Temporal Role for MyD88 in a Model of Brucella-Induced Arthritis and Musculoskeletal Inflammation

Brucella osteoarthritis: recent progress and future directions

Brucellosis is a common zoonosis, and Brucella osteoarthritis is the most common chronic complication of brucellosis. Development of brucellosis osteoarthritis involves multiple organs, tissues, and cells. Brucella grows and multiplies in intrinsic cells of the skeleton, including osteoblasts, osteocyte and osteoclasts, which results in sustained release of bacteria that leads to exacerbation of the immune response. Concurrently, activation of the immune system caused by invasion with Brucella may affect the dynamic balance of the skeleton. A variety of in vitro and in vivo models have been employed to study Brucella osteoarthritis, such as using bone marrow-derived macrophages to establish cell models and mice to develop animal models of Brucella osteoarthritis. However, limited studies on the molecular pathological mechanisms of Brucella osteoarthritis have been performed and inadequate animal models have been developed due to the challenging parameters of Brucella research. This paper reviews recent advances in the clinical features, molecular pathological mechanisms, and animal models of Brucella osteoarticular infections. This review underscores the complexity of the pathogenesis of Brucella osteoarticular infections and highlights inflammation as a contributing factor to bone loss caused by Brucella. Additionally, the significant proliferation of Brucella in skeletal resident cells also is an important factor leading to bone loss. A deeper understanding of the molecular pathological mechanism of Brucella osteoarthrosis and their animal models could provide robust support for the prevention and treatment of Brucella osteoarticular disease.

Metabolomic analysis of murine tissues infected with Brucella melitensis

Brucella is a gram negative, facultative intracellular bacterial pathogen that constitutes a substantial threat to human and animal health. Brucella can replicate in a variety of tissues and can induce immune responses that alter host metabolite availability. Here, mice were infected with B. melitensis and murine spleens, livers, and female reproductive tracts were analyzed by GC-MS to determine tissue-specific metabolic changes at one-, two- and four- weeks post infection. The most remarkable changes were observed at two-weeks post-infection when relative to uninfected tissues, 42 of 329 detected metabolites in reproductive tracts were significantly altered by Brucella infection, while in spleens and livers, 68/205 and 139/330 metabolites were significantly changed, respectively. Several of the altered metabolites in host tissues were linked to the GABA shunt and glutaminolysis. Treatment of macrophages with GABA did not alter control of B. melitensis infection, and deletion of the putative GABA transporter BMEI0265 did not alter B. melitensis virulence. While glutaminolysis inhibition did not affect control of B. melitensis in macrophages, glutaminolysis was required for macrophage IL-1β production in response to B. melitensis. In summary, these results indicate that Brucella infection alters host tissue metabolism and that these changes could have effects on inflammation and the outcome of infection.

Metabolomic Analysis of Murine Tissues Infected with Brucella melitensis

Brucella is a gram negative, facultative, intracellular bacterial pathogen that constitutes a substantial threat to human and animal health. Brucella can replicate in a variety of tissues and can induce immune responses that alter host metabolite availability. Here, mice were infected with B. melitensis and murine spleens, livers, and female reproductive tracts were analyzed by GC-MS to determine tissue-specific metabolic changes at one-, two- and four- weeks post infection. The most remarkable changes were observed at two-weeks post-infection when relative to uninfected tissues, 42 of 329 detected metabolites in reproductive tracts were significantly altered by Brucella infection, while in spleens and livers, 68/205 and 139/330 metabolites were significantly changed, respectively. Several of the altered metabolites in host tissues were linked to the GABA shunt and glutaminolysis. Treatment of macrophages with GABA did not alter control of B. melitensis infection, and deletion of the putative GABA transporter BMEI0265 did not alter B. melitensis virulence. While glutaminolysis inhibition did not affect control of B. melitensis in macrophages, glutaminolysis was required for macrophage IL-1β production in response to B. melitensis. In sum, these results indicate that Brucella infection alters host tissue metabolism and that these changes could have effects on inflammation and the outcome of infection.

Innate Lymphoid Cells and Interferons Limit Neurologic and Articular Complications of Brucellosis

Brucellosis is a globally significant zoonotic disease. Human patients with brucellosis develop recurrent fever and focal complications, including arthritis and neurobrucellosis. The current study investigated the role of innate lymphoid cells (ILCs) in the pathogenesis of focal brucellosis caused by Brucella melitensis. After footpad infection, natural killer cells and ILC1 cells both limited joint colonization by Brucella. Mice lacking natural killer cells, and in particular mice lacking all ILCs, also developed marked arthritis after footpad infection. Following pulmonary infection, mice lacking adaptive immune cells and ILCs developed arthritis, neurologic complications, and meningitis. Adaptive immune cells and ILCs both limited colonization of the brain by Brucella following pulmonary infection. Transcriptional analysis of Brucella-infected brains revealed marked up-regulation of genes associated with inflammation and interferon responses, as well as down-regulation of genes associated with neurologic function. Type II interferon deficiency resulted in colonization of the brain by Brucella, but mice lacking both type I and type II interferon signaling more rapidly developed clinical signs of neurobrucellosis, exhibited hippocampal neuronal loss, and had higher levels of Brucella in their brains than mice lacking type II interferon signaling alone. Collectively, these findings indicate ILCs and interferons play an important role in prevention of focal complications during Brucella infection, and that mice with deficiencies in ILCs or interferons can be used to study pathogenesis of neurobrucellosis.

MyD88-Dependent Glucose Restriction and Itaconate Production Control Brucella Infection

Brucellosis is one of the most common global zoonoses and is caused by facultative intracellular bacteria of the genus Brucella. Numerous studies have found that MyD88 signaling contributes to protection against Brucella, however the underlying mechanism has not been entirely defined. Here we show that MyD88 signaling in hematopoietic cells contributes both to inflammation and to control of Brucella melitensis infection in vivo. While the protective role of MyD88 in Brucella infection has often been attributed to promotion of IFN-γ production, we found that MyD88 signaling restricts host colonization by B. melitensis even in the absence of IFN-γ. In vitro, we show that MyD88 promotes macrophage glycolysis in response to B. melitensis. Interestingly, a B. melitensis mutant lacking the glucose transporter, GluP, was more highly attenuated in MyD88^-/- than in WT mice, suggesting MyD88 deficiency results in an increased availability of glucose in vivo which Brucella can exploit via GluP. Metabolite profiling of macrophages identified several metabolites regulated by MyD88 in response to B. melitensis, including itaconate. Subsequently, we found that itaconate has antibacterial effects against Brucella and also regulates the production of pro-inflammatory cytokines in B. melitensis-infected macrophages. Mice lacking the ability to produce itaconate were also more susceptible to B. melitensis in vivo. Collectively, our findings indicate that MyD88-dependent changes in host metabolism contribute to control of Brucella infection.

Brucella Outer Membrane Lipoproteins 19 and 16 Differentially Induce IL-18 Response or Pyroptosis in Human Monocytic Cells

BACKGROUND

Brucella species (B. spp.) are Gram-negative intracellular bacteria, causing severe inflammatory diseases in animals and humans. Two major lipoproteins (L19) and (L16) of Brucella outer membrane proteins (OMPs) were extensively explored in associating with inflammatory response of human monocytes (THP-1).

METHODS

Activated THP-1 cells induced with recombinant L19 and L16 were analyzed in comparison with unlipidated forms (U19 and U16) and lipopolysaccharide (LPS) of B. melitensis, respectively.

RESULTS

Secretion of inflammatory factors TNF-α, IL-6 and IL-1β was significantly increased from L19, L16 or both stimulated THP-1 cells. High secretion of IL-18 was detected only from L19-induced cells. Signaling of those cytokine responses was identified mainly through P38-MAPK pathway, and signaling of L19-induced IL-1β response was partly occurred via NF-κB. Exploration for different forms of IL-18 found that L19-induced production of active IL-18 (18 kD) was through up-regulating NLRP3 and activating caspase-1, while L16-induced production of inactive IL-18 fragments (15 kD and 16 kD) occurred through activating caspase-8/3. Additionally, L19 up-regulated phosphorylation of XIAP for inhibiting caspase-3 activity to cleave IL-18, while L16 activated caspase-3 for producing GSDME-N and leading to pyroptosis of THP-1 cells.

CONCLUSION

Brucella L19 and L16 differentially induce IL-18 response or pyroptosis in THP-1 cells, respectively.

B Cells Inhibit CD4+ T Cell-Mediated Immunity to Brucella Infection in a Major Histocompatibility Complex Class II-Dependent Manner

Brucella spp. are facultative intracellular bacteria notorious for their ability to induce a chronic, and often lifelong, infection known as brucellosis. To date, no licensed vaccine exists for prevention of human disease, and mechanisms underlying chronic illness and immune evasion remain elusive. We and others have observed that B cell-deficient mice challenged with Brucella display reduced bacterial burden following infection, but the underlying mechanism has not been clearly defined. Here, we show that at 1 month postinfection, B cell deficiency alone enhanced resistance to splenic infection ∼100-fold; however, combined B and T cell deficiency did not impact bacterial burden, indicating that B cells only enhance susceptibility to infection when T cells are present. Therefore, we investigated whether B cells inhibit T cell-mediated protection against Brucella Using B and T cell-deficient Rag1^-/- animals as recipients, we demonstrate that adoptive transfer of CD4⁺ T cells alone confers marked protection against Brucella melitensis that is abrogated by cotransfer of B cells. Interestingly, depletion of CD4⁺ T cells from B cell-deficient, but not wild-type, mice enhanced susceptibility to infection, further confirming that CD4⁺ T cell-mediated immunity against Brucella is inhibited by B cells. In addition, we found that the ability of B cells to suppress CD4⁺ T cell-mediated immunity and modulate CD4⁺ T cell effector responses during infection was major histocompatibility complex class II (MHCII)-dependent. Collectively, these findings indicate that B cells modulate CD4⁺ T cell function through an MHCII-dependent mechanism which enhances susceptibility to Brucella infection.

Interaction of Brucella abortus with Osteoclasts: a Step toward Understanding Osteoarticular Brucellosis and Vaccine Safety

Osteoarticular disease is a frequent complication of human brucellosis. Vaccination remains a critical component of brucellosis control, but there are currently no vaccines for use in humans, and no in vitro models for assessing the safety of candidate vaccines in reference to the development of bone lesions currently exist. While the effect of Brucella infection on osteoblasts has been extensively evaluated, little is known about the consequences of osteoclast infection. Murine bone marrow-derived macrophages were derived into mature osteoclasts and infected with B. abortus 2308, the vaccine strain S19, and attenuated mutants S19vjbR and B. abortus ΔvirB2 While B. abortus 2308 and S19 replicated inside mature osteoclasts, the attenuated mutants were progressively killed, behavior that mimics infection kinetics in macrophages. Interestingly, B. abortus 2308 impaired the growth of osteoclasts without reducing resorptive activity, while osteoclasts infected with B. abortus S19 and S19vjbR were significantly larger and exhibited enhanced resorption. None of the Brucella strains induced apoptosis or stimulated nitric oxide or lactose dehydrogenase production in mature osteoclasts. Finally, infection of macrophages or osteoclast precursors with B. abortus 2308 resulted in generation of smaller osteoclasts with decreased resorptive activity. Overall, Brucella exhibits similar growth characteristics in mature osteoclasts compared to the primary target cell, the macrophage, but is able to impair the maturation and alter the resorptive capacity of these cells. These results suggest that osteoclasts play an important role in osteoarticular brucellosis and could serve as a useful in vitro model for both analyzing host-pathogen interactions and assessing vaccine safety.

A genetics-led approach defines the drug target landscape of 30 immune-related traits

Most candidate drugs currently fail later-stage clinical trials, largely due to poor prediction of efficacy on early target selection1. Drug targets with genetic support are more likely to be therapeutically valid2,3, but the translational use of genome-scale data such as from genome-wide association studies for drug target discovery in complex diseases remains challenging4-6. Here, we show that integration of functional genomic and immune-related annotations, together with knowledge of network connectivity, maximizes the informativeness of genetics for target validation, defining the target prioritization landscape for 30 immune traits at the gene and pathway level. We demonstrate how our genetics-led drug target prioritization approach (the priority index) successfully identifies current therapeutics, predicts activity in high-throughput cellular screens (including L1000, CRISPR, mutagenesis and patient-derived cell assays), enables prioritization of under-explored targets and allows for determination of target-level trait relationships. The priority index is an open-access, scalable system accelerating early-stage drug target selection for immune-mediated disease.

IFN-γ-dependent nitric oxide suppresses Brucella-induced arthritis by inhibition of inflammasome activation

Brucellosis, caused by the intracellular bacterial pathogen Brucella, is a globally important zoonotic disease for which arthritis is the most common focal complication in humans. Wild-type mice infected systemically with Brucella typically do not exhibit arthritis, but mice lacking IFN-γ develop arthritis regardless of the route of Brucella infection. Here, we investigated mechanisms by which IFN-γ suppresses Brucella-induced arthritis. Several cell types, including innate lymphoid cells, contributed to IFN-γ production and suppression of joint swelling. IFN-γ deficiency resulted in elevated joint IL-1β levels, and severe joint inflammation that was entirely inflammasome dependent, and in particular, reliant on the NLRP3 inflammasome. IFN-γ was vital for induction of the nitric oxide producing enzyme, iNOS, in infected joints, and nitric oxide directly inhibited IL-1β production and inflammasome activation in Brucella-infected macrophages in vitro. During in vivo infection, iNOS deficiency resulted in an increase in IL-1β and inflammation in Brucella-infected joints. Collectively, this data indicate that IFN-γ prevents arthritis both by limiting Brucella infection, and by inhibiting excessive inflammasome activation through the induction of nitric oxide.

Caspase-1 and Caspase-11 Mediate Pyroptosis, Inflammation, and Control of Brucella Joint Infection

Brucellosis, caused by the intracellular bacterial pathogen Brucella, is a zoonotic disease for which arthritis is the most common focal complication in humans. Here we investigated the role of inflammasomes and their effectors, including interleukin-1 (IL-1), IL-18, and pyroptosis, on inflammation and control of infection during Brucella-induced arthritis. Early in infection, both caspase-1 and caspase-11 were found to initiate joint inflammation and proinflammatory cytokine production. However, by 1 week postinfection, caspase-1 and caspase-11 also contributed to control of Brucella joint infection. Inflammasome-dependent restriction of Brucella joint burdens did not require AIM2 (absent in melanoma 2) or NLRP3 (NLR family, pyrin domain containing 3). IL-1R had a modest effect on Brucella-induced joint swelling, but mice lacking IL-1R were not impaired in their ability to control infection of the joint by Brucella In contrast, IL-18 contributed to the initiation of joint swelling and control of joint Brucella infection. Caspase1/11-dependent cell death was observed in vivo, and in vitro studies demonstrated that both caspase-1 and caspase-11 induce pyroptosis, which limited Brucella infection in macrophages. Brucella lipopolysaccharide alone was also able to induce caspase-11-dependent pyroptosis. Collectively, these data demonstrate that inflammasomes induce inflammation in an IL-18-dependent manner and that inflammasome-dependent IL-18 and pyroptosis restrict Brucella infection.

Alternative strategies for vaccination to brucellosis

Brucellosis remains burdensome for livestock and humans worldwide. Better vaccines for protection are needed to reduce disease incidence. Immunity to brucellosis and barriers to protection are discussed. The benefits and limitations of conventional and experimental brucellosis vaccines are outlined, and novel vaccination strategies needed to ultimately protect against brucellosis are introduced.

Hematopoietic MyD88 and IL-18 are essential for IFN-γ-dependent restriction of type A Francisella tularensis infection

Francisella tularensis is a highly infectious intracellular bacterium that causes the potentially fatal disease tularemia. We used mice with conditional MyD88 deficiencies to investigate cellular and molecular mechanisms by which MyD88 restricts type A F. tularensis infection. F. tularensis-induced weight loss was predominately dependent on MyD88 signaling in nonhematopoietic cells. In contrast, MyD88 signaling in hematopoietic cells, but not in myeloid and dendritic cells, was essential for control of F. tularensis infection in tissue. Myeloid and dendritic cell MyD88 deficiency also did not markedly impair cytokine production during infection. Although the production of IL-12 or -18 was not significantly reduced in hematopoietic MyD88-deficient mice, IFN-γ production was abolished in these animals. In addition, neutralization studies revealed that control of F. tularensis infection mediated by hematopoietic MyD88 was entirely dependent on IFN-γ. Although IL-18 production was not significantly affected by MyD88 deficiency, IL-18 was essential for IFN-γ production and restricted bacterial replication in an IFN-γ-dependent manner. Caspase-1 was also found to be partially necessary for the production of IL-18 and IFN-γ and for control of F. tularensis replication. Our collective data show that the response of leukocytes to caspase-1-dependent IL-18 via MyD88 is critical, whereas MyD88 signaling in myeloid and dendritic cells is dispensable for IFN-γ-dependent control of type A F. tularensis infection.