Mechanisms of evasion of neutrophil killing by Anaplasma phagocytophilum

Innate immunity in rickettsial infections

Rickettsial agents are a diverse group of alpha-proteobacteria within the order Rickettsiales, which possesses two families with human pathogens, Rickettsiaceae and Anaplasmataceae. These obligate intracellular bacteria are most frequently transmitted by arthropod vectors, a first step in the pathogens' avoidance of host cell defenses. Considerable study of the immune responses to infection and those that result in protective immunity have been conducted. Less study has focused on the initial events and mechanism by which these bacteria avoid the innate immune responses of the hosts to survive within and propagate from host cells. By evaluating the major mechanisms of evading innate immunity, a range of similarities among these bacteria become apparent, including mechanisms to escape initial destruction in phagolysosomes of professional phagocytes, those that dampen the responses of innate immune cells or subvert signaling and recognition pathways related to apoptosis, autophagy, proinflammatory responses, and mechanisms by which these microbes attach to and enter cells or those molecules that trigger the host responses. To illustrate these principles, this review will focus on two common rickettsial agents that occur globally, Rickettsia species and Anaplasma phagocytophilum.

Microbes and the fate of neutrophils

Neutrophils or polymorphonuclear neutrophils (PMNs) are an important component of innate host defense. These phagocytic leukocytes are recruited to infected tissues and kill invading microbes. There are several general characteristics of neutrophils that make them highly effective as antimicrobial cells. First, there is tremendous daily production and turnover of granulocytes in healthy adults-typically 1011 per day. The vast majority (~95%) of these cells are neutrophils. In addition, neutrophils are mobilized rapidly in response to chemotactic factors and are among the first leukocytes recruited to infected tissues. Most notably, neutrophils contain and/or produce an abundance of antimicrobial molecules. Many of these antimicrobial molecules are toxic to host cells and can destroy host tissues. Thus, neutrophil activation and turnover are highly regulated processes. To that end, aged neutrophils undergo apoptosis constitutively, a process that contains antimicrobial function and proinflammatory capacity. Importantly, apoptosis facilitates nonphlogistic turnover of neutrophils and removal by macrophages. This homeostatic process is altered by interaction with microbes and their products, as well as host proinflammatory molecules. Microbial pathogens can delay neutrophil apoptosis, accelerate apoptosis following phagocytosis, or cause neutrophil cytolysis. Here, we review these processes and provide perspective on recent studies that have potential to impact this paradigm.

Epidemiological and Clinicopathological Features of Anaplasma phagocytophilum Infection in Dogs: A Systematic Review

Anaplasma phagocytophilum is a worldwide emerging zoonotic tick-borne pathogen transmitted by Ixodid ticks and naturally maintained in complex and incompletely assessed enzootic cycles. Several studies have demonstrated an extensive genetic variability with variable host tropisms and pathogenicity. However, the relationship between genetic diversity and modified pathogenicity is not yet understood. Because of their proximity to humans, dogs are potential sentinels for the transmission of vector-borne pathogens. Furthermore, the strong molecular similarity between human and canine isolates of A. phagocytophilum in Europe and the USA and the positive association in the distribution of human and canine cases in the USA emphasizes the epidemiological role of dogs. Anaplasma phagocytophilum infects and survives within neutrophils by disregulating neutrophil functions and evading specific immune responses. Moreover, the complex interaction between the bacterium and the infected host immune system contribute to induce inflammatory injuries. Canine granulocytic anaplasmosis is an acute febrile illness characterized by lethargy, inappetence, weight loss and musculoskeletal pain. Hematological and biochemistry profile modifications associated with this disease are unspecific and include thrombocytopenia, anemia, morulae within neutrophils and increased liver enzymes activity. Coinfections with other tick-borne pathogens (TBPs) may occur, especially with Borrelia burgdorferi, complicating the clinical presentation, diagnosis and response to treatment. Although clinical studies have been published in dogs, it remains unclear if several clinical signs and clinicopathological abnormalities can be related to this infection.

Anaplasma phagocytophilum AptA enhances the UPS, autophagy, and anti-apoptosis of host cells by PSMG3

Anaplasma phagocytophilum is an obligate intracellular bacterium and a common tick-borne infectious pathogen that can cause human granulocytic anaplasmosis (HGA). Effector proteins play an important role in the pathogenic mechanism of A. phagocytophilum, but the specifics of the disease mechanism are unclear. We studied the effector protein AptA (A. phagocytophilum toxin A) using yeast two hybrid assays to screen its interacting protein proteasome assembly chaperone 3 (PSMG3, PAC3), and identified new mechanisms for the pathogenicity of A. phagocytophilum in HEK293T cells. After AptA enters the host cell, it interacts with PSMG3 to enhance the activity of the proteasome, causing ubiquitination and autophagy in the host cell and thereby increasing cross-talk between the ubiquitination-proteasome system (UPS) and autophagy. AptA also reduces the apoptotic efficiency of the host cells. These results offer new clues as to the pathogenic mechanism of A. phagocytophilum and support the hypothesis that AptA interacts with host PSMG3.

Disruption of VirB6 Paralogs in Anaplasma phagocytophilum Attenuates Its Growth

Knowledge of the T4SS is derived from model systems, such as Agrobacterium tumefaciens. The structure of the T4SS in Rickettsiales differs from the classical arrangement. These differences include missing and duplicated components with structural alterations. Particularly, two sequenced virB6-4 genes encode unusual C-terminal structural extensions resulting in proteins of 4,322 (GenBank accession number AGR79286.1) and 9,935 (GenBank accession number ANC34101.1) amino acids. To understand how the T4SS is used in A. phagocytophilum, we describe the expression of the virB6 paralogs and explore their role as the bacteria replicate within its host cell. Conclusions about the importance of these paralogs for colonization of human and tick cells are supported by the deficient phenotype of an A. phagocytophilum mutant isolated from a sequence-defined transposon insertion library.

Many pathogenic bacteria translocate virulence factors into their eukaryotic hosts by means of type IV secretion systems (T4SS) spanning the inner and outer membranes. Genes encoding components of these systems have been identified within the order Rickettsiales based upon their sequence similarities to other prototypical systems. Anaplasma phagocytophilum strains are obligate intracellular, tick-borne bacteria that are members of this order. The organization of these components at the genomic level was determined in several Anaplasma phagocytophilum strains, showing overall conservation, with the exceptions of the virB2 and virB6 genes. The virB6 loci are characterized by the presence of four virB6 copies (virB6-1 through virB6-4) arranged in tandem within a gene cluster known as the sodB-virB operon. Interestingly, the virB6-4 gene varies significantly in length among different strains due to extensive tandem repeats at the 3′ end. To gain an understanding of how these enigmatic virB6 genes function in A. phagocytophilum, we investigated their expression in infected human and tick cells. Our results show that these genes are expressed by A. phagocytophilum replicating in both cell types and that VirB6-3 and VirB6-4 proteins are surface exposed. Analysis of an A. phagocytophilum mutant carrying the Himar1 transposon within the virB6-4 gene demonstrated that the insertion not only disrupted its expression but also exerted a polar effect on the sodB-virB operon. Moreover, the altered expression of genes within this operon was associated with the attenuated in vitro growth of A. phagocytophilum in human and tick cells, indicating the importance of these genes in the physiology of this obligate intracellular bacterium in such different environments.

IMPORTANCE Knowledge of the T4SS is derived from model systems, such as Agrobacterium tumefaciens. The structure of the T4SS in Rickettsiales differs from the classical arrangement. These differences include missing and duplicated components with structural alterations. Particularly, two sequenced virB6-4 genes encode unusual C-terminal structural extensions resulting in proteins of 4,322 (GenBank accession number AGR79286.1) and 9,935 (GenBank accession number ANC34101.1) amino acids. To understand how the T4SS is used in A. phagocytophilum, we describe the expression of the virB6 paralogs and explore their role as the bacteria replicate within its host cell. Conclusions about the importance of these paralogs for colonization of human and tick cells are supported by the deficient phenotype of an A. phagocytophilum mutant isolated from a sequence-defined transposon insertion library.

Repression of tick microRNA-133 induces organic anion transporting polypeptide expression critical for Anaplasma phagocytophilum survival in the vector and transmission to the vertebrate host

The microRNAs (miRNAs) are important regulators of gene expression. In this study, we provide evidence for the first time to show that rickettsial pathogen Anaplasma phagocytophilum infection results in the down-regulation of tick microRNA-133 (miR-133), to induce Ixodes scapularis organic anion transporting polypeptide (isoatp4056) gene expression critical for this bacterial survival in the vector and for its transmission to the vertebrate host. Transfection studies with recombinant constructs containing transcriptional fusions confirmed binding of miR-133 to isoatp4056 mRNA. Treatment with miR-133 inhibitor resulted in increased bacterial burden and isoatp4056 expression in ticks and tick cells. In contrast, treatment with miR-133 mimic or pre-mir-133 resulted in dramatic reduction in isoatp4056 expression and bacterial burden in ticks and tick cells. Moreover, treatment of ticks with pre-mir-133 affected vector-mediated A. phagocytophilum infection of murine host. These results provide novel insights to understand impact of modulation of tick miRNAs on pathogen colonization in the vector and their transmission to infect the vertebrate host.

This study provides novel evidence that shows that down-regulation of arthropod microRNA-133 leading to enhanced expression of organic anion transporting polypeptide is not only critical for rickettsial pathogen Anaplasma phagocytophilum survival in ticks but also for this bacterial transmission from vector to the vertebrate host. Understanding how pathogens manipulate vector-signaling repertoire for their benefit would lead to the development of strategies to block their transmission from vector to the vertebrate host.

What Is the Evolutionary Fingerprint in Neutrophil Granulocytes?

Over the years of evolution, thousands of different animal species have evolved. All these species require an immune system to defend themselves against invading pathogens. Nevertheless, the immune systems of different species are obviously counteracting against the same pathogen with different efficiency. Therefore, the question arises if the process that was leading to the clades of vertebrates in the animal kingdom-namely mammals, birds, amphibians, reptiles, and fish-was also leading to different functions of immune cells. One cell type of the innate immune system that is transmigrating as first line of defense in infected tissue and counteracts against pathogens is the neutrophil granulocyte. During the host-pathogen interaction they can undergo phagocytosis, apoptosis, degranulation, and form neutrophil extracellular traps (NETs). In this review, we summarize a wide spectrum of information about neutrophils in humans and animals, with a focus on vertebrates. Special attention is kept on the development, morphology, composition, and functions of these cells, but also on dysfunctions and options for cell culture or storage.

Anaplasma phagocytophilum infection in cats: A literature review to raise clinical awareness

PRACTICAL RELEVANCE

Granulocytic anaplasmosis is a disease in humans and animals caused by the Gram-negative bacterium Anaplasma phagocytophilum within the family Anaplasmataceae. The pathogen is transmitted by ticks of the Ixodes species. Infections with A phagocytophilum have often been described in dogs but reports on natural infections in cats are rare. An infection with A phagocytophilum should be considered as a differential diagnosis in cats if the history reveals tick infestation and/or outdoor access in combination with the relevant clinical signs.

GLOBAL IMPORTANCE

A phagocytophilum is also important in human medicine because of its zoonotic potential. Due to the risk of vector-borne infections for both feline and public health, cats should be protected with ectoparasiticides, especially in endemic areas.

AIM

The aim of this review is to give an overview of the published data and summarise the epidemiology, pathogenesis, diagnosis, clinical signs and therapy of feline granulocytic anaplasmosis. As clinical signs are vague and non-specific, this review aims to raise awareness of A phagocytophilum infection, both among clinicians, so that they consider testing potentially exposed cats, and scientists, in order to prompt further research.

EVIDENCE BASE

Sixteen publications describing 55 cats have been reviewed. Thirty-four cats were well diagnosed based on guidelines of the European Advisory Board on Cat Diseases and blood analyses were performed to varying extents for these cats. Because of the limited number of studies and a lack of knowledge in cats, clinical signs and blood analyses are compared with available data in dogs.

Polymicrobial Nature of Tick-Borne Diseases

Tick-borne diseases have doubled in the last 12 years, and their geographic distribution has spread as well. The clinical spectrum of tick-borne diseases can range from asymptomatic to fatal infections, with a disproportionate incidence in children and the elderly. In the last few years, new agents have been discovered, and genetic changes have helped in the spread of pathogens and ticks. Polymicrobial infections, mostly in Ixodes scapularis, can complicate diagnostics and augment disease severity. Amblyomma americanum ticks have expanded their range, resulting in a dynamic and complex situation, possibly fueled by climate change. To document these changes, using molecular biology strategies for pathogen detection, an assessment of 12 microbes (9 pathogens and 3 symbionts) in three species of ticks was done in Suffolk County, New York. At least one agent was detected in 63% of I. scapularis ticksBorrelia burgdorferi was the most prevalent pathogen (57% in adults; 27% in nymphs), followed by Babesia microti (14% in adults; 15% in nymphs), Anaplasma phagocytophilum (14% in adults; 2% in nymphs), Borrelia miyamotoi (3% in adults), and Powassan virus (2% in adults). Polymicrobial infections were detected in 22% of I. scapularis ticks, with coinfections of B. burgdorferi and B. microti (9%) and of B. burgdorferi and A. phagocytophilum (7%). Three Ehrlichia species were detected in 4% of A. americanum ticks. The rickettsiae constituted the largest prokaryotic biomass of all the ticks tested and included Rickettsia amblyommatis, Rickettsia buchneri, and Rickettsia montanensis The high rates of polymicrobial infection in ticks present an opportunity to study the biological interrelationships of pathogens and their vectors.IMPORTANCE Tick-borne diseases have increased in prevalence in the United States and abroad. The reasons for these increases are multifactorial, but climate change is likely to be a major factor. One of the main features of the increase is the geographic expansion of tick vectors, notably Amblyomma americanum, which has brought new pathogens to new areas. The clinical spectrum of tick-borne diseases can range from asymptomatic to fatal infections, with a disproportionate incidence in children and the elderly. In addition, new pathogens that are cotransmitted by Ixodes scapularis have been discovered and have led to difficult diagnoses and to disease severity. Of these, Borrelia burgdorferi, the agent of Lyme disease, continues to be the most frequently transmitted pathogen. However, Babesia microti, Borrelia miyamotoi (another spirochete), Anaplasma phagocytophilum, and Powassan virus are frequent cotransmitted agents. Polymicrobial infection has important consequences for the diagnosis and management of tick-borne diseases.

Ixodes scapularis Src tyrosine kinase facilitates Anaplasma phagocytophilum survival in its arthropod vector

Anaplasma phagocytophilum, the agent of human anaplasmosis, is an obligate intracellular bacterium that uses multiple survival strategies to persist in Ixodes scapularis ticks. Our previous study showed that A. phagocytophilum efficiently induced the tyrosine phosphorylation of several Ixodes proteins that includes extended phosphorylation of actin at tyrosine residue Y178. In order to identify the tyrosine kinase responsible for the A. phagocytophilum induced tyrosine phosphorylation of proteins, we combed the I. scapularis genome and identified a non-receptor Src tyrosine kinase ortholog. I. scapularis Src kinase showed high degree of amino acid sequence conservation with Dsrc from Drosophila melanogaster. We noted that at different developmental stages of I. scapularis ticks, larvae expressed significantly higher levels of src transcripts in comparison to the other stages. We found that A. phagocytophilum significantly reduced Src levels in unfed nymphs and in nymphs while blood feeding (48 h during feeding) in comparison to the levels noted to relative uninfected controls. However, A. phagocytophilum increased Src levels in fully engorged larvae and nymphs (48 h post feeding) and in vitro tick cells in comparison to the relative uninfected controls. Inhibition of Src kinase expression and activity by treatment with src-dsRNA or Src-inhibitor, respectively, significantly reduced A. phagocytophilum loads in ticks and tick cells. Overall, our study provides evidence for the important role of I. scapularis Src kinase in facilitating A. phagocytophilum colonization and survival in the arthropod vector.

Anaplasma phagocytophilum-Related Defects in CD8, NKT, and NK Lymphocyte Cytotoxicity

Human granulocytic anaplasmosis, caused by the tick-transmitted Anaplasma phagocytophilum, is not controlled by innate immunity, and induces a proinflammatory disease state with innate immune cell activation. In A. phagocytophilum murine infection models, hepatic injury occurs with production of IFNγ thought to be derived from NK, NKT cells, and CD8 T lymphocytes. Specific A. phagocytophilum ligands that drive inflammation and disease are not known, but suggest a clinical and pathophysiologic basis strikingly like macrophage activation syndrome (MAS) and hemophagocytic syndrome (HPS). We studied in vivo responses of NK, NKT, and CD8 T lymphocytes from infected animals for correlates of lymphocyte-mediated cytotoxicity and examined in vitro interactions with A. phagocytophilum-loaded antigen-presenting cells (APCs). Murine splenocytes were examined and found deficient in cytotoxicity as determined by CD107a expression in vitro for specific CTL effector subsets as determined by flow cytometry. Moreover, A. phagocytophilum-loaded APCs did not lead to IFNγ production among CTLs in vitro. These findings support the concept of impaired cytotoxicity with A. phagocytophilum presentation by APCs that express MHC class I and that interact with innate and adaptive immune cells with or after infection. The findings strengthen the concept of an enhanced proinflammatory phenotype, such as MAS and HPS disease states as the basis of disease and severity with A. phagocytophilum infection, and perhaps by other obligate intracellular bacteria.

Alternative Splicing of Differentiated Myeloid Cell Transcripts after Infection by Anaplasma phagocytophilum Impacts a Selective Group of Cellular Programs

Eukaryotic proteome diversity exceeds that encoded within individual genes, and results in part from alternative splicing events of pre-messenger RNA. The diversity of these splicing events can shape the outcome in development and differentiation of normal tissues, and is important in pathogenic circumstances such as cancer and some heritable conditions. A role for alternative splicing of eukaryotic genes in response to viral and intracellular bacterial infections has only recently been recognized, and plays an important role in providing fitness for microbial survival, while potentially enhancing pathogenicity. Anaplasma phagocytophilum survives within mammalian neutrophils by reshaping transcriptional programs that govern cellular functions. We applied next generation RNAseq to ATRA-differentiated HL-60 cells established to possess transcriptional and functional responses similar to A. phagocytophilum-infected human neutrophils. This demonstrated an increase in transcripts with infection and high proportion of alternatively spliced transcript events (ASEs) for which predicted gene ontology processes were in part distinct from those identified by evaluation of single transcripts or gene-level analyses alone. The alternative isoforms are not on average shorter, and no alternative splicing in genes encoding spliceosome components is noted. Although not evident at gene-level analyses, individual spliceosome transcripts that impact nearly all spliceosome components were significantly upregulated. How the distinct GO processes predicted by ASEs are regulated by infection and whether they are relevant to fitness or pathogenicity of A. phagocytophilum should be addressed in more detailed studies.

Anaplasma phagocytophilum infection modulates expression of megakaryocyte cell cycle genes through phosphatidylinositol-3-kinase signaling

Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis infects neutrophils and other cells from hematopoietic origin. Using human megakaryocytic cell line, MEG-01, we show that expression of cell cycle genes in these cells are altered upon A. phagocytophilum infection. Expression of several cell cycle genes in MEG-01 cells was significantly up regulated at early and then down regulated at later stages of A. phagocytophilum infection. Lactate dehydrogenase (LDH) assays revealed reduced cellular cytotoxicity in MEG-01 cells upon A. phagocytophilum infection. The levels of both PI3KCA (p110 alpha, catalytic subunit) and PI3KR1 (p85, regulatory subunit) of Class I PI3 kinases and phosphorylated protein kinase B (Akt/PKB) and inhibitory kappa B (IκB) were elevated at both early and late stages of A. phagocytophilum infection. Inhibition of PI3 kinases with LY294002 treatment resulted in significant reduction in the expression of tested cell cycle genes, A. phagocytophilum burden and phosphorylated Akt levels in these MEG-01 cells. Collectively, these results suggest a role for PI3K-Akt-NF-κB signaling pathway in the modulation of megakaryocyte cell cycle genes upon A. phagocytophilum infection.

Genome-Wide Anaplasma phagocytophilum AnkA-DNA Interactions Are Enriched in Intergenic Regions and Gene Promoters and Correlate with Infection-Induced Differential Gene Expression

Anaplasma phagocytophilum, an obligate intracellular prokaryote, infects neutrophils, and alters cardinal functions via reprogrammed transcription. Large contiguous regions of neutrophil chromosomes are differentially expressed during infection. Secreted A. phagocytophilum effector AnkA transits into the neutrophil or granulocyte nucleus to complex with DNA in heterochromatin across all chromosomes. AnkA binds to gene promoters to dampen cis-transcription and also has features of matrix attachment region (MAR)-binding proteins that regulate three-dimensional chromatin architecture and coordinate transcriptional programs encoded in topologically-associated chromatin domains. We hypothesize that identification of additional AnkA binding sites will better delineate how A. phagocytophilum infection results in reprogramming of the neutrophil genome. Using AnkA-binding ChIP-seq, we showed that AnkA binds broadly throughout all chromosomes in a reproducible pattern, especially at: (i) intergenic regions predicted to be MARs; (ii) within predicted lamina-associated domains; and (iii) at promoters ≤ 3000 bp upstream of transcriptional start sites. These findings provide genome-wide support for AnkA as a regulator of cis-gene transcription. Moreover, the dominant mark of AnkA in distal intergenic regions known to be AT-enriched, coupled with frequent enrichment in the nuclear lamina, provides strong support for its role as a MAR-binding protein and genome “re-organizer.” AnkA must be considered a prime candidate to promote neutrophil reprogramming and subsequent functional changes that belie improved microbial fitness and pathogenicity.

Characterization of neutrophils and macrophages from ex vivo-cultured murine bone marrow for morphologic maturation and functional responses by imaging flow cytometry

Neutrophils and macrophages differentiate from common myeloid progenitors in the bone marrow, where they undergo nuclear morphologic changes during maturation. During this process, both cell types acquire critical innate immune functions that include phagocytosis of pathogens, and for neutrophils the release of nuclear material called nuclear extracellular traps (NETs). Primary cells used to study these functions are typically purified from mature mouse tissues, but bone marrow-derived ex vivo cultures provide more abundant numbers of progenitors and functionally mature cells. Routine analyses of these cells use conventional microscopy and flow cytometry, which present limitations; microscopy is laborious and subjective, whereas flow cytometry lacks spatial resolution. Here we describe methods to generate enriched populations of neutrophils or macrophages from cryopreserved mouse bone marrow cultured ex vivo, and to use imaging flow cytometry that combines the resolution of microscopy with flow cytometry to analyze cells for morphologic features, phagocytosis, and NETosis.

Global DNA methylation changes and differential gene expression in Anaplasma phagocytophilum-infected human neutrophils

Background

Anaplasma phagocytophilum is an obligate intracellular prokaryotic pathogen that both infects and replicates within human neutrophils. The bacterium represses multiple antimicrobial functions while simultaneously increasing proinflammatory functions by reprogramming the neutrophil genome. Previous reports show that many observed phenotypic changes are in part explained by altered gene transcription. We recently identified that large chromosomal regions of the neutrophil genome are differentially expressed during A. phagocytophilum infection. Because of this, we sought to determine whether gene expression programs altered by infection were the result of changes in the host neutrophil DNA methylome.

Results

Within 24 h of infection, marked increases in DNA methylation were observed genome-wide as compared with mock-infected controls and pharmacologic inhibition of DNA methyltransferases resulted in decreased bacterial growth. New regions of DNA methylation were enriched at intron and exon junctions; however, intragenic methylation did not correlate with altered gene expression. In contrast, intergenic DNA methylation was associated with A. phagocytophilum-induced gene expression changes. Within the major histocompatibility complex locus on chromosome 6, a region with marked changes in infection-induced differential gene expression, new regions of methylation were localized to boundaries of active and inactive chromatin.

Conclusions

These data strongly suggest that A. phagocytophilum infection, in addition to altering histone structure, alters DNA methylation and the epigenome of its host cell to promote survival and replication, providing evidence that such bacterial infection can radically alter the epigenome of its host cell.

Electronic supplementary material

The online version of this article (doi:10.1186/s13148-015-0105-1) contains supplementary material, which is available to authorized users.

Pseudo-Pelger-Huët anomaly and granulocytic dysplasia associated with human granulocytic anaplasmosis

Pseudo-Pelger-Huët anomaly (PHA) refers to mono- or bi-lobed granulocytes, reportedly observed in patients with severe infections and inflammation or hematological malignancies including myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML). Dysplastic changes in granulocytes are typical manifestations in MDS and granulocytic leukemias. Here, we report the unique case of a patient found to have human granulocytic anaplasmosis (HGA), a tick-borne disease caused by Anaplasma phagocytophilum, a Gram-negative coccobacillus. This patient showed striking hematological manifestations including a large number of pseudo-PHA, a severe degree of left shift, and dysplastic granulocytes. These hematological presentations on the peripheral smear all resolved with doxycycline treatment, implying that the changes were most likely reactive manifestations secondary to HGA, rather than underlying hematological malignancies such as MDS or AML.

Bioinformatic and mass spectrometry identification of Anaplasma phagocytophilum proteins translocated into host cell nuclei

Obligate intracellular bacteria have an arsenal of proteins that alter host cells to establish and maintain a hospitable environment for replication. Anaplasma phagocytophilum secrets Ankyrin A (AnkA), via a type IV secretion system, which translocates to the nucleus of its host cell, human neutrophils. A. phagocytophilum-infected neutrophils have dramatically altered phenotypes in part explained by AnkA-induced transcriptional alterations. However, it is unlikely that AnkA is the sole effector to account for infection-induced transcriptional changes. We developed a simple method combining bioinformatics and iTRAQ protein profiling to identify potential bacterial-derived nuclear-translocated proteins that could impact transcriptional programming in host cells. This approach identified 50 A. phagocytophilum candidate genes or proteins. The encoding genes were cloned to create GFP fusion protein-expressing clones that were transfected into HEK-293T cells. We confirmed nuclear translocation of six proteins: APH_0062, RplE, Hup, APH_0382, APH_0385, and APH_0455. Of the six, APH_0455 was identified as a type IV secretion substrate and is now under investigation as a potential nucleomodulin. Additionally, application of this approach to other intracellular bacteria such as Mycobacterium tuberculosis, Chlamydia trachomatis and other intracellular bacteria identified multiple candidate genes to be investigated.

Effector bottleneck: microbial reprogramming of parasitized host cell transcription by epigenetic remodeling of chromatin structure

Obligate intracellular pathogenic bacteria evolved to manipulate their host cells with a limited range of proteins constrained by their compact genomes. The harsh environment of a phagocytic defense cell is one that challenges the majority of commensal and pathogenic bacteria; yet, these are the obligatory vertebrate homes for important pathogenic species in the Anaplasmataceae family. Survival requires that the parasite fundamentally alter the native functions of the cell to allow its entry, intracellular replication, and transmission to a hematophagous arthropod. The small genomic repertoires encode several eukaryotic-like proteins, including ankyrin A (AnkA) of Anaplasma phagocytophilum and Ank200 and tandem-repeat containing proteins of Ehrlichia chaffeensis that localize to the host cell nucleus and directly bind DNA. As a model, A. phagocytophilum AnkA appears to directly alter host cell gene expression by recruiting chromatin modifying enzymes such as histone deacetylases and methyltransferases or by acting directly on transcription in cis. While cis binding could feasibly alter limited ranges of genes and cellular functions, the complex and dramatic alterations in transcription observed with infection are difficult to explain on the basis of individually targeted genes. We hypothesize that nucleomodulins can act broadly, even genome-wide, to affect entire chromosomal neighborhoods and topologically associating chromatin domains by recruiting chromatin remodeling complexes or by altering the folding patterns of chromatin that bring distant regulatory regions together to coordinate control of transcriptional reprogramming. This review focuses on the A. phagocytophilum nucleomodulin AnkA, how it impacts host cell transcriptional responses, and current investigations that seek to determine how these multifunctional eukaryotic-like proteins facilitate epigenetic alterations and cellular reprogramming at the chromosomal level.

Anaplasma phagocytophilum, interferon gamma production and Stat1 signaling

Human granulocytic anaplasmosis is caused by the obligate intracellular bacterium, Anaplasma phagocytophilum. The proinflammatory cytokine, IFN-γ, is necessary for innate immunity and plays an important role in the induction of severe histopathology in A. phagocytophilum-infected mice, horses and humans. In this study, activation of signal transducer and activator of transcription (Stat) 1 phosphorylation associated with A. phagocytophilum infection was examined in mice and found to be markedly greater on day 7 post-infection than in mock-infected controls. This increase in phosphorylated Stat1 (pStat1) correlated significantly with IFN-γ production and inflammatory tissue injury. Because pStat1 operates as a transcription factor central to the generation of effectors of inflammatory injury, these data suggest that Stat1 signaling is involved in IFN-γ-mediated immunopathologic lesions and disease in A. phagocytophilum infection and could be an important target for intervention in this disease.

Neutrophils mediate immunopathology and negatively regulate protective immune responses during fatal bacterial infection-induced toxic shock

Ehrlichia chaffeensis is an obligate intracellular bacterium that infects primarily monocytes and macrophages and causes potentially fatal human monocytic ehrlichiosis (HME) that mimics toxic-shock-like syndrome in immunocompetent hosts. Early recruitment of neutrophils to the sites of infection is critical for the control of bacterial infection and inflammatory responses. We recently observed rapid and sustained neutrophil recruitment at a primary site of infection (peritoneum) following lethal murine ehrlichial infection compared to innocuous ehrlichial infection. We examined here the contribution of neutrophils to protective immunity or immunopathology during infection with monocytic Ehrlichia. Unexpectedly, depletion of neutrophils from lethally infected mice enhanced bacterial elimination, decreased immune-mediated pathology, and prolonged survival. Furthermore, compared to lethally infected sham controls, neutrophil depletion in infected mice resulted in amelioration of pathogenic responses, as evidenced by a decreased number of tumor necrosis factor alpha (TNF-α)-producing CD8(+) T cells, which is known to mediate immunopathology and toxic shock in a murine model of fatal ehrlichiosis. Although neutrophil depletion did not influence the number of CD4(+) Th1 cells and NKT cells producing gamma interferon (IFN-γ), it increased the ratio of IFN-γ- to IL-10-producing NKT cells as well as the ratio of IFN-γ to interleukin 10 (IL-10) transcripts in the liver. This may ameliorate the net suppressive effect of IL-10 on IFN-γ-mediated activation of infected macrophages and thus may account for the enhanced bacterial elimination. Finally, transcriptional analysis of gene expression in the liver indicated that neutrophils contribute to overproduction of cytokines and chemokines during fatal ehrlichiosis. In conclusion, these results revealed an unexpected role of neutrophils in supporting bacterial replication indirectly and promoting immunopathology during severe infection with an intracellular bacterium.

Adaptive immunity to Anaplasma pathogens and immune dysregulation: implications for bacterial persistence

Anaplasma marginale is an obligate intraerythrocytic bacterium that infects ruminants, and notably causes severe economic losses in cattle worldwide. Anaplasma phagocytophilum infects neutrophils and causes disease in many mammals, including ruminants, dogs, cats, horses, and humans. Both bacteria cause persistent infection - infected cattle never clear A. marginale and A. phagocytophilum can also cause persistent infection in ruminants and other animals for several years. This review describes correlates of the protective immune response to these two pathogens as well as subversion and dysregulation of the immune response following infection that likely contribute to long-term persistence. I also compare the immune dysfunction observed with intraerythrocytic A. marginale to that observed in other models of chronic infection resulting in high antigen loads, including malaria, a disease caused by another intraerythrocytic pathogen.

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.

Ixodes scapularis salivary gland protein P11 facilitates migration of Anaplasma phagocytophilum from the tick gut to salivary glands

Ixodes ticks harbour several human pathogens belonging to the order Rickettsiales, including Anaplasma phagocytophilum, the agent of human anaplasmosis. When ticks feed on A. phagocytophilum-infected mice, the pathogen enters the ticks' gut. The bacteria then migrate from the gut to infect the salivary glands of the ticks and are transmitted to the next host via the saliva. The molecular mechanisms that enable the migration of A. phagocytophilum from the gut to the salivary glands are poorly understood. Here we show that a secreted tick protein, P11, is important in this process. We show that P11 enables A. phagocytophilum to infect tick haemocytes, which are required for the migration of A. phagocytophilum from the gut to the salivary glands. Silencing of p11 impaired the A. phagocytophilum infection of tick haemocytes in vivo and consequently decreased pathogen infection of the salivary glands. In vitro experiments showed that P11 could bind to A. phagocytophilum and thus facilitate its infection of tick cells. This report provides new insights into A. phagocytophilum infection of ticks and reveals new avenues to interrupt the life cycle of Anaplasma and related Rickettsial pathogens.

Phagosome dynamics during phagocytosis by neutrophils

The neutrophil is a key player in immunity, and its activities are essential for the resolution of infections. Neutrophil-pathogen interactions usually trigger a large arsenal of antimicrobial measures that leads to the highly efficient killing of pathogens. In neutrophils, the phagocytic process, including the formation and maturation of the phagosome, is in many respects very different from that in other phagocytes. Although the complex mechanisms that coordinate the membrane traffic, oxidative burst, and release of granule contents required for the microbicidal activities of neutrophils are not completely understood, it is evident that they are unique and differ from those in macrophages. Neutrophils exhibit more rapid rates of phagocytosis and higher intensity of oxidative respiratory response than do macrophages. The phagosome maturation pathway in macrophages, which is linked to the endocytic pathway, is replaced in neutrophils by the rapid delivery of preformed granules to nonacidic phagosomes. This review describes the plasticity and dynamics of the phagocytic process with a special focus on neutrophil phagosome maturation.

Global proteomic analysis of two tick-borne emerging zoonotic agents: anaplasma phagocytophilum and ehrlichia chaffeensis

Anaplasma phagocytophilum and Ehrlichia chaffeensis are obligatory intracellular α-proteobacteria that infect human leukocytes and cause potentially fatal emerging zoonoses. In the present study, we determined global protein expression profiles of these bacteria cultured in the human promyelocytic leukemia cell line, HL-60. Mass spectrometric (MS) analyses identified a total of 1,212 A. phagocytophilum and 1,021 E. chaffeensis proteins, representing 89.3 and 92.3% of the predicted bacterial proteomes, respectively. Nearly all bacterial proteins (≥99%) with known functions were expressed, whereas only approximately 80% of “hypothetical” proteins were detected in infected human cells. Quantitative MS/MS analyses indicated that highly expressed proteins in both bacteria included chaperones, enzymes involved in biosynthesis and metabolism, and outer membrane proteins, such as A. phagocytophilum P44 and E. chaffeensis P28/OMP-1. Among 113 A. phagocytophilum p44 paralogous genes, 110 of them were expressed and 88 of them were encoded by pseudogenes. In addition, bacterial infection of HL-60 cells up-regulated the expression of human proteins involved mostly in cytoskeleton components, vesicular trafficking, cell signaling, and energy metabolism, but down-regulated some pattern recognition receptors involved in innate immunity. Our proteomics data represent a comprehensive analysis of A. phagocytophilum and E. chaffeensis proteomes, and provide a quantitative view of human host protein expression profiles regulated by bacterial infection. The availability of these proteomic data will provide new insights into biology and pathogenesis of these obligatory intracellular pathogens.

Anaplasma phagocytophilum AptA modulates Erk1/2 signalling

Anaplasma phagocytophilum causes human granulocytic anaplasmosis, one of the most common tick-borne diseases in North America. This unusual obligate intracellular pathogen selectively persists within polymorphonuclear leucocytes. In this study, using the yeast surrogate model we identified an A. phagocytophilum virulence protein, AptA (A. phagocytophilum toxin A), that activates mammalian Erk1/2 mitogen-activated protein kinase. This activation is important for A. phagocytophilum survival within human neutrophils. AptA interacts with the intermediate filament protein vimentin, which is essential for A. phagocytophilum-induced Erk1/2 activation and infection. A. phagocytophilum infection reorganizes vimentin around the bacterial inclusion, thereby contributing to intracellular survival. These observations reveal a major role for the bacterial protein, AptA, and the host protein, vimentin, in the activation of Erk1/2 during A. phagocytophilum infection.

Multiple mechanisms of NADPH oxidase inhibition by type A and type B Francisella tularensis

Ft is a facultative intracellular pathogen that infects many cell types, including neutrophils. In previous work, we demonstrated that the type B Ft strain LVS disrupts NADPH oxidase activity throughout human neutrophils, but how this is achieved is incompletely defined. Here, we used several type A and type B strains to demonstrate that Ft-mediated NADPH oxidase inhibition is more complex than appreciated previously. We confirm that phagosomes containing Ft opsonized with AS exclude flavocytochrome b(558) and extend previous results to show that soluble phox proteins were also affected, as indicated by diminished phosphorylation of p47(phox) and other PKC substrates. However, a different mechanism accounts for the ability of Ft to inhibit neutrophil activation by formyl peptides, Staphylococcus aureus, OpZ, and phorbol esters. In this case, enzyme targeting and assembly were normal, and impaired superoxide production was characterized by sustained membrane accumulation of dysfunctional NADPH oxidase complexes. A similar post-assembly inhibition mechanism also diminished the ability of anti-Ft IS to confer neutrophil activation and bacterial killing, consistent with the limited role for antibodies in host defense during tularemia. Studies of mutants that we generated in the type A Ft strain Schu S4 demonstrate that the regulatory factor fevR is essential for NADPH oxidase inhibition, whereas iglI and iglJ, candidate secretion system effectors, and the acid phosphatase acpA are not. As Ft uses multiple mechanisms to block neutrophil NADPH oxidase activity, our data strongly suggest that this is a central aspect of virulence.

Fitness and freezing: vector biology and human health

Microbes transmitted to mammals by arthropods contend with many factors that could impede survival. To survive, host fitness with infection must outweigh costs. In this issue of the JCI, Neelakanta et al. demonstrate that ticks infected with Anaplasma phagocytophilum show enhanced fitness against freezing injury owing to induced expression of tick "antifreeze glycoprotein." This allows A. phagocytophilum to successfully propagate and survive to cause disease in nonnatural hosts, such as humans. How an intracellular microbe with a small genome subverts host cell function for survival provides insight into the control of some cellular function programs and underscores how vector biology can have an impact on human health.

A morphological and molecular study of Anaplasma phagocytophilum transmission events at the time of Ixodes ricinus tick bite

BACKGROUND

Anaplasma phagocytophilum is the causative agent of human granulocytic anaplasmosis (HGA) in humans and tick-borne fever (TBF) in ruminants. The bacterium invades and replicates in phagocytes, especially in polymorphonuclear granulocytes.

METHODS

In the present study, skin biopsies and ticks (Ixodes ricinus) were collected from tick feeding lesions on 38 grazing lambs between two and three weeks after access to pastures. The histopathological changes associated with tick bites and A. phagocytophilum infection, were described. In addition the skin biopsies were examined by immunohistochemistry. Furthermore, samples from blood, skin biopsies and ticks were examined by serology, PCR amplification of msp2 (p44), genotyping of rrs (16S rRNA) variants, and compared with the results obtained from histological and immunohistochemical investigations.

RESULTS

Tick bites were associated with chronic and hyperplastic inflammatory skin lesions in this study. A. phagocytophilum present in skin lesions were mainly associated with neutrophils and macrophages. Bacteria were occasionally observed in the Tunica media and Tunica adventitia of small vessels, but were rarely found in association with endothelial cells. PCR and genotyping of organisms present in blood, ticks and skin biopsies suggested a haematogenous and a local spread of organisms at the tick attachment sites.

CONCLUSIONS

The present study describes different aspects of A. phagocytophilum infection at the site of tick bite, and indicates that A. phagocytophilum rarely associates with endothelium during the early pathogenesis of infection.

Anaplasma phagocytophilum and Ehrlichia chaffeensis: subversive manipulators of host cells

Anaplasma spp. and Ehrlichia spp. cause several emerging human infectious diseases. Anaplasma phagocytophilum and Ehrlichia chaffeensis are transmitted between mammals by blood-sucking ticks and replicate inside mammalian white blood cells and tick salivary-gland and midgut cells. Adaptation to a life in eukaryotic cells and transmission between hosts has been assisted by the deletion of many genes that are present in the genomes of free-living bacteria (including genes required for the biosynthesis of lipopolysaccharide and peptidoglycan), by the acquisition of a cholesterol uptake pathway and by the expansion of the repertoire of genes encoding the outer-membrane porins and type IV secretion system. Here, I review the specialized properties and other adaptations of these intracellular bacteria.

Impairment of gamma interferon signaling in human neutrophils infected with Anaplasma phagocytophilum

Anaplasma phagocytophilum, the causative agent of tick-borne human granulocytic anaplasmosis (HGA), is an intracellular bacterium which survives and multiplies inside polymorphonuclear neutrophil granulocytes (PMN). Increased bacterial burden in gamma interferon (IFN-gamma)-deficient mice suggested a major role of IFN-gamma in the control of A. phagocytophilum. Here we investigated whether infection of human PMN with A. phagocytophilum impairs IFN-gamma signaling thus facilitating intracellular survival of the bacterium. The secretion of the IFN-gamma-inducible chemokines IP-10/CXCL10 and MIG/CXCL9 was markedly inhibited in infected neutrophils. Molecular analyses revealed that, compared to uninfected PMN, A. phagocytophilum decreased the expression of the IFN-gamma receptor alpha-chain CD119, diminished the IFN-gamma-induced phosphorylation of STAT1, and enhanced the expression of SOCS1 and SOCS3 in PMN. Since IFN-gamma activates various antibacterial effector mechanisms of PMN, the impaired IFN-gamma signaling in infected cells likely contributes to the survival of A. phagocytophilum inside PMN and to HGA disease development.

The impact of vector-mediated neutrophil recruitment on cutaneous leishmaniasis

The dynamic process of pathogen transmission by the bite of an insect vector combines several biological processes that have undergone extensive co-evolution. Whereas the host response to an insect bite is only occasionally confronted with the parasitic pathogens that competent vectors might transmit, the transmitted parasites will always be confronted with the acute, wound-healing response that is initiated by the bite itself. Invariably, this response involves neutrophils. In the case of Leishmania, infection is initiated in the skin following the bite of an infected sand fly, suggesting that Leishmania must possess some means to survive their early encounter with recruited neutrophils at the bite site. Here, we review the literature regarding the impact of neutrophils on the outcome of infection with Leishmania, with special attention to the role of the sand fly bite.

Infection with Anaplasma phagocytophilum induces multilineage alterations in hematopoietic progenitor cells and peripheral blood cells

Infection with Anaplasma phagocytophilum, a gram-negative, lipopolysaccharide (LPS)-negative, obligate intracellular bacterium, results in multiple peripheral blood cytopenias. We hypothesized that infection with this organism would result in decreased bone marrow (BM) function and shifts in hematopoietic progenitor cells (HPCs) and lineage-committed cells in a well-established murine model of infection. HPCs and lineage-committed progenitors were enumerated in the BM and spleen during acute infection. BM cytokine production and BM CXCL12 expression were determined. Infection resulted in peripheral blood bicytopenia, marked decreases in the number of lineage-committed HPCs in the BM along with concurrent increases in the number of lineage-committed HPCs in the spleen, and a mixed, predominantly myelosuppressive BM cytokine environment. There was significant downregulation of CXCL12 in BM cells that may have been partially responsible for changes in HPC trafficking observed. Changes occurred in the absence of direct pathogen infection of BM cells. Hematopoietic lineage assessment demonstrated that there was loss of erythrocytes and B lymphocytes from the BM along with increased granulopoiesis. These changes were accompanied by splenomegaly due to lymphoid hyperplasia and increased hematopoiesis, most notably erythropoiesis. These changes largely mimic well-described inflammation and endotoxin-mediated effects on the BM and spleen; however, the numbers of peripheral blood neutrophils appear to be independently modulated as granulocytic hyperplasia does not result in neutrophilia. Our findings highlight a well-conserved series of events that we demonstrate can be instigated by an LPS-negative pathogen in the absence of an endotoxin-mediated acute proinflammatory response.

Delayed association of the NADPH oxidase complex with macrophage vacuoles containing the opportunistic pathogen Burkholderia cenocepacia

Burkholderia cenocepacia causes chronic lung infections in patients suffering from cystic fibrosis and chronic granulomatous disease. We have previously shown that B. cenocepacia survives intracellularly in macrophages within a membrane vacuole (BcCV) that delays acidification. Here, we report that after macrophage infection with live B. cenocepacia there is a approximately 6 h delay in the association of NADPH oxidase with BcCVs, while heat-inactivated bacteria are normally trafficked into NADPH oxidase-positive vacuoles. BcCVs in macrophages treated with a functional inhibitor of the cystic fibrosis transmembrane conductance regulator exhibited a further delay in the assembly of the NADPH oxidase complex at the BcCV membrane, but the inhibitor did not affect NADPH oxidase complex assembly onto vacuoles containing heat-inactivated B. cenocepacia or live Escherichia coli. Macrophages produced less superoxide following B. cenocepacia infection as compared to heat-inactivated B. cenocepacia and E. coli controls. Reduced superoxide production was associated with delayed deposition of cerium perhydroxide precipitates around BcCVs of macrophages infected with live B. cenocepacia, as visualized by transmission electron microscopy. Together, our results demonstrate that intracellular B. cenocepacia resides in macrophage vacuoles displaying an altered recruitment of the NADPH oxidase complex at the phagosomal membrane. This phenomenon may contribute to preventing the efficient clearance of this opportunistic pathogen from the infected airways of susceptible patients.

Human granulocytic anaplasmosis

Tick-borne infections have been recognized in the United States for more than a century. Patients who present with nonspecific fever after exposure to ticks should be evaluated by clinical examination and routine laboratory testing to determine if the illness is potentially a tick-borne infection. This article focuses on the diagnosis and management of human granulocytic anaplasmosis (HGA) caused by Anaplasma phagocytophilum.

Anaplasma phagocytophilum increases cathepsin L activity, thereby globally influencing neutrophil function

Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis, is an unusual obligate intracellular pathogen that persists in neutrophils. A. phagocytophilum increases the binding of a repressor, CCAAT displacement protein (CDP), to the gp91(phox) promoter, thereby diminishing the host oxidative burst. We now show that A. phagocytophilum infection also enhances the binding of CDP to the promoters of human neutrophil peptide 1 and C/EBPepsilon--molecules important for neutrophil defense and maturation--suggesting that this is a general strategy used by this pathogen to alter polymorphonuclear leukocyte function. To explore the mechanism by which A. phagocytophilum increases CDP activity, we assessed the effects of this microbe on cathepsin L, a protease that cleaves CDP into a form with increased DNA binding ability. A. phagocytophilum infection resulted in elevated cathepsin L activity and the proteolysis of CDP. Blocking the action of cathepsin L with a chemical inhibitor or small interfering RNA targeting of this molecule caused a marked reduction in the degree of A. phagocytophilum infection. These data demonstrate that increasing cathepsin L activity is a strategy used by A. phagocytophilum to alter CDP activity and thereby globally influence neutrophil function. As therapeutic options for A. phagocytophilum and related organisms are limited, these results also identify a cellular pathway that may be targeted for the treatment of A. phagocytophilum infection.

Understanding the multiple functions of Gr-1(+) cell subpopulations during microbial infection

The murine cell surface determinant Gr-1 is expressed at high level on neutrophils. Depletion of polymorphonuclear leukocytes with anti-Gr-1(+) monoclonal antibody results in increased susceptibility and dysregulated immunity to many microbial pathogens, a finding widely interpreted to indicate the importance of neutrophils during infection. Yet, in recent years it has become clear that additional cell types express the Gr-1 determinant, including dendritic cell and monocyte subpopulations. In this review, we evaluate current knowledge on the functional aspects of Gr-1(+) cell populations. We focus on infection with the opportunistic protozoan Toxoplasma gondii, a case where host survival depends on an intact Gr-1(+) cell population.

Neutrophil granulocytes as host cells and transport vehicles for intracellular pathogens: apoptosis as infection-promoting factor

Polymorphonuclear neutrophil granulocytes (PMN) are primary antimicrobial effector cells of the innate immune system and serve to destroy invading pathogens. Although most ingested microorganisms are killed readily inside PMN, several obligate or facultative intracellular pathogens survive even in this hostile environment. Extension of the life span of neutrophils is a general escape mechanism of pathogens residing in PMN. However, after 2-4 days, even infected neutrophils become apoptotic and are phagocytosed by macrophages. Since microbes entering macrophages via the uptake of infected apoptotic PMN may survive and multiply in macrophages, apoptotic neutrophils can serve as "Trojan horses" for certain pathogens. Interfering with activating signaling pathways appears to be another potent mechanism by which intracellular microorganisms suppress cellular activation in neutrophils. In addition to provide a short overview of the topic, the present review aims to summarize our own findings regarding the interaction between human neutrophils and intracellular pathogens as well as regarding the disease promoting role of apoptotic cells after infection with Leishmania major.

Structural mechanism of organic hydroperoxide induction of the transcription regulator OhrR

The Xanthomonas campestris transcription regulator OhrR contains a reactive cysteine residue (C22) that upon oxidation by organic hydroperoxides (OHPs) forms an intersubunit disulphide bond with residue C127'. Such modification induces the expression of a peroxidase that reduces OHPs to their less toxic alcohols. Here, we describe the structures of reduced and OHP-oxidized OhrR, visualizing the structural mechanism of OHP induction. Reduced OhrR takes a canonical MarR family fold with C22 and C127' separated by 15.5 A. OHP oxidation results in the disruption of the Y36'-C22-Y47' interaction network and dissection of helix alpha5, which then allows the 135 degrees rotation and 8.2 A translation of C127', formation of the C22-C127' disulphide bond, and alpha6-alpha6' helix-swapped reconfiguration of the dimer interface. These changes result in the 28 degrees rigid body rotations of each winged helix-turn-helix motif and DNA dissociation. Similar effector-induced rigid body rotations are expected for most MarR family members.

Human granulocytic anaplasmosis and macrophage activation

Patients with human granulocytic anaplasmosis present with fever, thrombocytopenia, leukopenia, and an elevated aspartate transaminase level. Clinical and histopathologic features of severe disease suggest macrophage activation. Twenty-nine patients with human granulocytic anaplasmosis had higher ferritin, interleukin-10, interleukin-12 p70, and interferon- gamma levels than did control subjects matched for age and sex; severity correlated with triglyceride, ferritin, and interleukin-12 p70 levels. Several severely affected patients had cases that fulfilled macrophage activation syndrome diagnostic criteria. Macrophage activation and excessive cytokine production may belie tissue injury associated with Ananplasma phagocytophilum infection.

Immune evasion and immunosuppression by Anaplasma phagocytophilum, the causative agent of tick-borne fever of ruminants and human granulocytic anaplasmosis

Anaplasma phagocytophilum, the causative agent of tick-borne fever (TBF) in sheep and cattle and human granulocytic anaplasmosis, has the unique ability to infect and multiply within neutrophils, eosinophils and monocytes, cells at the frontline of the immune system. Infection with A. phagocytophilum is also characterized by severe leukopenia due to lymphocytopenia, neutropenia and thrombocytopenia lasting for several days. By itself TBF does not cause high mortality rates but infected animals are more susceptible to other secondary infections, pregnant animals may abort and there is a severe reduction in milk yield in dairy cattle. The susceptibility to secondary infections can be attributed to the leukopenia that accompanies the disease and the organism's adverse effects on lymphocyte and neutrophil functions. One of its fascinating features is that it infects and actively grows in neutrophils by employing an array of mechanisms to subvert their bactericidal activity. These include its ability to inhibit phagosome-lysosome fusion, to suppress respiratory burst and to delay the apoptotic death of neutrophils. It is also able to survive within an apparently immune host by employing a complex mechanism of antigenic variation.

Degradation of p22phox and inhibition of superoxide generation by Ehrlichia chaffeensis in human monocytes

Ehrlichia chaffeensis is an obligate intracellular bacterium which replicates in monocytes or macrophages, the primary producers of reactive oxygen species (ROS). However, effects of ROS on E. chaffeensis infection and whether E. chaffeensis modulates ROS generation in host monocytes are unknown. Here, E. chaffeensis was shown to lose infectivity upon exposure to O(2)(-) or hydrogen peroxide. Upon incubation with human monocytes, E. chaffeensis neither induced O(2)(-) generation by human monocytes, nor colocalized with nicotinamide adenine dinucleotide phosphate (NADPH) oxidase components. Instead, it actively blocked O(2)(-) generation by monocytes stimulated with phorbol myristate acetate and caused the rapid degradation of p22(phox), a component of NADPH oxidase. These effects were not seen in neutrophil, which is another potent ROS generator, but a cell type that E. chaffeensis does not infect. Trypsin pretreatment of monocytes prevented the inhibition of O(2)(-) generation by E. chaffeensis. The degradation of p22(phox) by E. chaffeensis was specific to subsets of monocytes with bound and/or intracellular bacteria, and the degradation could be reduced by heat treatment of the bacterium, lipopolysaccharide pretreatment of monocytes, or the incubation with haemin. The degradation of p22(phox) by E. chaffeensis and its prevention by haemin or protease inhibitors also occurred in isolated monocyte membrane fractions, indicating that host cytoplasmic signalling is not required for these processes. The amount of gp91(phox) was stable under all conditions examined in this study. These findings point to a unique survival mechanism of ROS-sensitive obligate intraleucocytic bacteria that involves the destabilization of p22(phox) following the binding of bacteria to host cell surface proteins.

The expanding role of NADPH oxidases in health and disease: no longer just agents of death and destruction

The NADPH oxidase was originally identified as a key component of human innate host defence. In phagocytes, this enzyme complex is activated to produce superoxide anion and other secondarily derived ROS (reactive oxygen species), which promote killing of invading micro-organisms. However, it is now well-established that NADPH oxidase and related enzymes also participate in important cellular processes not directly related to host defence, including signal transduction, cell proliferation and apoptosis. These enzymes are present in essentially every organ system in the body and contribute to a multitude of physiological events. Although essential for human health, excess NADPH-oxidase-generated ROS can promote numerous pathological conditions. Herein, we summarize our current understanding of NADPH oxidases and provide an overview of how they contribute to specific human diseases.