Expression, Purification of toxoplasma ROP 18 Recombinant protein and its antigenic and immunogenic trials in mice

Abstract

Toxoplasma gondii is an obligate intracellular, apicomplexan parasite that infects all warm-blooded vertebrates, including mammals and birds. Human beings can be infected by ingestion of oocysts from cat feces or through the consumption of meat containing Toxoplasma gondii cysts. There are potential vaccines candidates among which ROP18 has its major role in host gene expression along with the modulatory effect on key regulators of the host immune system. Therefore in this study, ROP18 sequence was amplified from local T. gondii strain, recombinant ROP18 was expressed through recombinant DNA technology and this recombinant protein was then tested for its antigenicity and immunogenicity in a mouse model. Approximately 200 fecal samples were collected from domestic, wild and stray cats in and around city of Lahore, Pakistan. Oocysts of T. gondii from cat feces were identified by using light microscopy and flotation technique. The oocysts were measured by micrometry having diameter of 8-10 μm. Out of 200 fecal samples, only three were suspected for T. gondii through direct microscopic examination and flotation technique. From 3 fecal samples, genomic DNA was extracted using a stool DNA extraction kit. After DNA extraction, the 3 samples were confirmed and characterized by PCR and nested PCR by using B1 gene and SAG2 primer sets. Reference DNAs (RH) of toxoplasma were kindly provided by Dr. Henrik Vedel Nielsen (Statens Serum Institut, Denmark) and Dr. Jorge Enrique Gomez Marin (COLOMBIA, South America). For detection of the B1 gene of T. gondii, the diagnostic method was optimized to amplify a 529 base pair (bp) repetitive sequence by PCR using DNA extracted from cat feces. Then a nested PCR was employed using internal primers to amplify a 102 bp from 391 bp product. The SAG2 gene was targeted at 5 different regions to amplify 5 amplicons. Genotype analysis was done using SAG2 sequence by Dr. Jorge Enrique Gomez Marin using 10 different markers. For amplification of ROP18, 54 sequences of the ROP18 gene retrieved from Genbank (National Center for Biotechnology Information (NCBI)) We used Geneious R8.1.6 software for sequence alignment and creating consensus sequence from all 54 ROP18 sequences. Primers were designed manually from the consensus sequence of ROP18. Primer pair namely ROP18-F 5‟ATCTAGAATGTTTTCGGTACAGCGG3‟ and ROP18-R Reverse 5‟TTCGAATTCTGTGTGGAGATGTTCC3‟ were designed to have restriction sites XbaI and HindIII respectively. The rop18 sequence was first cloned in pGMT easy vector system and then subcloned in pET28. BL21 competent cells were transformed with pET28-ROP18 and rROP18 was expression using IPTG for induction. The rROP18 was quantified through protein quantification kit (BCA). The rROP18 was formulated into nanospheres using PLGA as coating material. The Swiss-Webster mice were inoculated either intranasal or subcutaneous with rROP18 with or without montanide as adjuvant 3 times with 2 week interval. The blood was collected 2 weeks after each immunization. The control groups were inoculated with PLGA I/n or montanide s/c. For western blotting, ROP18 protein was electrophoresed on SDS-PAGE and blots were immune-blotted with the sera of immunized or infected mice. Bound antibodies were detected through Goat anti-mouse IgG–alkaline phosphatase conjugated. For evaluation of humoral response, ELISA plate was coated overnight at 4°C with rROP18 protein at 5μg/ml in 50mM sodium carbonate buffer (pH 9.6) @ 100 μl/ well. The absorbance of each sample was measured at OD 405 nm using ELISA (Bio-Tek, E-800, USA). Comparisons of quantitative values in the different groups were performed using ANOVA test, after checking the homogeneity of variances. Comparisons between groups for the antibody titre were performed by Dunn multiple range tests test. Comparisons were considered significant when a probability of equality was less than 5% (P<0.05). It was observed that rROP18 in nanospheres administered intranasal elicited elevated responses of specific intestinal IgA and IgG2a as compared to other groups inoculated intranasally rROP18 alone or injected subcutaneously rROP18 adjuvanted in montanide. It was concluded that nanospheres of ROP18 would be a non-invasive approach to develop vaccination against toxoplasmosis. Further experiments are needed to conclude the cellular response of these nanospheres in a chronic mouse model.