Transcriptome analysis reveals strong and complex antiviral response in a mollusc.

Abstract Viruses are highly abundant in the oceans, and how filter-feeding molluscs without adaptive immunity defend themselves against viruses is not well understood. We studied the response of a mollusc Crassostrea gigasto Ostreid herpesvirus 1µVar (OsHV-1μVar) infections using transcriptome sequencing. OsHV-1μVar can replicate extremely rapidly after challenge of C. gigasas evidenced by explosive viral transcription and DNA synthesis, which peaked at 24 and 48 h post-inoculation, respectively, accompanied by heavy oystermortalities. At 120 h post-injection, however, viral gene transcription and DNA load, and oystermortality, were greatly reduced indicating an end of active infections and effective control of viral replication in surviving oysters. Transcriptome analysis of the host revealed strong and complex responsesinvolving the activation of all major innate immune pathways that are equipped with expanded and often novel receptors and adaptors. Novel Toll-like receptor (TLR) and MyD88-like genes lacking essential domains were highly up-regulated in the oyster, possibly interfering with TLR signal transduction. RIG-1/ MDA5receptors for viral RNA, interferon-regulatory factors, tissue necrosis factors and interleukin-17were highly activated and likely central to the oyster's antiviral response. Genes related to anti-apoptosis, oxidation, RNA and protein destruction were also highly up-regulated, while genes related to anti-oxidation were down-regulated. The oxidative burst induced by the up-regulation of oxidases and severe down-regulation of anti-oxidant genes may be important for the destruction of viral components, but may also exacerbate oystermortality. This study provides unprecedented insights into antiviral response in a mollusc. The mobilization and complex regulation of expanded innate immune-gene families highlights the oystergenome's adaptation to a virus-rich marine environment.