Characterization of a novel microfilarial antigen for diagnosis of Wuchereria bancrofti infections

Abstract

Background Lymphatic filariasis (LF) is a neglected tropical disease caused by the filarial nematodes Wuchereria bancrofti, Brugia malayi and Brugia timori. The Global Program to Eliminate LF uses mass drug administration (MDA) of anti-filarial drugs that clear microfilariae (Mf) from blood to interrupt transmission by mosquitos. New diagnostic tools are needed to assess the impact of MDA on bancroftian filariasis, because available serologic tests can remain positive after successful treatment. Methodology/Principal findings We identified Wb-bhp-1, which encodes a W. bancrofti homologue of BmR1, the B. malayi protein used in the Brugia Rapid antibody test for brugian filariasis. Wb-bhp-1 has a single exon that encodes a 16.3 kD protein (Wb-Bhp-1) with 45% amino acid identity to BmR1. Immunohistology shows that anti-Wb-Bhp-1 antibodies primarily bind to Mf. Plasma from 124 of 224 (55%) microfilaremic individuals had IgG4 antibodies to Wb-Bhp-1 by ELISA. Serologic reactivity to Wb-Bhp-1 varied widely with samples from different regions (sensitivity range 32–92%), with 77% sensitivity for 116 samples collected from microfilaremic individuals outside of sub-Saharan Africa. This variable sensitivity highlights the importance of validating new diagnostic tests for parasitic diseases with samples from different geographical regions. Individuals with higher Mf counts were more likely to have anti-Wb-Bhp-1 antibodies. Cross-reactivity was observed with a minority of plasma samples from people with onchocerciasis (17%) or loiasis (10%). We also identified, cloned and characterized BmR1 homologues from O. volvulus and L. loa that have 41% and 38% identity to BmR1, respectively. However, antibody assays with these antigens were not sensitive for onchocerciasis or loiasis. Conclusions Wb-Bhp-1 is a novel antigen that is useful for serologic diagnosis of bancroftian filariasis. Additional studies are needed to assess the value of this antigen for monitoring the success of filariasis elimination programs. Author summary Lymphatic filariasis (LF) is a highly disabling and stigmatizing disease caused by parasitic worms that are transmitted by mosquitoes. There is a coordinated global effort to eliminate LF based on mass drug administration (MDA) of donated anti-filarial medications. Improved methods are needed to determine when transmission of the infection has been interrupted in previously endemic areas so that MDA can be safely stopped. This paper reports the discovery and characterization of a novel W. bancrofti antigen, Wb-Bhp-1, which is a homologue of the Brugia malayi protein used in antibody tests to monitor filariasis elimination in areas of Asia where LF is caused by Brugia species. We show that a test for IgG4 antibodies to Wb-Bhp-1 was fairly specific for W. bancrofti infection. However, the sensitivity of this test varied by the geographic origin of the samples. Sensitivity was highest for samples collected in the Indo-Pacific region and lowest for samples collected in Côte d’Ivoire. Geographic differences in the parasite or the human immune responses to infection may account for this variability. This range in sensitivity highlights the importance of validating new diagnostic tests for parasitic diseases with samples from different geographical regions.