Inflammatory meningeal masses of unexplained origin. An ultrastructural and immunological study.

Two patients with inflammatory meningeal masses were studied. Lesions in both patients showed varying proportions of meningothelial and inflammatory components. The non-neoplastic nature of the inflammation was confirmed in one case by lymphocyte surface marker study, which showed T and B cells in one to four ratio, and by immunohistochemistry, which revealed polyclonal plasma cells. The abundant histiocytes contained muramidase and often enclosed intact lymphocytes or plasma cells within their cytoplasm, i.e., emperipolesis. Their surfaces bore slender interdigitating pseudopodia, intercellular junctions, and subplasmalemmal linear densities. The derivation of these histiocytes is uncertain: mononuclear phagocytes, meningothelial cells, and multipotential meningeal cells are all possible progenitors. A comparison with eleven similar reported cases reveals a tendency for inflammatory meningeal masses to occur in the young, as well as a predilection for posterior fossa involvement. They resemble the extranodal lesions of sinus histiocytosis with massive lymphadenopathy, as well as plasma cell granulomas or inflammatory pseudotumors of lung and other tissues. However, it is possible that these lesions represent a variant of meningioma in which an unusual immunological response has been evoked.