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Activation of autophagic programmed cell death and innate immune gene expression reveals immuno-competence of integumental epithelium in Bombyx mori infected by a dipteran parasitoid


In insects, the integument forms the primary barrier between the environment and internal milieu, but cellular and immune responses of the integumental epithelium to infection by micro- and macro-parasites are mostly unknown. We elucidated cellular and immune responses of the epithelium induced through infection by a dipteran endoparasitoid, Exorista bombycis in the economically important silkworm Bombyx mori. Degradative autophagic vacuoles, lamella-like bodies, a network of cytoplasmic channels with cellular cargo, and an RER network that opened to vacuoles were observed sequentially with increase in age after infection. This temporal sequence culminated in apoptosis, accompanied by the upregulation of the caspase gene and fragmentation of DNA. The infection significantly enhanced the tyrosine level and phenol oxidase activity in the integument. Proteomic analysis revealed enhanced expression of innate immunity components of toll and melanization pathways, cytokines, signaling molecules, chaperones, and proteolytic enzymes demonstrating diverse host responses. qPCR analysis revealed the upregulation of spatzle, BmToll, and NF kappa B transcription factors Dorsal and BmRel. NF kappa B inhibitor cactus showed diminished expression when Dorsal and BmRel were upregulated, revealing a negative correlation (R = -0.612). During melanization, prophenol oxidase 2 was expressed, a novel finding in integumental epithelium. The integument showed a low level of melanin metabolism and localized melanism in order to prevent the spreading of cytotoxic quinones. The gene-encoding proteolytic enzyme, beta-N-acetylglucosaminidase, was activated at 24 h post-infection, whereas chitinase, was activated at 96 h post-infection; however, most of the immune genes enhanced their expression in the early stages of infection. Thus the integument contributes to humoral immune responses that enhance resistance against macroparasite invasion.

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The authors thank the anonymous referees for critical comments, Mr. B. Srinivasa for uzi fly maintenance and silkworm rearing, and the Central Silk Board, Government of India, Bangalore for facilities.

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Correspondence to Appukuttan Nair R. Pradeep.

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Appukuttan Nair R. Pradeep and Jayaram Anitha are equal contributors

Authors acknowledge the financial support received from Department of Biotechnology, Government of India, New Delhi, in the form of a research project to ARP (BT/PR11722/PBD/19/197/2008 dated 11/6/2009). AJ and AKH are supported by DBT junior research fellowships.

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Fig. S1

a-d. Variations induced in cuticular morphology of B. mori larvae after infection by the parasitoid, E. bombycis. (a) Egg (x70) of E. bombycis is attached to the larval cuticle by a cementing substance (arrow). (b) Scanning electron micrograph of the cuticle where egg was attached showing blackening (arrow) of the area (x500) at 24 h after infection. (c) Control cuticle showing normal features such as hairs and microtrichia (x200). (d) Fifth instar larva showing translucent cuticle (bolded white arrow) adjoining the melanized cuticle ( black arrow) at 96 h after infection. (JPEG 44 kb)

Fig. S2

a-d. Variation in protein profile of integument of B. mori larvae after infection by the parasitoid, E. bombycis. (a) Total protein content showed significant reduction over ages after infection. (b) Tyrosine content showed significant increase at 24 h after infection compared to control. (c) PO activity was measured by optical density at 490 nm showed significant increase at 96 h after infection. (d) SDS-PAGE showed resolution of total protein extracted from integument at 72 h after infection. Two boxed bands at ~155 kDa and ~30 kDa showed enhanced expression after infection (arrows) and were cut and processed for mass spectrometry. Ctrl. – saline injected control; Inf. – infected; M - marker. (JPEG 37 kb)

Fig. S3

Differential expression of various host-response genes in integumental epithelium of B. mori larva after infection by the parasitoid E. bombycis. Total RNA was isolated from integument at 24 h intervals from the control and infected larvae and cDNA was synthesized by RT-PCR using oligo- d(T)23 primer. All genes were amplified as per the details given in Table 1. Number of cycles was optimized for each gene. β- actin was used as control housekeeping gene. Control – saline injected control; M- Marker. (JPEG 55 kb)

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Pradeep, A.R., Anitha, J., Awasthi, A.K. et al. Activation of autophagic programmed cell death and innate immune gene expression reveals immuno-competence of integumental epithelium in Bombyx mori infected by a dipteran parasitoid. Cell Tissue Res 352, 371–385 (2013).

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  • Host–parasite interaction
  • Apoptosis
  • Toll pathway activation
  • PPO2 expression
  • Bombyx mori (Insecta)