Inflammatory pseudotumor of lymph nodes. Additional observations and evidence for an inflammatory etiology.

Inflammatory pseudotumor of lymph nodes (IPT), a recently described benign cause of lymphadenopathy, was studied in 14 patients using paraffin-section immunohistochemistry. All biopsies showed a proliferation of spindle cells (small blood vessels, histiocytes, and "activated" fibroblasts resembling myofibroblasts) containing a mixture of inflammatory cells without atypia and involving the connective tissue framework (hilum and sinuses) of the lymph node. Ten patients had additional histologic features of IPT originally described (extranodal extension, obliterative vasculitis, and endothelial infiltration), and nine of these had associated fever of unknown origin, which in some was relieved by biopsy alone. Additional features observed focally in some cases but not previously described included lymph node parenchymal infarction, fibrinoid vascular necrosis, karyorrhexis, and involvement of only part of the lymph node. Immunostaining showed the lymphoid infiltrate to be predominantly of T-lineage (except for plasma cells), only a minority of which marked as T-helper cells. Numerous mononuclear cells resembling histiocytes were identified, some of which had a spindled shape but reacted with an antibody (KP1) of myelomonocytic specificity. Large fibroblastic cells expressed alpha-muscle actin but not desmin, similar to myofibroblasts in granulation tissue. The morphologic and immunohistologic features were similar to those in inflammatory pseudotumors of spleens and livers also studied, but the lack of simultaneous lymph node involvement argued against a common etiology. The findings suggest that the mass lesion of IPT is produced in response to localized lymph node inflammation or injury and further exclude hematolymphoid or mesenchymal neoplasia as a cause.