Does hyperthermia increase apoptosis in white spot syndrome virus (WSSV)-infected Litopenaeus vannamei?

Apoptosis plays a critical role in development and maintenance of multicellular organisms. It has also been described as an anti-viral mechanism in both insects and vertebrates. In fact, to escape the immune system and to increase their spread, some viruses such as baculovirus produce antiapoptotic molecules. Conversely, a recent report showing a positive correlation between the number of apoptotic cells and the severity of white spot syndrome virus (WSSV) infection in Penaeus monodon suggested that apoptosis might be the cause of death in viral-infected shrimp. Searching for the mechanisms involved in the beneficial effect of hyperthermia for WSSV-infected Litopenaeus vannamei (also called Penaeus vannamei) and considering that hyperthermia increases apoptosis in other experimental models, we investigated the presence of apoptosis by Tdt-mediated dUTP nick-end label (TUNEL), from 4 of 168 h in 3 groups of 50 L. vannamei juveniles. Group 1 consisted of experimentally infected shrimp (intramuscular injection of 3 × 10⁷ viral copies) kept at 25°C, Group 2 of similarly infected shrimp kept at 32°C and Group 3 of uninjected shrimp kept at 32°C. Apoptosis was found only in WSSV-infected individuals. Shrimp at 25°C were positive for apoptotic cells in 48 (16%) of their examined tissues or organs, compared to 62 (21%) for those at 32°C. Moreover, shrimp at 32°C also had a significantly higher overall mean apoptotic index (AI) than shrimp at 25°C (p < 0.05). Comparison of mean AI at 72, 96 and 120 h post-infection showed that individuals at 32°C presented a significantly higher values than those at 25°C. These results suggested that hyperthermia might facilitate apoptosis in WSSV-infected L. vannamei and might be one of the mechanisms responsible for increased survival of infected shrimp maintained at 32°C.