Single-cell transcriptome maps of myeloid blood cell lineages in Drosophila

Abstract

The Drosophila lymph gland, the larval hematopoietic organ comprised of prohemocytes and mature hemocytes, has been a valuable model for understanding mechanisms underlying hematopoiesis and immunity. Three types of mature hemocytes have been characterized in the lymph gland: plasmatocytes, lamellocytes, and crystal cells, which are analogous to vertebrate myeloid cells, yet molecular underpinnings of the lymph gland hemocytes have been less investigated. Here, we use single-cell RNA sequencing to comprehensively analyze heterogeneity of developing hemocytes in the lymph gland, and discover previously undescribed hemocyte types including adipohemocytes, stem-like prohemocytes, and intermediate prohemocytes. Additionally, we identify the developmental trajectory of hemocytes during normal development as well as the emergence of the lamellocyte lineage following active cellular immunity caused by wasp infestation. Finally, we establish similarities and differences between embryonically derived- and larval lymph gland hemocytes. Altogether, our study provides detailed insights into the hemocyte development and cellular immune responses at single-cell resolution.

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

“…We therefore used the same data analysed above but investigated gene expression within individual cells. 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 ( Cattenoz et al, 2020 ; Cho et al, 2020 ; Fu et al, 2020 ; Tattikota et al, 2020 ). Excluding cells that did not express the pan-hemocyte markers Hemese or Serpent ( Banerjee et al, 2019 ; Bernardoni et al, 1997 ), we grouped 19,344 larval hemocytes into nine clusters based on their transcriptomes ( Figure 3A ).…”

“…Our results support the hypothesis that natural selection favours constitutive defences when infection is common ( Boots and Best, 2018 ; Shudo and Iwasa, 2001 ; Westra et al, 2015 ) by demonstrating that an inducible immune response becomes constitutive when populations frequently encounter a parasite. The induced immune response to parasitoids relies on the differentiation of lamellocytes from embryonic-derived plasmatocytes, lymph gland prohemocytes ( Honti et al, 2014 ) and lymph gland plasmatocytes ( Cho et al, 2020 ). Evidence for immature lamellocytes during infection has previously come from patterns of marker gene expression ( Anderl et al, 2016 ; Honti et al, 2010 ) and recent scRNAseq data ( Cattenoz et al, 2020 ; Tattikota et al, 2020 ).…”

“…Crystal cells are a less abundant specialized cell type that produces the prophenoloxidase molecules PPO1 and PPO2, which are processed into enzymes required for the melanization of parasites and wound healing ( Dudzic et al, 2015 ). Infection can trigger an induced response, where these cell types proliferate, and cells called lamellocytes differentiate from circulating plasmatocytes, and prohemocytes and plasmatocytes in the lymph gland ( Cho et al, 2020 ; Honti et al, 2014 ). Lamellocytes are large flat cells with a specialised role in encapsulating and killing large parasites like parasitic wasps (parasitoids).…”

Immune Cell Production Is Targeted by Parasitoid Wasp Virulence in a Drosophila–Parasitoid Wasp Interaction

“…These data suggest that AsDen infection may be inhibiting a specific aspect of lamellocyte transdifferentiation or maturation, consistent with the finding that msn expression coincides with early morphological changes in transdifferentiating hemocytes [ 49 ]. Recent studies have uncovered a broader range of Drosophila hemocyte subtypes than previously appreciated [ 63 , 64 , 65 , 66 ], and future investigation into this complexity may help to unravel the specific effects of AsDen venom on host hemocytes and lamellocyte production.…”

Parasitoid wasp venom targets host immune cell production in aDrosophila-parasitoid interaction

“…Recent studies have uncovered a broader range of Drosophila hemocyte subtypes than previously appreciated [63][64][65][66], and future investigation into this complexity may help to unravel the specific effects of AsDen venom on host hemocytes and lamellocyte production.…”