Inhibition of immune pathway-initiating hemolymph protease-14 by Manduca sexta serpin-12, a conserved mechanism for the regulation of melanization and Toll activation in insects

Comprehensive Proteomics Analysis of the Hemolymph Composition of Sugar-Fed Aedes aegypti Female and Male Mosquitoes

In arthropods, hemolymph carries immune cells and solubilizes and transports nutrients, hormones, and other molecules that are involved in diverse physiological processes including immunity, metabolism, and reproduction. However, despite such physiological importance, little is known about its composition. We applied mass spectrometry-based label-free quantification approaches to study the proteome of hemolymph perfused from sugar-fed female and male Aedes aegypti mosquitoes. A total of 1403 proteins were identified, out of which 447 of them were predicted to be extracellular. In both sexes, almost half of these extracellular proteins were predicted to be involved in defense/immune response, and their relative abundances (based on their intensity-based absolute quantification, iBAQ) were 37.9 and 33.2%, respectively. Interestingly, among them, 102 serine proteases/serine protease-homologues were identified, with almost half of them containing CLIP regulatory domains. Moreover, proteins belonging to families classically described as chemoreceptors, such as odorant-binding proteins (OBPs) and chemosensory proteins (CSPs), were also highly abundant in the hemolymph of both sexes. Our data provide a comprehensive catalogue of A. aegypti hemolymph basal protein content, revealing numerous unexplored targets for future research on mosquito physiology and disease transmission. It also provides a reference for future studies on the effect of blood meal and infection on hemolymph composition.

The immune response mechanism of Nilaparvata lugens against a combined infection of rice ragged stunt virus and Metarhizium anisopliae

BACKGROUND

Previous studies of brown planthopper (BPH), Nilaparvata lugens, showed that carrying the plant pathogenic virus, rice ragged stunt virus (RRSV), enhanced the lethality of the entomopathogenic fungus, Metarhizium anisopliae (YTTR). The underlying mechanism for this was not established but a serine protease cascade was hypothesized to be involved.

RESULTS

Two immune response genes, NlKPI and NlVenomase, were identified and shown to be involved. The synthesized double-strand RNA (dsRNA) techniques used in this study to explore gene function revealed that treatment with dsRNA to silence either gene led to a higher BPH mortality from M. anisopliae infection than the dsRNA control treatment. NlKPI and NlVenomase play vital roles in BPH immunity to defend against alien pathogens. Both genes participate in the immune response process of BPH against co-infection with RRSV and M. anisopliae YTTR by regulating the expression of antimicrobial peptides and phenoloxidase activity.

CONCLUSION

Our study provided new targets for BPH biocontrol and laid a solid foundation for further research on the interaction of virus-insect-EPF (entomopathogenic fungus). © 2023 Society of Chemical Industry.

Invertebrate Immunity, Natural Transplantation Immunity, Somatic and Germ Cell Parasitism, and Transposon Defense

While the vertebrate immune system consists of innate and adaptive branches, invertebrates only have innate immunity. This feature makes them an ideal model system for studying the cellular and molecular mechanisms of innate immunity sensu stricto without reciprocal interferences from adaptive immunity. Although invertebrate immunity is evolutionarily older and a precursor of vertebrate immunity, it is far from simple. Despite lacking lymphocytes and functional immunoglobulin, the invertebrate immune system has many sophisticated mechanisms and features, such as long-term immune memory, which, for decades, have been exclusively attributed to adaptive immunity. In this review, we describe the cellular and molecular aspects of invertebrate immunity, including the epigenetic foundation of innate memory, the transgenerational inheritance of immunity, genetic immunity against invading transposons, the mechanisms of self-recognition, natural transplantation, and germ/somatic cell parasitism.

Parasitoid Serpins Evolve Novel Functions to Manipulate Host Homeostasis

Parasitoids introduce various virulence factors when parasitism occurs, and some taxa generate teratocytes to manipulate the host immune system and metabolic homeostasis for the survival and development of their progeny. Host-parasitoid interactions are extremely diverse and complex, yet the evolutionary dynamics are still poorly understood. A category of serpin genes, named CvT-serpins, was discovered to be specifically expressed and secreted by the teratocytes of Cotesia vestalis, an endoparasitoid of the diamondback moth Plutella xylostella. Genomic and phylogenetic analysis indicated that the C. vestalis serpin genes are duplicated and most of them are clustered into 1 monophyletic clade. Intense positive selection was detected at the residues around the P1-P1' cleavage sites of the Cv-serpin reactive center loop domain. Functional analyses revealed that, in addition to the conserved function of melanization inhibition (CvT-serpins 1, 16, 18, and 21), CvT-serpins exhibited novel functions, i.e. bacteriostasis (CvT-serpins 3 and 5) and nutrient metabolism regulation (CvT-serpins 8 and 10). When the host-parasitoid system is challenged with foreign bacteria, CvT-serpins act as an immune regulator to reprogram the host immune system through sustained inhibition of host melanization while simultaneously functioning as immune effectors to compensate for this suppression. In addition, we provided evidence that CvT-serpin8 and 10 participate in the regulation of host trehalose and lipid levels by affecting genes involved in these metabolic pathways. These findings illustrate an exquisite tactic by which parasitoids win out in the parasite-host evolutionary arms race by manipulating host immune and nutrition homeostasis via adaptive gene evolution and neofunctionalization.

Serpin-1a and serpin-6 regulate the Toll pathway immune homeostasis by synergistically inhibiting the Spätzle-processing enzyme CLIP2 in silkworm, Bombyx mori

The Toll receptor signaling pathway is an important innate immune response of insects to pathogen infection; its extracellular signal transduction involves serine protease cascade activation. However, excessive or constitutive activation of the Toll pathway can be detrimental. Hence, the balance between activation and inhibition of the extracellular protease cascade must be tightly regulated to achieve favorable outcomes. Previous studies have shown that serpins-serine protease inhibitors-negatively regulate insect innate immunity by inhibiting extracellular protease cascade signaling. Although the roles of serpins in insect innate immunity are well described, the physiological mechanisms underlying their synergistic effects remain poorly understand. Here, we characterize the molecular mechanism by which serpin-1a and serpin-6 synergistically maintain immune homeostasis of the silkworm Toll pathway under physiological and pathological conditions. Through in vitro biochemical assays and in vivo bioassays, we demonstrate that clip-domain serine protease 2 (CLIP2), as the Toll cascade-activating terminal protease, is responsible for processing proSpätzle1 to induce the expression of antimicrobial peptides. Further biochemical and genetic analyses indicate that constitutively expressed serpin-1a and inducible serpin-6 synergistically target CLIP2 to maintain homeostasis of the silkworm Toll pathway under physiological and pathological conditions. Taken together, this study provides new insights into the precise regulation of Toll cascade activation signals in insect innate immune responses and highlights the importance and complexity of insect immune homeostasis regulation.

Clip-SP1 cleavage activates downstream prophenoloxidase activating protease (PAP) in Plutella xylostella

Melanization is a component of the humoral immune defense of insects and is induced by serine protease-mediated phenoloxidase (PO) catalysis. Prophenoloxidase (PPO) in the midgut of Plutella xylostella is activated by the CLIP domain serine protease (clip-SP) in response to Bacillus thuringiensis (Bt) infection, but the detailed signaling cascade following this activation is unknown. Here, we report that activation of clip-SP enhances PO activity in the P. xylostella midgut by cleaving three downstream PPO-activating proteases (PAPs). First, the expression level of clip-SP1 was increased in the midgut after Bt8010 infection of P. xylostella. Then, purified recombinant clip-SP1 was able to activate three PAPs - PAPa, PAPb and PAP3 - which in turn enhanced their PO activity in the hemolymph. Furthermore, clip-SP1 showed a dominant effect on PO activity compared to the individual PAPs. Our results indicate that Bt infection induces the expression of clip-SP1, which is upstream of a signaling cascade, to efficiently activate PO catalysis and mediate melanization in the midgut of P. xylostella. And it provides a basis for studying the complex PPO regulatory system in the midgut during Bt infection.

Two venom serpins from the parasitoid wasp Microplitis mediator inhibit the host prophenoloxidase activation and antimicrobial peptide synthesis

Endoparasitoid wasps inject venom proteins into the hemocoel of host insects to ensure survival, growth, and development of their progenies by blocking host immunity. We previously identified ten serine protease inhibitors of the serpin superfamily in venom of the endoparasitoid wasp, Microplitis mediator, but it is unclear how these inhibitors may interact with host immune serine proteases. In this study, we investigated the functions of two serpins, MmvSPN-1 and MmvSPN-2, in the regulation of humoral immune responses in two hosts, the oriental armyworm Pseudaletia separate and the cotton bollworm Helicoverpa armigera, by dsRNA knockdown and biochemical assays using recombinant proteins. Knockdown of the two serpins resulted in increases in prophenoloxidase (PPO) activation and antimicrobial peptide (AMP) production in the hosts. After injection into the host hemocoel, the recombinant serpins inhibited PPO activation and AMP transcription. Mass spectrometry analysis of the pull-downs and in vitro reconstitution experiments revealed that HacSP29, a clip-domain serine protease in H. armigera, is the target of these two serpins. Therefore, these two inhibitors in the wasp venom may protect eggs from attacks by melanization and AMPs in the host insects.

Proteomic analysis of pharate pupal molting fluid from the tobacco hornworm, Manduca sexta

The insect cuticle is a key component of their success, being important for protection, communication, locomotion, and support. Conversely, as an exoskeleton, it also limits the size of the insect and must be periodically molted and a new one synthesized, to permit growth. To achieve this, the insect secretes a solution of chitinases, proteases and other proteins, known collectively as molting fluid, during each molting process to break down and recycle components of the old cuticle. Previous research has focused on the degradative enzymes in molting fluid and offered some characterization of their biochemical properties. However, identification of the specific proteins involved remained to be determined. We have used 2D SDS-PAGE and LC/MS-based proteomic analysis to identify proteins in the molting fluid of the tobacco hornworm, Manduca sexta, undergoing the larval to pupal molt. We categorized these proteins based on their proposed functions including chitin metabolism, proteases, peptidases, and immunity. This analysis complements previous reported work on M. sexta molting fluid and identifies candidate genes for enzymes involved in cuticle remodeling. Proteins classified as having an immune function highlight potential for molting fluid to act as an immune barrier to prevent infections during the cuticle degradation and ecdysis processes. Several proteins known to function in melanin synthesis as an immune response in hemolymph were present in molting fluid. We demonstrated that the bacterium Micrococcus luteus and the entomopathogenic fungus Beauveria bassiana can stimulate activation of phenoloxidase in molting fluid, indicating that the recognition proteins, protease cascade, and prophenoloxidase needed for melanin synthesis are present as a defense against infection during cuticle degradation. This analysis offers insights for proteins that may be important not only for molting in M. sexta but for insects in general.

A fat body transcriptome analysis of the immune responses of Rhodnius prolixus to artificial infections with bacteria

Background

Rhodnius prolixus is an important vector of Trypanosoma cruzi, the causal agent of Chagas disease in humans. Despite the medical importance of this and other triatomine vectors, the study of their immune responses has been limited to a few molecular pathways and processes. Insect immunity studies were first described for holometabolous insects such as Drosophila melanogaster, and it was assumed that their immune responses were conserved in all insects. However, study of the immune responses of triatomines and other hemimetabolous insects has revealed discrepancies between these and the Drosophila model.

Methods

To expand our understanding of innate immune responses of triatomines to pathogens, we injected fifth instar nymphs of R. prolixus with the Gram-negative (Gr−) bacterium Enterobacter cloacae, the Gram-positive (Gr+) bacterium Staphylococcus aureus, or phosphate-buffered saline (PBS), and evaluated transcript expression in the fat body 8 and 24 h post-injection (hpi). We analyzed the differential expression of transcripts at each time point, and across time, for each treatment.

Results

At 8 hpi, the Gr− bacteria-injected group had a large number of differentially expressed (DE) transcripts, and most of the changes in transcript expression were maintained at 24 hpi. In the Gr+ bacteria treatment, few DE transcripts were detected at 8 hpi, but a large number of transcripts were DE at 24 hpi. Unexpectedly, the PBS control also had a large number of DE transcripts at 24 hpi. Very few DE transcripts were common to the different treatments and time points, indicating a high specificity of the immune responses of R. prolixus to different pathogens. Antimicrobial peptides known to be induced by the immune deficiency pathway were induced upon Gr− bacterial infection. Many transcripts of genes from the Toll pathway that are thought to participate in responses to Gr+ bacteria and fungi were induced by both bacteria and PBS treatment. Pathogen recognition receptors and serine protease cascade transcripts were also overexpressed after Gr− bacteria and PBS injections. Gr- injection also upregulated transcripts involved in the metabolism of tyrosine, a major substrate involved in the melanotic encapsulation response to pathogens.

Conclusions

These results reveal time-dependent pathogen-specific regulation of immune responses in triatomines, and hint at strong interactions between the immune deficiency and Toll pathways.

Graphical abstract

Supplementary Information

The online version contains supplementary material, which is available at 10.1186/s13071-022-05358-9.

Serpin-4 Facilitates Baculovirus Infection by Inhibiting Melanization in Asian Corn Borer, Ostrinia furnacalis (Guenée)

Phenoloxidase (PO)-catalyzed melanization is a vital immune response in insects for defense against pathogen infection. This process is mediated by clip domain serine proteases and regulated by members of the serpin superfamily. We here revealed that the infection of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) significantly inhibited the PO activity in Ostrinia furnacalis hemolymph and induced the expression of O. furnacalis serpin-4. Addition of recombinant serpin-4 protein to O. furnacalis hemolymph resulted in a great increase of AcMNPV copies. Serpin-4 significantly suppressed the PO activity and the amidase activity in cleaving colorimetric substrate IEARpNA (IEARase activity) of hemolymph. Further experiments indicated it formed covalent complexes with three serine proteases (SP1, SP13 and SP105) and prevented them from cleaving their cognate downstream proteases in vitro. Altogether, O. furnacalis melanization restricted AcMNPV replication and serpin-4 facilitated AcMNPV infection by inhibiting serine proteases, SP1, SP13, and SP105 which were all involved in the melanization response.

Mutagenesis of both prophenoloxidases in the fall armyworm induces major defects in metamorphosis

Upon infection, the phenoloxidase system in arthropods is rapidly mobilized and constitutes a major defense system against invaders. The activation of the key enzymes prophenoloxidase (PPO) and their action in immunity through melanization and encapsulation of foreign bodies in hemolymph has been described in many insects. On the other hand, little is known about PPOs involvement in other essential functions related to insect development. In this paper, we investigated the function of the two PPOs of the crop pest, Spodoptera frugiperda (PPO1 and PPO2). We show that PPOs are mainly expressed in hemocytes with the PPO2 expressed at higher levels than the PPO1. In addition, these two genes are expressed in the same tissue and at the same stages of insect development. Through the generation of loss-of-function mutants by CRISPR/Cas9 method, we show that the presence of PPOs is essential for the normal development of the pupa and the survival of the insect.

Serine protease SP7 cleaves prophenoloxidase and is regulated by two serpins in Ostrinia furnacalis melanization

Melanization is an innate immune response in insects to defend against the invading pathogens and parasites. During melanization, prophenoloxidase (PPO) requires proteolytic activation by its upstream prophenoloxidase-activating protease (PAP). We here cloned a full-length cDNA for a serine protease, named as SP7, from Ostrinia furnacalis. The open reading frame of SP7 encodes 421-amino acid residue protein with a 19-residue signal peptide. qRT-PCR analysis showed that SP7 mRNA levels were significantly upregulated upon exposure to microbial infection. Recombinant SP7 zymogen was activated by serine protease SP2. The active SP7 could cleave O. furnacalis PPOs including PPO2, PPO1b and PPO3. Additionally, active SP7 could form covalent complexes with serine protease inhibitor serpin-3 and serpin-4. The activity of SP7 in cleaving a colorimetric substrate IEARpNA or O. furnacalis PPOs was efficiently blocked by either serpin-3 or serpin-4. Our work thus revealed that SP7 and SP2 partially constituted a PPO activation cascade in which SP7 was activated by SP2 and then likely worked as a PAP. SP7 was effectively regulated by serpin-3 and serpin-4. The results would allow further advances in the understanding of melanization mechanisms in O. furnacalis.

Haemocyte-mediated immunity in insects: Cells, processes and associated components in the fight against pathogens and parasites

The host defence of insects includes a combination of cellular and humoral responses. The cellular arm of the insect innate immune system includes mechanisms that are directly mediated by haemocytes (e.g., phagocytosis, nodulation and encapsulation). In addition, melanization accompanying coagulation, clot formation and wound healing, nodulation and encapsulation processes leads to the formation of cytotoxic redox-cycling melanin precursors and reactive oxygen and nitrogen species. However, demarcation between cellular and humoral immune reactions as two distinct categories is not straightforward. This is because many humoral factors affect haemocyte functions and haemocytes themselves are an important source of many humoral molecules. There is also a considerable overlap between cellular and humoral immune functions that span from recognition of foreign intruders to clot formation. Here, we review these immune reactions starting with the cellular mechanisms that limit haemolymph loss and participate in wound healing and clot formation and advancing to cellular functions that are critical in restricting pathogen movement and replication. This information is important because it highlights that insect cellular immunity is controlled by a multilayered system, different components of which are activated by different pathogens or during the different stages of the infection.

Immune properties of invertebrate phenoloxidases

Melanin production from different types of phenoloxidases (POs) confers immunity from a variety of pathogens ranging from viruses and microorganisms to parasites. The arthropod proPO expresses a variety of activities including cytokine, opsonin and microbiocidal activities independent of and even without melanin production. Proteolytic processing of proPO and its activating enzyme gives rise to several peptide fragments with a variety of separate activities in a process reminiscent of vertebrate complement system activation although proPO bears no sequence similarity to vertebrate complement factors. Pathogens influence proPO activation and thereby what types of immune effects that will be produced. An increasing number of specialised pathogens - from parasites to viruses - have been identified who can synthesise compounds specifically aimed at the proPO-system. In invertebrates outside the arthropods phylogenetically unrelated POs are participating in melanization reactions obviously aimed at intruders and/or aberrant tissues.

Sexual conflict drives micro- and macroevolution of sexual dimorphism in immunity

BACKGROUND

Sexual dimorphism in immunity is believed to reflect sex differences in reproductive strategies and trade-offs between competing life history demands. Sexual selection can have major effects on mating rates and sex-specific costs of mating and may thereby influence sex differences in immunity as well as associated host-pathogen dynamics. Yet, experimental evidence linking the mating system to evolved sexual dimorphism in immunity are scarce and the direct effects of mating rate on immunity are not well established. Here, we use transcriptomic analyses, experimental evolution and phylogenetic comparative methods to study the association between the mating system and sexual dimorphism in immunity in seed beetles, where mating causes internal injuries in females.

RESULTS

We demonstrate that female phenoloxidase (PO) activity, involved in wound healing and defence against parasitic infections, is elevated relative to males. This difference is accompanied by concomitant sex differences in the expression of genes in the prophenoloxidase activating cascade. We document substantial phenotypic plasticity in female PO activity in response to mating and show that experimental evolution under enforced monogamy (resulting in low remating rates and reduced sexual conflict relative to natural polygamy) rapidly decreases female (but not male) PO activity. Moreover, monogamous females had evolved increased tolerance to bacterial infection unrelated to mating, implying that female responses to costly mating may trade off with other aspects of immune defence, an hypothesis which broadly accords with the documented sex differences in gene expression. Finally, female (but not male) PO activity shows correlated evolution with the perceived harmfulness of male genitalia across 12 species of seed beetles, suggesting that sexual conflict has a significant influence on sexual dimorphisms in immunity in this group of insects.

CONCLUSIONS

Our study provides insights into the links between sexual conflict and sexual dimorphism in immunity and suggests that selection pressures moulded by mating interactions can lead to a sex-specific mosaic of immune responses with important implications for host-pathogen dynamics in sexually reproducing organisms.

CLIPB10 is a Terminal Protease in the Regulatory Network That Controls Melanization in the African Malaria Mosquito Anopheles gambiae

Humoral immune responses in animals are often tightly controlled by regulated proteolysis. This proteolysis is exerted by extracellular protease cascades, whose activation culminates in the proteolytic cleavage of key immune proteins and enzymes. A model for such immune system regulation is the melanization reaction in insects, where the activation of prophenoxidase (proPO) leads to the rapid formation of eumelanin on the surface of foreign entities such as parasites, bacteria and fungi. ProPO activation is tightly regulated by a network of so-called clip domain serine proteases, their proteolytically inactive homologs, and their serpin inhibitors. In Anopheles gambiae, the major malaria vector in sub-Saharan Africa, manipulation of this protease network affects resistance to a wide range of microorganisms, as well as host survival. However, thus far, our understanding of the molecular make-up and regulation of the protease network in mosquitoes is limited. Here, we report the function of the clip domain serine protease CLIPB10 in this network, using a combination of genetic and biochemical assays. CLIPB10 knockdown partially reversed melanotic tumor formation induced by Serpin 2 silencing in the absence of infection. CLIPB10 was also partially required for the melanization of ookinete stages of the rodent malaria parasite Plasmodium berghei in a refractory mosquito genetic background. Recombinant serpin 2 protein, a key inhibitor of the proPO activation cascade in An. gambiae, formed a SDS-stable protein complex with activated recombinant CLIPB10, and efficiently inhibited CLIPB10 activity in vitro at a stoichiometry of 1.89:1. Recombinant activated CLIPB10 increased PO activity in Manduca sexta hemolymph ex vivo, and directly activated purified M. sexta proPO in vitro. Taken together, these data identify CLIPB10 as the second protease with prophenoloxidase-activating function in An. gambiae, in addition to the previously described CLIPB9, suggesting functional redundancy in the protease network that controls melanization. In addition, our data suggest that tissue melanization and humoral melanization of parasites are at least partially mediated by the same proteases.

The CLIP-domain serine protease CLIPC9 regulates melanization downstream of SPCLIP1, CLIPA8, and CLIPA28 in the malaria vector Anopheles gambiae

The arthropod melanization immune response is activated by extracellular protease cascades predominantly comprised of CLIP-domain serine proteases (CLIP-SPs) and serine protease homologs (CLIP-SPHs). In the malaria vector, Anopheles gambiae, the CLIP-SPHs SPCLIP1, CLIPA8, and CLIPA28 form the core of a hierarchical cascade downstream of mosquito complement that is required for microbial melanization. However, our understanding of the regulatory relationship of the CLIP-SPH cascade with the catalytic CLIP-SPs driving melanization is incomplete. Here, we report on the development of a novel screen to identify melanization pathway components based on the quantitation of melanotic mosquito excreta, eliminating the need for microdissections or hemolymph enzymatic assays. Using this screen, we identified CLIPC9 and subsequent functional analyses established that this protease is essential for the melanization of both Escherichia coli and the rodent malaria parasite Plasmodium berghei. Mechanistically, septic infection with E. coli promotes CLIPC9 cleavage and both full-length and cleaved CLIPC9 localize to this bacterium in a CLIPA8-dependent manner. The steady state level of CLIPC9 in the hemolymph is regulated by thioester-containing protein 1 (TEP1), suggesting it functions downstream of mosquito complement. In support, CLIPC9 cleavage is inhibited following SPCLIP1, CLIPA8, and CLIPA28 knockdown positioning it downstream of the CLIP-SPH cascade. Moreover, like CLIPA8 and CLIPA28, CLIPC9 processing is negatively regulated by serine protease inhibitor 2 (SRPN2). This report demonstrates how our novel excretion-based approach can be utilized to dissect the complex protease networks regulating mosquito melanization. Collectively, our findings establish that CLIPC9 is required for microbial melanization in An. gambiae and shed light on how the CLIP-SPH cascade regulates this potent immune response.

Hemolymph protease-5 links the melanization and Toll immune pathways in the tobacco hornworm, Manduca sexta

Proteolytic activation of phenoloxidase (PO) and the cytokine Spätzle during immune responses of insects is mediated by a network of hemolymph serine proteases (HPs) and noncatalytic serine protease homologs (SPHs) and inhibited by serpins. However, integration and conservation of the system and its control mechanisms are not fully understood. Here we present biochemical evidence that PO-catalyzed melanin formation, Spätzle-triggered Toll activation, and induced synthesis of antimicrobial peptides are stimulated via hemolymph (serine) protease 5 (HP5) in Manduca sexta Previous studies have demonstrated a protease cascade pathway in which HP14 activates proHP21; HP21 activates proPAP2 and proPAP3, which then activate proPO in the presence of a complex of SPH1 and SPH2. We found that both HP21 and PAP3 activate proHP5 by cleavage at ESDR176*IIGG. HP5 then cleaves proHP6 at a unique site of LDLH112*ILGG. HP6, an ortholog of Drosophila Persephone, activates both proHP8 and proPAP1. HP8 activates proSpätzle-1, whereas PAP1 cleaves and activates proPO. HP5 is inhibited by Manduca sexta serpin-4, serpin-1A, and serpin-1J to regulate its activity. In summary, we have elucidated the physiological roles of HP5, a CLIPB with unique cleavage specificity (cutting after His) that coordinates immune responses in the caterpillar.