Immunoprofiling of Drosophila Hemocytes by Single-cell Mass Cytometry

Abstract Single-cell mass cytometry (SCMC) combines features of traditional flow cytometry (FACS) with mass spectrometry, making it possible to measure several parameters at the single-cell level for a complex analysis of biological regulatory mechanisms. In this study, we optimized SCMC to analyze hemocytes of the Drosophila innate immune system. We used metal-conjugated antibodies (H2, H3, H18, L1, L4, and P1 at the cell surface, intracellular 3A5 and L2) and anti-IgM (L6 at the cell surface) to detect the levels of antigens, while anti-GFP was used to detect crystal cells in the immune induced samples. We investigated the antigen expression profile of single cells and hemocyte populations in naive states, in immune induced states, in tumorous mutants bearing a driver mutation in the Drosophila homologue of Janus kinase (hopTum) and carrying deficiency of a tumor suppressor l(3)mbn1 gene, as well as in stem cell maintenance-defective hdcΔ84 mutant larvae. Multidimensional analysis enabled the discrimination of the functionally different major hemocyte subsets for lamellocytes, plasmatocytes, and crystal cells, and delineated the unique immunophenotype of Drosophila mutants. We have identified subpopulations of L2+/P1+ (l(3)mbn1), L2+/L4+/P1+ (hopTum) transitional phenotype cells in the tumorous strains and a subpopulation of L4+/P1+ cells upon immune induction. Our results demonstrated for the first time that SCMC, combined with multidimensional bioinformatic analysis, represents a versatile and powerful tool to deeply analyze the regulation of cell-mediated immunity of Drosophila.