Microbial danger signals control transcriptional induction of distinct MHC class I L lineage genes in Atlantic salmon

Abstract

Antigen processing and presentation by major histocompatibility complex (MHC) molecules is a cornerstone in vertebrate immunity. Like mammals, teleosts possess both classical MHC class I and multiple families of divergent MHC class I genes. However, while certain mammalian MHC class I-like molecules have proven to be integral in immune regulation against a broad array of pathogens, the biological relevance of the different MHC class I lineages in fish remains elusive. This work focuses on MHC class I L lineage genes and reveals unique regulatory patterns of six genes (<i>Sasa-lia, Sasa-lda, Sasa-lca, Sasa-lga, Sasa-lha</i>, and <i>Sasa-lfa</i>) in antimicrobial immunity of Atlantic salmon (<i>Salmo salar L.</i>). Using two separate <i>in vivo</i> challenge models with different kinetics and immune pathologies combined with <i>in vitro</i> stimulation using viral and bacterial TLR ligands, we show that <i>de novo</i> synthesis of different L lineage genes is distinctly regulated in response to various microbial stimuli. Prior to the onset of classical MHC class I gene expression, <i>lia</i> was rapidly and systemically induced <i>in vivo</i> by the single-stranded (ss) RNA virus salmonid alpha virus 3 (SAV3) but not in response to the intracellular bacterium <i>Piscirickettsia salmonis</i>. In contrast, <i>lga</i> expression was upregulated in response to both viral and bacterial stimuli. A role for distinct MHC class I L-lineage genes in anti-microbial immunity in salmon was further substantiated by a marked upregulation of <i>lia</i> and <i>lga</i> gene expression in response to type I IFNa stimulation <i>in vitro</i>. Comparably, <i>lha</i> showed no transcriptional induction in response to IFNa stimulation but was strongly induced in response to a variety of viral and bacterial TLR ligands. In sharp contrast, <i>lda</i> showed no response to viral or bacterial challenge. Similarly, induction of <i>lca</i>, which is predominantly expressed in primary and secondary lymphoid tissues, was marginal with the exception of a strong and transient upregulation in pancreas following SAV₃ challenge Together, these findings suggest that certain Atlantic salmon MHC class I L lineage genes play important and divergent roles in early anti-microbial response and that their regulation, in response to different activation signals, represents a system for selectively promoting the expression of distinct non-classical MHC class I genes in response to different types of immune challenges.