| Summary: | <p>Abstract</p> <p>Background</p> <p>Human monocytic ehrlichiosis (HME) and Rocky Mountain spotted fever (RMSF) are caused by <it>Ehrlichia chaffeensis </it>and <it>Rickettsia rickettsii</it>, respectively. The pathogenesis of RMSF relates to rickettsia-mediated vascular injury, but it is unclear in HME.</p> <p>Methods</p> <p>To study histopathologic responses in the lymphatic system for correlates of immune injury, lymph nodes from patients with HME (n = 6) and RMSF (n = 5) were examined. H&E-stained lymph node tissues were examined for five histopathologic features, including hemophagocytosis, cellularity, necrosis, and vascular congestion and edema. The relative proportions of CD68 macrophages, CD8 and CD4 T lymphocytes, and CD20 B lymphocytes were evaluated by immunohistochemical staining.</p> <p>Results</p> <p>Hemophagocytosis was similar in HME and RMSF, and was greater than in control cases (p = .015). Cellularity in HME was not different from controls, whereas RMSF lymph nodes were markedly less cellular (p < 0.002). <it>E. chaffeensis</it>-infected mononuclear phagocytes were infrequent compared to <it>R. rickettsii</it>-infected endothelial cells. More CD8 cells in lymph nodes were observed with HME (p < .001), but no quantitative differences in CD4 lymphocytes, macrophages, or B lymphocytes were identified.</p> <p>Conclusion</p> <p>Hemophagocytosis, CD8 T cell expansion, and the paucity of infected cells in HME, suggest that <it>E. chaffeensis </it>infection leads to macrophage activation and immune-mediated injury.</p>
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