The inflammasome adaptor <i>pycard</i> is essential for immunity against <i>Mycobacterium marinum</i> infection in adult zebrafish

Inflammasomes regulate the host response to intracellular pathogens including mycobacteria. We have previously shown that the course of Mycobacterium marinum infection in adult zebrafish (Danio rerio) mimics the course of tuberculosis in human. To investigate the role of the inflammasome adaptor pycard in zebrafish M. marinum infection, we produced two zebrafish knockout mutant lines for the pycard gene with CRISPR/Cas9 mutagenesis. Although the zebrafish larvae lacking pycard developed normally and had unaltered resistance against M. marinum, the loss of pycard led to impaired survival and increased bacterial burden in the adult zebrafish. Based on histology, immune cell aggregates, granulomas, were larger in pycard-deficient fish than in wild-type controls. Transcriptome analysis with RNA sequencing of a zebrafish haematopoietic tissue, kidney, suggested a role for pycard in neutrophil-mediated defence, haematopoiesis and myelopoiesis during infection. Transcriptome analysis of fluorescently labelled, pycard-deficient kidney neutrophils identified genes that are associated with compromised resistance, supporting the importance of pycard for neutrophil-mediated immunity against M. marinum. Our results indicate that pycard is essential for resistance against mycobacteria in adult zebrafish.