Experimental DNA vaccine against filariasis

Abstract

Filarial infections are major causes of morbidity in the tropics and sub-tropics, afflicting over 150 million people in about 80 countries, causing debilitating symptoms such as elephantiasis (lymphatic filariasis), dermatitis and blindness (onchocerciasis or river blindness). Current control of lymphatic filariais relies on mass drug treatment with diethylcarbamazine（DEC) and albendazole while ivermectin is used against onchocerciasis. Repeat treatment is frequently required and this highlights the possibility of development of drug resistance. In addition, risk of adverse reactions following treatment excludes some patients from control programmes. Such circumstances urgently call for the development of complementary control measures such as vaccination. DNA vaccines are novel type of subunit vaccine in which production of the immunizing antigen is induced in host cells after introduction of a plasmid or recombinant viral vector containing the gene that encodes the antigen. DNA vaccines are relatively simple and cheap to produce and their stability makes them particularly suitable for use in remote regions that lack the cold-chain storage facilities required of conventional vaccines. Filarial nematodes are tissue-dwelling parasites that survive for many years in immunocompetent hosts. It is proposed that this longevity is, in part, due to the capacity of the parasites to modulate potentially lethal Th2 responses of their hosts. Consequently, the efficiency of a filarial vaccine may depend on how well it circumvents filarial-driven immunosuppression. To test this hypothesis, a series of DNA vaccines were developed and tested in the Litomosoides sigmodontis-mouse model of filarial infections. The L. sigmodontis Abundant Larval Transcript-1 (Ls-ALT) and Cysteine Protease Inhibitor (Ls-CPI) genes were cloned and genetically engineered to ablate their immunomodulatory properties by deleting the acid domain and by site mutation, respectively. In addition, the L. sigmodontis Venom Allergen Homologue (Ls-VAH) and Thioredoxin Peroxidase (Ls-TPX) were used in their native forms as vaccine targets. To improve immunisation and antigen processing by the host, these parasite genes were fused to a DNA sequence encoding an antibody that specifically binds dendritic cell surface protein (αDEC205 single chain Fv). DNA plasmids carrying mutated forms and/or anti-DEC205 were then co-administered with plasmids encoding the Th2 promoting cytokine IL4, and/or antigen-presenting cell activating MIP1α and Flt3L. Mice immunized with mutated forms (ADDALT and CPImu) of parasite antigens produced more specific antibody post-challenge and showed strongly increased lymphocyte stimulation above controls immunized with the native form. The immune response was further enhanced when plasmids encoding IL4, MIP1α, Flt3L and anti-DEC-205 forms were co-administered, resulting in production of a Th2/IgG1 phenotype. Significant reduction of worm burden (82.3%) was achieved by a cocktail vaccination which combined the ADDALT and CPImu candidates. Mice immunized with Ls-VAH and Ls-TPX DNA carried by αDEC205 elicited Th2-biased responses with up-regulated IgG1 and IgE antibodies as well as enhanced IL5, IL4, and IL13 and diminished IFNγ production compared to controls. The immune responses were further driven towards the Th2/IgG1 phenotype when Ls-VAH and Ls-TPX were injected with plasmids encoding ADDALT and CPImu and with the adjuvants Flt3L, MIP1α and IL4. This resulted in reduction in worm burden of 55.7% (cocktail vaccine containing Ls-VAH) and 41.6% (cocktail vaccine containing Ls-TPX) respectively in vaccinated animals.