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Abstract
The gene encoding the Wuchereria bancrofti orthologue of the Brugia malayi-derived
diagnostic antigen SXP1 was identified from a W. bancrofti L3 cDNA library and characterized.
The Wb-sxp-1 cDNA encoded a basic protein with a calculated molecular mass of 20.8
kDa. Wb-SXP-1 was 85% identical to the SXP1 protein described from B. malayi (Bm-SXP-1).
The Wb-SXP-1 sequence also showed significant identity with proteins described from
B. pahangi, Onchocerca volvulus, Acanthochilonema vitea, Ascaris suum, Loa loa, Litomosoides
sigmodontis and Caenorhabditis elegans. The presence of a number of invariant and
conserved residues in all of these nematode-derived molecules suggests that Wb-SXP-1
is a member of a new protein family. A recombinant form of Wb-SXP-1 was produced and
it was determined that the anti-Wb-SXP-1 antibody response in patients with W. bancrofti
infections was restricted to the IgG4 subclass. An anti-Wb-SXP-1 IgG4 ELISA was developed
and this assay was found to be 100% sensitive for patients with patent W. bancrofti
infection. Sera from individuals experiencing chronic pathology, endemic normals or
patients with non-filarial nematode infections had no detectable IgG4 against Wb-SXP-1.
While patients with patent Onchocerca volvulus infections were uniformly negative
in the Wb-SXP-1 assay, 40% of sera from patent Loa loa infections were positive. When
Bm-SXP-1 was used as the antigen under identical conditions, the assay was 88% specific
for patent W. bancrofti infections and the antigen was recognized by antibodies from
both O. volvulus and L. loa infections. The results strongly suggested that, for certain
diagnostic filarial antigens, the use of same-species molecules can enhance the specificity
of diagnostic tests.