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. 2019 Mar:267:9-16.
doi: 10.1016/j.vetpar.2019.01.004. Epub 2019 Jan 29.

Characterization of a novel cathepsin L-like protease from Taenia solium metacestodes for the immunodiagnosis of porcine cysticercosis

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Characterization of a novel cathepsin L-like protease from Taenia solium metacestodes for the immunodiagnosis of porcine cysticercosis

Nancy León-Janampa et al. Vet Parasitol. 2019 Mar.

Abstract

Porcine cysticercosis is an endemic parasitic disease caused by infection with Taenia solium that is found predominantly in developing countries. In order to aid in the development of simple diagnostic approaches, identification and characterization of potential new antigens for immunodiagnostic purposes is desired. The cysteine protease family has previously been found to have important immunodiagnostic properties. These proteases are expressed as zymogens which contain a signal peptide, pro-peptide, and an active domain. Subsequent catalytic cleavage of the pro-peptide converts these zymogens into enzymes. With the use of bioinformatic tools we identified an active domain of a novel cathepsin L-like cysteine protease (TsolCL) in the T. solium genome. The TsolCL gene includes 705 nucleotides (nt) within a single intron and a 633 nt exonic sequence encoding an active protein of 211 amino acids. Sequence alignment and phylogenetic analysis suggest that the TsolCL gene is closely related to genes found in Echinoccocus granulosus and E. multiloculars. In addition, TsolCL was found to have a 61.9%-99.0% similarity to other cathepsin L proteins found in other helminths and mammals. We cloned, expressed, purified, and characterized the recombinant active TsolCL (27 kDa) using the baculovirus-insect cell expression system. TsolCL showed cysteine protease enzymatic activity with the capacity to hydrolyze the Z-Phe-Arg-AMC substrate as well as bovine serum albumin. However, TsolCL was not able to hydrolyze human immunoglobulin. In addition, TsolCL has cathepsin L conserved amino acid residues in the catalytic site (Gln8, Cys14, His159, Asn179 and Trp181) and the motif GCNGG. Using ELISA, TsolCL was able to distinguish circulating IgG antibodies between healthy animals and naturally infected pigs with cysticercosis, showing a moderate sensitivity of 83.33% (40/48; 95% CI: [69.8%-92.5 %]), and a specificity of 83.78% (31/37; 95% CI: [67.9%-93.8%]). In conclusion, a novel cathepsin L-like cysteine protease from a T. solium metacestode was expressed successfully in Baculovirus system and was evaluated as a candidate antigen to diagnose porcine cysticercosis using the ELISA immunoassay.

Keywords: Baculovirus expression vector system; Cathepsin L; Cysticercosis; Immunodiagnosis; Taenia solium.

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Conflict of interest statement

Disclosure:

The authors have no conflict of interest to declare.

Figures

Figure 1:
Figure 1:
Multiple sequence aligment of the TsolCL amino acid sequence with cathepsin L cysteine proteases from other species using CLC Sequence Viewer software. The red box shows the conserved motif GCNGG from cathepsin L-like cysteine protease. Asterisks (*) represent the residues from the protease active site (Gln 8, Cys 14, His 159, Asn 179, and Trp 181).
Figure 2:
Figure 2:
Phylogenetic tree based on the deduced amino acid sequence of TsolCL and cathepsin L-like cysteine protease from other species. The unrooted maximum likelihood tree with 1000 bootstrap replicates was constructed using the MEGA6 program. Each sequence presents its accession number together with the name of the species. The arrow indicates the TsolCL protease reported in this study.
Figure 3:
Figure 3:
Catalytic activity of purified TsolCL expressed in insect cells. (A) TsolCL catalytic activity curves on the specific substrate AMC052. It is observed that the catalytic activity of TsolCL and papain increases over time. However, this is inhibited when using E64. TsolCL activity using BSA, in (B) citrate buffer with a pH of 5.2 (Lanes 1: protein molecular weight marker, 2: BSA + TsolCL + E64 reaction, 3: BSA, and 4: BSA + TsolCL reaction) and (C) phosphate buffer pH 6.5 (Lanes 1: protein molecular weight marker; 2: BSA, 3: BSA + TsolCL reaction, and 4: BSA + TsolCL + E64 reaction). It is seen that TsolCL can degrade BSA in citrate buffer and phosphate buffer at acidic pHs. However, note that TsolCL was unable to degrade human IgG (data not shown).
Figure 4:
Figure 4:
Plot representing the PP (cut-off: 90.4%) of positive and negative results, obtained from TsolCL ELISA test. Using sera from non-infected and infected pigs with cysticercosis in a 1/400 dilution.
Figure 5:
Figure 5:
Statistical analysis. (A) ROC curve for TsolCL ELISA test, ratio of true positive versus false positive rate with area under ROC curve of 0.895. (B) Probability cutoff between sensitivity (83.33%) and specificity (83.78%) of TsolCL ELISA test.

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