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Background  The recombinant antigen  Bm R1 has been extensively employed in both ELISA and immunochromatographic rapid dipstick (Brugia Rapid) formats for the specific and sensitive detection of IgG4 antibodies against the lymphatic filarial parasites  Brugia malayi  and  Brugia timori . In sera of individuals infected with  Wuchereria bancrofti  the IgG4 reactivity to  Bm R1 is variable, and cross-reactivity of sera from individuals infected with  Onchocerca volvulus  or  Loa loa  was observed only in single cases. In order to characterize the homologs of the  Bm R1 antigen in  W. bancrofti  (Wb- BmR1 ),  O. volvulus  (Ov- Bm R1) and  L. loa  (Ll- Bm R1) the cDNA sequences were identified, the protein expressed and the antibody reactivity of patients' sera was studied.    Methods  PCR methodology was used to identify the cDNA sequences from cDNA libraries and/or genomic DNA of  W. bancrofti ,  O. volvulus  and  L. loa . The clones obtained were sequenced and compared to the cDNA sequence of  Bm R1. Ov- Bm R1 and Ll- Bm R1 were expressed in  E. coli  and tested using an IgG4-ELISA with 262 serum samples from individuals with or without  B. malayi ,  W. bancrofti ,  O .  volvulus  and  L. loa  infections or various other parasitic infections.  Bm R1, Ov- Bm R1 and Ll- Bm R1 were also tested for reactivity with the other three IgG subclasses in patients' sera.    Results  Wb- Bm R1 was found to be identical to  Bm R1. Ov- Bm R1 and Ll- Bm R1 were found to be identical to each other and share 99.7% homology with  Bm R1. The pattern of IgG4 recognition of all serum samples to  Bm R1, Ov- Bm R1 and Ll- Bm R1 were identical. This included weak IgG4 reactivities demonstrated by  L. loa - and  O. volvulus -infected patients tested with Ov- Bm R1 and Ll- Bm R1 (or  Bm R1). With respect to reactivity to other IgG subclasses, sera from  O. volvulus - and  L. loa -infected patients showed positive reactions (when tested with  Bm R1, Ov- Bm R1 or Ll-BmR1 antigens) only with IgG1. No reactivity was observed with IgG2 or with IgG3. Similarly, ELISAs to detect reactivity to other anti-filarial IgG subclasses antibodies showed that sera from individuals infected with  B. malayi  or  W. bancrofti  (active infections as well as patients with chronic disease) were positive with  Bm R1 only for IgG1 and were negative when tested with IgG2 and with IgG3 subclasses.    Conclusions  This study demonstrates that homologs of the  Bm R1 antigen are present in  W. bancrofti ,  O. volvulus  and  L. loa  and that these antigens are highly conserved. Recognition of this antigen by patients' sera is similar with regard to IgG1, IgG2 and IgG3, but different for IgG4 antibodies. We conclude that the  Bm R1 antigen is suitable for detection of IgG4 antibodies in brugian filariasis. However, its homologs are not suitable for IgG4-based diagnosis of other filarial infections.

Homologs of the Brugia malayi diagnostic antigen BmR1 are present in other filarial parasites but induce different humoral immune responses.