Identification and Functional Analysis of Cytokine-Like Protein CLEC-47 in Caenorhabditis elegans

A variety of effector proteins contribute to host defense in Caenorhabditis elegans. However, beyond lytic enzymes and antimicrobial peptides and proteins, little is known about the exact function of these infection-related effectors. This study set out to identify pathogen-dependent cytokine-like molecules, focusing on C-type lectin domain-containing proteins (CLECs). In total, 38 CLECs that are differentially regulated in response to bacterial infections have been previously identified by microarray and transcriptome sequencing (RNA-seq) analyses in C. elegans. We successfully cloned 18 of these 38 CLECs and chose to focus on CLEC-47 because, among these 18 cloned CLECs, it was the smallest protein and was recombinantly expressed at the highest levels in prokaryotic cells examined by SDS-PAGE. Quantitative real-time PCR (qRT-PCR/qPCR) showed that the expression of clec-47 was induced by a variety of Gram-positive bacterial pathogens, including Enterococcus faecium, Staphylococcus aureus, and Cutibacterium acnes, but was suppressed by the Gram-negative bacteria Klebsiella pneumoniae and Pseudomonas aeruginosa. By expressing CLEC-47 in HEK 293 cells, we showed that CLEC-47 is released into the culture media, which the Golgi apparatus inhibitors (brefeldin A [BFA] and GolgiStop) could block. Purified recombinant CLEC-47 (maltose binding protein [MBP]-CLEC-47-His) did not display antimicrobial activity against ESKAPE pathogen isolates but bound directly to murine macrophage J774A.1 cells. Recombinant CLEC-47 attracted and recruited J774A.1 cells in a chemotaxis assay. In addition, qPCR studies and enzyme-linked immunosorbent assays (ELISAs) showed that CLEC-47 activates J774A.1 cells in a dose- and time-dependent manner to express the proinflammatory cytokines tumor necrosis factor alpha (TNF-α), interleukin-1β (IL-1β), IL-6, and Macrophage Inflammatory Protein 2 (MIP-2). Moreover, C. elegans, fed with CLEC-47-expressing Escherichia coli, demonstrated enhanced expression of several antimicrobial proteins (CNC-1, CNC-2, CPR-1, and CPR-2) as well as the detoxification protein MTL-1. These data suggest that CLEC-47 functions as a novel cytokine-like signaling molecule and exemplify how the study of infection-related effectors in C. elegans can help elucidate the evolution of immune responses. IMPORTANCE A variety of effector proteins contribute to host defense in the nematode Caenorhabditis elegans. However, little is known about the exact function of these infection-related effectors beyond lytic enzymes and antimicrobial peptides and proteins. This study set out to identify pathogen-dependent cytokine-like molecules, and we focus on the C-type lectin domain-containing proteins (CLECs). Our data suggest that CLEC-47 functions as a novel cytokine-like signaling molecule and exemplify how the study of infection-related effectors in nematodes can help elucidate the evolution of immune responses.