Combined cellular and proteomics approach suggest differential processing of native and foreign vibrios in the sponge Halichondria panicea

Phagocytosis is a conserved cellular mechanism for food uptake, defense and general animal-microbe interactions in metazoans. How the discrimination and subsequent processing of different microbes in marine invertebrates is facilitated remains largely unknown. Thereto, we combined a recently developed phagocytic assay with proteomics analysis to compare the phagocytic activity of the sponge Halichondria panicea upon encounter with a native (i.e., H. panicea isolate; Hal 281) and a foreign (i.e., Nematostella vectensis isolate; NJ 1) Vibrio. The sponge cell fraction was recovered after Vibrio exposure of 30 and 60 min and used for cellular (fluorescence-activated cell sorting and microscopy) and proteomics analyses. While the number of phagocytic active cells was similar between the isolates (p = 0.19), the distribution of vibrios over cell types differed (p = 0.02) with the tendency for a higher accumulation of foreign vibrios in small phagocytic cells than the native vibrios. Initially, both vibrios elicited a mitochondrial proteomic response (e.g., TBXASI, CASP7), whereas after 60 min differentially abundant proteins were dominated by lysosomal proteins (e.g., AGA, FUCA, SCARB2) in response to the native compared to immune-related proteins (e.g., AQP9, H2A, SAMHDI) in response to the foreign vibrio. H. panicea may discriminate native and foreign vibrios post-cellular internalization into choanocytes, followed by differential cellular and molecular processing: Native vibrios are readily transferred to archeocytes and elicit a digestive response, whereas foreign vibrios further accumulate before being transferred and provoke an immune response. These findings provide a mechanism for immune specificity in sponges. ImportanceMetazoans recognize and discriminate between native (i.e., associated and/or commonly encountered) and foreign microbes. In marine invertebrates, the mechanisms of microbial discrimination and immune specificity are not well understood. Phagocytosis is a conserved cellular process from amoeba to humans that facilitates the ingestion and digestion of microbial cells and likely plays a role in this discrimination. To elucidate the molecular and cellular basis of immune specificity in marine invertebrates, we examined the differential phagocytic processing of native and foreign Vibrio isolates in a marine sponge. Our findings revealed that both vibrios provoke a generalist mitochondrial protein-mediated response, while native vibrios trigger a digestive response, and foreign vibrios elicit an immune response after being internalized into sponge cells. This study uncovers a mechanism for immune specificity in an early-divergent metazoan and provides a valuable model for studying the evolution of immunity and its role in animal-microbe interactions.