Dual role for Fcγ receptors in host defense and disease in Borrelia burgdorferi-infected mice

A human secretome library screen reveals a role for Peptidoglycan Recognition Protein 1 in Lyme borreliosis

Lyme disease, the most common vector-borne illness in North America, is caused by the spirochete Borrelia burgdorferi. Infection begins in the skin following a tick bite and can spread to the hearts, joints, nervous system, and other organs. Diverse host responses influence the level of B. burgdorferi infection in mice and humans. Using a systems biology approach, we examined potential molecular interactions between human extracellular and secreted proteins and B. burgdorferi. A yeast display library expressing 1031 human extracellular proteins was probed against 36 isolates of B. burgdorferi sensu lato. We found that human Peptidoglycan Recognition Protein 1 (PGLYRP1) interacted with the vast majority of B. burgdorferi isolates. In subsequent experiments, we demonstrated that recombinant PGLYRP1 interacts with purified B. burgdorferi peptidoglycan and exhibits borreliacidal activity, suggesting that vertebrate hosts may use PGLYRP1 to identify B. burgdorferi. We examined B. burgdorferi infection in mice lacking PGLYRP1 and observed an increased spirochete burden in the heart and joints, along with splenomegaly. Mice lacking PGLYRP1 also showed signs of immune dysregulation, including lower serum IgG levels and higher levels of IFNγ, CXCL9, and CXCL10.Taken together, our findings suggest that PGLYRP1 plays a role in the host’s response to B. burgdorferi and further demonstrate the utility of expansive yeast display screening in capturing biologically relevant interactions between spirochetes and their hosts.

Lyme disease is the most common vector-borne illness in North America and is caused by the spirochete Borrelia burgdorferi. The disease starts with a tick bite that leads to a skin rash and inflammation in other organs of the body, such as hearts and joints. B. burgdorferi uses many strategies to evade detection and persist in the human host. It is important to have efficient methods to be able to identify the various components of the immune system that interact with B. burgdorferi to better understand the disease process, but few currently exist. In this study, we used a novel yeast display screening assay of over 1,000 human immune proteins probed against several isolates of Borrelia to uncover biologically relevant interactions for the Lyme disease pathogen. We identified Peptidoglycan Recognition Protein 1 (PGLYRP1), an innate immune protein important in defense against bacteria, as a major candidate from this screen. We validated the interaction of PGLYRP1 with Borrelia and were able to use PGLYRP1-deficient mice as a model to understand the role of this protein in Lyme disease pathogenesis. Our study demonstrates the potential implications of yeast screens in uncovering important host-pathogen interactions.

Regulation of macrophage activity by surface receptors contained within Borrelia burgdorferi-enriched phagosomal fractions

Macrophages mediate the elimination of pathogens by phagocytosis resulting in the activation of specific signaling pathways that lead to the production of cytokines, chemokines and other factors. Borrelia burgdorferi, the causative agent of Lyme disease, causes a wide variety of pro-inflammatory symptoms. The proinflammatory capacity of macrophages is intimately related to the internalization of the spirochete. However, most receptors mediating this process are largely unknown. We have applied a multiomic approach, including the proteomic analysis of B. burgdorferi-containing phagosome-enriched fractions, to identify surface receptors that are involved in the phagocytic capacity of macrophages as well as their inflammatory output. Sucrose gradient protein fractions of human monocyte-derived macrophages exposed to B. burgdorferi contained the phagocytic receptor, CR3/CD14 highlighting the major role played by these proteins in spirochetal phagocytosis. Other proteins identified in these fractions include C-type lectins, scavenger receptors or Siglecs, of which some are directly involved in the interaction with the spirochete. We also identified the Fc gamma receptor pathway, including the binding receptor, CD64, as involved both in the phagocytosis of, and TNF induction in response to B. burgdorferi in the absence of antibodies. The common gamma chain, FcγR, mediates the phagocytosis of the spirochete, likely through Fc receptors and C-type lectins, in a process that involves Syk activation. Overall, these findings highlight the complex array of receptors involved in the phagocytic response of macrophages to B. burgdorferi.

Macrophages eliminate infecting microorganisms through the concerted action of surface receptors and signaling molecules. As a consequence, these cells produce a series of soluble factors that participate in the inflammatory response during infections. The composition of the full complement of receptors that participate in the recognition and internalization of the causative agent of Lyme disease, Borrelia burgdorferi, is largely unknown. We have analyzed the protein composition of phagosomes containing B. burgdorferi from human macrophages and identified a series of surface proteins that may be involved in the process. Through the use of gene silencing techniques, we have determined the participation of several of these receptors both in the internalization of the bacterium and the subsequent inflammatory response. Among these, we have identified the Fc gamma receptor pathway as involved in this process in the absence of antibodies. We have also identified receptors that are directly involved in the attachment of B. burgdorferi, while others seem to have an accessory role in the internalization and/or induction of proinflammatory cytokines in response to the spirochete. These data clarify the complex array of interactions between macrophages and B. burgdorferi and shed light on the overall response to this infectious agent.

Protozoan Parasite Babesia microti Subverts Adaptive Immunity and Enhances Lyme Disease Severity

Lyme disease is the most prominent tick-borne disease in the United States. Co-infections with the tick-transmitted pathogens Babesia microti and Borrelia burgdorferi sensu stricto are becoming a serious health problem. B. burgdorferi is an extracellular spirochete that causes Lyme disease while B. microti is a protozoan that infects erythrocytes and causes babesiosis. Testing of donated blood for Babesia species is not currently mandatory due to unavailability of an FDA approved test. Transmission of this protozoan by blood transfusion often results in high morbidity and mortality in recipients. Infection of C3H/HeJ mice with B. burgdorferi and B. microti individually results in inflammatory Lyme disease and display of human babesiosis-like symptoms, respectively. Here we use this mouse model to provide a detailed investigation of the reciprocal influence of the two pathogens on each other during co-infection. We show that B. burgdorferi infection attenuates parasitemia in mice while B. microti subverts the splenic immune response, such that a marked decrease in splenic B and T cells, reduction in antibody levels and diminished functional humoral immunity, as determined by spirochete opsonophagocytosis, are observed in co-infected mice compared to only B. burgdorferi infected mice. Furthermore, immunosuppression by B. microti in co-infected mice showed an association with enhanced Lyme disease manifestations. This study demonstrates the effect of only simultaneous infection by B. burgdorferi and B. microti on each pathogen, immune response and on disease manifestations with respect to infection by the spirochete and the parasite. In our future studies, we will examine the overall effects of sequential infection by these pathogens on host immune responses and disease outcomes.

Borrelia burgdorferi peptidoglycan is a persistent antigen in patients with Lyme arthritis

Lyme disease, caused by the spirochete Borrelia burgdorferi, is the most common vector-borne disease in North America. If early infection is untreated, it can result in late-stage manifestations, including arthritis. Although antibiotics are generally effective at all stages of the disease, arthritis may persist in some patients for months to several years despite oral and intravenous antibiotic treatment. Excessive, dysregulated host immune responses are thought to play an important role in this outcome, but the underlying mechanisms are not completely understood. This study identifies the B. burgdorferi peptidoglycan, a major component of the cell wall, as an immunogen likely to contribute to inflammation during infection and in cases of postinfectious Lyme arthritis.

Lyme disease is a multisystem disorder caused by the spirochete Borrelia burgdorferi. A common late-stage complication of this disease is oligoarticular arthritis, often involving the knee. In ∼10% of cases, arthritis persists after appropriate antibiotic treatment, leading to a proliferative synovitis typical of chronic inflammatory arthritides. Here, we provide evidence that peptidoglycan (PG), a major component of the B. burgdorferi cell envelope, may contribute to the development and persistence of Lyme arthritis (LA). We show that B. burgdorferi has a chemically atypical PG (PG^Bb) that is not recycled during cell-wall turnover. Instead, this pathogen sheds PG^Bb fragments into its environment during growth. Patients with LA mount a specific immunoglobulin G response against PG^Bb, which is significantly higher in the synovial fluid than in the serum of the same patient. We also detect PG^Bb in 94% of synovial fluid samples (32 of 34) from patients with LA, many of whom had undergone oral and intravenous antibiotic treatment. These same synovial fluid samples contain proinflammatory cytokines, similar to those produced by human peripheral blood mononuclear cells stimulated with PG^Bb. In addition, systemic administration of PG^Bb in BALB/c mice elicits acute arthritis. Altogether, our study identifies PG^Bb as a likely contributor to inflammatory responses in LA. Persistence of this antigen in the joint may contribute to synovitis after antibiotics eradicate the pathogen. Furthermore, our finding that B. burgdorferi sheds immunogenic PG^Bb fragments during growth suggests a potential role for PG^Bb in the immunopathogenesis of other Lyme disease manifestations.

Phagocytic Receptors Activate Syk and Src Signaling during Borrelia burgdorferi Phagocytosis

Phagocytosis of the Lyme disease-causing pathogen Borrelia burgdorferi has been shown to be important for generating an inflammatory response to the pathogen. As a result, understanding the mechanisms of phagocytosis has been an area of great interest in the field of Lyme disease. Several cell surface receptors that participate in B. burgdorferi phagocytosis have been reported, including the scavenger receptor MARCO and integrin α3β1. We sought to define the mechanisms by which these receptors mediate phagocytosis and to identify signaling pathways activated downstream of these receptors upon contact with B. burgdorferi We identified both Syk and Src signaling pathways as ones that participate in B. burgdorferi phagocytosis and the resulting cytokine activation. In our studies, we found that both MARCO and integrin β1 play a role in the activation of the Src kinase pathway. However, only integrin β1 participates in the activation of Syk. Interestingly, the integrin activates Syk without the help of the signaling adaptor Dap12 or FcRγ. Thus, we report that multiple pathways participate in B. burgdorferi internalization and that different cell surface receptors act simultaneously in cooperation and independently to mediate phagocytosis.

Selenoprotein K regulation of palmitoylation and calpain cleavage of ASAP2 is required for efficient FcγR-mediated phagocytosis

Effective activation of macrophages through phagocytic Fcγ receptors (FcγR) has been shown to require selenoprotein K (Selk). We set out to determine whether the FcγR-mediated uptake process itself also requires Selk and potential underlying mechanisms. Macrophages from Selk knockout (KO) mice were less efficient compared with wild-type (WT) controls in engulfing IgG-coated fluorescent beads. Using LC-MS/MS to screen for Selk-binding partners involved in FcγR-mediated phagocytosis, we identified Arf-GAP with SH3 domain, ANK repeat, and PH domain-containing protein 2 (ASAP2). Coimmunoprecipitation assays confirmed interactions between Selk and ASAP2. Selk was required for ASAP2 to be cleaved by calpain-2 within the Bin/Amphiphysin/Rvs (BAR) domain of ASAP2. BAR domains promote membrane association, which was consistent with our data showing that Selk deficiency led to retention of ASAP2 within the phagocytic cup. Because Selk was recently identified as a cofactor for the palmitoylation of certain proteins, we investigated whether ASAP2 was palmitoylated and whether this was related to its cleavage by calpain-2. Acyl/biotin exchange assays and MALDI-TOF analysis showed that cysteine-86 in ASAP2 was palmitoylated in WT, but to a much lesser extent in KO, mouse macrophages. Inhibitors of either palmitoylation or calpain-2 cleavage and rescue experiments with different versions of Selk demonstrated that Selk-dependent palmitoylation of ASAP2 leads to cleavage by calpain-2 within the BAR domain, which releases this protein from the maturing phagocytic cup. Overall, these findings identify ASAP2 as a new target of Selk-dependent palmitoylation and reveal a new mechanism regulating the efficiency of FcγR-mediated phagocytosis.