1. IntroductionPolydnaviruses (PDVs) are large double-stranded circular DNA (dsDNA) viruses that can be classified into bracoviruses (BVs) and ichnoviruses (IVs) according to their mutualistic relationship with braconid and ichneumonid wasps [1,2]. PDVs persist in a given wasp genome as proviruses with two distinct parts: structural core genes that are critical for the formation of virion particles, and proviral segments that serve as templates for the production of the virulence genes in its secondary host, primarily an insect in the Lepidoptera order [3]. These virulence genes alter host physiology, growth, and development, facilitating the survival of wasp offspring [1,4,5,6,7]. They are classified into gene families and contribute greatly to host immunosuppression and growth regulation [6,8,9]. Several BV virulence genes have been demonstrated to disrupt multiple elements of a host’s immune system. For example, two genes in Microplitis demolitor BV (MdBV), Egf 1.0 and Egf 1.5, which carry an epidermal growth-factor-like motif, have been demonstrated to inhibit melanization by targeting phenoloxidase-activating proteinase (PAP) [10,11]. Two members of the BV ankyrin gene family function as mimics of IκB proteins, and target the host nuclear factor-κB pathway to suppress the expression of antimicrobial peptides [12,13]. Recently, Microplitis bicoloratus bracovirus was found to induce apoptosis in host hemocytes by activating caspase-3 and suppressing the immune response by reducing the phosphorylation level of components of the pI3K/AKT signaling pathway [14,15]. Notably, the function of many other PDV genes is unknown.In vertebrates, protein tyrosine phosphatases (PTPs), which catalyze the removal of the phosphate group from a tyrosine residue in a target protein, are indispensable and specific modulators of cellular signaling, thus regulating many processes, such as cell convergence and extension [16], cell differentiation [17,18,19,20], the signal transduction pathways in immunity [21,22,23], and metabolism-related pathways [24]. The structurally conserved PTP domain is key to membership in the PTP family, and three PTP groups have been categorized to date: (i) classical PTPs, (ii) PTPs with dual-specificity, and (iii) low-molecular-weight PTPs [25]. The PTP gene family has been found in the most extensively sequenced BV genome and constitutes the largest PDV gene family, except for the recently sequenced CinsBV of the Chelonus insularis wasp that does not contain PTP genes [26,27,28,29,30,31,32,33,34]. The BV PTP gene family expanded via classical mechanisms, including segmental duplication, tandem duplication, and dispersed duplication [35]. Bracovirus PTPs belong to the classical PTP family, and are characterized by their highly comparable PTP domain to vertebrate PTPs, with some carrying other functional domains [36]. Despite their sequence similarity, in contrast to classical PTPs, it is unclear whether the bracovirus PTPs are functional because they are highly divergent and lack conserved domains for substrate specificity, as proven by the fact that some BV PTPs are functional tyrosine phosphatases, whereas others are not [37,38]. Several BV PTPs have been demonstrated to suppress host immunity. For example, PTP-H2 and PTP-H3 of MdBV showed tyrosine phosphatase activity and involvement in the inhibition of phagocytosis by S2 cells [38]; Richard et al. [39] confirmed that the phosphatase activity of PTP-H2 in MdBV is critical for the induction of cell apoptosis. Hemocytes expressing PTP1 derived from Cotesia vestalis bracovirus (CvBV) exhibit a significant reduction in both cell spreading and encapsulation [40]. In addition, recent research has shown that a PTP from Toxoneuron nigriceps BV participates in the blockade of ecdysteroidogenesis, possibly by disrupting the phosphorylation balance of key proteins in the MAPK and PI3K ecdysone pathways, altering host growth and development [41,42]. However, the role of most BV PTPs in host–parasitoid interactions has not been well elucidated.Cotesia vestalis (Haliday) is a major natural enemy of Plutella xylostella (L.) (Lepidoptera: Plutellidae), a destructive pest of Brassica crops worldwide [43]. Our previous studies showed that the CvBV genome is composed of 30 DNA segments that encode 218 genes in 13 gene families, with CvBV-PTP constituting the largest gene family (with 33 PTP genes) [44]. In this study, we focused on characterizing the immunosuppressive role of CvBV-PTP genes. We found a PTP gene, designated CvBV_12-6, encoded by the 6th gene in the 12th segment of the CvBV genome. CvBV_12-6 was verified to show phosphatase activity. Ectopic expression of CvBV_12-6 in the D. melanogaster model suppressed the phenoloxidase (PO) activity of hemolymph and increased the susceptibility to Staphylococcus aureus. Furthermore, we found that CvBV_12-6 inhibited cellular immunity and humoral immunity in the P. xylostella host by reducing the hemocyte viability and suppressing the PO activity in the host hemolymph. 2. Materials and Methods 2.1. Insect RearingPlutella xylostella and its endoparasitoid Cotesia vestalis were reared as previously described [45]. They were maintained in a 14 h light/10 h dark photoperiod at 25 ± 1 °C with 65% relative humidity. Adult P. xylostella and C. vestalis were fed a 20% honey/water (V/V) solution. To ensure a high parasitization rate, middle third instar host individuals were exposed to a single female wasp within a test tube until oviposition was observed. W1118 wild-type and Bloomington Hml-GAL4 (BDSC_30140) D. melanogaster stock was used in this study, and the flies were reared on standard cornmeal/yeast/agar medium at 18 °C. 2.2. Sequence AnalysisAccording to the annotated CvBV genome, the sequences of CvBV_12-6 were downloaded. The signal sequence was predicted by SignalP 6.0 (https://services.healthtech.dtu.dk/service.php?SignalP, accessed on 2 January 2021). Multiple sequence alignment was performed with DNAMAN (V10.0.2.128) software. 2.3. RNA Extraction and ORF CloningRNA was extracted from five P. xylostella larvae at 12 h, 24 h, 48 h, and 96 h post parasitization (pp), respectively. TRIzol reagent (Invitrogen, Carlsbad, CA, USA) was used to isolate total RNA according to the manufacturer’s instructions. RNA purity was determined with a NanoDrop® spectrophotometer (Thermo Fisher, Waltham, MA, USA). A SuperScriptTM III Reverse Transcriptase kit (Vazyme, Nanjing, China) was used to synthesize a complementary DNA (cDNA) library. The primers used for PCR are shown in Table 1. PCR was performed under the following cycling conditions: 94 °C for 2 min, 35 cycles of 95 °C for 30 s, 60 °C (according to different primers inducing change) for 30 s, and 72 °C for 15 s, and then 72 °C for 10 min. Each PCR product was cleaned with a Wizard® SV Gel and PCR Clean-Up System (Promega, Madison, WI, USA) kit, and then cloned into a pGEM-T Easy Vector. Recombinant vectors were transferred into the E. coli TG1 strain, and positive clones were sequenced by Sangon (Shanghai, China). The sequenced fragments were assembled by SeqMan software (DNASTAR, 7.1.0.44 version), and then aligned with the CvBV genome with BLASTP (http://www.ncbi.nlm.nih.gov/, accessed on 2 February 2021). 2.4. Expression Analyses by qPCRTo analyze gene expression levels in different developmental stages, five P. xylostella larvae were collected as RNA samples 6 h, 12 h, 24 h, 48 h, 72 h, 96 h, and 120 h after parasite exposure. qPCR was performed as previously described [46]. A ReverTra Ace qPCR RT kit (Toyobo, Osaka, Japan) was used to synthesize cDNAs. The primers used for qPCR are shown in Table 1. β-Tubulin (GenBank accession No. EU127912) from P. xylostella was used as the reference gene. qPCRs were performed on a CFX Connect real-time system (Bio–Rad, Hercules, CA, USA) using THUNDERBIRD qPCR Mix (Toyobo, Osaka, Japan). Each qPCR for a given gene and time point was performed using at least three biological replicates under the following cycling conditions: 95 °C for 60 s, 40 cycles of 95 °C for 15 s, and 60 °C for 30 s. The results were analyzed via the 2-ΔΔCT method.

Similarly, to detect the tissue preference of CvBV_12-6 in a host, hemocytes and tissues, i.e., central nervous system (CNS), fat body, midgut, cuticle, testis, silk gland, and malpighian tubule tissues, from different developmental stages of parasitized larvae (30 larvae per sample) were dissected in phosphate-buffered saline (PBS) and then subjected to qPCR. The qPCR program was the same as that described above. All assays were performed at least three times.

2.5. Recombinant Baculovirus ConcentrationThe pFASTBAC-HTb (Invitrogen, San Diego, CA, USA) vector for baculovirus expression in P. xylostella hemocytes was modified by inserting the open reading frame (ORF) sequence of CvBV_12-6 with an HA tag using conventional molecular biology techniques. The GFP gene was used as the negative control, and the sequence was cloned from a pRSET-eGFP vector (Thermo, USA, V35320). The primers used are shown in Table 1. Recombinant nucleopolyhedroviruses (NPVs), NPV-CvBV_12-6 or NPV-GFP, were produced with a Bac-to-Bac Baculovirus Expression System (Invitrogen, San Diego, CA, USA) according to the manufacturer’s instructions. The recombinant NPVs were concentrated by centrifugation as described by [47] and then resuspended in 100 μL of PBS buffer (pH 7.4). The titer of the generated high-titer virus stock was determined by viral plaque assay according to the Bac-to-Bac Baculovirus Expression System manufacturer instructions (Invitrogen, San Diego, CA, USA). An overexpression experiment was performed by injecting 1 × 105 copies of NPV-CvBV_12-6 or NPV-GFP into third instar P. xylostella larvae. 2.6. Tyrosine Phosphatase Assay and Western Blotting

For convenient protein collection, the recombinant baculovirus NPV-CvBV_12-6 and NPV-GFP (5 × 107 copies) were expressed in Sf9 cells (5 × 106 cells), which were cultured at 27 °C in a cell incubator. All the cells were collected 2 days later and washed with PBS at least three times. The clean cells were lysed with a reagent (Promega, Madison, WI, USA), and the following procedure was performed according to the instructions of a tyrosine phosphatase assay kit (Promega, Madison, WI, USA). The activity of tyrosine phosphatase depends on the concentration of free phosphate, which can be reflected by the absorbance read at 630 nm.

The recombinant CvBV_12-6 protein from Sf9 cells was detected by western blot using an HA monoclonal antibody (Sangon, Shanghai, China), and an anti-Actin monoclonal antibody (Sangon, Shanghai, China) was used as the internal control. Samples were diluted in 5× protein sodium dodecyl sulfate–polyacrylamide gel electrophoresis loading buffer (Sangon, Shanghai, China) and then boiled for 10 min. The proteins in the samples separated in the denaturing polyacrylamide gel were transferred to a polyvinylidene difluoride membrane. After blocking and washing, the membranes were incubated with primary antibodies against HA or actin (1: 10,000) for 2 h at room temperature. The membranes were then incubated with horseradish peroxidase-conjugated goat anti-mouse IgG secondary antibody diluted 1:3000 in Tris-buffered saline with Tween-20. After washing five times, the membranes were incubated with enhanced chemiluminescence western blot substrate (Promega, Madison, WI, USA).

2.7. Transgenic Fly ConstructionThe GAL4/UAS (upstream activator sequence) binary expression system was used to study the function of CvBV_12-6 in D. melanogaster [48]. The open reading frame (ORF) sequence of CvBV_12-6 was cloned into a pUAST-attb vector [49]. A transgenic fly carrying the UAS-CvBV_12-6 gene was obtained by phiC31 integrase-mediated insertion into the attP2 landing site locus on the third chromosome. We used Hml-GAL4 (BDSC_30140), an expression driver of lymph glands and circulating hemocytes, to drive CvBV_12-6 expression in the D. melanogaster hemocytes [50]. Thirty adult Hml-GAL4 strain males and 200 adult UAS-CvBV_12-6 strain virgins were selected, mated for 1 day, and transferred to a new food bottle every 2 h. Larvae from crosses of Hml-GAL4 and W1118 were used as controls. 2.8. RNA InterferenceDouble-stranded RNA (dsRNA) specific for CvBV_12-6 and GFP was produced and purified using a T7 RiboMAXTM Express kit (Promega, Madison, WI, USA). The primers used for dsRNA synthesis are shown in Table 1. The synthesis products were confirmed by running dsRNA on an agarose gel; before parasitization, 1 µg of CvBV_12-6 or GFP dsRNA was injected into third instar P. xylostella larvae using an Eppendorf Femto jet device (Eppendorf, Germany). The efficiency of the interference was determined individually by qPCR 12 h, 24 h, 36 h, and 48 h pp. 2.9. Hemocyte Count

The hemolymph of P. xylostella was collected 24 h after RNAi or injection of NPVs. A trypan blue stock solution was diluted to generate a 1× work solution, and 19.5 µL of the trypan blue work solution was mixed with 0.5 μL of hemolymph. This reagent was then added to a cell counting plate, and Countstar 1.0 software was used to determine the cell number and viability automatically using 6–15 μm as the initial detection parameter.

2.10. PO Activity AssaysPO activity was measured as previously described [51] with a minor modification to the procedure described by Goldsworthy et al. [52]. Approximately 5 μL of hemolymph from approximately 30 P. xylostella larvae 24 h post treatment was added to 95 μL of anticoagulant buffer (100 mmol/L glucose; 62 mmol/L NaCl; 30 mmol/L trisodium citrate; 26 mmol/L citric acid, and 10 mmol/L ethylenediaminetetraacetic acid, pH 4.6). To remove the hemocytes, the diluted hemolymph was centrifuged at 1000× g for 5 min. A Better Bradford AssayTM kit (Thermo, USA) was used to determine the protein concentration of each diluted cell-free hemolymph sample. Subsequently, 20 µL of heat-killed Micrococcus luteus (optical density (OD) = 0.5) was added to the diluted hemolymph and incubated for 20 min at room temperature. The mixture was then used to immediately measure the PO activity. PO activity was measured spectrophotometrically by recording the formation of dopachrome from L-dihydroxyphenylalanine (L-DOPA). Each assay was established in a 96-well plate with a total volume of 200 μL (with 140 μL of L-DOPA [3 g/L] and 60 μL of diluted cell-free hemolymph). The absorbance at 490 nm was measured once every 5 min for a total of 60 min. PO activity is expressed as a change in absorbance at 490 nm per mg protein per min. Each assay was performed with at least three biological replicates.

As for D. melanogaster, 30 adult Hml-GAL4 strain males and 200 adult UAS-CvBV_12-6 or W1118 strain virgins were crossed and laid eggs in different bottles. The offspring in one bottle were used as a replicate. The hemolymph was collected from approximately 30 third instar offspring larvae and PO activity was measured as described above. The assay was replicated three times.

2.11. Survival AssayA survival assay was performed as previously described [53]. Staphylococcus aureus is a gram-positive bacteria that is among the most frequent cause of morbidity and mortality due to infection worldwide [54]. The strength of the immune response of D. melanogaster larvae was measured to determine the risk of S. aureus infection. S. aureus was grown overnight at 37 °C with shaking at 250 r/min. Cultures were centrifuged at 1000× g for 1 min. The bacteria were resuspended in sterile PBS until an OD600 of 0.4 was obtained. Seven days post eclosion, a male fly was injected with 40 nL of bacterial resuspension using an Eppendorf Femto jet device (Eppendorf, Germany) with a microcontroller (Narishige, Japan). Experiments were performed in triplicate (n = 20 flies per treatment). Injections were performed under a Stemi 2000-C microscope (Zeiss, Germany). Flies were reared at 25 °C after injection. Flies that died within 6 h were considered dead and removed from the final count. Flies were transferred every day to provide new food sources, and fly death was recorded every 12 h for 3 days. 2.12. Statistical Analysis

All the data were calculated as the mean ± SD. The significant difference between samples was determined by one-way analysis of variance (ANOVA) with Tukey’s test, and the significance threshold was set to a p value < 0.05. Log rank tests were performed to determine whether the survival curves between different treatment groups were significantly different.

4. DiscussionPolydnavirus-mediated disruption of cellular and humoral immunity renders parasitized lepidopteran larvae suitable for the development of wasp larvae as well as makes them susceptible to opportunistic infections [8]. In the P. xylostella–C. vestalis system, a series of PDV genes are delivered into the host genome to inhibit the host immune response and contribute greatly to successful parasitization [6,8,9]. In this study, we characterized a PTP gene, CvBV_12-6, encoded by CvBV, and explored its role in the immune response of the host.Previous research had identified 10 conserved motifs in 113 vertebrate PTP domains, and these structural motifs were shown to work together to regulate the dephosphorylation process precisely [36]. The proposed role of each motif during dephosphorylation has been well characterized: Motifs 2 to 7 play key roles in stabilizing the secondary structure of the PTP domain; Motifs 1 and 8 are required for substrate recognition; Motifs 9 and 10 participate in substrate binding, catalysis, and optimal hydrolysis, respectively [36,37]. In our research, multiple sequence alignment showed that CvBV_12-6 carries ten motifs, with most diverging from the motifs of classical vertebrate PTPs, except for Motif 2 and Motif 3 (Figure 1A). We speculate that this divergence probably evolved in the context of parasitism, which allowed parasites to recognize different substrates or similar substrates in different host species. In addition to this study, other BV PTPs, in which the motifs divergence from the classical motifs, have been demonstrated to exhibit tyrosine phosphatase activity; these PTPs include PTP-H2 and PTP-H3 from MdBV [38]. Notably, while four residues (D9, N12, S14, and I15) in Motif 9 of CvBV_12-6 differ from those at these sites in classical PTPs, conservation of the cysteine (C6) site suggests that the PTP functions as a phosphatase, as described in previous papers [37,56,57]. This evidence suggests that whether bracovirus PTPs exhibit phosphatase activity does not entirely depend on the conservation of the motifs, but on the conservation of some important amino acid residues.The expression of CvBV_12-6 in different tissues reveals that CvBV_12-6 was expressed in all the tested tissues with the highest transcript abundance in the hemocytes, which are critical for many immune responses in insects [58]. Hence, we infer that enriching CvBV_12-6 in hemocytes may have modulated the host immune response. As an effective system to study proteins of unknown function, the D. melanogaster model has been employed in the study of the function of BV virulence genes [59], and we used this model in our study. As a nonmodal organism, no effective genetic manipulation has been performed to explore functional proteins in P. xylostella, especially exogenous gene products of PDV that integrate into the host genome after embryonic development. In this study, RNAi was chosen as the optimal system for testing the effect of virulence factors. Although gene knockdown is difficult to achieve in Lepidoptera, the effectiveness of PDV gene interference is relatively high, with a 70–80%, and sometimes a 90%, decrease in target gene expression [60,61]. In addition, it is a technical challenge to overexpress genes in Lepidoptera, and previous studies have used baculovirus as a transient expression system to analyze CvBV-gene cellular immune responses [40,62,63]. Therefore, we took advantage of the Bac-to-Bac baculovirus expression system, which enables the rapid and efficient expression of target genes. NPV was injected into the P. xylostella larvae and infected the tissues, leading to the expression of the CvBV_12-6 protein. To exclude the effect of baculovirus infection on P. xylostella, we strictly examined the response of hemocytes within 24 h, in which CvBV_12-6 was highly expressed, and there were no obvious symptoms caused by baculovirus infection. Additionally, the negative control NPV-GFP was an important reference for the interpretation of the results. Taken together, the results of this study show evidence for the potential function of bracovirus PTPs.There are some commonalities and differences among the function of CvBV_12-6 in D. melanogaster and P. xylostella. The difference is that CvBV_12-6 did not change the viability of hemocytes in D. melanogaster, but impaired that in the host P. xylostella, which may have been a result of the genetic difference between these two species. Concerning a decrease in host hemocyte viability, some BV PTPs have been shown to induce apoptosis in the Sf21 cell line via caspase activation [39]. However, this does not seem to be the cause for CvBV_12-6 because the total number of host hemocytes was not affected. Cell death and survival involve multiple processes; therefore, more experiments are needed to explore the role of CvBV_12-6 in inducing host cell death. CvBV_12-6 inhibited melanization reactions in both insects, consistent with early findings after C. vestalis parasitization [55,64] and CvBV injection [55]. Although the mechanism via which CvBV_12-6 regulates host melanization remains unclear, the regulation of phosphorylation levels during melanization in D. melanogaster has been extensively studied. In D. melanogaster, p38 MAPKs are preferentially activated in response to a wide variety of stress stimuli, and thereafter phosphorylate various substrates to regulate cellular immune and stress responses [65,66,67]. Sekine et al. found that p38 functions in the dopamine synthesis pathway to activate the melanization reaction and may be involved in immune and stress responses [68]. Therefore, we propose that CvBV_12-6 may inhibit melanization by negatively regulating the MAPK pathway or directly suppressing the survival of hemocytes in the P. xylostella host. Furthermore, our previous studies have shown that CLP genes in CvBV [55] and a trypsin inhibitor-like protein [69] in teratocytes functioned as melanization inhibitors, indicating multistep regulation of the host PO pathway mediated by parasitic wasp-associated factors.

In summary, we identified a PTP gene in C. vestalis BV, CvBV_12-6, that showed tyrosine phosphatase activity. CvBV_12-6 inhibited both the cellular and humoral immunity of P. xylostella by reducing the hemocyte viability or suppressing phenoloxidase/melanization responses individually, and it also showed functional immunosuppression in the ectopic host D. melanogaster. Our results not only provide evidence for an inhibitory role played by a newly characterized gene family (PTP) in the host melanization response, but also expand our knowledge about the mechanisms by which parasitoids regulate the humoral immunity of their hosts.