A critical role for the TLR signaling adapter Mal in alveolar macrophage-mediated protection against Bordetella pertussis

Imperative role of adaptor proteins in macrophage toll-like receptor signaling pathways

Macrophages are integral part of the body's defense against pathogens and serve as vital regulators of inflammation. Adaptor molecules, featuring diverse domains, intricately orchestrate the recruitment and transmission of inflammatory responses through signaling cascades. Key domains involved in macrophage polarization include Toll-like receptors (TLRs), Src Homology2 (SH2) and other small domains, alongside receptor tyrosine kinases, crucial for pathway activation. This review aims to elucidate the enigmatic role of macrophage adaptor molecules in modulating macrophage activation, emphasizing their diverse roles and potential therapeutic and investigative avenues for further exploration.

Combined regulation of pro-inflammatory cytokines production by STAT3 and STAT5 in a model of B. pertussis infection of alveolar macrophages

Bordetella pertussis is a highly contagious respiratory pathogen responsible for whooping-cough or pertussis. Despite high vaccination coverage worldwide, this gram-negative bacterium continues to spread among the population. B. pertussis is transmitted by aerosol droplets from an infected individual to a new host and will colonize its upper respiratory tract. Alveolar macrophages (AMs) are effector cells of the innate immune system that phagocytose B. pertussis and secrete both pro-inflammatory and antimicrobial mediators in the lungs. However, understanding their role in B. pertussis pathogenesis at the molecular level is hampered by the limited number of primary AMs that can be collected in vivo. In order to decipher the regulation of innate response induced by B. pertussis infection, we used for the first time self-renewing, non-transformed cells, called Max Planck Institute (MPI) cells, which are phenotypically and functionally very close to pulmonary AMs. Using optimized infection conditions, we characterized the entry and the clearance of B. pertussis within MPI macrophages. We showed that under these conditions, MPI cells exhibit a pro-inflammatory phenotype with the production of TNF, IL-1β, IL-6 and MIP-2α, similarly to primary AMs purified from broncho-alveolar fluids of mice. In addition, we explored the yet uncharacterized role of the signal transduction activator of transcription (STAT) proteins family in the innate immune response to B. pertussis infection and showed for the first time the parallel regulation of pro-inflammatory cytokines by STAT3 and STAT5 in MPI macrophages infected by B. pertussis. Altogether, this work highlights the interest of using MPI cells for experiments optimization and preliminary data acquisition to understand B. pertussis interaction with AMs, and thus significantly reduce the number of animals to be sacrificed.

Next-Generation Pertussis Vaccines Based on the Induction of Protective T Cells in the Respiratory Tract

Immunization with current acellular pertussis (aP) vaccines protects against severe pertussis, but immunity wanes rapidly after vaccination and these vaccines do not prevent nasal colonization with Bordetella pertussis. Studies in mouse and baboon models have demonstrated that Th1 and Th17 responses are integral to protective immunity induced by previous infection with B. pertussis and immunization with whole cell pertussis (wP) vaccines. Mucosal Th17 cells, IL-17 and secretory IgA (sIgA) are particularly important in generating sustained sterilizing immunity in the nasal cavity. Current aP vaccines induce potent IgG and Th2-skewed T cell responses but are less effective at generating Th1 and Th17 responses and fail to prime respiratory tissue-resident memory T (TRM) cells, that maintain long-term immunity at mucosal sites. In contrast, a live attenuated pertussis vaccine, pertussis outer membrane vesicle (OMV) vaccines or aP vaccines formulated with novel adjuvants do induce cellular immune responses in the respiratory tract, especially when delivered by the intranasal route. An increased understanding of the mechanisms of sustained protective immunity, especially the role of respiratory TRM cells, will facilitate the development of next generation pertussis vaccines that not only protect against pertussis disease, but prevent nasal colonization and transmission of B. pertussis.

Paradoxical Roles of the MAL/Tirap Adaptor in Pathologies

Toll-like receptors (TLRs) are at the forefront of pathogen recognition ensuring host fitness and eliciting protective cellular and humoral responses. Signaling pathways downstream of TLRs are tightly regulated for preventing collateral damage and loss of tolerance toward commensals. To trigger effective intracellular signaling, these receptors require the involvement of adaptor proteins. Among these, Toll/Interleukin-1 receptor domain containing adaptor protein (Tirap or MAL) plays an important role in establishing immune responses. Loss of function of MAL was associated with either disease susceptibility or resistance. These opposite effects reveal paradoxical functions of MAL and their importance in containing infectious or non-infectious diseases. In this review, we summarize the current knowledge on the signaling pathways involving MAL in different pathologies and their impact on inducing protective or non-protective responses.

Activation of Human NK Cells by Bordetella pertussis Requires Inflammasome Activation in Macrophages

Pertussis is a highly contagious respiratory infection caused by the bacterium Bordetella pertussis. Humans are the only known natural reservoir of B. pertussis. In mice, macrophages and NK cells have a key role in confining B. pertussis to the respiratory tract. However, the mechanisms underlying this process, particularly during human infections, remain unclear. Here we characterized the activation of human macrophages and NK cells in response to B. pertussis and unraveled the role of inflammasomes in this process. NLRP3 inflammasome activation by B. pertussis in human macrophage-like THP-1 cells and primary monocyte-derived macrophages (mo-MΦ) was shown by the visualization of ASC-speck formation, pyroptosis, and the secretion of caspase-mediated IL-1β and IL-18. In contrast to macrophages, stimulation of human CD56⁺CD3⁻ NK cells by B. pertussis alone did not result in activation of these cells. However, co-culture of B. pertussis-stimulated mo-MΦ and autologous NK cells resulted in high amounts of IFNγ secretion and an increased frequency of IL-2Rα⁺ and HLA-DR⁺ NK cells, indicating NK cell activation. This activation was significantly reduced upon inhibition of inflammasome activity or blocking of IL-18 in the mo-MΦ/NK cell co-culture. Furthermore, we observed increased secretion of proinflammatory cytokines in the B. pertussis-stimulated mo-MΦ/NK co-culture compared to the mo-MΦ single culture. Our results demonstrate that B. pertussis induces inflammasome activation in human macrophages and that the IL-18 produced by these cells is required for the activation of human NK cells, which in turn enhances the pro-inflammatory response to this pathogen. Our data provides a better understanding of the underlying mechanisms involved in the induction of innate immune responses against B. pertussis. These findings contribute to the knowledge required for the development of improved intervention strategies to control this highly contagious disease.

Superior B. pertussis Specific CD4+ T-Cell Immunity Imprinted by Natural Infection

Pertussis remains endemic in vaccinated populations due to waning of vaccine-induced immunity and insufficient interruption of transmission. Correlates of long-term protection against whooping cough remain elusive but increasing evidence from experimental models indicates that the priming of particular lineages of B. pertussis (Bp) specific CD4+ T cells is essential to control bacterial load. Critical hallmarks of these protective CD4+ T cell lineages in animals are suggested to be their differentiation profile as Th1 and Th17 cells and their tissue residency. These features seem optimally primed by previous infection but insufficiently or only partially by current vaccines. In this review, evidence is sought indicating whether infection also drives such superior Bp specific CD4+ T cell lineages in humans. We highlight key features of effector immunity downstream of Th1 and Th17 cell cytokines that explain clearing of primary Bp infections in naïve hosts, and effective prevention of infection in convalescent hosts during secondary challenge. Outstanding questions are put forward that need answers before correlates of human Bp infection-primed CD4+ T cell immunity can be used as benchmark for the development of improved pertussis vaccines.

Functional Programming of Innate Immune Cells in Response to Bordetella pertussis Infection and Vaccination

Despite widespread vaccination, B. pertussis remains one of the least controlled vaccine-preventable diseases. Although it is well known that acellular and whole cell pertussis vaccines induce distinct immune functionalities in memory cells, much less is known about the role of innate immunity in this process. In this review, we provide an overview of the known differences and similarities in innate receptors, innate immune cells and inflammatory signalling pathways induced by the pertussis vaccines either licensed or in development and compare this to primary infection with B. pertussis. Despite the crucial role of innate immunity in driving memory responses to B. pertussis, it is clear that a significant knowledge gap remains in our understanding of the early innate immune response to vaccination and infection. Such knowledge is essential to develop the next generation of pertussis vaccines with improved host defense against B. pertussis.

Immunological Signatures after Bordetella pertussis Infection Demonstrate Importance of Pulmonary Innate Immune Cells

Effective immunity against Bordetella pertussis is currently under discussion following the stacking evidence of pertussis resurgence in the vaccinated population. Natural immunity is more effective than vaccine-induced immunity indicating that knowledge on infection-induced responses may contribute to improve vaccination strategies. We applied a systems biology approach comprising microarray, flow cytometry and multiplex immunoassays to unravel the molecular and cellular signatures in unprotected mice and protected mice with infection-induced immunity, around a B. pertussis challenge. Pre-existing systemic memory Th1/Th17 cells, memory B-cells, and mucosal IgA specific for Ptx, Vag8, Fim2/3 were detected in the protected mice 56 days after an experimental infection. In addition, pre-existing high activity and reactivation of pulmonary innate cells such as alveolar macrophages, M-cells and goblet cells was detected. The pro-inflammatory responses in the lungs and serum, and neutrophil recruitment in the spleen upon an infectious challenge of unprotected mice were absent in protected mice. Instead, fast pulmonary immune responses in protected mice led to efficient bacterial clearance and harbored potential new gene markers that contribute to immunity against B. pertussis. These responses comprised of innate makers, such as Clca3, Retlna, Glycam1, Gp2, and Umod, next to adaptive markers, such as CCR6+ B-cells, CCR6+ Th17 cells and CXCR6+ T-cells as demonstrated by transcriptome analysis. In conclusion, besides effective Th1/Th17 and mucosal IgA responses, the primary infection-induced immunity benefits from activation of pulmonary resident innate immune cells, achieved by local pathogen-recognition. These molecular signatures of primary infection-induced immunity provided potential markers to improve vaccine-induced immunity against B. pertussis.

Roads to the development of improved pertussis vaccines paved by immunology

Current acellular pertussis vaccines have various shortcomings, which may contribute to their suboptimal efficacy and waning immunity in vaccinated populations. This calls for the development of new pertussis vaccines capable of inducing long-lived protective immunity. Immunization with whole cell pertussis vaccines and natural infection with Bordetella pertussis induce distinct and more protective immune responses when compared with immunization with acellular pertussis vaccines. Therefore, the immune responses induced with whole cell vaccine or after infection can be used as a benchmark for the development of third-generation vaccines against pertussis. Here, we review the literature on the immunology of B. pertussis infection and vaccination and discuss the lessons learned that will help in the design of improved pertussis vaccines.

To develop improved pertussis vaccines capable of inducing long-lived protective immunity, lessons have to be learned from immunology of Bordetella pertussis infection and current vaccination.