Immature sinus histiocytes. Their identification as a novel B-cell population.

The true nature of cells of "immature sinus histiocytosis" (ISH) is uncertain because they lack the typical features of normal histiocytes when analyzed by enzyme cytochemistry or electron microscopy. In the present study the antigenic profile of ISH cells has been analyzed by immunohistologic techniques in six cases of Piringer's lymphadenitis with the use of a large panel of monoclonal and polyclonal antibodies reactive with the major cell types of the hematolymphoid system. The results obtained indicate that ISH cells consistently lack markers found on cells of the monocyte/macrophage series, myeloid cells, interdigitating reticulum cells, follicular dendritic reticulum cells, T cells, or Ki-1-positive cells. They constantly express B-cell antigens and HLA-DR and (on a variable proportion of cells) surface immunoglobulin. The application of antibodies reactive with different B-cell subsets showed that the cells of ISH do not correspond to any previously described B-cell population, eg, pre-B cells, germinal center cells, follicular mantle lymphocytes, or marginal zone cells. Furthermore, ISH cells and germinal center cells are found in association with clearly different cell types. These findings indicate that ISH cells represent a B-cell population at a previously undescribed differentiation stage, occurring only under certain circumstances (eg, in toxoplasmosis or AIDS). It is proposed that the term "immature sinus histiocytosis" be replaced by "B-cell sinus reaction."