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

Abstract

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 immediateearly (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 thirdinstar 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.

A haemocyte tropism for an arbovirus

“…In several lepidopteran species, insect blood cells, called haemocytes, facilitate amplification of baculoviruses as well as virus dissemination from tracheoles to secondary organs (Engelhard et al, 1994;Keddie et al, 1989). Resistance to baculovirus infection in some species is attributed to haemocytes avoiding infection, and also exhibiting apoptosis-like behaviour (Clarke & Clem, 2002;Feng et al, 2007;Trudeau et al, 2001). …”

Functional analysis of the putative antiapoptotic genes, p49 and iap4, of Spodoptera litura nucleopolyhedrovirus with RNAi

“…Apoptosis is used by insects as one of the host defence strategies against virus infection (Clem et al, 1991;Zhang et al, 2002;Clarke & Clem, 2003;Feng et al, 2007). Antiapoptotic proteins encoded by viral genomes play a key role in suppressing the apoptotic response (Clem, 2007).…”

“…The family Baculoviridae comprises the genera Nucleopolyhedrovirus (NPV) and Granulovirus (GV) (Theilmann et al, 2005). Baculoviruses usually have limited host ranges and recent studies suggest that apoptosis plays a role in host range restriction (Zhang et al, 2002;Clarke & Clem, 2003;Feng et al, 2007).…”

“…The family Baculoviridae comprises the genera Nucleopolyhedrovirus (NPV) and Granulovirus (GV) (Theilmann et al, 2005). Baculoviruses usually have limited host ranges and recent studies suggest that apoptosis plays a role in host range restriction (Zhang et al, 2002;Clarke & Clem, 2003;Feng et al, 2007).Apoptosis is a type of programmed cell death which is characterized frequently by nuclear condensation and fragmentation, accompanied by vigorous blebbing of the cytoplasm membrane (Kerr et al, 1972;Wyllie et al, 1980). The mechanism of apoptosis is evolutionarily conserved.…”

Highly passage of Spodoptera litura cell line causes its permissiveness to baculovirus infection

“…The haemolymph of the infected larvae at 72 h of p.i. identified by observation of optical microscope was collected (Feng et al 2007). In the following, we briefly described the method.…”