The role of a recombinant hybrid protein based ELISA for the serodiagnosis of Onchocerca volvulus

AIMS

Onchocerca volvulus infection is traditionally diagnosed by examination of skin snips for the presence of microfilariae. A disadvantage of this method is the low sensitivity particularly with light or prepatent infection. Serodiagnosis using recombinant-antigen-based assays may provide a more sensitive diagnostic test. An ELISA based on a recombinant antigen OvH3 has previously been validated using sera from endemic areas. This study investigated the role of this ELISA-based assay for use in the serodiagnosis of onchocerciasis in non-endemic areas.

METHODS

The ELISA-based assay was tested on sera from untreated patients with known onchocerciasis and on untreated and treated patients with definite or probable onchocerciasis identified from a hospital diagnostic database. The assay was also tested on sera from patients with other helminthic infections to determine the sensitivity and specificity of this assay in a tertiary referral laboratory dealing with sera from a variety of patients.

RESULTS

The sensitivity and specificity of the OvH3 assay were 93.2% and 93.5%, respectively, when tested on non-endemic patients with clinical diagnosis of onchocerciasis.

CONCLUSIONS

This study demonstrates the potential role of the assay as a sensitive and specific test for use in the serodiagnosis of onchocerciasis in a reference laboratory dealing with sera from patients in non-endemic setting.

Localization of T and B cell stimulating domains of the immunodominant 33-kDa protein of Onchocerca volvulus (Ov33)

The localization of T and B cell epitopes on a well characterized 33-kDa protein of the filarial nematode Onchocerca volvulus (Ov33) was studied using peripheral blood mononuclear cells (PBMC) and sera from a total of 52 onchocerciasis patients with the generalized form of infection. A proportion of the PBMC samples proliferated in response to recombinant Ov33-GST fusion protein and to fusion free Ov33-6xHis. Proliferative responses of patient PBMC to seven truncated Ov33-6xHis polypeptides and to three synthetic peptides revealed at least one major and two minor T cell epitopes in the protein. The dominant T cell stimulating domain was localized between amino acids 113 and 143. ELISA studies with the Ov33-GST fusion protein revealed that patient sera contained Ov33-specific IgG1, IgG4, IgE, and IgM antibodies. Analysis of the IgG4 response with 10 truncated Ov33 polypeptides identified four B cell stimulating domains in the N-terminal, central, and C-terminal region of the molecule. The B cell domain recognized by the majority of sera was localized between amino acids 113 and 143. The data indicate that this region of the protein is the major T and B cell stimulating domain of Ov33 and might be relevant for vaccine development and for improved immunodiagnosis of onchocerciasis.

The roles of IgG and defined antigens in cytoadherence and cytotoxicity reactions to onchocercal microfilariae

Cytoadherence and cytotoxicity reactions against larval stages of Onchocerca volvulus may be protective or pathogenic. To elucidate the inherent mechanisms, we examined a panel of human, rabbit and mouse antisera to O. volvulus antigens. Human IgG, but not IgM from onchocerciasis sera promoted the adherence of leucocytes to microfilariae (mf) that resulted in the killing of the mf in the presence of heat labile serum factors. Polyclonal and monoclonal antibodies to three recombinant antigens of O. volvulus (OV7I, OV25 and O13) were assayed for their abilities to induce cellular adherence and/or cytotoxicity of nodular mf. These antibodies were found to promote cytoadherence but showed negligible cytotoxicity effects. We conclude that IgG antibodies from onchocerciasis sera are implicated in the adherence to and killing of mf by patients' leucocytes.

Three new DPB1 alleles identified in a Bantu-speaking population from central Cameroon

HLA-DPB1 genotyping of 241 individuals from an African Bantu-speaking population in central Cameroon using sequence-specific oligonucleotide probes identified five individuals with novel probe hybridization patterns. DNA sequence analysis of the second exon of the DPB1 alleles from these five individuals identified three new alleles, *6001, *6101N, and *6201. DPB1*6001, found in two individuals, contains a single nucleotide change that results in a polar amino acid, asparagine, at residue 65; this position in the beta 1 domain is occupied by a nonpolar amino acid in all other reported DPB1 alleles. DPB1*6101N, found in one individual, contains a single base mutation that results in a premature termination codon at position 67. DPB1*6201, found in two individuals, is characterized by the apparent motif shuffling that has been hypothesized to be responsible for the majority of DPB1 sequence polymorphism. These new sequences shed additional light on the potential mechanisms by which allelic diversity is generated at the HLA-DPB1 locus.

Cell-mediated and monoclonal antibody-dependent killing of Onchocerca volvulus microfilariae

To identify the target antigens implicated in the adherence and killing of microfilariae (mf) by leucocytes, we incubated nodular mf and leucocytes in the presence of anti-cuticular monoclonal antibodies (MOVs) and fresh serum. Leucocyte donors were patients with a mean age of 37 years (with 0-1 mf/snip), who had lived in the endemic village studied for at least 10 years. After 16-20 h of incubation, up to 74% of the mf could be seen with 10 or more cells adhering to them. By 36-40 h up to 54% of the mf had been killed by the leucocytes in the presence of a cocktail of monoclonal antibodies termed MOV GIV. The degree of killing in control experiments with the monoclonal antibody MOV 1 remained lower (P less than 0.05), ranging from 0.0 to 4.5% of mf with initial viability of 90-95%. Western blotting revealed MOV GIV prominent target antigens of 10.5, 18.0, 23.5 and 27 kDa in crude surface extracts of female O. volvulus. The detected antigens may play a role in host protection.