Humoral responses to a secretory Onchocerca volvulus protein: differences in the pattern of antibody isotypes to recombinant Ov20/OvS1 in generalizedand hyperreactive onchocerciasis

The Onchocerca volvulus secretory protein Ov20/OvS1 represents a dominant a ntigen expressed in the infective larvae, microfilariae and adult stages of the parasite. The humoral responses to this protein have not yet been anal ysed in the polar clinical and immunological forms of onchocerciasis. Analy sis by ELISA of class and subclass antibodies to Ov20/OvS1 in per-sons with the generalized or the hyperreactive form of onchocerciasis revealed simil ar strong responses of IgG1, IgG4 and IgM antibody levels in both forms of onchocerciasis and significant differences were observed in the IgE and IgA antibody classes. Computation of the ratios of antibodies showed that pers ons with the generalized form exhibited significantly higher ratios of IgG4 to IgG1, IgG4 to IgE, and IgM to IgE than patients with the hyperreactive form,. To investigate the isotype recognition of antigenic sites oil Ov20/O vS1 protein, three recombinantly expressed fragments (F1-3) of Ov20/OvS1 we re probed rising sera which strongly reacted with intact recombinant Ov20/O vS1. Epitope(s) on F1 comprising amino acid residues 1-63 were significantl y recognized by IgG1 min IgE, while IgM recognized epitopes on all three fr agments. The strongest reaction of IgM occurred with epitope(s) formed by r esidues 105-171 (F3). In contrast, IgG4 type antibodies were not reactive w ith either of the three OvS1 fragments, but they reacted with intact Ov20/O vS1 protein. Generalized onchocerciasis, unable to eliminate microfilariae, and hyperreactive onchocerciasis, with a high potency to eliminate or to r educe parasite loads, can be distinguished by a distinct pattern of isotype responses to Ov20/OvS1.