Immature sinus histiocytosis. Light- and electron-microscopic features, immunologic phenotype, and relationship with marginal zone lymphocytes.

The light-microscopic, ultrastructural, and immunohistochemical features of immature sinus histiocytosis were studied in 10 lymph nodes with the histologic picture of toxoplasmic lymphadenitis and compared with the features of lymphoid cells present in the marginal zone of the splenic white pulp. Areas of immature sinus histiocytosis consisted largely of medium-sized lymphoid cells with markedly irregular nuclei and abundant pale cytoplasm. Using a panel of monoclonal antibodies, the predominating lymphoid cells were found to carry the B-cell phenotype B1+Ba1-sIgM+sIgD-OKIa1+. Admixed were variable numbers of larger, blastic lymphoid cells, small lymphocytes, histiocytic elements, and polymorphonuclear granulocytes. The marginal zone of the splenic white pulp was composed of a similar mixture of cells, and marginal-zone lymphocytes demonstrated an analogous immunohistochemical phenotype. Our results indicate that immature sinus histiocytes are B-lymphoid cells that are closely related to marginal zone lymphocytes. As such, immature sinus histiocytes may have a role similar to that of marginal-zone lymphocytes, which have been claimed to transport antigens or immune complexes toward the follicular center or to serve as precursors of plasma cells. We suggest that immature sinus histiocytosis represents an abnormal expansion of the marginal zone, normally present at the sinusoidal pole of lymphoid follicles. The reason for this marginal-zone hyperplasia, recognized as immature sinus histiocytosis in a variety of reactive lymph node conditions, may be a maturation arrest in the normal development of immature sinus histiocytes into small, sIgM+ sIgD+ lymphocytes.