Temporal specificity and heterogeneity of Drosophila immune cells

Abstract

Immune cells provide defense against non‐self and have recently been shown to also play key roles in diverse processes such as development, metabolism, and tumor progression. The heterogeneity of Drosophila immune cells (hemocytes) remains an open question. Using bulk RNA sequencing, we find that the hemocytes display distinct features in the embryo, a closed and rapidly developing system, compared to the larva, which is exposed to environmental and metabolic challenges. Through single‐cell RNA sequencing, we identify fourteen hemocyte clusters present in unchallenged larvae and associated with distinct processes, e.g., proliferation, phagocytosis, metabolic homeostasis, and humoral response. Finally, we characterize the changes occurring in the hemocyte clusters upon wasp infestation, which triggers the differentiation of a novel hemocyte type, the lamellocyte. This first molecular atlas of hemocytes provides insights and paves the way to study the biology of the Drosophila immune cells in physiological and pathological conditions.

Identification of functionally-distinct macrophage subpopulations regulated by efferocytosis inDrosophila

“…Non-uniform expression has also been reported for plasmatocyte genes such as hemolectin [59], hemese, nimrod [60,61], croquemort [26], TGF-β family members [26] and the iron transporter malvolio [62], though some of these differences are likely due to incomplete differentiation from a pro-hemocyte state [25]. Recent transcriptional profiling approaches via scRNA-seq have suggested the existence of distinct larval blood cell populations in Drosophila [27,28]. One study interpreted this data as reflecting different progenitor/differentiation states [27]; another identified a number of potentially different functional groups, including more activated cell populations displaying expression signatures reflective of active Toll and JNK signalling [28].…”

“…Recent transcriptional profiling approaches via scRNA-seq have suggested the existence of distinct larval blood cell populations in Drosophila [27,28]. One study interpreted this data as reflecting different progenitor/differentiation states [27]; another identified a number of potentially different functional groups, including more activated cell populations displaying expression signatures reflective of active Toll and JNK signalling [28]. Our identification of developmentally-regulated subpopulations, coupled with this recent evidence from larvae, points to heterogeneity within the plasmatocyte lineage.…”

“…Additionally, modes of migration to sites of injury are similar in embryos and pupae (directional migration [19,63]), but larval cells are captured from circulation via adhesion [64]. These functional differences are reflected in molecular differences between embryonic and larval blood cells revealed via bulk RNA-seq [28], with reprogramming within larvae potentially explaining why our VT enhancer-labelled subpopulations are absent at that stage. Transcriptional changes are also associated with steroid hormone-mediated signalling in pupae [37], which may drive re-emergence of subpopulations in time for metamorphosis.…”

“…Hints that Drosophila plasmatocytes may exhibit heterogeneity exist in the literature with variation in marker expression observed in larval hemocytes [25] and non-uniform expression of TGF-β homologues upon injury or infection in adults [26]. Recent single-cell RNA-sequencing (scRNA-seq) experiments performed on larval hemocytes demonstrated the presence of multiple clusters of cells, which were interpreted as representing either different stages of differentiation or functional groupings [27,28]. However, the in vivo identification of subtypes and insights into the roles and specification mechanisms of potential macrophage subtypes in Drosophila has not yet been described.…”

Constitutive activation of cellular immunity underlies the evolution of resistance to infection

“…The global changes in hemocyte transcriptomes could result from a change in the relative proportion of cell types in the hemocyte population, or from a shift in gene expression within existing cell types. The three classical Drosophila hemocyte types were originally defined morphologically, but emerging single cell transcriptomic data suggests that Drosophila hemocyte populations are more complex than this 16–19 . Excluding cells that did not express the pan-hemocyte markers Hemese or Serpent 20,21 , we grouped 19,344 larval hemocytes into nine clusters based on their transcriptomes (Fig.…”