Comparison of Ehrlichia chaffeensis recombinant proteins for serologic diagnosis of human monocytotropic ehrlichiosis

Diagnosis of human monocytotropic ehrlichiosis (HME) generally depends on s erology that detects the antibody response to immunodominant proteins of Eh rlichia chaffeensis. Protein immunoblotting was used to evaluate the reacti on of the antibodies in patients' sera with the recombinant E. chaffeensis 120- and 28-kDa proteins as well as the 106- and the 37-kDa proteins. The c loning of the genes encoding the latter two proteins is described in this r eport. Immunoelectron microscopy demonstrated that the 106-kDa protein is l ocated at the surfaces of ehrlichiae and on the intramorular fibrillar stru ctures associated with E. chaffeensis. The 37-kDa protein is homologous to the iron-binding protein of gram-negative bacteria. Forty-two serum samples from patients who were suspected to have HME were tested by immunofluoresc ence (IFA) using E. chaffeensis antigen and by protein immunoblotting using recombinant E. chaffeensis proteins expressed in Escherichia coli. Thirty- two serum samples contained IFA antibodies at a titer of 1:64 or greater. T he correlation of IFA and recombinant protein immunoblotting was 100% for t he 120-kDa protein, 41% for the 28-kDa protein, 9.4% for the 106-kDa protei n, and 0% for the 37-kDa protein. None of the recombinant antigens yielded false-positive results. All the sera reactive with the recombinant 28- or t he 106-kDa proteins also reacted with the recombinant 120-kDa protein.