Wucherria bancrofti glutathione S-Transferase: Insights into the 2.3 Å resolution X-ray structure and function, a therapeutic target for human lymphatic filariasis

The notoriety of parasitic nematode survival is directly related to chronic pathogenicity, which is evident in human lymphatic filariasis. It is a disease of poverty which causes severe disability affecting more than 120 million people worldwide. These nematodes down-regulate host immune system through a myriad of strategies that includes secretion of antioxidant and detoxification enzymes like glutathione-S-transferases (GSTs). Earlier studies have shown Wuchereria bancrofti GST to be a potential therapeutic target. Parasite GSTs catalyse the conjugation of glutathione to xenobiotic and other endogenous electrophiles and are essential for their long-term survival in lymph tissues. Hence, the crystal structure of WbGST along with its cofactor GSH at 2.3 Å resolution was determined. Structural comparisons against host GST reveal distinct differences in the substrate binding sites. The parasite xenobiotic binding site is more substrate/solvent accessible. The structure also suggests the presence of putative non-catalytic binding sites that may permit sequestration of endogenous and exogenous ligands. The structure of WbGST also provides a case for the role of the π-cation interaction in stabilizing catalytic Tyr compared to stabilization interactions described for other GSTs. Hence, the obtained information regarding crucial differences in the active sites will support future design of parasite specific inhibitors. Further, the study also evaluates the inhibition of WbGST and its variants by antifilarial diethylcarbamazine through kinetic assays.

Immunoadjuvant effect of diethylcarbamazine in experimental filariasis

Lymphatic filariasis caused by tissue dwelling nematodes is endemic in 73 countries and drugs have been administered to control or stop the infection. Resurgence of the infection after mass drug administration necessitates the study of several parasite antigens or adjuvants for vaccine developments. In this study, diethylcarbamazine (DEC) was evaluated for its efficacy as adjuvant against the filarial parasite; Brugia malayi microfilariae (mf) by combining with the Escherichia coli expressed recombinant BmShp-1 protein. Shp-1 is one of the sheath proteins expressed by adult female and microfilarial stage of the filarial parasite. Hence, immunoprophylactic efficacy of Shp-1 using DEC and alum adjuvants was compared in BALB/c mice model by an in situ micropore chamber method. Shp-1 antibody titre was high when the mice were immunized with Shp-1 along with DEC and they exhibited balanced Th1/Th2 profile. DEC also induced significantly high T-cell proliferation (P<0.001) when stimulated with Shp-1 compared to alum. Significantly high percentage protection against B. malayi microfilariae was observed in Shp-1+DEC immunized mice groups (P<0.05) and hence it is concluded that the need of repeated drug administration can be controlled when there is a possibility of developing protective immunity in the host against mf by vaccination.

Comparison of immunogenicity, protective efficacy of single and cocktail DNA vaccine of Brugia malayi abundant larval transcript (ALT-2) and thioredoxin peroxidase (TPX) in mice

Although DNA vaccines have several advantages over conventional vaccines, antibody production and protection are often not adequate, particularly in single plasmid vaccine formulation. In the present study we evaluated the efficacy of a cocktail vaccine based on plasmids encoding larval (L3) stage-specific Brugia malayi abundant larval transcript (BmALT-2) and antioxidant detoxification enzyme B. malayi thioredoxin peroxidase (BmTPX) to induce antibodies, protective efficacy and cell-mediated immune response in mice. Mice immunized with cocktail DNA vaccines containing the pVAX ALT-2+TPX developed higher titers of anti-BmALT-2+TPX (1:5000) antibodies, compared to the mice immunized with single DNA vaccine of pVAX ALT-2 or pVAX TPX (1:2000). Correlating with this, the mice administered with cocktail vaccine induced up to 78% of cytotoxicity against B. malayi mf. This cytotoxicity was high compared to 34% induced by the pVAX-ALT2 or 37% by pVAX-TPX. Moreover, cocktail vaccination of mice resulted in significantly higher level of cellular proliferative response associated with raised levels of IFN-gamma that skewed towards Th1 type of response compared to vaccination using either of the components. Taken together, these data suggest that the combination of two or more antigens maybe an effective vaccine development strategy to improve protection and immunogenicity against human lymphatic filariasis.