Characterization of hemocytes and hematopoietic cells of a freshwater crayfish based on single-cell transcriptome analysis

Unveiling a new hemocyte subpopulation in white shrimp (Penaeus vannamei) and the characterization of immune response in hemocyte subpopulation

Hemocytes are the primary cellular components of the shrimp immune system, playing a crucial role in host defense. However, a comprehensive understanding of their subpopulations and specific functions remains incomplete. In this study, four hemocyte subpopulations, designated as H1, H2, H3, and H4, were identified in Penaeus vannamei using transmission electron microscopy (TEM) and flow cytometry. The H1 subpopulation was the most abundant cells, the smallest in size, lacked granules, and had the highest nucleus-to-cytoplasm (N:C) ratio, identifying it as prohemocytes (immature cells). The H2 subpopulation fits the criteria of hyaline hemocytes. They are relatively small, have a large nucleus, and contain no or very few cytoplasmic granules. The H3 subpopulation was the least abundant cell. These cells are larger than HCs, have a moderate N:C ratio, and contain a few granules, identifying them as semi-granulocytes. The H4 subpopulation, representing granulocytes, had the largest cell size and the lowest N:C ratio and was characterized by the presence of large granules in the cytoplasm. Non-specific immune responses were investigated through various parameters and gene expression profiling. Each hemocyte subpopulation exhibited distinct immune functions. Prohemocytes strongly expressed notch-1, suggesting a role in hemocyte proliferation. Hyalinocytes exhibited strong phagocytic activity and produced superoxide anions. Semigranulocytes exhibited high expression of lysozyme and anti-lipopolysaccharide factor. Granulocytes showed high expression of propo-1, propo-2, and antimicrobial peptide genes. Following Vibrio parahaemolyticus injection, the H1 subpopulation significantly increased at 6 h post-infection before returning to baseline levels, whereas the H4 subpopulation followed an opposite trend. These findings suggest that both H1 and H4 hemocytes play critical roles in the immune response against V. parahaemolyticus.

Specific host factors determine resistance in a North American crayfish to the crayfish plague, Aphanomyces astaci

The crayfish plague is caused by the oomycete Aphanomyces astaci with North American crayfish (for example Pacifastacus leniusculus and Procambarus clarkii) serving as carriers and vectors for this pathogen. This poses a constant threat to native crayfish in Europe, Asia, South America and Australia, which all are highly susceptible to this pathogen. In this study we now show how the symbiotic balance between the pathogen and its host are maintained at the molecular level. The host factors involved in this balance between the pathogen, A. astaci and the host, P. leniusculus, are one glycine-rich antimicrobial peptide (GRP) that is specifically active against A. astaci, but not to other microorganisms and two Kazal proteinase inhibitors (KPI2 and KPILA) inhibit secreted A. astaci proteases by binding to subtilisin enzymes from the pathogen. Accordingly, the expression of GRP, KPI2, KPILA, as well as proPO mRNAs increases following A. astaci infection. Silencing GRP, or KPI2 + KPILA mRNAs results in death of the crayfish from infection. Over time, this host-pathogen relationship has evolved to allow resistant crayfish to coexist with A. astaci in their cuticle for life, provided critical components remain unaltered by environmental changes or other pathogens. It is unclear whether a similar relationship could develop between currently susceptible crayfish and A. astaci.

An overview of recent progress in the molecular mechanisms and key biological macromolecules involved in limb regeneration of decapods

Understanding the molecular mechanisms of limb regeneration in decapods can significantly enhance aquaculture production by improving survival and growth, as well as facilitating the development of lab-grown crustacean meat as a sustainable protein source. This review explores the molecular mechanisms of decapod limb regeneration, focusing on the key signaling pathways, genes, and proteins involved in this process. The initial stages of regeneration involve immune response and hemolymph coagulation, which are regulated via signaling pathways such as Toll, MAPK, IMD, and JAK/STAT. Subsequent stages, including blastema formation and limb growth, are regulated by signaling pathways such as Wnt, Hippo, Hedgehog, Ecdysteroid, TGF-β, Notch, Insulin-like, Fibroblast Growth Factor, Epidermal Growth Factor, and BMP. This review also discusses the interplay among environmental factors, nutrition, and hormonal signaling in regeneration and how these elements influence regenerative capability. Furthermore, this review highlights existing research gaps in decapod regeneration and suggests future research directions. This review aims to bridge existing gaps in decapod regeneration research and guide future studies toward potential breakthroughs in aquaculture practices.

The hematopoietic tissue of the freshwater crayfish, Pacifastacus leniusculus: organization and expression analysis

The hematopoietic tissue (HPT) and anterior proliferation center (APC) are the main hemocyte-producing organs of the freshwater crayfish, Pacifastacus leniusculus. To deepen our understanding of immune responses to various pathogens, it is essential to identify distinct hemocyte subpopulations with specific functions and to further explore how these cells are generated. Here we provide an in-depth histological study of the HPT and APC in order to localize cell types in different developmental stages, and to provide some information regarding the hemocyte differentiation in the crayfish. We localized mRNA expression of previously identified genes in the HPT/APC and hemocytes by RNA-FISH. The expression of hemolectin and transglutaminase 1 was shown to be co-localized in a high number of the HPT cells, while transglutaminase 2 was expressed in different cell types mainly associated with epithelium or endothelium. Furthermore, by double RNA-FISH for hemolectin and a previously unidentified PDGF-like factor, combined with immunostaining for prophenoloxidase, we could identify several different subtypes of hemocytes, indicating that the immune function of hemocytes in crayfish is more diversified and complex than previously appreciated.

Expression of C/EBP and Kr-h1 transcription factors under immune stimulation in the noble crayfish

Transcription factors (TFs) have an important role in the regulation of the gene expression network. The role of TFs in the immune response of freshwater crayfish is poorly understood, but leveraging the regulatory mechanisms of immune response could augment the resistance against the invasive oomycete pathogen, Aphanomyces astaci. Previous studies indicated that the TFs CCAAT/enhancer-binding protein (C/EBP) and putative Krüppel homolog-1 protein (Kr-h1) might play a role in immune and stress response of the noble crayfish (Astacus astacus). Here, we aimed to further characterise these two gene products to gain a better understanding of their evolutionary origin, domain organisation and expression patterns across different crayfish tissues. Furthermore, we conducted an immune stimulation experiment to observe the potential changes in the gene expression of C/EBP and Kr-h1 under immune challenge in different crayfish tissues. Our results showed that both C/EBP and Kr-h1 are closely related to other C/EBPs and Kr-h1s in Malacostraca. Gene expression analysis revealed that both TFs are present in all analysed tissues, with higher expression of C/EBP in the gills and Kr-h1 in the abdominal muscle. Immune stimulation with laminarin (mimicking β-1-3-glucan in the oomycete cell wall) showed an activation of the crayfish immune system, with an overall increase in the total haemocyte count (THC) compared to untreated control and crayfish buffered saline (CBS) treatment. On the gene expression level, an up-regulation of the C/EBP gene was detected in the laminarin treated group in hepatopancreas and heart, while no changes were observed for the Kr-h1 gene. Our results indicate an early change in C/EBP expression in multiple tissues during immune stimulation and suggest its involvement in the immune response of the noble crayfish.

Comparison analysis of circulating hemocytes in decapod crustaceans

Hemocytes are the primary immune cells of crustaceans. Few comparison studies have been done among different crustaceans and some key parameters of circulating hemocytes have not been investigated. Here, we compared the circulating hemocytes in six decapod crustaceans, Cherax quadrinatus, Procambarus clarkii, Penaeus vannamei, Penaeus monodon, Eriocheir sinensis, and Scylla paramamosain. Although the hemocytes of different species vary in size, they share common morphological characteristics. Based on their morphological features, circulating hemocytes can be basically classified into granular cells (GCs), semi-granular cells (SGCs), and hyaline cells (HCs). In the six decapods analyzed in this study, the proportion of GCs varied from 10 % to 30 %. P. vannamei, P. monodon, and P. clarkii had fewer GCs in circulation than the other three species. Correspondingly, proliferation was detected only in a small portion of cells in P. vannamei, P. monodon, and P. clarkii under physical conditions. The hemocyte renewal rates for P. clarkii, E. sinensis, and C. quadrinatus were 6.1 %, 5.1 %, and 1.5 % per day, while no steady new hemocyte production was found in S. paramamosain within six days. These data give a general picture of the similarities and differences of circulating hemocytes in decapods and provide a base for an in-depth study of their immune system.

Back to the future: Forgotten protocols for optimizing the isolation of arthropod haemocytes

Consideration is given to previous and more recent protocols for harvesting arthropod haemocytes from Galleria, Drosophila, mosquitoes, Limulus and crustaceans. The optimal harvesting of these cells is essential for meaningful studies of invertebrate immunity in vitro. The results of such experiments, however, have often been flawed due to a lack of understanding of the fragile nature of arthropod haemocytes on exposure to bacterial lipopolysaccharides, resulting in the aggregation and loss of cell types during haemolymph clotting. This article emphasizes that although there are similarities between mammalian neutrophils and arthropod haemocytes, the protocols required for the successful harvesting of these cells vary significantly. The various stages for the successful harvesting of arthropod haemocytes are described in detail and should provide invaluable advice to those requiring both high cell viability and recovery of the different cell types for subsequent experimentation.

Longitudinal tracking of hemocyte populations in vivo indicates lineage relationships and supports neural progenitor identity in adult neurogenesis

Adult neurogenesis, which takes place in both vertebrate and invertebrate species, is the process by which new neurons are born and integrated into existing functional neural circuits, long after embryonic development. Most studies in mammals suggest that self-renewing stem cells are the source of the new neurons, although the extent of self-renewal is a matter of debate. In contrast, research in the crayfish Procambarus clarkii has demonstrated that the neural progenitors producing adult-born neurons are capable of both self-renewing and consuming (non-self-renewing) divisions. However, self-renewing divisions are relatively rare, and therefore the production of adult-born neurons depends heavily on progenitors that are not replenishing themselves. Because the small pool of neural progenitors in the neurogenic niche is never exhausted throughout the long lives of these animals, we hypothesized that there must also be an extrinsic source of these cells. It was subsequently demonstrated that the neural progenitors originate in hemocytes (blood cells) produced by the immune system that travel in the circulation before ultimately integrating into niches where the neural lineage begins. The current study examines the developmental lineage of the three hemocyte types - hyaline (HC), semigranular (SGC) and granular (GC) cells - with the goal of understanding the origins of the progenitor cells that produce adult-born neurons. Longstanding qualitative metrics for hemocyte classification were validated quantitatively. Then, in a longitudinal study, proliferation markers were used to label the hemocytes in vivo, followed by sampling the circulating hemocyte population over the course of two months. Hemolymph samples were taken at intervals to track the frequencies of the different hemocyte types. These data reveal sequential peaks in the relative frequencies of HCs, SGCs and GCs, which were identified using qualitative and quantitative measures. These findings suggest that the three hemocyte types comprise a single cellular lineage that occurs in the circulation, with each type as a sequential progressive stage in hemocyte maturation beginning with HCs and ending with GCs. When combined with previously published data, this timeline provides additional evidence that HCs serve as the primary neural progenitor during adult neurogenesis in P. clarkii.

The melano-macrophage: The black leukocyte of fish immunity

Melanin and the process of melanin synthesis or melanogenesis have central roles in the immune system of insects, and production of melanin-synthesizing enzymes from their haemocytes may be induced following activation through danger signals. Melanin-containing macrophage-like cells have been extensively studied in amphibians and they are also present in reptiles. In fish, melano-macrophages are especially recognized with respect to melano-macrophage centres (MMCs), hypothesized to be analogues of germinal centres in secondary lymphoid organs of mammals and some birds. Melano-macrophages are in addition present in several inflammatory conditions, in particular melanised focal changes, or black spots, in the musculature of farmed Atlantic salmon, Salmo salar. Melanins are complex compounds that may be divided into different forms which all have the ability to absorb and scatter light. Other functions include the quenching of free radicals and a direct effect on the immune system. According to the common view held in the pigment cell community, vertebrate melanin synthesis with melanosome formation may only occur in cells of ectodermal origin. However, abundant information suggests that also myeloid cells of ectothermic vertebrates may be classified as melanocytes. Here, we discuss these opposing views and review relevant literature. Finally, we review the current status on the research concerning melanised focal muscle changes that represent the most severe quality problem in Norwegian salmon production, but also other diseases where melano-macrophages play important roles.

Single-cell RNA-seq revealed heterogeneous responses and functional differentiation of hemocytes against white spot syndrome virus infection in Litopenaeus vannamei

Shrimp hemocytes are the vital immune cells participating in innate immune response to defend against viruses. However, the lack of specific molecular markers for shrimp hemocyte hindered the insightful understanding of their functional clusters and differential roles in combating microbial infections. In this study, we used single-cell RNA sequencing to map the transcriptomic landscape of hemocytes from the white spot syndrome virus (WSSV)-infected Litopenaeus vannamei and conjointly analyzed with our previous published single-cell RNA sequencing technology data from the healthy hemocytes. A total of 16 transcriptionally distinct cell clusters were identified, which occupied different proportions in healthy and WSSV-infected hemocytes and exerted differential roles in antiviral immune response. Following mapping of the sequencing data to the WSSV genome, we found that all types of hemocytes could be invaded by WSSV virions, especially the cluster 8, which showed the highest transcriptional levels of WSSV genes and exhibited a cell type-specific antiviral response to the viral infection. Further evaluation of the cell clusters revealed the delicate dynamic balance between hemocyte immune response and viral infestation. Unsupervised pseudo-time analysis of hemocytes showed that the hemocytes in immune-resting state could be significantly activated upon WSSV infection and then functionally differentiated to different hemocyte subsets. Collectively, our results revealed the differential responses of shrimp hemocytes and the process of immune-functional differentiation post-WSSV infection, providing essential resource for the systematic insight into the synergistic immune response mechanism against viral infection among hemocyte subtypes.

IMPORTANCE

Current knowledge of shrimp hemocyte classification mainly comes from morphology, which hinder in-depth characterization of cell lineage development, functional differentiation, and different immune response of hemocyte types during pathogenic infections. Here, single-cell RNA sequencing was used for mapping hemocytes during white spot syndrome virus (WSSV) infection in Litopenaeus vannamei, identifying 16 cell clusters and evaluating their potential antiviral functional characteristics. We have described the dynamic balance between viral infestation and hemocyte immunity. And the functional differentiation of hemocytes under WSSV stimulation was further characterized. Our results provided a comprehensive transcriptional landscape and revealed the heterogeneous immune response in shrimp hemocytes during WSSV infection.

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.

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.

scRNA-seq Analysis of Hemocytes of Penaeid Shrimp Under Virus Infection

The classification of cells in non-model organisms has lagged behind the classification of cells in model organisms that have established cluster of differentiation marker sets. To reduce fish diseases, research is needed to better understand immune-related cells, or hemocytes, in non-model organisms like shrimp and other marine invertebrates. In this study, we used Drop-seq to examine how virus infection affected the populations of hemocytes in kuruma shrimp, Penaeus japonicus, which had been artificially infected with a virus. The findings demonstrated that virus infection reduced particular cell populations in circulating hemolymph and inhibited the expression of antimicrobial peptides. We also identified the gene sets that are likely to be responsible for this reduction. Additionally, we identified functionally unknown genes as novel antimicrobial peptides, and we supported this assumption by the fact that these genes were expressed in the population of hemocytes that expressed other antimicrobial peptides. In addition, we aimed to improve the operability of the experiment by conducting Drop-seq with fixed cells as a source and discussed the impact of methanol fixation on Drop-seq data in comparison to previous results obtained without fixation. These results not only deepen our understanding of the immune system of crustaceans but also demonstrate that single-cell analysis can accelerate research on non-model organisms.

Hemocyte coagulation and phagocytic behavior in early stages of injury in crayfish (Arthropoda: Decapoda) affect their morphology

Crustacean hemocytes are important mediators of immune functions such as coagulation and phagocytosis. We employed an in situ approach to investigate the ultrastructural behavior of hemocytes during coagulation and phagocytosis in the early stages after injury caused by leg amputation, using transmission electron microscopy technique in marbled crayfish Procambarus virginalis. Hemocytes underwent drastic morphological changes during coagulation. The morphology of the cytoplasmic granules changed from electron-dense to electron-lucent forms in an expanding manner. The transformed granules containing amorphous electron-lucent material were observed to merge and discharge their contents into extracellular space for coagulation. We also observed that the contents of the nucleus participate in the process of coagulation. In addition, leg amputation induced extensive muscle degeneration and necrotic tissues were avidly taken up by the phagocytic hemocytes containing distinct phagosomes. Interestingly, we observed for the first time how the digested contents of phagocytized necrotic tissues are incorporated into granules and other cellular components that change the cell morphology by increasing the granularity of the hemocytes. Nevertheless, the degranulation of hemocytes during coagulation can also reduce their granularity. Given that morphological traits are important criteria for hemocyte classification, these morphological changes that occur during coagulation and phagocytosis must be taken into account.