title: Lymph node hemophagocytosis in rickettsial diseases: a pathogenetic role for CD8 T lymphocytes in human monocytic ehrlichiosis (HME)?
abstract: BACKGROUND
  Human monocytic ehrlichiosis (HME) and Rocky Mountain spotted fever (RMSF) are caused by Ehrlichia chaffeensis and Rickettsia rickettsii, respectively. The pathogenesis of RMSF relates to rickettsia-mediated vascular injury, but it is unclear in HME.
  
  
  METHODS
  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.
  
  
  RESULTS
  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). E. chaffeensis-infected mononuclear phagocytes were infrequent compared to R. rickettsii-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.
  
  
  CONCLUSIONS
  Hemophagocytosis, CD8 T cell expansion, and the paucity of infected cells in HME, suggest that E. chaffeensis infection leads to macrophage activation and immune-mediated injury.
authors.affiliation: Division of Medical Microbiology, Department of Pathology, The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA. kdierbe1@jhmi.edu
authors.forename: Kerry L
authors.initials: KL
authors.lastname: Dierberg
authors.forename: J Stephen
authors.initials: JS
authors.lastname: Dumler
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D018414:CD8-Positive T-Lymphocytes / Q000276:immunology*; D002648:Child; D018469:Ehrlichia chaffeensis / Q000254:growth & development*; D016873:Ehrlichiosis / Q000276:immunology* / Q000382:microbiology / Q000473:pathology*; D005260:Female; D006801:Humans; D008198:Lymph Nodes / Q000276:immunology* / Q000473:pathology; D008262:Macrophage Activation / Q000276:immunology; D008297:Male; D008875:Middle Aged; D010587:Phagocytosis / Q000276:immunology; D012284:Rickettsia rickettsii / Q000254:growth & development; D012373:Rocky Mountain Spotted Fever / Q000276:immunology / Q000382:microbiology / Q000473:pathology