A panel of seven recombinant
antigens, derived from Ehrlichia phagocytophila (the agent of human granulocytic
ehrlichiosis), was evaluated by class-specific
enzyme-linked
immunosorbent assays (ELISAs) for utility in the diagnosis of the
infection. Fourteen genomic fragments, obtained by serologic expression screening, contained open reading frames (ORFs) encoding 16
immunodominant antigens. Eleven of these
antigens were members of the major
surface protein (MSP) multigene family. Alignment of their predicted
protein sequences revealed a pattern of conserved sequences, which contained short direct repeats, flanking a variable region. In addition, two genomic clones contained two and three MSP ORFs, respectively, indicating that these genes are clustered in tandem copies. The implications for this pattern of both genomic and
protein arrangements in antigenic variations of MSPs and in their utilities in a diagnostic assay are discussed. In addition to two MSP recombinant
antigens (rHGE-1 and -3) and a fusion
protein of these
antigens (rErf-1), five further recombinants were evaluated by ELISA. Two of these
antigens (rHGE-14 and -15) were novel, while a third (rHGE-2), with no known function, has been described. The final two recombinant
antigens (rHGE-9 and -17) represent overlapping segments of the
ankyrin gene (ank). The addition of rHGE-9 ELISA data resulted in the detection of 78% (21 of 27) of acute-phase sera. When serologic data for all recombinants are combined, 96.2% (26 of 27) of convalescent-phase patient serum samples and 85.2% (23 of 27) of acute-phase patient serum samples are detected, indicating the potential of these
antigens for use in the development of a rapid serologic assay for the detection of E. phagocytophila
infection.