Monoclonal antibody-based immunohistochemical diagnosis of rickettsialpox:The macrophage is the principal target

Cutaneous biopsies of five eschars and two rash lesions from five patients from New York City with documented rickettsialpox were examined by immunohi stochemical methods with a monoclonal antibody directed against spotted fev er group rickettsial lipopolysaccharide for the presence and cellular locat ion of Rickettsia akari. Rickettsiae were identified in all of the five pat ients, with good concordance of results for the same biopsy tissues with pr eviously reported results by the direct immunofluorescence method. In contr ast with immunofluorescence, which did not reveal the location of the organ isms, immunohistochemical examination demonstrated R. akari to be in periva scular cells, morphologically resembling macrophages, Evaluation with doubl e staining for rickettsiae and either CD68 or Factor VIII-related antigen r evealed that the predominant infected cell type was CD68-positive macrophag es, and only a rare rickettsia was detected in vascular endothelium, the ma jor target cell for other rickettsioses. These results provide a diagnostic method for rickettsialpox and other spotted fever group rickettsioses and indicate that the elucidation of the pathogenesis of rickettsialpox must ta ke into account that its target cell differs from that of Rocky Mountain sp otted fever, boutonneuse fever, louse-borne typhus fever, and murine typhus .