was less virulent in the acute pneumonia model. In conclusion, our results showed that pneumococcal serine proteases contributed significantly to pneumococcal colonization but played only a minor role in pneumonia and invasive diseases. Because colonization is a prerequisite for invasive diseases and transmission, these enzymes could be promising candidates for the development of antimicrobials to reduce pneumococcal transmission.

Steptococcus pneumoniae (pneumococcus) represents a common colonizer of the human upper respiratory tract (URT). However, under certain conditions, for example following viral infections, or in indiciduals with a weakened immune system, including young children, elderly and immunocompromised persons, it can cause a wide range of life-threatening diseases, such as pneumonia, meningitis or sepsis. Based on the polysaccharide capsule that surrounds the bacterium, pneumococci are classified into so far 98 different serotypes. Prevention of S. pneumoniae infections was achieved by the development of pneumococcal polysaccharide-based (PPSV) vaccines. However, these vaccines have important limitations, including high manufacturing costs and restricted serotype coverage facilitating replacement by non-vaccine serotypes. Aiming for the development of a serotype-independent vaccine, the potential of surface-exposed and highly conserved pneumococcal lipoproteins was evaluated for being targeted as a future protein-based vaccine. Therefore, selected lipoproteins were examined i) for their surface abundance and accessibility, ii) for their presence in clinically relevant S. pneumoniae strains, and iii) for their immunogenicity. Finally, based on these initial screenings, the most promising candidates were selected to analyze their protective efficacy in a moude model of colonization. DacB and PnrA were identified as highly abundant lipoproteins on the pneumococcal surface. They showed to be immunogenic both during natural infection using convalescent patient sera and when given to mice as a subunit vaccine formulation. Following intranasal immunization and challenge of mice with two heterologous S. pneumoniae strains, both proteins reduced the pneumococcal load in the nasopharynx. The protection correlated with increased production of IL-17A indicative for a Th17-mediated immunity, which is strongly suggested to play a critical role in preventing pneumococcal colonization and infection. Lipoproteins are triggering innate receptors on antigen-presenting cells, thereby linking innate with adaptive immune responses. Therefore, lipidated proteins were evaluated for their potential to be used as an adjuvant for vaccination. Lipidation clearly enhanced humoral immune responses to DacB and PnrA without the need of an additional adjuvant. However, an additional adjuvant was required to confer protection against pneumococcal colonization. In conclusion, Lipoproteins are interesting candidates for future protein-based vaccine strategies because they are highly conserved, abundant and immunogenic. PnrA and DacB were identified as potential candidates, since they induced protection against pneumococcal colonization, which in turn may lead to a decline in infections and transmission.

Streptococcus pneumoniae remains a significant global threat, with existing vaccines having important limitations such as restricted serotype coverage and high manufacturing costs. Pneumococcal lipoproteins are emerging as promising vaccine candidates due to their surface exposure and conservation across various serotypes. While prior studies have explored their potential in mice, data in a human context and insights into the impact of the lipid moiety remain limited. In the present study, we examined the immunogenicity of two pneumococcal lipoproteins, DacB and MetQ, both in lipidated and non-lipidated versions, by stimulation of primary human immune cells. Immune responses were assessed by the expression of common surface markers for activation and maturation as well as cytokines released into the supernatant. Our findings indicate that in the case of MetQ lipidation was crucial for activation of human antigen-presenting cells such as dendritic cells and macrophages, while non-lipidated DacB demonstrated an intrinsic potential to induce an innate immune response. Nevertheless, immune responses to both proteins were enhanced by lipidation. Interestingly, following stimulation of dendritic cells with DacB, LipDacB and LipMetQ, cytokine levels of IL-6 and IL-23 were significantly increased, which are implicated in triggering potentially important Th17 cell responses. Furthermore, LipDacB and LipMetQ were able to induce proliferation of CD4+ T cells indicating their potential to induce an adaptive immune response. These findings contribute valuable insights into the immunogenic properties of pneumococcal lipoproteins, emphasizing their potential role in vaccine development against pneumococcal infections.