Activation of natural killer cells in arthritis-susceptible but not arthritis-resistant mouse strains following Borrelia burgdorferi infection

Immune Response to Borrelia: Lessons from Lyme Disease Spirochetes

The mammalian host responds to infection with Borrelia spirochetes through a highly orchestrated immune defense involving innate and adaptive effector functions aimed toward limiting pathogen burdens, minimizing tissue injury, and preventing subsequent reinfection. The evolutionary adaptation of Borrelia spirochetes to their reservoir mammalian hosts may allow for its persistence despite this immune defense. This review summarizes our current understanding of the host immune response to B. burgdorferi sensu lato, the most widely studied Borrelia spp. and etiologic agent of Lyme borreliosis. Pertinent literature will be reviewed with emphasis on in vitro, ex vivo and animal studies that influenced our understanding of both the earliest responses to B. burgdorferi as it enters the mammalian host and those that evolve as spirochetes disseminate and establish infection in multiple tissues. Our focus is on the immune response of inbred mice, the most commonly studied animal model of B. burgdorferi infection and surrogate for one of this pathogen's principle natural reservoir hosts, the white-footed deer mouse. Comparison will be made to the immune responses of humans with Lyme borreliosis. Our goal is to provide an understanding of the dynamics of the mammalian immune response during infection with B. burgdorferi and its relation to the outcomes in reservoir (mouse) and non-reservoir (human) hosts.

A joint effort: The interplay between the innate and the adaptive immune system in Lyme arthritis

Articular joints are a major target of Borrelia burgdorferi, the causative agent of Lyme arthritis. Despite antibiotic treatment, recurrent or persistent Lyme arthritis is observed in a significant number of patients. The host immune response plays a crucial role in this chronic arthritic joint complication of Borrelia infections. During the early stages of B. burgdorferi infection, a major hinder in generating a proper host immune response is the lack of induction of a strong adaptive immune response. This may lead to a delayed hyperinflammatory reaction later in the disease. Several mechanisms have been suggested that might be pivotal for the development of Lyme arthritis and will be highlighted in this review, from molecular mimicry of matrix metallopeptidases and glycosaminoglycans, to autoimmune responses to live bacteria, or remnants of Borrelia spirochetes in joints. Murine studies have suggested that the inflammatory responses are initiated by innate immune cells, but this does not exclude the involvement of the adaptive immune system in this dysregulated immune profile. Genetic predisposition, via human leukocyte antigen-DR isotype and microRNA expression, has been associated with the development of antibiotic-refractory Lyme arthritis. Yet the ultimate cause for (antibiotic-refractory) Lyme arthritis remains unknown. Complex processes of different immune cells and signaling cascades are involved in the development of Lyme arthritis. When these various mechanisms are fully been unraveled, new treatment strategies can be developed to target (antibiotic-refractory) Lyme arthritis more effectively.

T Cells Exacerbate Lyme Borreliosis in TLR2-Deficient Mice

Infection of humans with the spirochete, Borrelia burgdorferi, causes Lyme borreliosis and can lead to clinical manifestations such as arthritis, carditis, and neurological conditions. Experimental infection of mice recapitulates many of these symptoms and serves as a model system for the investigation of disease pathogenesis and immunity. Innate immunity is known to drive the development of Lyme arthritis and carditis, but the mechanisms driving this response remain unclear. Innate immune cells recognize B. burgdorferi surface lipoproteins primarily via toll-like receptor (TLR)2; however, previous work has demonstrated TLR2^−/− mice had exacerbated disease and increased bacterial burden. We demonstrate increased CD4 and CD8 T cell infiltrates in B. burgdorferi-infected joints and hearts of C3H TLR2^−/− mice. In vivo depletion of either CD4 or CD8 T cells reduced Borrelia-induced joint swelling and lowered tissue spirochete burden, whereas depletion of CD8 T cells alone reduced disease severity scores. Exacerbation of Lyme arthritis correlated with increased production of CXCL9 by synoviocytes, and this was reduced with CD8 T cell depletion. These results demonstrate T cells can exacerbate Lyme disease pathogenesis and prolong disease resolution possibly through dysregulation of inflammatory responses and inhibition of bacterial clearance.

Outer surface protein OspC is an antiphagocytic factor that protects Borrelia burgdorferi from phagocytosis by macrophages

Outer surface protein C (OspC) is one of the major lipoproteins expressed on the surface of Borrelia burgdorferi during tick feeding and the early phase of mammalian infection. OspC is required for B. burgdorferi to establish infection in both immunocompetent and SCID mice and has been proposed to facilitate evasion of innate immune defenses. However, the exact biological function of OspC remains elusive. In this study, we showed that the ospC-deficient spirochete could not establish infection in NOD-scid IL2rγ(null) mice that lack B cells, T cells, NK cells, and lytic complement. The ospC mutant also could not establish infection in anti-Ly6G-treated SCID and C3H/HeN mice (depletion of neutrophils). However, depletion of mononuclear phagocytes at the skin site of inoculation in SCID and C3H/HeN mice allowed the ospC mutant to establish infection in vivo. In phagocyte-depleted mice, the ospC mutant was able to colonize the joints and triggered neutrophilia during dissemination. Furthermore, we found that phagocytosis of green fluorescent protein (GFP)-expressing ospC mutant spirochetes by murine peritoneal macrophages and human THP-1 macrophage-like cells, but not in PMN-HL60, was significantly higher than parental wild-type B. burgdorferi strains, suggesting that OspC has an antiphagocytic property. In addition, overproduction of OspC in spirochetes also decreased the uptake of spirochetes by murine peritoneal macrophages. Together, our findings provide evidence that mononuclear phagocytes play a key role in clearance of the ospC mutant and that OspC promotes spirochetes' evasion of macrophages during early Lyme borreliosis.

Hamster and murine models of severe destructive Lyme arthritis

Arthritis is a frequent complication of infection in humans with Borrelia burgdorferi. Weeks to months following the onset of Lyme borreliosis, a histopathological reaction characteristic of synovitis including bone, joint, muscle, or tendon pain may occur. A subpopulation of patients may progress to a chronic, debilitating arthritis months to years after infection which has been classified as severe destructive Lyme arthritis. This arthritis involves focal bone erosion and destruction of articular cartilage. Hamsters and mice are animal models that have been utilized to study articular manifestations of Lyme borreliosis. Infection of immunocompetent LSH hamsters or C3H mice results in a transient synovitis. However, severe destructive Lyme arthritis can be induced by infecting irradiated hamsters or mice and immunocompetent Borrelia-vaccinated hamsters, mice, and interferon-gamma- (IFN-γ-) deficient mice with viable B. burgdorferi. The hamster model of severe destructive Lyme arthritis facilitates easy assessment of Lyme borreliosis vaccine preparations for deleterious effects while murine models of severe destructive Lyme arthritis allow for investigation of mechanisms of immunopathology.

Interval-specific congenic lines reveal quantitative trait Loci with penetrant lyme arthritis phenotypes on chromosomes 5, 11, and 12

The observation that Borrelia burgdorferi-induced arthritis is severe in C3H mice and milder in C57BL/6 (B6) mice has allowed a forward genetics approach for the identification of genetic elements that regulate the arthritis response. Quantitative trait loci (QTL) on five chromosomes (Chr) were identified previously in segregating crosses between C3H and B6 mice and collectively designated B. burgdorferi arthritis-associated (Bbaa) QTL. Reciprocal interval-specific congenic lines (ISCL) that encompass Bbaa1, Bbaa2-Bbaa3, Bbaa4, Bbaa6, and Bbaa12 on Chr 4, 5, 11, 12, and 1, respectively, have now been generated. Bidirectional transfer of the arthritis severity phenotype in association with Bbaa2-Bbaa3 and Bbaa4 was observed, and unidirectional transfer with the B6 allele of Bbaa6 was noted. These findings confirm the existence of polymorphic loci within Bbaa2-Bbaa3, Bbaa4, and Bbaa6 that regulate the severity of B. burgdorferi-induced arthritis. ISCL were used to assess the regulation of a previously identified interferon transcriptional profile associated with severe disease in C3H mice. The regulation of this transcriptional signature was found to be independent of penetrant Bbaa QTL, both in joint tissues and in isolated macrophages. These results clearly demonstrate the utility of forward genetics for the discovery of novel genes and pathways involved in the regulation of the severity of Lyme arthritis and predict the involvement of regulatory elements not evident from other experimental approaches.

A critical role for type I IFN in arthritis development following Borrelia burgdorferi infection of mice

Gene expression analysis previously revealed a robust IFN-responsive gene induction profile that was selectively up-regulated in Borrelia burgdorferi-infected C3H mice at 1 wk postinfection. This profile was correlated with arthritis development, as it was absent from infected, mildly arthritic C57BL/6 mice. In this report we now demonstrate that profile induction in infected C3H scid mice occurs independently of B or T lymphocyte infiltration in the joint tissue. Additionally, type I IFN receptor-blocking Abs, but not anti-IFN-gamma Abs, dramatically reduced arthritis, revealing a critical but previously unappreciated role for type I IFN in Lyme arthritis development. Certain examined IFN-inducible transcripts were also significantly diminished within joint tissue of mice treated with anti-IFNAR1, whereas expression of other IFN-responsive genes was more markedly altered by anti-IFN-gamma treatment. These data indicate that induction of the entire IFN profile is not necessary for arthritis development. These findings further tie early type I IFN induction to Lyme arthritis development, a connection not previously made. Bone marrow-derived macrophages readily induced IFN-responsive genes following B. burgdorferi stimulation, and this expression required a functional type I IFN receptor. Strikingly, induction of these genes was independent of TLRs 2,4, and 9 and of the adapter molecule MyD88. These data demonstrate that the extracellular pathogen B. burgdorferi uses a previously unidentified receptor and a pathway traditionally associated with viruses and intracellular bacteria to initiate transcription of type I IFN and IFN-responsive genes and to initiate arthritis development.

Evolutionary struggles between NK cells and viruses

Natural killer (NK) cells are well recognized for their ability to provide a first line of defence against viral pathogens and they are increasingly being implicated in immune responses against certain bacterial and parasitic infections. Reciprocally, viruses have devised numerous strategies to evade the activation of NK cells and have influenced the evolution of NK-cell receptors and their ligands. NK cells contribute to host defence by their ability to rapidly secrete cytokines and chemokines, as well as to directly kill infected host cells. In addition to their participation in the immediate innate immune response against infection, interactions between NK cells and dendritic cells shape the nature of the subsequent adaptive immune response to pathogens.

Lymphocyte subset numbers in cerebrospinal fluid: comparison of tick-borne encephalitis and neuroborreliosis

OBJECTIVE

The aim of this study was to analyze lymphocyte subset numbers in cerebrospinal fluid (CSF) from patients with tick-borne encephalitis (TBE) and acute neuroborreliosis.

METHODS

CSF lymphocyte subsets were enumerated in 42 TBE and nine neuroborreliosis patients using flow cytometry.

RESULTS

The CSF numbers of CD4+, CD8+, HLA-DR+ and total-T lymphocytes, B lymphocytes, and NK cells were all greater in neuroborreliosis patients than in TBE patients. Neuroborreliosis patients showed positive correlation of CSF protein levels with the numbers of CD4+, HLA-DR+ and total-T lymphocytes. Also, the numbers of CSF B lymphocytes correlated positively with intrathecal Borrelia burgdorferi-specific IgG antibodies. Conversely, TBE patients demonstrated intrathecal protein levels that correlated positively with all investigated CSF lymphocyte subsets.

CONCLUSION

These results suggest an intensive recruitment of lymphocyte subsets into the central nervous system (CNS) during acute neuroborreliosis, whereas TBE is characterized by a lower accumulation of lymphocyte subsets in the CSF.

Bone-marrow chimeras reveal hemopoietic and nonhemopoietic control of resistance to experimental Lyme arthritis

Both genetic resistance and susceptibility to development of experimental Lyme arthritis are mediated by the innate immune response. To determine whether this process is mainly controlled by hemopoietic or nonhemopoietic cells, we created bone marrow (BM) chimeric mice between arthritis-resistant DBA/2J (DBA) and arthritis-susceptible C3H/HeJ (C3H) mice and infected them with Borrelia burgdorferi. Both sets of BM chimeric mice, C3H donors into DBA recipients (C-->D) and DBA donors into C3H recipients (D-->C), as well as DBA sham chimeric mice (D-->D) were resistant to the development of experimental Lyme arthritis as measured by ankle swelling and arthritis severity scores. Only the C3H sham chimeric mice (C-->C) developed severe arthritis. These results indicate that independent and nonoverlapping mechanisms exist in hemopoietic and nonhemopoietic cellular compartments that can provide protection against arthritic pathology.

Experimental lyme arthritis in the absence of interleukin-4 or gamma interferon

Genetic resistance and susceptibility to experimental Lyme arthritis have been linked with the production of interleukin-4 (IL-4) or gamma interferon (IFN-gamma), respectively. To determine the absolute requirement for these cytokines in disease outcome, we compared arthritis development in wild-type, IL-4-deficient (IL-4 degrees ), and IFN-gamma-deficient (IFN-gamma degrees ) mice. While susceptible C3H mice developed swelling of ankle joints during the second week of infection, this swelling was exacerbated in C3H IFN-gamma degrees mice. Their arthritis severity scores at day 21, however, were similar. Resolution of arthritis was also similar between C3H and C3H IFN-gamma degrees mice. Arthritis-resistant DBA mice did not develop ankle swelling during the experimental period. There were no differences in ankle swelling or arthritis severity scores between control DBA mice and DBA IL-4 degrees mice at any of the time points tested. While the presence of spirochetes in various tissues was similar among all strains at day 21, DBA IL-4 degrees mice had a higher presence of spirochetes in blood, heart, and spleen than the DBA, C3H, and C3H IFN-gamma degrees mice did at day 60. DBA IL-4 degrees mice also had impaired ability to produce Borrelia-specific antibody responses, especially immunoglobulin G1. Thus, while IFN-gamma and IL-4 are not absolutely required for arthritis susceptibility or resistance, the production of IL-4 does appear to play an important role in Borrelia-specific antibody production and spirochete clearance.