Diversity of secretion systems associated with virulence characteristics of the classical bordetellae

The Love and Hate Relationship between T5SS and Other Secretion Systems in Bacteria

Bacteria have existed on Earth for billions of years, exhibiting ubiquity and involvement in various biological activities. To ensure survival, bacteria usually release and secrete effector proteins to acquire nutrients and compete with other microorganisms for living space during long-term evolution. Consequently, bacteria have developed a range of secretion systems, which are complex macromolecular transport machines responsible for transporting proteins across the bacterial cell membranes. Among them, one particular secretion system that stands out from the rest is the type V secretion system (T5SS), known as the "autotransporter". Bacterial activities mediated by T5SS include adherence to host cells or the extracellular matrix, invasion of host cells, immune evasion and serum resistance, contact-dependent growth inhibition, cytotoxicity, intracellular flow, protease activity, autoaggregation, and biofilm formation. In a bacterial body, it is not enough to rely on T5SS alone; in most cases, T5SS cooperates with other secretion systems to carry out bacterial life activities, but regardless of how good the relationship is, there is friction between the secretion systems. T5SS and T1SS/T2SS/T3SS/T6SS all play a synergistic role in the pathogenic processes of bacteria, such as nutrient acquisition, pathogenicity enhancement, and immune modulation, but T5SS indirectly inhibits the function of T4SS. This could be considered a love-hate relationship between secretion systems. This paper uses the systematic literature review methodology to review 117 journal articles published within the period from 1995 to 2024, which are all available from the PubMed, Web of Science, and Scopus databases and aim to elucidate the link between T5SS and other secretion systems, providing clues for future prevention and control of bacterial diseases.

Genomic evidence and virulence properties decipher the extra-host origin of Bordetella bronchiseptica

Until recently, members of the classical Bordetella species comprised only pathogenic bacteria that were thought to live exclusively in warm-blooded animals. The close phylogenetic relationship of Bordetella with Achromobacter and Alcaligenes, which include primarily environmental bacteria, suggests that the ancestral Bordetellae were probably free-living. Eventually, the Bordetella species evolved to infect and live within warm-blooded animals. The modern history of pathogens related to the genus Bordetella started towards the end of the 19th century when it was discovered in the infected respiratory epithelium of mammals, including humans. The first identified member was Bordetella pertussis, which causes whooping cough, a fatal disease in young children. In due course, B. bronchiseptica was recovered from the trachea and bronchi of dogs with distemper. Later, a second closely related human pathogen, B. parapertussis, was described as causing milder whooping cough. The classical Bordetellae are strictly host-associated pathogens transmitted via the host-to-host aerosol route. Recently, the B. bronchiseptica strain HT200 has been reported from a thermal spring exhibiting unique genomic features that were not previously observed in clinical strains. Therefore, it advocates that members of classical Bordetella species have evolved from environmental sources. This organism can be transmitted via environmental reservoirs as it can survive nutrient-limiting conditions and possesses a motile flagellum. This study aims to review the molecular basis of origin and virulence properties of obligate host-restricted and environmental strains of classical Bordetella.

T cell reactivity to Bordetella pertussis is highly diverse regardless of childhood vaccination

The incidence of whooping cough due to Bordetella pertussis (BP) infections has increased recently. It is believed that the shift from whole-cell pertussis (wP) vaccines to acellular pertussis (aP) vaccines may be contributing to this rise. While T cells are key in controlling and preventing disease, nearly all knowledge relates to antigens in aP vaccines. A whole-genome mapping of human BP-specific CD4+ T cell responses was performed in healthy vaccinated adults and revealed unexpected broad reactivity to hundreds of antigens. The overall pattern and magnitude of T cell responses to aP and non-aP vaccine antigens are similar regardless of childhood vaccination, suggesting that asymptomatic infections drive the pattern of T cell reactivity in adults. Lastly, lack of Th1/Th2 polarization to non-aP vaccine antigens suggests these antigens have the potential to counteract aP vaccination Th2 bias. These findings enhance our insights into human T cell responses to BP and identify potential targets for next-generation pertussis vaccines.

Genome-wide characterization of T cell responses to Bordetella pertussis reveals broad reactivity and similar polarization irrespective of childhood vaccination profiles

The incidence of whooping cough (pertussis), the respiratory disease caused by Bordetella pertussis (BP) has increased in recent years, and it is suspected that the switch from whole-cell pertussis (wP) to acellular pertussis (aP) vaccines may be a contributing factor to the rise in morbidity. While a growing body of evidence indicates that T cells play a role in the control and prevention of symptomatic disease, nearly all data on human BP-specific T cells is related to the four antigens contained in the aP vaccines, and data detailing T cell responses to additional non-aP antigens, are lacking. Here, we derived a full-genome map of human BP-specific CD4+ T cell responses using a high-throughput ex vivo Activation Induced Marker (AIM) assay, to screen a peptide library spanning over 3000 different BP ORFs. First, our data show that BP specific-CD4+ T cells are associated with a large and previously unrecognized breadth of responses, including hundreds of targets. Notably, fifteen distinct non-aP vaccine antigens were associated with reactivity comparable to that of the aP vaccine antigens. Second, the overall pattern and magnitude of CD4+ T cell reactivity to aP and non-aP vaccine antigens was similar regardless of aP vs wP childhood vaccination history, suggesting that the profile of T cell reactivity in adults is not driven by vaccination, but rather is likely driven by subsequent asymptomatic or sub-clinical infections. Finally, while aP vaccine responses were Th1/Th2 polarized as a function of childhood vaccination, CD4+ T cell responses to non-aP BP antigens vaccine responses were not, suggesting that these antigens could be used to avoid the Th2 bias associated with aP vaccination. Overall, these findings enhance our understanding of human T cell responses against BP and suggest potential targets for designing next-generation pertussis vaccines.

Pertactin-Deficient Bordetella pertussis, Vaccine-Driven Evolution, and Reemergence of Pertussis

Recent reemergence of pertussis (whooping cough) in highly vaccinated populations and rapid expansion of Bordetella pertussis strains lacking pertactin (PRN), a common acellular vaccine antigen, have raised the specter of vaccine-driven evolution and potential return of what was once the major killer of children. The discovery that most circulating B. pertussis strains in the United States have acquired new and independent disruptive mutations in PRN is compelling evidence of strong selective pressure. However, the other 4 antigens included in acellular vaccines do not appear to be selected against so rapidly. We consider 3 aspects of PRN that distinguish it from other vaccine antigens, which might, individually or collectively, explain why only this antigen is being precipitously eliminated. An understanding of the increase in PRN-deficient strains should provide useful information for the current search for new protective antigens and provide broader lessons for the design of improved subunit vaccines.

Cytotoxicity of the effector protein BteA was attenuated in Bordetella pertussis by insertion of an alanine residue

Bordetella bronchiseptica and Bordetella pertussis are closely related respiratory pathogens that evolved from a common bacterial ancestor. While B. bronchiseptica has an environmental reservoir and mostly establishes chronic infections in a broad range of mammals, B. pertussis is a human-specific pathogen causing acute pulmonary pertussis in infants and whooping cough illness in older humans. Both species employ a type III secretion system (T3SS) to inject a cytotoxic BteA effector protein into host cells. However, compared to the high BteA-mediated cytotoxicity of B. bronchiseptica, the cytotoxicity induced by B. pertussis BteA (Bp BteA) appears to be quite low and this has been attributed to the reduced T3SS gene expression in B. pertussis. We show that the presence of an alanine residue inserted at position 503 (A503) of Bp BteA accounts for its strongly attenuated cytotoxic potency. The deletion of A503 from Bp BteA greatly enhanced the cytotoxic activity of B. pertussis B1917 on mammalian HeLa cells and expression of Bp BteAΔA503 was highly toxic to Saccharomyces cerevisiae cells. Vice versa, insertion of A503 into B. bronchiseptica BteA (Bb BteA) strongly decreased its cytotoxicity to yeast and HeLa cells. Moreover, the production of Bp BteAΔA503 increased virulence of B. pertussis B1917 in the mouse model of intranasal infection (reduced LD50) but yielded less inflammatory pathology in infected mouse lungs at sublethal infectious doses. This suggests that A503 insertion in the T3SS effector Bp BteA may represent an evolutionary adaptation that fine-tunes B. pertussis virulence and host immune response.

Pertussis remains the least-controlled vaccine-preventable infectious disease and the mechanisms by which Bordetella pertussis subverts defense mechanisms of human airway mucosa remain poorly understood. We found that B. pertussis had the cytotoxic activity of its type III secretion system-delivered effector BteA strongly attenuated by insertion of an alanine residue at position 503 as compared to the BteA homologue of the animal pathogen B. bronchiseptica. This functional adaptation reduced the capacity of B. pertussis to suppress host inflammatory response and may contribute to an acute course of the pulmonary form of human infant pertussis.

Using Cryo-EM to Investigate Bacterial Secretion Systems

Bacterial secretion systems are responsible for releasing macromolecules to the extracellular milieu or directly into other cells. These membrane complexes are associated with pathogenicity and bacterial fitness. Understanding of these large assemblies has exponentially increased in the last few years thanks to electron microscopy. In fact, a revolution in this field has led to breakthroughs in characterizing the structures of secretion systems and other macromolecular machineries so as to obtain high-resolution images of complexes that could not be crystallized. In this review, we give a brief overview of structural advancements in the understanding of secretion systems, focusing in particular on cryo-electron microscopy, whether tomography or single-particle analysis. We describe how such techniques have contributed to knowledge of the mechanism of macromolecule secretion in bacteria and the impact they will have in the future.

Bordetella pertussis population dynamics and phylogeny in Japan after adoption of acellular pertussis vaccines

Bordetella pertussis, the causative agent of whooping cough, has experienced a resurgence in the past 15 years, despite the existence of both whole-cell and acellular vaccines. Here, we performed whole genome sequencing analysis of 149 clinical strains, provided by the National Institute of Infectious Diseases (NIID), Japan, isolated in 1982–2014, after Japan became the first country to adopt acellular vaccines against B. pertussis. Additionally, we sequenced 39 strains provided by the Konan Kosei Hospital in Aichi prefecture, Japan, isolated in 2008–2013. The genome sequences afforded insight into B. pertussis genome variability and population dynamics in Japan, and revealed that the B. pertussis population in Japan was characterized by two major clades that divided more than 40 years ago. The pertactin gene was disrupted in about 20 % of the 149 NIID isolates, by either a deletion within the signal sequence (ΔSS) or the insertion of IS element IS481 (prn :: IS481). Phylogeny suggests that the parent clones for these isolates originated in Japan. Divergence dating traced the first generation of the pertactin-deficient mutants in Japan to around 1990, and indicated that strains containing the alternative pertactin allele prn2 may have appeared in Japan around 1974. Molecular clock data suggested that observed fluctuations in B. pertussis population size may have coincided with changes in vaccine usage in the country. The continuing failure to eradicate the disease warrants an exploration of novel vaccine compositions.

Discriminating Self and Non-Self by RNA: Roles for RNA Structure, Misfolding, and Modification in Regulating the Innate Immune Sensor PKR

Pathogens are recognized by the innate immune system in part via their unique and complex RNA signatures. A key sensor in human innate immunity is the RNA-activated protein kinase, protein kinase R (PKR), which has two double-stranded RNA (dsRNA) binding motifs (dsRBMs) at its N-terminus. Early studies described PKR as being activated potently by long stretches of perfect dsRNA, a signature typical of viruses. More recently, we and others have found that PKR is also activated by RNAs having structural defects such as bulges and internal loops. This Account describes advances in our understanding of the ability of PKR to detect diverse foreign RNAs and how that recognition plays significant roles in discriminating self from non-self. The experiments discussed employ a wide range of techniques including activation assays, native polyacrylamide gel electrophoresis (PAGE), protein footprinting, and small-angle X-ray scattering (SAXS). We discuss how misfolding and dimerization of RNA lead to activation of PKR. We also present recent findings on the activation of PKR by varied bacterial functional RNAs including ribozymes and riboswitches, which are among the few structured RNAs known to interact with PKR in a site-specific manner. Molecular models for how these structured RNAs activate PKR are provided. Studies by SAXS revealed that PKR straightens bent RNAs. Most external and internal RNA cellular modifications introduced in vitro and found naturally, such as the m7G cap and m6A group, abrogate activation of PKR, but other modifications, such as 5'-ppp and 2'-fluoro groups, are immunostimulatory and potential anticancer agents. Genome-wide studies of RNA folding in vitro and in vivo have provided fresh insights into general differences in RNA structure among bacteria, viruses, and human. These studies suggest that in vivo, cellular human RNAs are less folded than once thought, unwound by helicases, destabilized by m6A modifications, and often bound up with proteins, all conditions known to abrogate activation of PKR. It thus appears that non-self RNAs are detected as unmodified, naked RNAs with appreciable secondary and tertiary structure. Observation that PKR is activated by structured but otherwise diverse RNAs is consistent both with the broad-spectrum nature of innate immunity and the nonspecific recognition of RNA by the dsRBM family. These findings provide a possible explanation for the apparent absence of protein-free structured human RNAs, such as ribozymes and riboswitches.