Abstract
The family Baculoviridae encompasses a large number of invertebrate viruses, mainly infecting caterpillars of the order Lepidoptera. The baculovirus Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) induces physiological and behavioral changes in its host Spodoptera exigua, as well as immunological responses, which may affect virus transmission. Here we show that the SeMNPV-encoded protein tyrosine phosphatase 2 (PTP2) induces mild apoptosis in Spodoptera frugiperda (Sf) 21 cells upon transient expression. Transient expression of a catalytic-site mutant of ptp2 did not lead to apoptosis, indicating that the phosphatase activity of PTP2 is needed to induce apoptosis. We also found that the caspase level (indicator of apoptosis) was higher in cells transfected with the ptp2 gene than in cells transfected with the catalytic mutant. Adding a caspase inhibitor reduced the level of ptp2-induced apoptosis. Moreover, deletion of the ptp2 gene from the viral genome prevented the induction of apoptosis in S. exigua hemocytes. The virus titer and virulence indices (the viral infectivity and the time to death) were not affected by deletion of the ptp2 gene. However, the viral occlusion body yield from S. exigua larvae infected with the mutant virus lacking the ptp2 gene was much lower than the yield from larvae infected with the wild-type (WT) virus. We hypothesize that the observed pro-apoptotic effects of PTP2 are the result of PTP2-mediated immune suppression in larvae, which consequently leads to higher viral occlusion body yields.
Highlights
Apoptosis is an active process of programmed cell death that is involved in immunity, normal development and cell differentiation [1]
Since the protein tyrosine phosphatase gene from the polydnavirus Microplitis demolitor bracovirus (MdBV) showed pro-apoptotic effects on host cells, we hypothesized that Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) ptp2 might function in a similar way
Cell blebbing and apoptotic bodies were observed in cells transfected with pIB-SePTP2, but to a lesser extent compared with cells treated with
Summary
Apoptosis is an active process of programmed cell death that is involved in immunity, normal development and cell differentiation [1]. It is characterized by morphological changes, including cell blebbing and cell shrinkage, chromosomal DNA fragmentation, as well as energy-dependent biochemical changes [1,2]. Viruses have developed a range of strategies to counteract host-induced apoptosis [3], including the expression of anti-apoptotic genes. It was found that the polydnavirus Microplitis demolitor bracovirus (MdBV) induced apoptosis in immune cells of Spodoptera frugiperda caterpillars and that this activity contributed to the immunosuppression of the host [5]. Suderman et al [5] found that the protein tyrosine phosphatase-H2 (PTP-H2) from MdBV induced apoptosis in S. frugiperda
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