Borrelia burgdorferi clinical isolates induce human innate immune responses that are not dependent on genotype

The Interaction of Borrelia burgdorferi with Human Dendritic Cells: Functional Implications

Dendritic cells bridge the innate and adaptive immune responses by serving as sensors of infection and as the primary APCs responsible for the initiation of the T cell response against invading pathogens. The naive T cell activation requires the following three key signals to be delivered from dendritic cells: engagement of the TCR by peptide Ags bound to MHC molecules (signal 1), engagement of costimulatory molecules on both cell types (signal 2), and expression of polarizing cytokines (signal 3). Initial interactions between Borrelia burgdorferi, the causative agent of Lyme disease, and dendritic cells remain largely unexplored. To address this gap in knowledge, we cultured live B. burgdorferi with monocyte-derived dendritic cells (mo-DCs) from healthy donors to examine the bacterial immunopeptidome associated with HLA-DR. In parallel, we examined changes in the expression of key costimulatory and regulatory molecules as well as profiled the cytokines released by dendritic cells when exposed to live spirochetes. RNA-sequencing studies on B. burgdorferi-pulsed dendritic cells show a unique gene expression signature associated with B. burgdorferi stimulation that differs from stimulation with lipoteichoic acid, a TLR2 agonist. These studies revealed that exposure of mo-DCs to live B. burgdorferi drives the expression of both pro- and anti-inflammatory cytokines as well as immunoregulatory molecules (e.g., PD-L1, IDO1, Tim3). Collectively, these studies indicate that the interaction of live B. burgdorferi with mo-DCs promotes a unique mature DC phenotype that likely impacts the nature of the adaptive T cell response generated in human Lyme disease.

Borrelia burgdorferi Engages Mammalian Type I IFN Responses via the cGAS-STING Pathway

Borrelia burgdorferi, the etiologic agent of Lyme disease, is a spirochete that modulates numerous host pathways to cause a chronic, multisystem inflammatory disease in humans. B. burgdorferi infection can lead to Lyme carditis, neurologic complications, and arthritis because of the ability of specific borrelial strains to disseminate, invade, and drive inflammation. B. burgdorferi elicits type I IFN (IFN-I) responses in mammalian cells and tissues that are associated with the development of severe arthritis or other Lyme-related complications. However, the innate immune sensors and signaling pathways controlling IFN-I induction remain unclear. In this study, we examined whether intracellular nucleic acid sensing is required for the induction of IFN-I to B. burgdorferi. Using fluorescence microscopy, we show that B. burgdorferi associates with mouse and human cells in culture, and we document that internalized spirochetes colocalize with the pattern recognition receptor cyclic GMP-AMP synthase (cGAS). Moreover, we report that IFN-I responses in mouse macrophages and murine embryonic fibroblasts are significantly attenuated in the absence of cGAS or its adaptor stimulator of IFN genes (STING), which function to sense and respond to intracellular DNA. Longitudinal in vivo tracking of bioluminescent B. burgdorferi revealed similar dissemination kinetics and borrelial load in C57BL/6J wild-type, cGAS-deficient, or STING-deficient mice. However, infection-associated tibiotarsal joint pathology and inflammation were modestly reduced in cGAS-deficient compared with wild-type mice. Collectively, these results indicate that the cGAS-STING pathway is a critical mediator of mammalian IFN-I signaling and innate immune responses to B. burgdorferi.

Borreliae Part 1: Borrelia Lyme Group and Echidna-Reptile Group

Simple Summary

Borreliae are spirochaetes, which represent a heterogeneous phylum within bacteria. Spirochaetes are indeed distinguished from other bacteria for their spiral shape, which also characterizes Borreliae. This review describes briefly the organization of the phylum Spirocheteales with a digression about its pathogenicity and historical information about bacteria isolation and characterization. Among spirochaetes, Borrelia genus is here divided into three groups, namely the Lyme group (LG), the Echidna-Reptile group (REPG) and the Relapsing Fever group (RFG). Borreliae Part 1 deals with Lyme group and Echidna-Reptile group Borreliae, while the subject of Borreliae Part 2 is Relapsing Fever group and unclassified Borreliae. Lyme group Borreliae is organized here in sections describing ecology, namely tick vectors and animal hosts, epidemiology, microbiology, and Borrelia genome organization and antigen characterization. Furthermore, the main clinical manifestations in Lyme borreliosis are also described. Although included in the Lyme group due to their particular clinical features, Borrelia causing Baggio Yoshinari syndrome and Borrelia mayonii are described in dedicated paragraphs. The Borrelia Echidna-Reptile group has been recently characterized including spirochaetes that apparently are not pathogenic to humans, but infect reptiles and amphibians. The paragraph dedicated to this group of Borreliae describes their vectors, hosts, geographical distribution and their characteristics.

Abstract

Borreliae are divided into three groups, namely the Lyme group (LG), the Echidna-Reptile group (REPG) and the Relapsing Fever group (RFG). Currently, only Borrelia of the Lyme and RF groups (not all) cause infection in humans. Borreliae of the Echidna-Reptile group represent a new monophyletic group of spirochaetes, which infect amphibians and reptiles. In addition to a general description of the phylum Spirochaetales, including a brief historical digression on spirochaetosis, in the present review Borreliae of Lyme and Echidna-Reptile groups are described, discussing the ecology with vectors and hosts as well as microbiological features and molecular characterization. Furthermore, differences between LG and RFG are discussed with respect to the clinical manifestations. In humans, LG Borreliae are organotropic and cause erythema migrans in the early phase of the disease, while RFG Borreliae give high spirochaetemia with fever, without the development of erythema migrans. With respect of LG Borreliae, recently Borrelia mayonii, with intermediate characteristics between LG and RFG, has been identified. As part of the LG, it gives erythema migrans but also high spirochaetemia with fever. Hard ticks are vectors for both LG and REPG groups, but in LG they are mostly Ixodes sp. ticks, while in REPG vectors do not belong to that genus.

Microfluidic Assays for Probing Neutrophil-Borrelia Interactions in Blood During Lyme Disease

Human neutrophils are highly sensitive to the presence of Borrelia burgdorferi (Bb), the agent of Lyme disease (LD), in tissues. Although Bb is also found in the blood of LD patients, far less is known about how neutrophils respond to Bb in the presence of blood. In this study, we employed microfluidic tools to probe the interaction between human neutrophils and Bb and measured the activation of human neutrophils in blood samples from patients. We found that neutrophils migrate vigorously toward Bb in the presence of serum, and this process was complement-dependent. Preventing complement factor 5 cleavage or blocking complement receptors decreased neutrophil's ability to interact with Bb. We also found that spiking Bb directly into the blood from healthy donors induced spontaneous neutrophil motility. This response to Bb was also complement-dependent. Preventing complement factor 5 cleavage decreased spontaneous neutrophil motility in Bb-spiked blood. Moreover, we found that neutrophils in blood samples from acute LD patients displayed spontaneous motility patterns similar to those observed in Bb-spiked samples. Neutrophil motility was more robust in blood samples from LD patients than that measured in healthy and ill controls, validating the utility of the microfluidic assay for the study of neutrophil-Bb interactions in the presence of blood.

Borrelia miyamotoi Activates Human Dendritic Cells and Elicits T Cell Responses

The spirochete Borrelia miyamotoi has recently been shown to cause relapsing fever. Like the Lyme disease agent, Borrelia burgdorferi, B. miyamotoi is transmitted through the bite of infected ticks; however, little is known about the response of the immune system upon infection. Dendritic cells (DCs) play a central role in the early immune response against B. burgdorferi We investigated the response of DCs to two different strains of B. miyamotoi using in vitro and ex vivo models and compared this to the response elicited by B. burgdorferi. Our findings show that B. miyamotoi is phagocytosed by monocyte-derived DCs, causing upregulation of activation markers and production of proinflammatory cytokines in a similar manner to B. burgdorferi. Recognition of B. miyamotoi was demonstrated to be partially mediated by TLR2. DCs migrated out of human skin explants upon inoculation of the skin with B. miyamotoi. Finally, we showed that B. miyamotoi-stimulated DCs induced proliferation of naive CD4⁺ and CD8⁺ T cells to a larger extent than B. burgdorferi. In conclusion, we show in this study that DCs respond to and mount an immune response against B. miyamotoi that is similar to the response to B. burgdorferi and is able to induce T cell proliferation.

Outer surface protein polymorphisms linked to host-spirochete association in Lyme borreliae

Lyme borreliosis is caused by multiple species of the spirochete bacteria Borrelia burgdorferi sensu lato. The spirochetes are transmitted by ticks to vertebrate hosts, including small- and medium-sized mammals, birds, reptiles, and humans. Strain-to-strain variation in host-specific infectivity has been documented, but the molecular basis that drives this differentiation is still unclear. Spirochetes possess the ability to evade host immune responses and colonize host tissues to establish infection in vertebrate hosts. In turn, hosts have developed distinct levels of immune responses when invaded by different species/strains of Lyme borreliae. Similarly, the ability of Lyme borreliae to colonize host tissues varies among different spirochete species/strains. One potential mechanism that drives this strain-to-strain variation of immune evasion and colonization is the polymorphic outer surface proteins produced by Lyme borreliae. In this review, we summarize research on strain-to-strain variation in host competence and discuss the evidence that supports the role of spirochete-produced protein polymorphisms in driving this variation in host specialization. Such information will provide greater insights into the adaptive mechanisms driving host and Lyme borreliae association, which will lead to the development of interventions to block pathogen spread and eventually reduce Lyme borreliosis health burden.

MRP8/14 does not contribute to dissemination or inflammation in a murine model of Lyme borreliosis

Myeloid-related protein (MRP)8 and MRP14 form a complex (MRP8/14) that is released by activated neutrophils and monocytes during infection. MRP8/14 has been shown to have bacteriostatic activity in vitro against Borrelia burgdorferi, the spirochete that causes Lyme borreliosis. Furthermore, levels of MRP8/14 have been shown to be elevated in the joints of patients with Lyme arthritis. We hypothesized that MRP8/14 has a protective effect during B. burgdorferi infection. To determine the role of MRP8/14 in the immune response to B. burgdorferi, we studied the course of B. burgdorferi infection in wildtype (wt) and mrp14^-/- mice. In addition, we studied the response of leukocytes from mice lacking MRP8/14 to B. burgdorferi ex vivo. We demonstrated similar levels of B. burgdorferi dissemination, cytokine and immunoglobulin production in infected wt and mrp14^-/- mice after 21 days. Neutrophils and monocytes lacking MRP8/14 were undiminished in their ability to become activated or phagocytose B. burgdorferi. In conclusion, we did not find a central role of MRP8/14 in the immune response against B. burgdorferi. As the levels of MRP8/14 in the serum of infected mice were low, we speculate that MRP8/14 is not released in levels great enough to influence the course of B. burgdorferi infection.

Coxiella burnetii isolates originating from infected cattle induce a more pronounced proinflammatory cytokine response compared to isolates from infected goats and sheep

Coxiella burnetii is the causative agent of Q fever. Although the prevalence of C. burnetii in cattle is much higher than in goats and sheep, infected cattle are rarely associated with human outbreaks. We investigated whether the immune response of humans differs after contact with C. burnetii isolates from different host origins or with different multilocus variable number of tandem repeat analysis (MLVA) genotypes. Cytokine responses were measured in human peripheral blood mononuclear cells (PBMCs) stimulated with 16 C. burnetii isolates with known MLVA genotype from goats, sheep, cattle, acute and chronic Q fever patients. Coxiella burnetii isolates originating from cattle induce significantly more IL-1β, TNF-α and IL-22 than the isolates from goats, sheep or chronic Q fever patients. Comparing the cytokine induction of the isolates based on their MVLA genotype did not reveal differences in response between the MLVA genotypes. The proinflammatory cytokine response induced in human PBMCs by C. burnetii isolates from cattle may explain the low incidence of human Q fever outbreaks caused by cattle. The cytokine profile of PBMCs stimulated with C. burnetii isolates from chronic Q fever patients resembles isolates from goats. Furthermore, cytokine responses seem to be depending on host origin than on MLVA genotype.

Investigating Human Dendritic Cell Immune Responses to Borrelia burgdorferi

Dendritic cells (DCs) are professional antigen-presenting cells that recognize and phagocytose pathogens, and help to orchestrate adaptive immune responses to combat them. DCs are abundant in the skin where Borrelia burgdorferi first enters the body during a tick bite, and are thus critical in determining the initial stages of the innate and adaptive immune responses against Borrelia. Here, we describe two methods to study the response of DCs to Borrelia; an in vitro approach using monocyte-derived DCs (moDCs) and an ex vivo approach using a human skin model.

Differences in Genotype, Clinical Features, and Inflammatory Potential of Borrelia burgdorferi sensu stricto Strains from Europe and the United States

Strains from the United States are more virulent and have greater inflammatory potential.

Borrelia burgdorferi sensu stricto isolates from patients with erythema migrans in Europe and the United States were compared by genotype, clinical features of infection, and inflammatory potential. Analysis of outer surface protein C and multilocus sequence typing showed that strains from these 2 regions represent distinct genotypes. Clinical features of infection with B. burgdorferi in Slovenia were similar to infection with B. afzelii or B. garinii, the other 2 Borrelia spp. that cause disease in Europe, whereas B. burgdorferi strains from the United States were associated with more severe disease. Moreover, B. burgdorferi strains from the United States induced peripheral blood mononuclear cells to secrete higher levels of cytokines and chemokines associated with innate and Th1-adaptive immune responses, whereas strains from Europe induced greater Th17-associated responses. Thus, strains of the same B. burgdorferi species from Europe and the United States represent distinct clonal lineages that vary in virulence and inflammatory potential.

Borrelia burgdorferi Induces TLR2-Mediated Migration of Activated Dendritic Cells in an Ex Vivo Human Skin Model

Borrelia burgdorferi is transmitted into the skin of the host where it encounters and interacts with two dendritic cell (DC) subsets; Langerhans cells (LCs) and dermal DCs (DDCs). These cells recognize pathogens via pattern recognition receptors, mature and migrate out of the skin into draining lymph nodes, where they orchestrate adaptive immune responses. In order to investigate the response of skin DCs during the early immunopathogenesis of Lyme borreliosis, we injected B. burgdorferi intradermally into full-thickness human skin and studied the migration of DCs out of the skin, the activation profile and phenotype of migrated cells. We found a significant increase in the migration of LCs and DDCs in response to B. burgdorferi. Notably, migration was prevented by blocking TLR2. DCs migrated from skin inoculated with higher numbers of spirochetes expressed significantly higher levels of CD83 and produced pro-inflammatory cytokines. No difference was observed in the expression of HLA-DR, CD86, CD38, or CCR7. To conclude, we have established an ex vivo human skin model to study DC-B. burgdorferi interactions. Using this model, we have demonstrated that B. burgdorferi-induced DC migration is mediated by TLR2. Our findings underscore the utility of this model as a valuable tool to study immunity to spirochetal infections.