Primary culture of the region of the amebocyte-producing organ of the snail Biomphalaria glabrata, the intermediate host of Schistosoma mansoni

Single cell RNA sequencing reveals hemocyte heterogeneity in Biomphalaria glabrata: Plasticity over diversity

The freshwater snail Biomphalaria glabrata is an intermediate host of Schistosoma mansoni, the agent of human intestinal schistosomiasis. However, much is to be discovered about its innate immune system that appears as a complex black box, in which the immune cells (called hemocytes) play a major role in both cellular and humoral response towards pathogens. Until now, hemocyte classification has been based exclusively on cell morphology and ultrastructural description and depending on the authors considered from 2 to 5 hemocyte populations have been described. In this study, we proposed to evaluate the hemocyte heterogeneity at the transcriptomic level. To accomplish this objective, we used single cell RNA sequencing (scRNAseq) technology coupled to a droplet-based system to separate hemocytes and analyze their transcriptome at a unique cell level in naive Biomphalaria glabrata snails. We were able to demonstrate the presence of 7 hemocyte transcriptomic populations defined by the expression of specific marker genes. As a result, scRNAseq approach showed a high heterogeneity within hemocytes, but provides a detailed description of the different hemocyte transcriptomic populations in B. glabrata supported by distinct cellular functions and lineage trajectory. As a main result, scRNAseq revealed the 3 main population as a super-group of hemocyte diversity but, on the contrary, a great hemocytes plasticity with a probable capacity of hemocytes to engage to different activation pathways. This work opens a new field of research to understand the role of hemocytes particularly in response to pathogens, and towards S. mansoni parasites.

Interaction of Biomphalaria cells in primary cultures with Schistosoma mansoni sporocysts

INTRODUCTION:

Biomphalaria snails may display varying levels of susceptibility to Schistosoma mansoni infection. We have been developing an in vitro model to study the interaction between the snail and the parasite, using tissue-derived cell cultures from Biomphalaria.

METHODS:

The digestive gland- and kidney-derived cells from primary cultures of resistant (B. tenagophila Taim) and susceptible (B. tenagophila HM and B. glabrata BH) strains of Biomphalaria were exposed to S. mansoni sporocysts.

RESULTS:

S. mansoni sporocysts were surrounded and encapsulated exclusively by cells derived from the digestive gland (DG) of B. tenagophila Taim. The process was followed by a marked decrease in the number of free sporocysts in the culture medium. The morphological characteristics of DG-derived cells in culture have been described.

CONCLUSIONS:

Cells derived from DG (but not SK) primary cultures of B. tenagophila Taim may participate in S. mansoni sporocyst control.

Hemócitos de Bradybaena similaris e Megalobulimus abbreviatus (Gastropoda, Stylommatophora)

Os hemócitos atuam no sistema de defesa contra organismos invasores e partículas estranhas, auxiliando o reconhecimento do que é próprio do corpo dos grastrópodes e o que não é. São escassas as informações e estudos sobre os hemócitos em espécies de moluscos saudáveis (sem infecções), principalmente em Bradybaena similaris (Fèrussac, 1821) and Megalobulimus abbreviatus (Bequaert, 1948). Portanto, este trabalho tem como objetivos a caracterização e quantificação dos hemócitos presentes na hemolinfa destas duas espécies. Neste trabalho, foram identificados três tipos celulares na hemolinfa de ambas espécies: as células redondas, hialinócitos e granulócitos. Os três tipos de hemócitos foram medidos e foi calculada a média do diâmetro total e do núcleo para cada um deles. Para B. similaris, o diâmetro médio das células redondas foi de 10,7 µm, dos hialinócitos foi de 20 µm e dos granulócitos de 25,4 µm. Para M. abbreviatus, o diâmetro médio foi de 11,7 µm para as células redondas, de 21,5 µm para os hialinócitos e de 30,5 µm para os granulócitos. Embora os hialinócitos possuam médias parecidas entre B. similaris e M. abbreviatus, foram detectadas diferenças significativas do diâmetro celular total e diâmetro do núcleo (p<0,0001) dessas células entre as espécies estudadas. A densidade média de células por ml, sem distinção de tipo celular foi de 197.813 células/ml para M. abbreviatus, e de 416.333 células/ml para B. similaris. Diferentemente de outros gastrópodes, os hemócitos mais frequentes em M. abbreviatus e em B. similaris foram os hialinócitos.

Multicellular spheroid formation and evolutionary conserved behaviors of apple snail hemocytes in culture

A hemocyte primary culture system for Pomacea canaliculata in a medium mimicking hemolymphatic plasma composition was developed. Hemocytes adhered and spread onto culture dish in the first few hours after seeding but later began forming aggregates. Time-lapse video microscopy showed the dynamics of the early aggregation, with cells both entering and leaving the aggregates. During this period phagocytosis occurs and was quantified. Later (>4 h), hemocytes formed large spheroidal aggregates that increased in size and also merged with adjacent spheroids (24-96 h). Large single spheroids and spheroid aggregates detach from the bottom surface and float freely in the medium. Correlative confocal, transmission electron and phase contrast microscopy showed a peculiar organization of the spheroids, with a compact core, an intermediate zone with large extracellular lacunae and an outer zone of flattened cells; also, numerous round cells emitting cytoplasmic extensions were seen attaching to the spheroids' smooth surface. Dual DAPI/propidium iodide staining revealed the coexistence of viable and non-viable cells within aggregates, in varying proportions. DNA concentration increased during the first 24 h of culture and stabilized afterward. BrdU incorporation also indicated proliferation. Spontaneous spheroid formation in culture bears interesting parallels with spheroidal hemocyte aggregates found in vivo in P. canaliculata, and also with spheroids formed by tumoral or non-tumoral mammalian cells in vitro.

Schistosoma mansoni in susceptible and resistant snail strains Biomphalaria tenagophila: in vivo tissue response and in vitro hemocyte interactions

Schistosomiasis is a parasitic disease that is highly prevalent, especially in developing countries. Biomphalaria tenagophila is an important invertebrate host of Schistosoma mansoni in Brazil, with some strains (e.g. Cabo Frio) being highly susceptible to the parasite, whereas others (e.g. Taim) are completely resistant to infection. Therefore, B. tenagophila is an important research model for studying immune defense mechanisms against S. mansoni. The internal defense system (IDS) of the snail comprises hemocytes and hemolymph factors acting together to recognize self from non-self molecular patterns to eliminate the threat of infection. We performed experiments to understand the cellular defenses related to the resistance and/or susceptibility of B. tenagophila to S. mansoni. During the early stages of infection, fibrous host cells of both snail strains were arranged as a thin layer surrounding the sporocysts. However, at later stages of infection, the cellular reactions in resistant snails were increasingly more intense, with thicker layers surrounding the parasites, in contrast to susceptible strains. All parasites were damaged or destroyed inside resistant snails after 10 h of infection. By contrast, parasites inside susceptible snails appeared to be morphologically healthy. We also performed experiments using isolated hemocytes from the two strains interacting with sporocysts. Hemocyte attachment started as early as 1 h after initial infection in both strains, but the killing of sporocysts was exclusive to hemocytes from the resistant strain and was time course dependent. The resistant strain was able to kill all sporocysts. In conclusion, our study revealed important aspects of the initial process of infection related to immune defense responses of strains of B. tenagophila that were resistant to S. mansoni compared with strains that were susceptible. Such information is relevant for the survival or death of the parasites and so is important in the development of control measures against this parasite.

Morphological characterization of hemocytes from Biomphalaria glabrata and Biomphalaria straminea

Biomphalaria glabrata and Biomphalaria straminea have been identified as intermediate hosts for Schistosoma mansoni. Several studies have found two cell types in the hemolymph of B. glabrata (hyalinocytes and granulocytes). However, there are no studies describing the hemocytes of B. straminea. With the aim of further describing the hemocyte subsets in B. glabrata and B. straminea, we conducted a detailed study using optical microscopy and transmission electron microscopy. Based on the morphological characteristics of the cells, we identified the same types of hemocytes in two species of molluscs, namely: blast-like cells, granulocytes, type I hyalinocytes, type II hyalinocytes and type III hyalinocytes. Blast-like cells had a spherical profile with a central nucleus filling almost the whole cell. Granulocytes were characterized by presenting variable numbers of granules. Type I hyalinocytes were the most abundant cell type and displayed various cytoplasmic projections. Type II and type III hyalinocytes had never previously been reported. They were few in number and were characterized by having an eccentric nucleus. From these results, it is concluded that there are five types of cells in the hemolymph of B. glabrata and B. straminea. Further studies are now needed to identify the role of these hemocytes in the immune response of these snails.

Behaviour of albino and melanic variants of Biomphalaria glabrata Say, 1818 (Mollusca: Planorbidae) following infection by Schistosoma mansoni Sambon, 1907

The behaviour of the albino and melanic variants of Biomphalaria glabrata of Belo Horizonte (MG. Brazil) was studied comparatively, in terms of their respective susceptibilities to infection by Schistosoma mansoni of the same origin, through observation of the elimination of cercariae for a three-month period and the calculation of mortality and infection rates, in control and in infected snails. The number of amoebocytes, granulocytes and hyalinocytes in the circulating hemolymph during different periods of infection was analyzed. The evolution of the infection in the tissues was observed by means of histological cross-sections. The melanic variant showed greater susceptibility to infection and a higher mortality rate. The albino variant showed a higher number of circulating amoebocytes, both granulocytes and hyalinocytes. A higher number of degenerated sporocysts were seen in the histological cross-sections of the albino variant. The results suggest that the melanic variant of B. glabrata was more susceptible to infection by S. mansoni than was the albino variant.

In vitro mitotic responses of the amebocyte-producing organ of Biomphalaria glabrata to extracts of Schistosoma mansoni

Amebocyte-producing organs (APOs) of Biomphalaria glabrata were maintained in nonnutritive saline with, or without, extracts of miracidia and adults of Schistosoma mansoni, and examined histologically. The hematopoietic cells remained viable and showed measurable mitotic activity for up to 6 days, with little evidence of tissue death. APOs accumulated fluid and became swollen by as soon as 24 hr, but no cell exomigration was observed. Parasite extracts elicited an increase in the number of dividing cells in the APO, suggesting that the extract may directly stimulate a response from the hematopoietic cells by providing either nutrients or mitogenic growth factors.

Transmission control of schistosomiasis mansoni by introduction of a resistant strain of Biomphalaria tenagophila in areas where transmission is maintained by this species

Biomphalaria tenagophila Taim lineage has proved to be consistently resistant to Schistosoma mansoni. Several published works have shown that this resistance is due to the innate defence system of that strain, and in cross-breedings with susceptible strains the Taim lineage presents dominant character. These findings led to the hypothesis that, introducing this strain in areas where transmission of schistosomiasis is maintained by this species, the introduced lineage would perform cross-breeding with the local snails, thus generating offsprings resistant to the parasites. The perspectives of the proposed approach, as well as some preliminary results and problems related to the first introduction without a previous use of molluscicide are discussed.