Lipidation of Pneumococcal Antigens Leads to Improved Immunogenicity and Protection

Lipoprotein Signal Peptide as Adjuvants: Leveraging Lipobox-Driven TLR2 Activation in Modern Vaccine Design

Toll-like receptor 2 (TLR2) signaling is a pivotal component of immune system activation, and it is closely linked to the lipidation of bacterial proteins. This lipidation is guided by bacterial signal peptides (SPs), which ensure the precise targeting and membrane anchoring of these proteins. The lipidation process is essential for TLR2 recognition and the activation of robust immune responses, positioning lipidated bacterial proteins as potent immunomodulators and adjuvants for vaccines against bacterial-, viral-, and cancer-related antigens. The structural diversity and cleavage pathways of bacterial SPs are critical in determining lipidation efficiency and protein localization, influencing their immunogenic potential. Recent advances in bioinformatics have significantly improved the prediction of SP structures and cleavage sites, facilitating the rational design of recombinant lipoproteins optimized for immune activation. Moreover, the use of SP-containing lipobox motifs, as adjuvants to lipidate heterologous proteins, has expanded the potential of vaccines targeting a broad range of pathogens. However, challenges persist in expressing lipidated proteins, particularly within heterologous systems. These challenges can be addressed by optimizing expression systems, such as engineering E. coli strains for enhanced lipidation. Thus, lipoprotein signal peptides (SPs) demonstrate remarkable versatility as adjuvants in vaccine development, diagnostics, and immune therapeutics, highlighting their essential role in advancing immune-based strategies to combat diverse pathogens.

Lipoteichoic Acids Are Essential for Pneumococcal Colonization and Membrane Integrity

INTRODUCTION

The hydrophilic, polymeric chain of the lipoteichoic acid (LTA) of the Gram-positive pathobiont Streptococcus pneumoniae is covalently linked to the glycosylglycerolipid α-d-glucopyranosyl-(1,3)-diacylglycerol by the LTA ligase TacL, leading to its fixation in the cytoplasmic membrane. Pneumococcal LTA, sharing identical repeating units with the wall teichoic acids (WTA), is dispensable for normal growth but required for full virulence in invasive infections.

METHODS

Mutants deficient in TacL and complemented strains constructed were tested for their growth, resistance against oxidative stress, and susceptibility against antimicrobial peptides. Further, the membrane fluidity of pneumococci, their capability to adhere to lung epithelial cells, and virulence in a Galleria mellonella as well as intranasal mouse infection model were assessed.

RESULTS

In the present study, we indicate that LTA is already indispensable for pneumococcal adherence to human nasopharyngeal cells and colonization in an intranasal mouse infection model. Mutants deficient for TacL did not show morphological defects. However, our analysis of pneumococcal membranes in different serotypes showed an altered membrane fluidity and surface protein abundance of lipoproteins in mutants deficient for LTA but not WTA. These mutants had a decreased membrane fluidity, exhibited higher amounts of lipoproteins, and showed an increased susceptibility to antimicrobial peptides. In complemented mutant strains, this defect was fully restored.

CONCLUSION

Taken together, LTA is crucial for colonization and required to effectively protect pneumococci from innate immune defence mechanisms by maintaining the membrane integrity.

Lipidation of pneumococcal proteins enables activation of human antigen-presenting cells and initiation of an adaptive immune response

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.

Expression conditions and characterization of a novelly constructed lipoprotein intended as a vaccine to prevent human Haemophilus influenzae infections

Bacterial lipoproteins are structurally divided into two groups, based on their lipid moieties: diacylated (present in Gram-positive bacteria) and triacylated (present in some Gram-positive and most Gram-negative bacteria). Diacylated and triacylated lipid moieties differ by a single amide-linked fatty acid chain. Lipoproteins induce host innate immune responses by the mammalian Toll-like receptor 2 (TLR2). In this study, we added a lipid moiety to recombinant OMP26, a native nonlipidated (NL) membrane protein of Haemophilus influenzae, and characterized it extensively under different expression conditions using flow cytometry, LC/MS, and MALDI-TOF. We also investigated the ability of NL and lipidated (L) OMP26 to induce in vitro stimulation of HEK Blue-hTLR2-TR1 and hTLR-TLR6 cells. Our L-OMP26 was predominantly expressed in diacylated form, so we employed an additional gene copy of apolipoprotein N-acetyltransferase enzyme (Lnt)-rich Escherichia coli strain that further acylates the diacyl lipoproteins to enhance the production of triacylated L-OMP26. The diacyl and triacyl versions of L-OMP26, intended as a vaccine for use in humans, were characterized and evaluated as protein vaccine components in a mouse model. We found that the diacyl and triacyl L-OMP26 protein formulations differed markedly in their immune-stimulatory activity, with diacylated L-OMP26 stimulating higher adaptive immune responses compared with triacylated L-OMP26 and both stimulating higher adaptive immune response compared to NL-OMP26. We also constructed and characterized an L-OMP26φNL-P6 fusion protein, where NL-P6 protein (a commonly studied H. influenzae vaccine candidate) was recombinantly fused to L-OMP26. We observed a similar pattern of lipidation (predominantly diacylated) in the L-OMP26φNL-P6 fusion protein.

Lipidation of Haemophilus influenzae Antigens P6 and OMP26 Improves Immunogenicity and Protection against Nasopharyngeal Colonization and Ear Infection

Nontypeable Haemophilus influenzae (NTHi) causes respiratory infections that lead to high morbidity and mortality worldwide, encouraging development of effective vaccines. To achieve a protective impact on nasopharyngeal (NP) colonization by NTHi, enhanced immunogenicity beyond that achievable with recombinant-protein antigens is likely to be necessary. Adding a lipid moiety to a recombinant protein would enhance immunogenicity through Toll-like receptor 2 signaling of antigen-presenting cells and Th17 cell response in the nasal-associated lymphoid tissue (NALT). We investigated effects of lipidation (L) of recombinant proteins P6 and OMP26 compared to nonlipidated (NL) P6 and OMP26 and as fusion constructs (L-OMP26ϕNL-P6 and L-P6ϕNL-OMP26) in a mouse model. After intraperitoneal or intranasal vaccination, antibody responses were compared and protection from NP colonization and middle ear infection were assessed. L-P6 and L-OMP26 induced approximately 10- to 100-fold-higher IgG antibody levels than NL-P6 and NL-OMP26. Fusion constructs significantly increased IgG antibody to both target proteins, even though only one of the proteins was lipidated. NP colonization and middle ear bullae NTHi density was 1 to 4 logs lower following vaccination with L-P6 and L-OMP26 than with NL-P6 and NL-OMP26. Fusion constructs also resulted in a 1- to 3-log-lower NTHi density following vaccination. NALT cells from mice vaccinated with lipidated protein constructs had higher levels of interleukin-17 (IL-17), IL-22, and CD4⁺ T-cell memory. Passive transfer of sera from L-OMP26ϕNL-P6-vaccinated mice to recipient infant mice reduced NP colonization and ear bulla NTHi density. We conclude that L-P6, L-OMP26, and fusion constructs generate enhanced antibody responses and protection from NP colonization and middle ear infection by NTHi in mice.

Platelets, Bacterial Adhesins and the Pneumococcus

Systemic infections with pathogenic or facultative pathogenic bacteria are associated with activation and aggregation of platelets leading to thrombocytopenia and activation of the clotting system. Bacterial proteins leading to platelet activation and aggregation have been identified, and while platelet receptors are recognized, induced signal transduction cascades are still often unknown. In addition to proteinaceous adhesins, pathogenic bacteria such as Staphylococcus aureus and Streptococcus pneumoniae also produce toxins such as pneumolysin and alpha-hemolysin. They bind to cellular receptors or form pores, which can result in disturbance of physiological functions of platelets. Here, we discuss the bacteria-platelet interplay in the context of adhesin–receptor interactions and platelet-activating bacterial proteins, with a main emphasis on S. aureus and S. pneumoniae. More importantly, we summarize recent findings of how S. aureus toxins and the pore-forming toxin pneumolysin of S. pneumoniae interfere with platelet function. Finally, the relevance of platelet dysfunction due to killing by toxins and potential treatment interventions protecting platelets against cell death are summarized.

Recombinant Antigen of Type 2 Porcine Reproductive and Respiratory Syndrome Virus (PRRSV-2) Promotes M1 Repolarization of Porcine Alveolar Macrophages and Th1 Type Response

The polarization status of porcine alveolar macrophages (PAMs) determines the infectivity of porcine reproductive and respiratory syndrome virus (PRRSV). PRRSV infection skews macrophage polarization toward an M2 phenotype, followed by T-cells inactivation. CD163, one of the scavenger receptors of M2 macrophages, has been described as a putative receptor for PRRSV. In this study, we examined two types of PRRSV-2-derived recombinant antigens, A1 (g6Ld10T) and A2 (lipo-M5Nt), for their ability to mediate PAM polarization and T helper (Th1) response. A1 and A2 were composed of different combination of ORF5, ORF6, and ORF7 in full or partial length. To enhance the adaptive immunity, they were conjugated with T cells epitopes or lipidated elements, respectively. Our results showed that CD163⁺ expression on PAMs significantly decreased after being challenged with A1 but not A2, followed by a significant increase in pro-inflammatory genes (TNF-α, IL-6, and IL-12). In addition, next generation sequencing (NGS) data show an increase in T-cell receptor signaling in PAMs challenged with A1. Using a co-culture system, PAMs challenged with A1 can induce Th1 activation by boosting IFN-γ and IL-12 secretion and TNF-α expression. In terms of innate and T-cell-mediated immunity, we conclude that A1 is regarded as a potential vaccine for immunization against PRRSV infection due to its ability to reverse the polarization status of PAMs toward pro-inflammatory phenotypes, which in turn reduces CD163 expression for viral entry and increases immunomodulation for Th1-type response.

Cognizance of posttranslational modifications in vaccines: A way to enhanced immunogenicity

Vaccination is a significant advancement or preventative strategy for controlling the spread of various severe infectious and noninfectious diseases. The purpose of vaccination is to stimulate or activate the immune system by injecting antigens, i.e., either whole microorganisms or using the pathogen's antigenic part or macromolecules. Over time, researchers have made tremendous efforts to reduce vaccine side effects or failure by developing different strategies combining with immunoinformatic and molecular biology. These newly designed vaccines are composed of single or several antigenic molecules derived from a pathogenic organism. Although, whole‐cell vaccines are still in use against various diseases but due to their ineffectiveness, other vaccines like DNA‐based, RNA‐based, and protein‐based vaccines, with the addition of immunostimulatory agents, are in the limelight. Despite this, many researchers escape the most common fundamental phenomenon of protein posttranslational modifications during the development of vaccines, which regulates protein functional behavior, evokes immunogenicity and stability, etc. The negligence about post translational modification (PTM) during vaccine development may affect the vaccine's efficacy and immune responses. Therefore, it becomes imperative to consider these modifications of macromolecules before finalizing the antigenic vaccine construct. Here, we have discussed different types of posttranslational/transcriptional modifications that are usually considered during vaccine construct designing: Glycosylation, Acetylation, Sulfation, Methylation, Amidation, SUMOylation, Ubiquitylation, Lipidation, Formylation, and Phosphorylation. Based on the available research information, we firmly believe that considering these modifications will generate a potential and highly immunogenic antigenic molecule against communicable and noncommunicable diseases compared to the unmodified macromolecules.

Understanding host immune responses to pneumococcal proteins in the upper respiratory tract to develop serotype-independent pneumococcal vaccines

Introduction: Nasopharyngeal colonization is a precondition for mucosal and invasive pneumococcal disease. Prevention of colonization may reduce pneumococcal transmission and disease incidence. Therefore, several protein-based pneumococcal vaccines are currently under investigation. Areas covered: We aimed to better understand the host immune responses to pneumococcal proteins in the upper respiratory tract (URT) that could facilitate the development of serotype-independent pneumococcal vaccines. English peer-reviewed papers reporting immunological mechanisms involved in host immune response to pneumococcal proteins in the URT were retrieved through a PubMed search using the terms 'pneumococcal proteins,' 'nasopharyngeal colonization' and/or 'cellular/humoral host immune response.' Expert opinion: Although pneumococcal protein antigens induce humoral immune responses, as well as IL-17A-mediated immunity, none of them, when used as single antigen, is sufficient to control and broadly protect against pneumococcal colonization. Novel vaccines should contain multiple conserved protein antigens to activate both arms of the immune system and evoke protection against the whole spectrum of pneumococcal variants by reducing, rather than eradicating, pneumococcal carriage. The highest efficacy would likely be achieved when the vaccine is intranasally applied, inducing mucosal immunity and enhancing the first line of defense by restricting pneumococcal density in the URT, which in turn will lead to reduced transmission and protection against disease.