Lymph node hemophagocytosis in rickettsial diseases: a pathogenetic role for CD8 T lymphocytes in human monocytic ehrlichiosis (HME)?
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.
Dierberg
Kerry L
KL
Division of Medical Microbiology, Department of Pathology, The Johns Hopkins University School of Medicine, Baltimore, Maryland, USA. kdierbe1@jhmi.edu
Dumler
J Stephen
JS
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