c-Jun NH2-terminal kinase 2 inhibits gamma interferon production during Anaplasma phagocytophilum infection

Ceramide-1-phosphate is a regulator of Golgi structure and is co-opted by the obligate intracellular bacterial pathogen Anaplasma phagocytophilum

Many intracellular pathogens structurally disrupt the Golgi apparatus as an evolutionarily conserved promicrobial strategy. Yet, the host factors and signaling processes involved are often poorly understood, particularly for Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis. We found that A. phagocytophilum elevated cellular levels of the bioactive sphingolipid, ceramide-1-phosphate (C1P), to promote Golgi fragmentation that enables bacterial proliferation, conversion from its non-infectious to infectious form, and productive infection. A. phagocytophilum poorly infected mice deficient in ceramide kinase, the Golgi-localized enzyme responsible for C1P biosynthesis. C1P regulated Golgi morphology via activation of a PKCα/Cdc42/JNK signaling axis that culminates in phosphorylation of Golgi structural proteins, GRASP55 and GRASP65. siRNA-mediated depletion of Cdc42 blocked A. phagocytophilum from altering Golgi morphology, which impaired anterograde trafficking of trans-Golgi vesicles into and maturation of the pathogen-occupied vacuole. Cells overexpressing phosphorylation-resistant versions of GRASP55 and GRASP65 presented with suppressed C1P- and A. phagocytophilum-induced Golgi fragmentation and poorly supported infection by the bacterium. By studying A. phagocytophilum, we identify C1P as a regulator of Golgi structure and a host factor that is relevant to disease progression associated with Golgi fragmentation.IMPORTANCECeramide-1-phosphate (C1P), a bioactive sphingolipid that regulates diverse processes vital to mammalian physiology, is linked to disease states such as cancer, inflammation, and wound healing. By studying the obligate intracellular bacterium Anaplasma phagocytophilum, we discovered that C1P is a major regulator of Golgi morphology. A. phagocytophilum elevated C1P levels to induce signaling events that promote Golgi fragmentation and increase vesicular traffic into the pathogen-occupied vacuole that the bacterium parasitizes. As several intracellular microbial pathogens destabilize the Golgi to drive their infection cycles and changes in Golgi morphology is also linked to cancer and neurodegenerative disorder progression, this study identifies C1P as a potential broad-spectrum therapeutic target for infectious and non-infectious diseases.

Anaplasma phagocytophilum Induces TLR- and MyD88-Dependent Signaling in In Vitro Generated Murine Neutrophils

Anaplasma phagocytophilum is a tick-transmitted obligate intracellular Gram-negative bacterium that replicates in neutrophils. It elicits febrile disease in humans and in animals. In a mouse model, elimination of A. phagocytophilum required CD4⁺ T cells, but was independent of IFN-γ and other classical antibacterial effector mechanisms. Further, mice deficient for immune recognition and signaling via Toll-like receptor (TLR) 2, TLR4 or MyD88 were unimpaired in pathogen control. In contrast, animals lacking adaptor molecules of Nod-like receptors (NLR) such as RIP2 or ASC showed delayed clearance of A. phagocytophilum. In the present study, we investigated the contribution of further pattern recognition receptor (PRR) pathways to the control of A. phagocytophilum in vivo. Mice deficient for the NLR NOD2 had elevated bacterial loads in the early phase of infection, but were unimpaired in pathogen elimination. In contrast, animals lacking adaptor proteins of different C-type lectin receptors (CLR) such as DAP12, Fc-receptor γ-chain (FcRγ) and SYK controlled A. phagocytophilum as efficiently as wild-type mice. Further, we investigated which PRR pathways are involved in the sensing of A. phagocytophilum by in vitro generated Hoxb8 murine neutrophils. In vitro, recognition of A. phagocytophilum by murine neutrophils was dependent on TLR- and MyD88 signaling. However, it remained intact in the absence of the NLR NOD1, NOD2 and NALP3 and of the CLR adaptor molecules DAP12 and FcRγ. From these results, we conclude that TLR rather than NLR or CLR are critical for the detection of A. phagocytophilum by neutrophils although in vivo defective TLR-signaling is compensated probably because of the redundancy of the immune system.

JNK2 modulates the CD1d-dependent and -independent activation of iNKT cells

Invariant natural killer T (iNKT) cells play critical roles in autoimmune, anti-tumor, and anti-microbial immune responses, and are activated by glycolipids presented by the MHC class I-like molecule, CD1d. How the activation of signaling pathways impacts antigen (Ag)-dependent iNKT cell activation is not well-known. In the current study, we found that the MAPK JNK2 not only negatively regulates CD1d-mediated Ag presentation in APCs, but also contributes to CD1d-independent iNKT cell activation. A deficiency in the JNK2 (but not JNK1) isoform enhanced Ag presentation by CD1d. Using a vaccinia virus (VV) infection model known to cause a loss in iNKT cells in a CD1d-independent, but IL-12-dependent manner, we found the virus-induced loss of iNKT cells in JNK2 KO mice was substantially lower than that observed in JNK1 KO or wild-type (WT) mice. Importantly, compared to WT mice, JNK2 KO mouse iNKT cells were found to express less surface IL-12 receptors. As with a VV infection, an IL-12 injection also resulted in a smaller decrease in JNK2 KO iNKT cells as compared to WT mice. Overall, our work strongly suggests JNK2 is a negative regulator of CD1d-mediated Ag presentation and contributes to IL-12-induced iNKT cell activation and loss during viral infections.

Differential Susceptibility of Male Versus Female Laboratory Mice to Anaplasma phagocytophilum Infection

Human granulocytic anaplasmosis (HGA) is a debilitating, non-specific febrile illness caused by the granulocytotropic obligate intracellular bacterium called Anaplasma phagocytophilum. Surveillance studies indicate a higher prevalence of HGA in male versus female patients. Whether this discrepancy correlates with differential susceptibility of males and females to A. phagocytophilum infection is unknown. Laboratory mice have long been used to study granulocytic anaplasmosis. Yet, sex as a biological variable (SABV) in this model has not been evaluated. In this paper, groups of male and female C57Bl/6 mice that had been infected with A. phagocytophilum were assessed for the bacterial DNA load in the peripheral blood, the percentage of neutrophils harboring bacterial inclusions called morulae, and splenomegaly. Infected male mice exhibited as much as a 1.85-fold increase in the number of infected neutrophils, which is up to a 1.88-fold increase in the A. phagocytophilum DNA load, and a significant increase in spleen size when compared to infected female mice. The propensity of male mice to develop a higher level of A. phagocytophilum infection is relevant for studies utilizing the mouse model. This stresses the importance of including SABV and aligns with the observed higher incidence of infection in male versus female patients.

Interferon-γ-dependent control of Anaplasma phagocytophilum by murine neutrophil granulocytes

Background

Anaplasma phagocytophilum is a Gram-negative obligate intracellular bacterium that is transmitted by ticks of the Ixodes ricinus complex. It replicates in neutrophils and elicits febrile disease in humans and animals. Because of its striking tropism for neutrophils, A. phagocytophilum has been used as a model organism to study the immune response against obligate intracellular pathogens. In mice, the control of A. phagocytophilum in the early phase of infection is dependent on natural killer cell-derived interferon-γ (IFN-γ). In contrast, the final elimination strictly requires CD4⁺ T-cells. It is a matter of debate, whether neutrophils serve only as host cells or as killer cells as well.

Results

To study this, we used in vitro generated murine neutrophils with defects in major antimicrobial molecules such as NADPH-oxidase (gp91^phox−/−), myeloperoxidase (MPO^−/−) and inducible nitric oxide synthase (iNOS^−/−). However, bacterial growth in gene-deficient neutrophils was comparable to that in wild-type cells. Whereas gp91^phox and MPO expression remained unchanged, the infection led to an induction of iNOS. In neutrophils stimulated with IFN-γ, bacterial growth was significantly impaired, and iNOS was induced. However, the antibacterial effect of IFN-γ was still seen in iNOS^−/− neutrophils.

Conclusion

Thus, murine in vitro generated neutrophils stimulated with IFN-γ seem to act as killer cells by an iNOS-independent mechanism.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2274-6) contains supplementary material, which is available to authorized users.

Stat1 negatively regulates immune-mediated injury with Anaplasma phagocytophilum infection

Human granulocytic anaplasmosis (HGA) is caused by the obligate intracellular bacterium Anaplasma phagocytophilum. Our data previously demonstrated that A. phagocytophilum induces an immunopathologic response by activating IFN-γ production through the Stat1 signaling pathway. In this study, we investigated the broader role of Stat1 signaling in the host response to infection with A. phagocytophilum. In Stat1 knockout (KO) compared with wild-type mice, A. phagocytophilum infection was more highly pathogenic as characterized by the unanticipated development of clinical signs in mice including markedly increased splenomegaly, more severe inflammatory splenic and hepatic histopathology, >100-fold higher blood and splenic bacterial loads, and more elevated proinflammatory cytokine/chemokine responses in serum. CD4(+) and CD8(+) T lymphocyte populations were significantly expanded in spleens of A. phagocytophilum-infected Stat1 KO mice compared with wild-type mice. The leukocyte infiltrates in the livers and spleens of A. phagocytophilum-infected Stat1 KO mice also contained expansions in neutrophil and monocyte/macrophage populations. Importantly, A. phagocytophilum-infected Stat1 KO mice did not demonstrate induction of inducible NO synthase in splenocytes. These results show that Stat1 plays an important role in controlling bacterial loads but also by unexpectedly providing an undefined mechanism for dampening of the immunopathologic response observed with A. phagocytophilum infection.

Anaplasma phagocytophilum: deceptively simple or simply deceptive?

Anaplasma phagocytophilum is an obligate intracellular rickettsial pathogen transmitted by ixodid ticks. This bacterium colonizes myeloid and nonmyeloid cells and causes human granulocytic anaplasmosis--an important immunopathological vector-borne disease in the USA, Europe and Asia. Recent studies uncovered novel insights into the mechanisms of A. phagocytophilum pathogenesis and immunity. Here, we provide an overview of the underlying events by which the immune system responds to A. phagocytophilum infection, how this pathogen counteracts host immunity and the contribution of the tick vector for microbial transmission. We also discuss current scientific gaps in the knowledge of A. phagocytophilum biology for the purpose of exchanging research perspectives.

Anaplasma phagocytophilum dihydrolipoamide dehydrogenase 1 affects host-derived immunopathology during microbial colonization

Anaplasma phagocytophilum is a tick-borne rickettsial pathogen that provokes an acute inflammatory response during mammalian infection. The illness caused by A. phagocytophilum, human granulocytic anaplasmosis, occurs irrespective of pathogen load and results instead from host-derived immunopathology. Thus, characterizing A. phagocytophilum genes that affect the inflammatory process is critical for understanding disease etiology. By using an A. phagocytophilum Himar1 transposon mutant library, we showed that a single transposon insertion into the A. phagocytophilum dihydrolipoamide dehydrogenase 1 gene (lpda1 [APH_0065]) affects inflammation during infection. A. phagocytophilum lacking lpda1 revealed enlargement of the spleen, increased splenic extramedullary hematopoiesis, and altered clinicopathological abnormalities during mammalian colonization. Furthermore, LPDA1-derived immunopathology was independent of neutrophil infection and correlated with enhanced reactive oxygen species from NADPH oxidase and nuclear factor (NF)-κB signaling in macrophages. Taken together, these findings suggest the presence of different signaling pathways in neutrophils and macrophages during A. phagocytophilum invasion and highlight the importance of LPDA1 as an immunopathological molecule.

Receptor interacting protein-2 contributes to host defense against Anaplasma phagocytophilum infection

The Gram-negative obligate intracellular bacterium Anaplasma phagocytophilum is the causative agent of human granulocytic anaplasmosis (HGA), an emerging tick-borne infectious disease occurring worldwide. HGA is generally self-limiting; however, the underlying mechanisms, particularly the innate immune pathways that mediate the immune clearance of A. phagocytophilum, are less understood. We herein report an unexpected role for Receptor interacting protein-2 (Rip2), the adaptor protein for the Nod-like receptors (NLRs), Nod1/Nod2, in the host immune response against A. phagocytophilum infection. Although A. phagocytophilum genome is reported to lack the genes encoding the known ligands of Nod1 and Nod2, its infection upregulated the transcription of Rip2 in human primary neutrophils. Our results revealed that Rip2-deficient mice had significantly higher bacterial load than wild-type controls throughout the infection period. In addition, the Rip2-deficient mice took strikingly longer duration to clear A. phagocytophilum infection. Detailed analysis identified that interferon gamma (IFNγ) and interleukin (IL)-18 but not IL-12, macrophage inflammatory protein-2, and KC response were diminished in A. phagocytophilum-challenged Rip2-deficient mice. Together, these results revealed that Rip2 plays important roles in the immune control of A. phagocytophilum and may contribute to our understanding of the host response to Rickettsiales.

The biological basis of severe outcomes in Anaplasma phagocytophilum infection

Anaplasma phagocytophilum causes granulocytic anaplasmosis, an acute disease in humans that is also often subclinical. However, 36% are hospitalized, 7% need intensive care, and the case fatality rate is 0.6%. The biological basis for severe disease is not understood. Despite A. phagocytophilum's mechanisms to subvert neutrophil antimicrobial responses, whether these mechanisms lead to disease is unclear. In animals, inflammatory lesions track with IFNγ and IL-10 expression and infection of Ifng(-/-) mice leads to increased pathogen load but inhibition of inflammation. Suppression of STAT signaling in horses impacts IL-10 and IFN-γ expression, and also suppresses disease severity. Similar inhibition of inflammation with infection of NKT-deficient mice suggests that innate immune responses are key for disease. With severe disease, tissues can demonstrate hemophagocytosis, and measures of macrophage activation/hemophagocytic syndromes (MAS/HPS) support the concept of human granulocytic anaplasmosis as an immunopathologic disease. MAS/HPS are related to defective cytotoxic lymphocytes that ordinarily diminish inflammation. Pilot studies in mice show cytotoxic lymphocyte activation with A. phagocytophilum infection, yet suppression of cytotoxic responses from both NKT and CD8 cells, consistent with the development of MAS/HPS. Whether severity relates to microbial factors or genetically determined diversity in human immune and inflammatory response needs more investigation.

Mechanisms of obligatory intracellular infection with Anaplasma phagocytophilum

Anaplasma phagocytophilum persists in nature by cycling between mammals and ticks. Human infection by the bite of an infected tick leads to a potentially fatal emerging disease called human granulocytic anaplasmosis. A. phagocytophilum is an obligatory intracellular bacterium that replicates inside mammalian granulocytes and the salivary gland and midgut cells of ticks. A. phagocytophilum evolved the remarkable ability to hijack the regulatory system of host cells. A. phagocytophilum alters vesicular traffic to create an intracellular membrane-bound compartment that allows replication in seclusion from lysosomes. The bacterium downregulates or actively inhibits a number of innate immune responses of mammalian host cells, and it upregulates cellular cholesterol uptake to acquire cholesterol for survival. It also upregulates several genes critical for the infection of ticks, and it prolongs tick survival at freezing temperatures. Several host factors that exacerbate infection have been identified, including interleukin-8 (IL-8) and cholesterol. Host factors that overcome infection include IL-12 and gamma interferon (IFN-γ). Two bacterial type IV secretion effectors and several bacterial proteins that associate with inclusion membranes have been identified. An understanding of the molecular mechanisms underlying A. phagocytophilum infection will foster the development of creative ideas to prevent or treat this emerging tick-borne disease.

Type IV secretion in the obligatory intracellular bacterium Anaplasma phagocytophilum

Anaplasma phagocytophilum is an obligatory intracellular bacterium that infects neutrophils, the primary host defence cells. Consequent effects of infection on host cells result in a potentially fatal systemic disease called human granulocytic anaplasmosis. Despite ongoing reductive genome evolution and deletion of most genes for intermediary metabolism and amino acid biosynthesis, Anaplasma has also experienced expansion of genes encoding several components of the type IV secretion (T4S) apparatus. Two A. phagocytophilum T4S effector molecules are currently known; Anaplasma translocated substrate 1 (Ats-1) and ankyrin repeat domain-containing protein A (AnkA) have C-terminal positively charged amino acid residues that are recognized by the T4S coupling protein, VirD4. AnkA and Ats-1 contain eukaryotic protein motifs and are uniquely evolved in the family Anaplasmataceae; Ats-1 contains a mitochondria-targeting signal. They are abundantly produced and secreted into the host cytoplasm, are not toxic to host cells, and manipulate host cell processes to aid in the infection process. At the cellular level, the two effectors have distinct subcellular localization and signalling in host cells. Thus in this obligatory intracellular pathogen, the T4S system has evolved as a host-subversive survival factor.

Absence of mitogen-activated protein kinase family member c-Jun N-terminal kinase-2 enhances resistance to Toxoplasma gondii

The function of mitogen-activated protein kinase (MAPK) family member c-Jun N-terminal kinase (JNK)-2 in resistance and pathology during infection has not been greatly studied. Here, we employed Jnk2(-/-) mice to investigate the role of JNK2 in resistance and immunity during oral infection with the protozoan pathogen Toxoplasma gondii. We found increased host resistance in the absence of JNK2 as determined by lower parasite burden and increased host survival. Lack of JNK2 also correlated with decreased neutrophil recruitment to the intestinal mucosa and less pathology in the small intestine. In the absence of JNK2, IL-12 production was slightly but significantly increased in restimulated splenocyte populations as well as in purified splenic dendritic cell cultures. These results provide evidence that expression of JNK2 plays a role in T. gondii-induced immunopathology, at the same time in promoting susceptibility to this parasitic pathogen.

The elimination of Anaplasma phagocytophilum requires CD4+ T cells, but is independent of Th1 cytokines and a wide spectrum of effector mechanisms

Anaplasma phagocytophilum is a Gram-negative, obligate intracellular bacterium that exhibits a striking tropism for neutrophils. When we depleted mice of neutrophils, we found that murine susceptibility to anaplasmal infection was dependent on their presence. While serving as sites of bacterial replication, neutrophils do not seem to act as efficient killer cells in A. phagocytophilum infection, because mice deficient for antimicrobial effectors of neutrophils such as myeloperoxidase, granulocyte elastase, and cathepsin G were fully competent in pathogen elimination. To identify components of the immune system other than neutrophils that control A. phagocytophilum, we studied the course of infection in several gene-deficient mouse strains. IFN-gamma production by NK cells was important for initial defense, but not critical for pathogen elimination. In contrast, bacterial clearance was strictly dependent on CD4(+) T cells, but unexpectedly achieved in the absence of perforin, Fas/FasL and major Th1 cytokines such as IL-12, IFN-gamma, and MCP-1. These findings provide a novel paradigm for the control of an intracellular pathogen, which appears to be strikingly different from the CD4(+) T cell-, IL-12-, and IFN-gamma-dependent immunity to other intracellular bacteria.