Rickettsia parkeri hijacks tick hemocytes to manipulate cellular and humoral transcriptional responses

TNF signaling mediates cellular immune function and promotes malaria parasite killing in the mosquito Anopheles gambiae

Tumor Necrosis Factor-α (TNF-α) is a proinflammatory cytokine and a master regulator of immune cell function in vertebrates. While previous studies have implicated TNF signaling in invertebrate immunity, the roles of TNF in mosquito innate immunity and vector competence have yet to be explored. Herein, we confirm the identification of a conserved TNF-α pathway in Anopheles gambiae consisting of the TNF-α ligand, Eiger, and its cognate receptors Wengen and Grindelwald. Through gene expression analysis, RNAi, and in vivo injection of recombinant TNF-α, we provide direct evidence for the requirement of TNF signaling in regulating mosquito immune cell function by promoting granulocyte midgut attachment, increased granulocyte abundance, and oenocytoid rupture. Moreover, our data demonstrate that TNF signaling is an integral component of anti-Plasmodium immunity that limits malaria parasite survival. Together, our data support the existence of a highly conserved TNF signaling pathway in mosquitoes that mediates cellular immunity and influences Plasmodium infection outcomes, offering potential new approaches to interfere with malaria transmission by targeting the mosquito host.

Ultrastructural characterization and quantification of hemocytes in engorged female Amblyomma sculptum ticks

Amblyomma sculptum (formerly Amblyomma cajennense) ticks have been implicated in the transmission of pathogens that cause diseases in animals and humans. Their wide geographic distribution and high impact on animal health and zoonotic disease transmission highlight the importance of studying and implementing effective control measures to mitigate the risks associated with this tick species. The aim of this study was to quantify and characterize the morphology and the ultrastructure of different types of hemocytes in the hemolymph in engorged A. sculptum females fed on rabbits. The hemolymph samples were collected by perforation of the cuticle in the dorsal region. Hemocyte types, sizes, and differential counts were determined using light microscopy, while ultrastructural analysis of hemocytes was performed using transmission electron microscopy. The average number of total hemocytes in the hemolymph was 1024 ± 597.6 cells µL-1. Five morphologically distinct cell types were identified in A. sculptum females: prohemocytes (6 % ± 8.8), plasmatocytes (10 % ± 7.7), granulocytes (78 % ± 12.2), spherulocytes (5 % ± 4.48), and oenocytoids (1 % ± 1.6). In general, prohemocytes were the smallest hemocytes. The ultrastructural morphology of A. sculptum hemocytes described in the present study agrees with the findings for other hard ticks. This is the first study to investigate ultrastructural characteristics of hemocytes of female A. sculptum ticks.

Mosquito immune cells enhance dengue and Zika virus dissemination in Aedes aegypti

Mosquito-borne viruses cause more than 400 million annual infections and place over half of the world's population at risk. Despite this importance, the mechanisms by which arboviruses infect the mosquito host and disseminate to tissues required for transmission are not well understood. Here, we provide evidence that mosquito immune cells, known as hemocytes, play an integral role in the dissemination of dengue virus (DENV) and Zika virus (ZIKV) in the mosquito Aedes aegypti. We establish that phagocytic hemocytes are a focal point for virus infection and demonstrate that these immune cell populations facilitate virus dissemination to the ovaries and salivary glands. Additional transfer experiments confirm that virus-infected hemocytes confer a virus infection to non-infected mosquitoes more efficiently than free virus in acellular hemolymph, revealing that hemocytes are an important tropism to enhance virus dissemination in the mosquito host. These data support a "trojan horse" model of virus dissemination where infected hemocytes transport virus through the hemolymph to deliver virus to mosquito tissues required for transmission and parallels vertebrate systems where immune cell populations promote virus dissemination to secondary sites of infection. In summary, this study significantly advances our understanding of virus infection dynamics in mosquitoes and highlights conserved roles of immune cells in virus dissemination across vertebrate and invertebrate systems.

Tick hemocytes have a pleiotropic role in microbial infection and arthropod fitness

Uncovering the complexity of systems in non-model organisms is critical for understanding arthropod immunology. Prior efforts have mostly focused on Dipteran insects, which only account for a subset of existing arthropod species in nature. Here we use and develop advanced techniques to describe immune cells (hemocytes) from the clinically relevant tick Ixodes scapularis at a single-cell resolution. We observe molecular alterations in hemocytes upon feeding and infection with either the Lyme disease spirochete Borrelia burgdorferi or the rickettsial agent Anaplasma phagocytophilum. We reveal hemocyte clusters exhibiting defined signatures related to immunity, metabolism, and proliferation. Depletion of phagocytic hemocytes affects hemocytin and astakine levels, two I. scapularis hemocyte markers, impacting blood-feeding, molting behavior, and bacterial acquisition. Mechanistically, astakine alters hemocyte proliferation, whereas hemocytin affects the c-Jun N-terminal kinase (JNK) signaling pathway in I. scapularis. Altogether, we discover a role for tick hemocytes in immunophysiology and provide a valuable resource for comparative biology in arthropods.

Critical roles of Rickettsia parkeri outer membrane protein B (OmpB) in the tick host

Rickettsia parkeri is a pathogen of public health concern and transmitted by the Gulf Coast tick, Amblyomma maculatum. Rickettsiae are obligate intracellular bacteria that enter and replicate in diverse host cells. Rickettsial outer membrane protein B (OmpB) functions in bacterial adhesion, invasion, and avoidance of cell-autonomous immunity in mammalian cell infection, but the function of OmpB in arthropod infection is unknown. In this study, the function of R. parkeri OmpB was evaluated in the tick host. R. parkeri wild-type and R. parkeri ompBSTOP::tn (non-functional OmpB) were capillary fed to naïve A. maculatum ticks to investigate dissemination in the tick and transmission to vertebrates. Ticks exposed to R. parkeri wild-type had greater rickettsial loads in all organs than ticks exposed to R. parkeri ompBSTOP::tn at 12 h post-capillary feeding and after 1 day of feeding on host. In rats that were exposed to R. parkeri ompBSTOP::tn-infected ticks, dermal inflammation at the bite site was less compared to R. parkeri wild-type-infected ticks. In vitro, R. parkeri ompBSTOP::tn cell attachment to tick cells was reduced, and host cell invasion of the mutant was initially reduced but eventually returned to the level of R. parkeri wild-type by 90 min post-infection. R. parkeri ompBSTOP::tn and R. parkeri wild-type had similar growth kinetics in the tick cells, suggesting that OmpB is not essential for R. parkeri replication in tick cells. These results indicate that R. parkeri OmpB functions in rickettsial attachment and internalization to tick cells and pathogenicity during tick infection.

Tick innate immune responses to hematophagy and Ehrlichia infection at single-cell resolution

Introduction

Ticks rely on robust cellular and humoral responses to control microbial infection. However, several aspects of the tick's innate immune system remain uncharacterized, most notably that of the immune cells (called hemocytes), which are known to play a significant role in cellular and humoral responses. Despite the importance of hemocytes in regulating microbial infection, our understanding of their basic biology and molecular mechanisms remains limited. Therefore, we believe that a more detailed understanding of the role of hemocytes in the interactions between ticks and tick-borne microbes is crucial to illuminating their function in vector competence and to help identify novel targets for developing new strategies to block tick-borne pathogen transmission.

Methods

This study examined hemocytes from the lone star tick (Amblyomma americanum) at the transcriptomic level using the 10X genomics single-cell RNA sequencing platform to analyze hemocyte populations from unfed, partially blood-fed, and Ehrlichia chaffeensis-infected ticks. The functional role of differentially expressed hemocyte markers in hemocyte proliferation and Ehrlichia dissemination was determined using an RNA interference approach.

Results and discussion

Our data exhibit the identification of fourteen distinct hemocyte populations. Our results uncover seven distinct lineages present in uninfected and Ehrlichia-infected hemocyte clusters. The functional characterization of hemocytin, cystatin, fibronectin, and lipocalin demonstrate their role in hemocyte population changes, proliferation, and Ehrlichia dissemination.

Conclusion

Our results uncover the tick immune responses to Ehrlichia infection and hematophagy at a single-cell resolution. This work opens a new field of tick innate immunobiology to understand the role of hemocytes, particularly in response to prolonged blood-feeding (hematophagy), and tick-microbial interactions.

Ehrlichia chaffeensis Co-Opts Phagocytic Hemocytes for Systemic Dissemination in the Lone Star Tick, Amblyomma americanum

INTRODUCTION

Hematophagous arthropods can acquire and transmit several pathogens of medical importance. In ticks, the innate immune system is crucial in the outcome between vector-pathogen interaction and overall vector competence. However, the specific immune response(s) elicited by the immune cells known as hemocytes remains largely undefined in Ehrlichia chaffeensis and its competent tick vector, Amblyomma americanum.

METHODS

We utilized injection of clodronate liposome to deplete tick granulocytes combined with infection with E. chaffeensis to demonstrate their essential role in microbial infection.

RESULTS

Here, we show that granulocytes, professional phagocytic cells, are integral in eliciting immune responses against commensal and pathogen infection. The chemical depletion of granulocytes led to decreased phagocytic efficiency of tissue-associated hemocytes. We demonstrate that E. chaffeensis can infect circulating hemocytes, and both cell-free plasma and hemocytes from E. chaffeensis-infected ticks can establish Ehrlichia infection in recipient ticks. Lastly, we provide evidence to show that granulocytes play a dual role in E. chaffeensis infection. Depleting granulocytic hemocytes increased Ehrlichia load in the salivary gland and midgut tissues. In contrast, granulocyte depletion led to a reduced systemic load of Ehrlichia.

CONCLUSION

This study has identified multiple roles for granulocytic hemocytes in the control and systemic dissemination of E. chaffeensis infection.

PERK-mediated antioxidant response is key for pathogen persistence in ticks

A crucial phase in the life cycle of tick-borne pathogens is the time spent colonizing and persisting within the arthropod. Tick immunity is emerging as a key force shaping how transmissible pathogens interact with the vector. How pathogens remain in the tick despite immunological pressure remains unknown. In persistently infected Ixodes scapularis, we found that Borrelia burgdorferi (causative agent of Lyme disease) and Anaplasma phagocytophilum (causative agent of granulocytic anaplasmosis) activate a cellular stress pathway mediated by the endoplasmic reticulum receptor PKR-like ER kinase (PERK) and the central regulatory molecule eIF2α. Disabling the PERK pathway through pharmacological inhibition and RNA interference (RNAi) significantly decreased microbial numbers. In vivo RNAi of the PERK pathway not only reduced the number of A. phagocytophilum and B. burgdorferi colonizing larvae after a bloodmeal but also significantly reduced the number of bacteria that survive the molt. An investigation into PERK pathway-regulated targets revealed that A. phagocytophilum and B. burgdorferi induce activity of the antioxidant response regulator, nuclear factor erythroid 2-related factor 2 (Nrf2). Tick cells deficient for nrf2 expression or PERK signaling showed accumulation of reactive oxygen and nitrogen species in addition to reduced microbial survival. Supplementation with antioxidants rescued the microbicidal phenotype caused by blocking the PERK pathway. Altogether, our study demonstrates that the Ixodes PERK pathway is activated by transmissible microbes and facilitates persistence in the arthropod by potentiating an Nrf2-regulated antioxidant environment. IMPORTANCE Recent advances demonstrate that the tick immune system recognizes and limits the pathogens they transmit. Innate immune mediators such as antimicrobial peptides and reactive oxygen/nitrogen species are produced and restrict microbial survival. It is currently unclear how pathogens remain in the tick, despite this immune assault. We found that an antioxidant response controlled by the PERK branch of the unfolded protein response is activated in ticks that are persistently infected with Borrelia burgdorferi (Lyme disease) or Anaplasma phagocytophilum (granulocytic anaplasmosis). The PERK pathway induces the antioxidant response transcription factor, Nrf2, which coordinates a gene network that ultimately neutralizes reactive oxygen and nitrogen species. Interfering with this signaling cascade in ticks causes a significant decline in pathogen numbers. Given that innate immune products can cause collateral damage to host tissues, we speculate that this is an arthropod-driven response aimed at minimizing damage to "self" that also inadvertently benefits the pathogen. Collectively, our findings shed light on the mechanistic push and pull between tick immunity and pathogen persistence within the arthropod vector.

Tick hemocytes have pleiotropic roles in microbial infection and arthropod fitness

Uncovering the complexity of systems in non-model organisms is critical for understanding arthropod immunology. Prior efforts have mostly focused on Dipteran insects, which only account for a subset of existing arthropod species in nature. Here, we describe immune cells or hemocytes from the clinically relevant tick Ixodes scapularis using bulk and single cell RNA sequencing combined with depletion via clodronate liposomes, RNA interference, Clustered Regularly Interspaced Short Palindromic Repeats activation (CRISPRa) and RNA-fluorescence in situ hybridization (FISH). We observe molecular alterations in hemocytes upon tick infestation of mammals and infection with either the Lyme disease spirochete Borrelia burgdorferi or the rickettsial agent Anaplasma phagocytophilum. We predict distinct hemocyte lineages and reveal clusters exhibiting defined signatures for immunity, metabolism, and proliferation during hematophagy. Furthermore, we perform a mechanistic characterization of two I. scapularis hemocyte markers: hemocytin and astakine. Depletion of phagocytic hemocytes affects hemocytin and astakine levels, which impacts blood feeding and molting behavior of ticks. Hemocytin specifically affects the c-Jun N-terminal kinase (JNK) signaling pathway, whereas astakine alters hemocyte proliferation in I. scapularis. Altogether, we uncover the heterogeneity and pleiotropic roles of hemocytes in ticks and provide a valuable resource for comparative biology in arthropods.

Ehrlichia chaffeensis co-opts phagocytic hemocytes for systemic dissemination in the Lone Star tick, Amblyomma americanum

Hematophagous arthropods can acquire and transmit several pathogens of medical importance. In ticks, the innate immune system is crucial in the outcome between vector-pathogen interaction and overall vector competence. However, the specific immune response(s) elicited by the immune cells known as hemocytes remains largely undefined in Ehrlichi a chaffeensis and its competent tick vector, Amblyomma americanum . Here, we show that granulocytes, professional phagocytic cells, are integral in eliciting immune responses against commensal and pathogen infection. The chemical depletion of granulocytes led to decreased phagocytic efficiency of tissues-associated hemocytes. We demonstrate E. chaffeensis can infect circulating hemocytes, and both cell-free plasma and hemocytes from E. chaffeensis- infected ticks can establish Ehrlichia infection in recipient ticks. Lastly, we provide evidence to show granulocytes play a dual role in E. chaffeensis infection. Depleting granulocytic hemocytes increased Ehrlichia load in the salivary gland and midgut tissues. In contrast, granulocyte depletion led to a reduced systemic load of Ehrlichia . This study has identified multiple roles for granulocytic hemocytes in the control and systemic dissemination of E. chaffeensis infection.

Clodronate Liposome-Mediated Phagocytic Hemocyte Depletion Affects the Regeneration of the Cephalic Tentacle of the Invasive Snail, Pomacea canaliculata

After amputation, granular hemocytes infiltrate the blastema of regenerating cephalic tentacles of the freshwater snail Pomacea canaliculata. Here, the circulating phagocytic hemocytes were chemically depleted by injecting the snails with clodronate liposomes, and the effects on the cephalic tentacle regeneration onset and on Pc-Hemocyanin, Pc-transglutaminase (Pc-TG) and Pc-Allograft Inflammatory Factor-1 (Pc-AIF-1) gene expressions were investigated. Flow cytometry analysis demonstrated that clodronate liposomes targeted large circulating hemocytes, resulting in a transient decrease in their number. Corresponding with the phagocyte depletion, tentacle regeneration onset was halted, and it resumed at the expected pace when clodronate liposome effects were no longer visible. In addition to the regeneration progress, the expressions of Pc-Hemocyanin, Pc-TG, and Pc-AIF-1, which are markers of hemocyte-mediated functions like oxygen transport and immunity, clotting, and inflammation, were modified. After the injection of clodronate liposomes, a specific computer-assisted image analysis protocol still evidenced the presence of granular hemocytes in the tentacle blastema. This is consistent with reports indicating the large and agranular hemocyte population as the most represented among the professional phagocytes of P. canaliculata and with the hypothesis that different hemocyte morphologies could exert diverse biological functions, as it has been observed in other invertebrates.

PERK-mediated antioxidant response is key for pathogen persistence in ticks

A crucial phase in the lifecycle of tick-borne pathogens is the time spent colonizing and persisting within the arthropod. Tick immunity is emerging as a key force shaping how transmissible pathogens interact with the vector. How pathogens remain in the tick despite immunological pressure remains unknown. In persistently infected Ixodes scapularis , we found that Borrelia burgdorferi (Lyme disease) and Anaplasma phagocytophilum (granulocytic anaplasmosis) activate a cellular stress pathway mediated by the endoplasmic reticulum receptor PERK and the central regulatory molecule, eIF2α. Disabling the PERK pathway through pharmacological inhibition and RNAi significantly decreased microbial numbers. In vivo RNA interference of the PERK pathway not only reduced the number of A. phagocytophilum and B. burgdorferi colonizing larvae after a bloodmeal, but also significantly reduced the number of bacteria that survive the molt. An investigation into PERK pathway-regulated targets revealed that A. phagocytophilum and B. burgdorferi induce activity of the antioxidant response regulator, Nrf2. Tick cells deficient for nrf2 expression or PERK signaling showed accumulation of reactive oxygen and nitrogen species in addition to reduced microbial survival. Supplementation with antioxidants rescued the microbicidal phenotype caused by blocking the PERK pathway. Altogether, our study demonstrates that the Ixodes PERK pathway is activated by transmissible microbes and facilitates persistence in the arthropod by potentiating an Nrf2-regulated antioxidant environment.