A whole adult T. solium was obtained from a patient with taeniasis from a taeniasis-endemic area in Yajiang, Ganzi, Sichuan Province. To generate T. solium larvae, each healthy piglet was infected by feeding 5 pieces of mature gravid proglottids from the worm at the Animal Experimental Center of Zunyi Medical University. There were 12 piglets in this infection experiment, including 10 piglets in the infection group and 2 piglets in the control group. The healthy piglets were confirmed to be pathogen-free before infection and raised under standard conditions (15). The infected piglets were euthanized 2–3 months postinfection, and Cysticercus cellulosae were harvested from the muscle tissue. After obtaining the parasite, we observed its structure under a microscope.

The T. solium was washed 3 times with phosphate-buffered saline (PBS), cut into pieces using scissors, ground and divided into 1.5 ml centrifuge tubes, and centrifuged at 4°C and 12,000 r/min for 5 min, and the supernatant was collected. The C. cellulosae were collected and washed three times with physiological saline and PBS, and then homogenized in a homogenizer (70 Hz/s, 10 s/time) at 4°C until completely lysed. The supernatants were collected, and their protein concentration was measured using a Bradford protein assay kit (Beyotime, China) according to the manufacturer's instructions and stored at −80°C until further use.

Each protein sample was first ground with liquid nitrogen into cell powder and then transferred into a 5-ml centrifuge tube. Next, four volumes of lysis buffer (8 M urea; Sigma, China) and 1% protease inhibitor (Calbiochem, Germany) were added to the cell powder, followed by sonication three times on ice using a high-intensity ultrasonic processor (Scientz); the remaining debris was removed by centrifugation at 12,000 g for 10 min at 4°C. Finally, the supernatant was collected and the protein concentration was determined using a BCA kit (Beyotime, China) according to the manufacturer's instructions.

The protein solution was added with dithiothreitol to a final concentration of 5 mM and incubated for 30 min at 56°C. Then, iodoacetamide was added to a final concentration of 11 mM and incubated for 15 min at room temperature in the dark. Finally, the urea concentration of the sample was diluted to less than 2 M. Trypsin was added at a mass ratio of 1:50 (trypsin: protein) and digested overnight at 37°C. The trypsin was added at a mass ratio of 1:100 and was digested for 4 h and continued for 4 h.

The tryptic peptides were separated into fractions by high pH reverse-phase HPLC, using a Thermo Betasil C18 column (5-μm particles, 10-mm inner diameter, 250-mm length). Briefly, peptides were first separated by a gradient spanning 8 to 32% acetonitrile (pH 9.0) over 60 min, which generated sixty fractions. Then, these peptides were pooled into six fractions and dried by vacuum centrifuging.

The tryptic peptides were dissolved in 0.1% formic acid (solvent A) and directly loaded onto a homemade reversed-phase analytical column (15-cm length, 75-μm inner diameter.). The gradient consisted of an increase from 6 to 23% solvent B (0.1% formic acid in 98% acetonitrile) over 26 min, followed by 23 to 35% over 8 min and climbing to 80% in 3 min, then holding it at 80% for the last 3 min, all performed at a constant flow rate of 400 nl/min on an EASY-nLC 1000 UPLC system. The peptides were subjected to a nanoelectrospray ionization (NSI) source, followed by tandem mass spectrometry (MS/MS) in a Q ExactiveTM Plus (Thermo) coupled online to the UPLC system. The electrospray voltage applied was 2.0 kV; the m/z scan range was 350 to 1,800 for each full scan, and intact peptides were detected in the Orbitrap at a resolution of 70,000. Peptides were then selected for MS/MS by using an NCE set to 28, with fragments detected in the Orbitrap at a resolution of 17,500. A data-dependent procedure was used, one that alternated between a single MS scan followed by 20 MS/MS scans with a 15.0-s dynamic exclusion. The automatic gain control (AGC) was set to 5E4. The fixed first mass was set to 100 m/z.

The secondary mass spectrum data were searched using Maxquant (version 1.5.2.8, http://www.maxquant.org/). For the retrieve parameter settings, the database is Taenia_asiatica_60517_PR_20190708 (10,328 sequences). Trypsin/P was specified as the cleavage enzyme, permitting up to four missing cleavages. The mass tolerance for precursor ions was set to 20 ppm in the “First search” and 5 ppm in the “Main search,” while the mass tolerance for fragment ions was set to 0.02 Da. Carbamidomethyl on Cys was specified as the fixed modification and acetylation modification and oxidation on Met were specified as the variable modifications. The false discovery rate (FDR) was adjusted to < 1% and a minimum score for modified peptides was set a priori to >40.

According to MS/MS analysis, the total peptide identified was 143,698, the specific peptide was 19,210, and a total of 3,658 proteins were identified, of which 2,481 contained quantitative information (Tables 1, 2). Relative standard deviation (RSD) was used to estimate repeatability, and a lower RSD (n = 3) value increases the accuracy of the repeatability (Figure 1). A corrected P-value of 0.05 and minimum fold-change of 2 were set as threshold criteria for determining the significant differential expression of a protein. In comparing the T. solium adults and larvae, we found 293 upregulated proteins (adults) and 265 downregulated proteins (cysticerci) (Supplementary Table S1).

The identified proteins were classified according to their molecular function, cellular component, and BP. Among these differentially expressed proteins, many were found to be involved in cellular processes, biological regulation, metabolic processes, multicellular organismal processes, developmental processes, and immune system processes. The predominant molecular functions are related to binding (e.g., protein binding) and catalytic activity (hydrolase and oxidoreductase activity). Regarding BPs, the predominant number of proteins is involved in metabolic and cellular processes. For cellular components, our GO results indicated that the identified proteins are primarily associated with cell parts, intracellular, and protein-containing complexes.

The forty downregulated (cysticercus stage) proteins were most enriched in terms of regulation of immune system process at biological function; such as protein kinase C, E3 ubiquitin-protein ligase, SH2 domain-containing protein, annexin, cAMP-dependent protein kinase regulatory, and fatty acid binding protein (Supplementary Table S2). Both annexin and human plasminogen proteins are known for their strong antigenicity, hence, their use as diagnostic antigens. In addition, 25 other proteins involved as immune effectors were found (Supplementary Table S3).

The results revealed that most downregulated proteins were enriched in the regulation of protein localization to membrane, bicarbonate transport, neuroepithelial cell differentiation, and mechanoreceptor differentiation, all under the BP category. The molecular function was enriched for cytoskeletal protein binding and actin binding. The cellular component was enriched in terms of the intracellular vesicle, cytoplasmic vesicle, plasma membrane region, and secretory vesicle (Figure 2A).

The upregulated (adult stage) proteins were mainly enriched in terms of regulation of metabolic processes in biological function (Figure 3A). Most proteins are involved in the metabolic process, of which nineteen of these proteins have been studied in other parasites, all of which are related to the growth, development, and metabolism of parasites, and only 2 proteins are related to immune function (Supplementary Tables S4, S5).

In the cysticercus stage, the KEGG enrichments included lysosome, glutathione metabolism, porphyrin and chlorophyll metabolisms, and 2-oxocarboxylic acid metabolism (Figure 2B, Table 3). Isocitrate dehydrogenase [NADP], NAD_binding_2 domain-containing protein, 6PGD domain-containing protein, and phospholipid hydroperoxide glutathione were involved in glutathione metabolism; 5-aminolevulinate synthase, porphobilinogen deaminase, coproporphyrinogen III oxidase, and protoporphyrinogen oxidase were involved in porphyrin and chlorophyll metabolisms; isocitrate dehydrogenase [NADP], aconitate hydratase, and citrate synthase were involved in 2-oxocarboxylic acid metabolism; ADP ribosylation factor-binding protein GGA1, beta-galactosidase, Prosaposin a preproprotein, and LITAF domain-containing protein were involved in the lysosome.

In the adult stage, the KEGG enrichments included ribosome, pyrimidine metabolism, and ribosome biogenesis in eukaryotes. There were 18 proteins involved in the ribosome, CTP synthase, PNP_UDP_1 domain-containing protein, and Thymidylat_synt domain-containing protein were associated with pyrimidine metabolism; ribonucleoside-diphosphate reductase subunit and ribonucleoside-diphosphate reductase were involved in glutathione metabolism and pyrimidine metabolism. 39S ribosomal protein L3, 60S ribosomal protein L3, Ribosomal_L2_C domain-containing protein, 40S ribosomal protein S4, Ribosomal protein L11, Ribosomal S15a protein, Ribosomal protein l5, large subunit ribosomal protein l7e, and 39S ribosomal protein L12 were associated with the ribosome biogenesis in eukaryotes (Figure 3B, Table 4).

According to GO annotation and KEGG enrichments, we analyzed 9 out of 20 differentially expressed proteins with PRM instead of traditional western blotting. The quantitative information of PRM protein was calculated according to the ion peak area of the peptide. Four proteins were identified in adults (Figure 4A), namely, S1 motif domain-containing protein, Cullin-1, Protein kinase domain-containing protein, and proliferating cell nuclear antigen. Four proteins were identified in the larvae (Figure 4B), namely, citrate synthase, glutamate synthase, synaptotagmin protein 4, and neurogenic locus notch protein.

Through a functional enrichment analysis, we found 25 proteins involving the immune process in the larva. It included known proteins such as thioredoxin peroxidase, calcium-binding protein 39, fatty acid-binding protein FABP2, and protein kinase C. Calcium-binding protein is involved in various regulatory functions of host invasion by parasites, and these mainly include members of the calmodulin family (CAM), the calcineurin B-like (CBL) family, and the calcium-dependent protein kinase (CDPK) family: all of these are highly conserved helix–loop–helix structure, namely, the EF-hand domain. In Plasmodium and Toxoplasma, phosphatidylinositol can also be stimulated by ligands binding to surface receptors (such as G-protein binding receptors [GPCR]), to produce calcium signals, thereby stimulating multiple cellular pathways (28). Fatty acid-binding proteins (FABPs) are a family of proteins with binding of triclabendazole (29), anti-oxidant activity, and immunomodulation (30). It is used as a potential drug target in Echinococcus granulosus and schistosomiasis (31, 32). Some proteins have been studied for immunity in other parasitic worms. For example, the Trichinella spiralis ES product thioredoxin peroxidase-2 induces macrophages toward an M2-like phenotype, both in vivo and in vitro, and CD4⁺T cells increased in number after immunization of mice with rTsTPX2 and mediate the expulsion of the worm from the host to protect them, thus suggesting TsTPX2 is a potential vaccine candidate against trichinosis (33). Also, thioredoxin peroxidase was significantly recognized by melioidosis-positive sera in the cysticercus stage, for which strong immunogenic properties render it an anticipated vaccine target (34). Pyruvate kinase is a crucial glycolytic enzyme that has been characterized in Clonorchis sinensis (35) and it promotes the development of Th1 and inhibition of dendritic cells (DCs).

Some researchers revealed that fructose-1-6-bisphosphate aldolase (FBPA) proteins of T. solium cyst fluid were mainly enriched PI3K-Akt signaling pathways which are thought to play important roles in proliferation, migration, invasion, and drug resistance (36, 37). FBPA is a crucial glycolytic enzyme and a plasminogen-binding protein and is involved in parasitic motion and invasion by connecting surface adhesion proteins to the actin-myosin of parasites (38). Specifically, FBPA binds to the surface of the cell membrane, and then binds to plasminogen, using the activity of hydrolyzing surface-related proteins to help invade host cells; hence, it is also considered a potential vaccine candidate or chemotherapy target for Trichinella spiralis and Giardia infections (39–41).

Furthermore, antigens and immunomodulatory proteins observed in cysticercus are able to play a special mechanism of adaptation at this developmental stage, one that has evolved to evade the host immune system, which is a prerequisite for establishing a successful infection. We have only discussed the known proteins, and there are still several unknown proteins with specific functions that need to be verified.

Lacking a digestive tract, T. solium instead absorbs nutrients from its host across the body wall. The cortex absorbs all kinds of nutrients by means of diffusion and active transport, and it also has the function of secreting and resisting the destruction of the host's digestive juice. Their main energy source, carbohydrates, can be catabolized by aerobic respiration or by two complementary anaerobic pathways, namely, lactate fermentation and malate dismutation (4). In this study, we found the upregulated proteins, most of the proteins are related to metabolism. Following PRM analysis, we found that the expression of four proteins (S1 motif domain-containing protein, Cullin-1, protein kinase domain-containing protein, and proliferating cell nuclear antigen) was consistently elevated, and several proteins whose functions are lacking in the study of the porcine tapeworm have important values in other parasite fields, which provides us with different insights into the tapeworm. Cullin protein is the most typical ubiquitin ligase family and a key tumor-associated protein, which is able to promote the proliferation of tumor cells and also can be used as a marker and therapeutic target for tumor prognosis. According to the GO analysis, the Cullin protein is also involved in cell cycle progression, signal transduction and transcription, intracellular metabolism, and the corresponding response to stimuli. However, this protein has not been studied in parasites and could be a research target to be verified in the next experiments (42–44). Proliferating cell nuclear antigen (PCNA) is a protein that acts as a processivity factor for DNA polymerase δ in eukaryotic cells and was originally identified as an antigen expressed in the nuclei of cells during the DNA synthesis phase of the cell cycle (45, 46). The major vault protein is a ribonucleoprotein that is highly conserved in lower and higher eukaryotes, and its function is not clear (47). Some studies have found that Schistosoma mansoni participates in the adaptation mechanism of parasitic hosts in the process of infection.

Topoisomerase is a critical type of enzyme for overcoming the problem of chromosome topology that arises during DNA replication, transcription, recombination, and mitosis, and they are involved in cell growth, tissue development, and cell differentiation (48–50). These enzymes participate in a variety of DNA metabolic processes; in the parasitic infection, the inhibition of topoisomerase II by different ketone benzene and furan derivatives in cells hinders the basic metabolic process of cells and eventually leads to apoptosis. In view of the parasite DNA topoisomerase II, these compounds can be used as potential antiparasitic drugs. Schistosome parasite miRNA mediates the activity of the frizzled protein (frizzled-related protein 1), which increases liver fibrosis, in that FRZB2 is a secreted frizzled protein-related protein, which can competitively bind to specific frizzled protein receptors to suppress the signal transduction of Wnt, thus affecting the severity of liver fibrosis. The FRZB2 gene knockdown affected S. japonicum morphology, development, survival, as well as reproductive capacity (51, 52). Plasmodium glyceraldehyde-3-phosphate dehydrogenase functions as a glycolytic enzyme and also as a host plasminogen-binding protein, thereby interacting with the host's fibrinolytic system to establish an important mechanism for a parasite's invasion, growth, and development. At the same time, it can be used as a potential diagnostic biomarker for a variety of parasites, such as Plasmodium, T. solium, E. granulosus, filariasis, S. mansoni, and Babesia microti (53–57). The distinct mechanisms of cell cycle arrest are associated with the upregulating or downregulating of TbPCNA. Deregulating the intra-parasite levels of TbPCNA is a potential strategy for therapeutically exploiting this target in the bloodstream form of Trypanosoma brucei, which shows that PCNA is related to that parasite's growth and development (58).

Adults only survive in the host's intestines and escape the immune response without causing excessive damage to the host. In our study, we found that there are few proteins related to immunity, and most of them are related to growth and development, which also reflects the difference between adults and larvae to the host.