Lymphatic filariasis is a tropical disease, with over 40 million people seriously incapacitated due to lymphangitis and elephantiasis. Over 99% of infections are caused by the nematode Wuchereria bancrofti. Expressed sequence tag (EST) analysis of filarial genome identified novel infective larval (L3) stage-specific antigen, abundant larval transcript (ALT-2), which was shown to be highly essential for parasite establishment and survival in the host. The unique structural features and immunological characteristics of ALT-2 indicate the presence of critical motifs that needs to be explored in natural human infection for understanding and management of the disease. In order to dissect the epitopes of ALT protein recognized in humans, eight peptides spanning the entire protein sequence were selected based on in-silico epitope prediction and synthesized. Analysis of the reactivity of W. bancrofti ALT-2 synthetic peptides with clinical sera (n = 40) from endemic areas identified epitopes recognized by putatively immune sera, of which two comprise the highly variable acidic domain (21-60). Interestingly, our study also revealed crucial epitopes recognized by microfilaremic (MF) sera with significantly high IgG4 isotype (p < 0.001), implicated in immunomodulation. The epitope peptides identified were highly specific for MF sera and, thus, have the potential to be exploited as diagnostic markers.

The chromadorea abundant larval transcript (ALT) family of proteins contains ALT one of the most studied putative vaccine candidate in experimental filariasis. This study reports the characterization of Wuchereria bancrofti 20/22 (Wb20/22) as a member of chromadorea, the ALT family of proteins from the L3 stage of W. bancrofti. The high reactivity with serum from the endemic normal (EN) population suggests that Wb20/22 could be a target of elicit protective immunity. The glutamic acid-rich region of Wb20/22 was predicted to harbour the longest linear B-cell epitope by insilico prediction tools. The significance of this region was revealed by studying the mutant form of Wb20/22, without acidic domain (WOAD) which was cloned, and the immune response was compared with Wb20/22. The signal sequence of Wb20/22 was also an immunodominant region, and mutant construct without signal sequence (WOSS) was cloned and characterized. The peak antibody titre elicited by WOAD was higher than Wb20/22 or WOSS, which pointed to the immunomodulatory role of glutamic acid-rich region. Wb20/22 elicited very high levels of IL-10 and diminished levels of IL-4 and IL-5 which could be the reason for low antibody titre. The prophylactic efficacy of WOAD conferred protection (62·26%) which was higher than Wb20/22 (49·82%) and WOSS (54·78%).

Helminth parasites use antioxidant defence strategies for survival during oxidative stress due to free radicals in the host. Accordingly, tissue-dwelling filarial parasites counteract host responses by releasing a number of antioxidants. Targeting these redox regulation proteins together, would facilitate effective parasite clearance. Here, we report the combined effect of protective immune responses trigged by recombinant Wuchereria bancrofti thioredoxin (WbTRX) and thioredoxin peroxidase (WbTPX) in an experimental filarial model. The expression of WbTRX and WbTPX in different stages of the parasite and their cross-reactivity were analysed by enzyme-linked immunosorbent assay (ELISA). The immunogenicity of recombinant proteins and their protective efficacy were studied in animal models when immunized in single or cocktail mode. The antigens showed cross-reactive epitopes and induced high humoral and cellular immune responses in mice. Further, parasite challenge against Brugia malayi L3 larvae in Mastomys coucha conferred significant protection of 57% and 62% against WbTRX and WbTPX respectively. The efficacy of L3 clearance was significantly higher (71%) (P < 0.001) when the antigens were immunized together, showing a synergistic effect in multiple-mode vaccination. Hence, the study suggests WbTRX and WbTPX to be attractive vaccine candidates when immunized together and provides a tandem block for parasite elimination in the control of lymphatic filariasis.

Although multiple vaccine strategy for lymphatic filariasis has provided tremendous hope, the choice of antigens used in combination has determined its success in the previous studies. Multiple antigens comprising key vaccine candidates from different life cycle stages would provide a promising strategy if the antigenic combination is chosen by careful screening. In order to analyze one such combination, we have used a chimeric construct carrying the well studied B. malayi antigens thioredoxin (BmTRX) and venom allergen homologue (BmVAH) as a fusion protein (TV) and evaluated its immune responses in mice model. The efficacy of fusion protein vaccine was explored in comparison with the single antigen vaccines and their cocktail. In mice, TV induced significantly high antibody titer of 1,28,000 compared to cocktail vaccine TRX+VAH (50,000) and single antigen vaccine TRX (16,000) or VAH (50,000). Furthermore, TV elicited higher level of cellular proliferative response together with elevated levels of IFN-γ, IL-4 and IL-5 indicating a Th1/Th2 balanced response. The isotype antibody profile showed significantly high level of IgG1 and IgG2b confirming the balanced response elicited by TV. Immunization with TV antigen induced high levels of both humoral and cellular immune responses compared to either cocktail or antigen given alone. The result suggests that TV is highly immunogenic in mice and hence the combination needs to be evaluated for its prophylactic potential.