Abundant Larval Transcript (ALT) from Filarial Parasites

The Abundant Larval Transcript products were discovered in the filarial parasite, Brugia malayi and a number of other filarial nematode species. They are among the top candidates for a future vaccine against filariasis, a major Neglected Tropical Disease.          

The human filarial parasite Brugia malayi can be maintained in the laboratory through Aedes aegypti mosquitoes in which parasites develop to the L3 stage, and infect the Mongolian word (gerbil) Meriones unguiculatus. In the Maizels lab, analysis of L3 mRNA, using primers for the trans-spliced leader (SL) attached to a subpopulation of messages, revealed high expression of two novel products, ALT-1 and ALT-2 with 79% amino acid identity to each other (Gregory et al 1997).

ALT-1 and ALT-2 proteins, expressed as recombinants in E. coli, were tested for vaccine potential in Meriones and found to elicit a 76% reduction in adult worm numbers (Gregory et al 2000). The two genes share a suggested promoter sequence that is functional when tested by transfection into C. elegans (Gomez-Escobar et al 2002).

The biological function of the ALTs may be immunomodulatory; macrophages infected with transgenic ALT-expressing Leishmania mexicana show heightened expression of SOCS1 (an inhibitor of IFNγ signalling) and GATA3 (a pivotal transcription factor for type 2 immunity). As a result, ALT-expressing Leishmania parasites are more resistant to macrophage immune attack (Gomez-Escobar et al 2005).

ALT protein have 21-aa signal peptides, a variable N-terminal acidic domain, and a more conserved C-terminal domain.  Studies in the Grieve laboratory identified similar products in the dog heart worm Dirofilaria immitis (Frank et al 1995, 1999), and later homologues have ben reported in other filariae, for example Acanthocheilonema viteae (Av-ALT, Pogonka et al 1999) and Wuchereria bancrofti (Wb-ALT-2, Aparnaa 2014).  Experimental development of B. malayi ALT-2 in filarial vaccines continues in other laboratories (eg Ganapathy 2014, Khatri et al 2018).

References (Maizels Lab)

Gomez-Escobar, N., Gregory, W.F., Britton, C., Murray, L., Corton, C., Hall, N., Daub, J., Blaxter, M.L. and Maizels, R.M. (2002). Abundant larval transcript-1 and -2 genes from Brugia malayi: diversity of genomic environments but conservation of 5′ promoter sequences functional in Caenorhabditis elegans. Mol Biochem Parasitol 125(1-2): 59-71. doi: 10.1016/s0166-6851(02)00219-0

Gomez-Escobar, N., Bennett, C., Prieto-Lafuente, L., Aebischer, T., Blackburn, C.C. and Maizels, R.M. (2005). Heterologous expression of the filarial nematode alt gene products reveals their potential to inhibit immune function. BMC Biol 3: 8. doi: 10.1186/1741-7007-3-8

Gregory, W.F., Atmadja, A.K., Allen, J.E. and Maizels, R.M. (2000). The abundant larval transcript-1 and -2 genes of Brugia malayi encode stage-specific candidate vaccine antigens for filariasis. Infect Immun 68(7): 4174-4179. doi: 10.1128/IAI.68.7.4174-4179.2000

Gregory, W.F., Blaxter, M.L. and Maizels, R.M. (1997). Differentially expressed, abundant trans-spliced cDNAs from larval Brugia malayi. Mol Biochem Parasitol 87(1): 85-95. doi: 10.1016/s0166-6851(97)00050-9

Other Laboratories’ References

Aparnaa, R., Mahalakshmi, N., Harini, A., Jeyaprita, P.J., Anugraha, G., Amdare, N.P., Khatri, V.K., Reddy, M.V. and Kaliraj, P. (2014). Wuchereria bancrofti 20/22 a homologue of abundant larval transcript L3 stage filarial antigen: molecular and immunological characterization. Parasite Immunol 36(10): 475-484. doi: 10.1111/pim.12120

Frank, G.R. and Grieve, R.B. (1995). Purification and characterization of three larval excretory-secretory proteins of Dirofilaria immitis. Molecular and Biochemical Parasitology 75: 221-229. doi: 

Frank, G.R., Wisnewski, N., Brandt, K.S., Carter, C.R.D., Jennings, N.S. and Selkirk, M.E. (1999). Molecular cloning of the 22-24 kDa excretory-secretory 22U protein of Dirofilaria immitis and other filarial nematode parasites. Molecular and Biochemical Parasitology 98: 297-302.

Ganapathy, M., Perumal, A., Mohan, C., Palaniswamy, H. and Perumal, K. (2014). Immunogenicity of Brugia malayi Abundant Larval Transcript-2, a potential filarial vaccine candidate expressed in tobacco. Plant Cell Rep 33(1): 179-188. doi: 10.1007/s00299-013-1521-3

Khatri, V., Chauhan, N., Vishnoi, K., von Gegerfelt, A., Gittens, C. and Kalyanasundaram, R. (2018). Prospects of developing a prophylactic vaccine against human lymphatic filariasis – evaluation of protection in non-human primates. Int J Parasitol 48(9-10): 773-783. doi: 10.1016/j.ijpara.2018.04.002

Pogonka, T., Oberlander, U., Marti, T. and Lucius, R. (1999). Acanthocheilonema viteae: characterization of a molt-associated excretory/secretory 18-kDa protein. Experimental Parasitology 93: 73-81.