Evidence for cell apoptosis suppressing white spot syndrome virus replication in Procambarus clarkii at high temperature

In shrimp, higher water temperatures (~32°C) can suppress the ability of white spot syndrome virus (WSSV) to replicate and cause mortality, but the mechanisms remain unclear. To investigate whether cell apoptosis might be involved, a Tdt-mediated dUTP nick-end label (TUNEL) method was used to assess levels of chromosomal DNA fragmentation in hepatopancreas and gill cells of Procambarus clarkii crayfish infected with WSSV and maintained at either 32 ± 1°C or 24 ± 1°C. Based on relative cell numbers with yellow-green colored TUNEL-positive nuclei, the apoptotic index was elevated in WSSV-infected crayfish maintained at 32°C. In gill tissue sections examined by transmission electron microscope, cells with nuclei displaying apoptotic bodies or marginated, condensed and fragmented chromatin without concurrent cell cytoplasm damage were also more prevalent. Flow cytometry sorting of annexin-stained cells showed apoptosis to be most prevalent in granular haemocytes, and assays for caspase-3 activity showed it to be most elevated in hepatopancreas tissue. Despite these indicators of cell apoptosis but consistent with WSSV replication being restricted at elevated temperatures, no increases in transcription of the viral anti-apoptosis genes ORF390 and ORF222 were detected by RT-PCR in shrimp maintained at 32°C, possibly due to the elevated levels of cellular apoptosis.