Apoptosis is induced in the haemolymph and fat body of Spodoptera exigua larvae upon oral inoculation with Spodoptera litura nucleopolyhedrovirus

Spodoptera exigua multinucleopolyhedrovirus (SeMNPV) and Spodoptera litura nucleopolyhedrovirus (SpltNPV) are genetically similar, but the larvae of S. exigua are not susceptible to SpltNPV. The aim of this study was to identify whether any process was inhibiting SpltNPV infection at some point. S. exigua larvae infected with a high concentration of wild-type SpltNPV by oral inoculation produced a fatal infection in second- or third-instar S. exigua, but the dead larvae did not undergo liquefaction; in contrast, fourth-instar infected larvae remained healthy. RT-PCR analysis of total RNA from infected second-instar larvae targeting immediate-early (ie-0), early (dnapol), late (chit) and very late (polh) genes suggested that SpltNPV initiated infection in the non-susceptible hosts. Total DNA extracted from the haemocytes of infected larvae showed DNA ladders characteristic of apoptosis. Sections of tissue from infected third-instar larvae of S. exigua at 96 h post- inoculation, stained with haematoxylin and eosin, revealed a highly disrupted morphology in the fat body. Apoptosis in fat body tissue was detected using terminal deoxynucleotidyltransferase-mediated fluorescein-dUTP nick end labelling (TUNEL) assays. In situ hybridization revealed the presence of viral DNA within the TUNEL-positive area, indicating viral infection in this tissue. These results suggest that apoptosis limits viral propagation by reducing the number of SpltNPV-infected haemocytes and fat body cells and inhibits disseminated viral infection.