Abstract
A wide range of hemocyte types exist in insects but a full definition of the different subclasses is not yet established. The current knowledge of the classification of silkworm hemocytes mainly comes from morphology rather than specific markers, so our understanding of the detailed classification, hemocyte lineage and functions of silkworm hemocytes is very incomplete. Bombyx mori nucleopolyhedrovirus (BmNPV) is a representative member of the baculoviruses and a major pathogen that specifically infects silkworms (Bombyx mori) and causes serious losses in sericulture industry. Here, we performed single-cell RNA sequencing (scRNA-seq) of hemocytes in BmNPV and mock-infected larvae to comprehensively identify silkworm hemocyte subsets and determined specific molecular and cellular characteristics in each hemocyte subset before and after viral infectmadion. A total of 20 cell clusters and their potential marker genes were identified in silkworm hemocytes. All of the hemocyte clusters were infected by BmNPV at 3 days after inoculation. Interestingly, BmNPV infection can cause great changes in the distribution of hemocyte types. The cells appearing in the infection group mainly belong to prohemocytes (PR), while plasmatocytes (PL) and granulocytes (GR) are very much reduced. Furthermore, we found that BmNPV infection suppresses the RNA interference (RNAi) and immune response in the major hemocyte types. In summary, our results revealed the diversity of silkworm hemocytes and provided a rich resource of gene expression profiles for a systems-level understanding of their functions in the uninfected condition and as a response to BmNPV.
Highlights
Insects have an efficient and potent innate immune system to discriminate and respond to invading pathogenic microorganism [1]
We used the 10x Genomics platforms to perform 3′ scRNA-seq on pooled hemocytes collected from Bombyx mori nucleopolyhedrovirus (BmNPV)-infected silkworm and phosphate-buffered saline (PBS)-treated controls (Figure 1A)
The clusters with the highest numbers of Differentially Expressed Genes (DEGs) and enrichment in immune-related host response processes correspond to the clusters that are predominant in control hemocytes
Summary
Insects have an efficient and potent innate immune system to discriminate and respond to invading pathogenic microorganism [1]. The innate immune system of insects is divided into humoral defenses that include the production of soluble effector molecules (like antimicrobial peptides) and cellular defenses like phagocytosis and encapsulation that are mediated by hemocytes [2]. In Lepidopteran insects such as the silkworm, hemocytes are generally classified into five major subsets including prohemocytes (PR), plasmatocytes (PL), granulocytes (GR), spherulocytes (SP), and oenocytoids (OE) based on morphology and function [2,3,4]. Plasmatocytes and granulocytes are the only hemocyte subsets capable of adhering to foreign surfaces, and together usually comprise more than 50% of the hemocytes in circulation in the larval stages [3]. Plasmatocytes and granulocytes are involved in most cellular defense responses [2, 3]. Oenocytoids are in rich in prophenoloxidase and mainly participate in melanization, whereas the functions of spherulocytes are unknown [6, 7]
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