The inflammasome adaptor pycard is essential for immunity against Mycobacterium marinum infection in adult zebrafish

Inflammasome regulates 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 knock-out mutant lines for the pycard gene with CRISPR/Cas9 mutagenesis. While the zebrafish larvae lacking pycard develop normally and have 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 compared to wild-type controls. Transcriptome analysis with RNA sequencing of a zebrafish haematopoietic tissue, kidney, suggests 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.