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

Abstract

IntroductionBlood-feeding arthropods rely on robust cellular and humoral immunity to control pathogen invasion and replication. Tick hemocytes produce factors that can facilitate or suppress microbial infection and pathogenesis. Despite the importance of hemocytes in regulating microbial infection, understanding of their basic biology and molecular mechanisms remains limited.MethodsHere we combined histomorphology and functional analysis to identify five distinct phagocytic and non-phagocytic hemocyte populations circulating within the Gulf Coast tick Amblyomma maculatum.Results and discussionDepletion of phagocytic hemocytes using clodronate liposomes revealed their function in eliminating bacterial infection. We provide the first direct evidence that an intracellular tick-borne pathogen, Rickettsia parkeri, infects phagocytic hemocytes in Am. maculatum to modify tick cellular immune responses. A hemocyte-specific RNA-seq dataset generated from hemocytes isolated from uninfected and R. parkeri-infected partially blood-fed ticks generated ~40,000 differentially regulated transcripts, >11,000 of which were immune genes. Silencing two differentially regulated phagocytic immune marker genes (nimrod B2 and eater-two Drosophila homologs), significantly reduced hemocyte phagocytosis.ConclusionTogether, these findings represent a significant step forward in understanding how hemocytes regulate microbial homeostasis and vector competence.

Tick hemocytes have pleiotropic roles in microbial infection and arthropod fitness

“…Bulk RNA-seq of hemocytes originating from the tick Amblyomma maculatum showed differential expression of hemocytin and astakine during infection with the intracellular bacterium Rickettsia parkeri ( 31 ). Our scRNA-seq revealed downregulation of hemocytin and astakine expression during A. phagocytophilum and B. burgdorferi infection (Figure 3D, Supplementary Dataset 9).…”

“…We observed an increased expression of genes and expansion of hemocyte subtypes involved in metabolism, cell proliferation and development following a blood meal (Figures 1-2), suggesting participation of these immune cells in the feeding or molting process. To investigate this observation, we manipulated hemocyte numbers in I. scapularis using clodronate liposomes (CLD), which have been used to deplete phagocytic hemocytes in flies, mosquitoes and, more recently, ticks ( 31, 36, 37 ). We found that the CLD treatment reduced the total number of hemocytes in nymphs (Figure 7A).…”

Tick Innate Immune Responses to Hematophagy andEhrlichiaInfection at Single-Cell Resolution

“…Three independent cohorts of adult female lone star ticks ( Amblyomma americanum) representing unfed, blood-fed, and Ehrlichia -infected (Arkansas strain) ticks were prepared for hemolymph collection. Hemolymph was collected from these three cohorts using our published method with slight modifications (Adegoke et al, 2023). Collected hemolymph was resuspended in Leibovitz’s L-15 Medium supplemented with 50% BSA.…”

“…Hemocyte mediates several immune processes ranging from phagocytosis, nodulation, melanization, and the release of antimicrobial peptides (Feitosa et Tick hemocytes exhibit remarkable heterogeneity and complexity, comprising multiple distinct subpopulations with unique functional attributes. Traditional microscopy approaches have provided valuable insights into the overall composition and morphology of tick hemocytes (Binnington and Obenchain, 1982;Adegoke et al, 2023), leading to the identification of five types: prohemocytes, plasmatocytes, granulocytes, spherulocytes, and oenocytoids (Binnington and Obenchain, 1982;Adegoke et al, 2023). Interestingly, several studies have reported fewer hemocyte types in other tick species (Sonenshine and Roe, 2013;Borovičková and Hypša, 2005).…”

“…Interestingly, several studies have reported fewer hemocyte types in other tick species (Sonenshine and Roe, 2013; Borovičková and Hypša, 2005). Transcriptional studies of tick hemocytes (Kotsyfakis et al, 2015; Adegoke et al, 2023) have revealed important information regarding the molecular regulation of tick hemocytes during physiological processes such as blood feeding or microbial infection. Nevertheless, these techniques often fail to capture the full spectrum of cellular diversity and functional heterogeneity at the single-cell level, limiting our understanding of the intricate cellular immune responses accompany blood feeding or infection.. Single-cell RNA sequencing (scRNA-seq) has redefined our understanding of hemocyte populations in invertebrate models such as Drosophila (Cattenoz et al, 2020; Cho et al, 2020; Tattikota et al, 2020), mosquito (Severo et al, 2018; Raddi et al, 2020; Kwon et al, 2021), silkworm and moth (Feng et al, 2021; Feng et al, 2022), ticks (Rolandelli et al, 2023) and aquatic invertebrates (Koiwai et al, 2021; Pichon et al, 2022; Cui et al, 2022).…”