Perturbation of gut bacteria induces a coordinated cellular immune response in the purple sea urchin larva

Abstract

The purple sea urchin (Strongylocentrotus purpuratus) genome sequence contains a complex repertoire of genes encoding innate immune recognition proteins and homologs of important vertebrate immune regulatory factors. To characterize how this immune system is deployed within an experimentally tractable, intact animal, we investigate the immune capability of the larval stage. Sea urchin embryos and larvae are morphologically simple and transparent, providing an organism-wide model to view immune response at cellular resolution. Here we present evidence for immune function in five mesenchymal cell types based on morphology, behavior and gene expression. Two cell types are phagocytic; the others interact at sites of microbial detection or injury. We characterize immune-associated gene markers for three cell types, including a perforin-like molecule, a scavenger receptor, a complement-like thioester-containing protein and the echinoderm-specific immune response factor 185/333. We elicit larval immune responses by (1) bacterial injection into the blastocoel and (2) seawater exposure to the marine bacterium Vibrio diazotrophicus to perturb immune state in the gut. Exposure at the epithelium induces a strong response in which pigment cells (one type of immune cell) migrate from the ectoderm to interact with the gut epithelium. Bacteria that accumulate in the gut later invade the blastocoel, where they are cleared by phagocytic and granular immune cells. The complexity of this coordinated, dynamic inflammatory program within the simple larval morphology provides a system in which to characterize processes that direct both aspects of the echinoderm-specific immune response as well as those that are shared with other deuterostomes, including vertebrates.

Cdc42 controls primary mesenchyme cell morphogenesis in the sea urchin embryo

“…To further assess the specificity of Cdc42 in PMC filopodia formation and eliminate the possibility of non-specific effects on actin-based motility, we examined Cdc42 inhibition on pigment cells, a population of non-skeletogenic mesenchymal cells that play a role in innate immune responses and display pseudopodial motility (Ch Ho et al, 2016). Pigment cells in control embryos were found subjacent to the ectodermal layer with multiple cellular protrusions (Fig.…”

IL17 factors are early regulators in the gut epithelium during inflammatory response to Vibrio in the sea urchin larva

“…Larvae have several types of immune cells (Ch Ho et al, 2016) including a granular cell population known as ‘pigment cells’ and a heterogeneous suite of several types of ‘blastocoelar cells’ that populate the body cavity (blastocoel) (Solek et al, 2013; Tamboline and Burke, 1992; Gibson and Burke, 1985). Collectively, these cells mediate the larval immune response through surveillance-like motility, phagocytosis, expression of immune effectors and regulatory cell-cell interactions (Ch Ho et al, 2016). The simplicity and optical transparency of the sea urchin larva allows visualization and quantification of the immune response on an organism-wide scale at single-cell resolution.…”

Bacterial Exposure Mediates Developmental Plasticity and Resistance to Lethal Vibrio lentus Infection in Purple Sea Urchin (Strongylocentrotus purpuratus) Larvae

“…Echinoderms have been important research organisms in cell and developmental biology from the work of the nineteenth century embryologists to modern day (16)(17)(18)(19). Ilya Metchnikoff first described cellular immunity in echinoderm larvae in the late 1800s (20), and recently, researchers have returned to these organisms with modern genomic, experimental, and imaging techniques, describing the specialized immune cell types of echinoderm larvae and their functions in response to diverse challenges [e.g., (21)(22)(23)(24); reviewed in (17,19,25)]. In parallel, embryonic, larval, and adult echinoderm bacterial microbiomes have been characterized in several species [e.g., (22,(26)(27)(28)(29)(30)(31); reviewed in (19)].…”

“…Ilya Metchnikoff first described cellular immunity in echinoderm larvae in the late 1800s (20), and recently, researchers have returned to these organisms with modern genomic, experimental, and imaging techniques, describing the specialized immune cell types of echinoderm larvae and their functions in response to diverse challenges [e.g., (21)(22)(23)(24); reviewed in (17,19,25)]. In parallel, embryonic, larval, and adult echinoderm bacterial microbiomes have been characterized in several species [e.g., (22,(26)(27)(28)(29)(30)(31); reviewed in (19)]. These studies focused on bacteria acquired in natural seawater or in the wild, but major questions that remain are to what degree artificial seawater-raised laboratory animals recapitulate the microbiota of their wild counterparts (32,33) and how these differences Abbreviations: FSW, artificial filtered seawater; P/S, 0.2 µm-filtered artificial seawater + 100 U/mL penicillin, 100 µg/mL streptomycin; AEW, artificial filtered seawater + 20% (v/v) adult-exposed tank water; CFU, colony-forming unit; dpf, days post fertilization; NSW, natural seawater; hpf, hours post fertilization; OTU, operational taxonomic unit; PCoA, principle coordinate analysis.…”

“…Vibrios are a widespread genus of aquatic Gammaproteobacteria that are pathogenic in a diverse array of organisms, including larval sea urchins (e.g., V. diazotrophicus, V. cyclitrophicus, and V. splendidus) (22), adult sea urchins (e.g., V. diazotrophicus, V. anguillarum) (40), and humans (e.g., V. cholerae, V. vulnificus, and V. parahemolyticus) (41). In S. purpuratus larvae, exposure to V. diazotrophicus induces numerous changes in immune cell behavior: the gut epithelium thickens, occluding the lumen, and specialized immunocytes become active-most notably pigment cells, which round up, migrate from the ectoderm into the blastocoel and to the gut surface, and form associations with other immune cells, including globular and filopodial cells (19,22). In later infection, V. diazotrophicus invades the blastocoel and is phagocytosed by a subclass of phagocytic cells (22).…”