Marginal zone B cells, monocytoid B cells, and the follicular microenvironment. Determinants of morphologic features in a subset of low-grade B-cell lymphomas.

Abstract

Splenic marginal zone B cells and nodal monocytoid B cells are cytologically similar to one another. However, recent studies have demonstrated that they are likely to be distinct, functionally different B-cell subpopulations that bear a relationship to B-cell follicles. Enhanced understanding of the biology of normal marginal zone and monocytoid B cells has helped to explain some unusual previously described features of low-grade B-cell lymphomas. One such study is reported by Abou-Elella and colleagues in this issue of the Journal, and it addresses the issue of marginal zone B-cell differentiation in follicular lymphomas. 1 Marginal zone B lymphocytes are easiest to recognize in the B-cell follicles of the splenic white pulp. 2,3 The B-cell follicles are composed of a germinal center surrounded by a rim of mantle zone lymphocytes, which is in turn surrounded by a concentric ring of marginal zone lymphocytes. All of the cells in the B-cell follicle are associated with follicular dendritic cells, and they are intermixed with a small population of T cells. Marginal zone lymphocytes are medium-sized and have round nuclei, partially clumped chromatin, inconspicuous nucleoli, moderately abundant pale staining cytoplasm, and indistinct cytoplasmic membranes. In peripheral lymph nodes, the marginal zone B cells are often inapparent because they are mixed with the mantle zone cells. 2 However, in intra-abdominal lymph nodes, halos of marginal zone B cells can surround the follicular mantle cells, similar to the architectural arrangement of these cells in the spleen. Most marginal zone B cells express CD19 and CD20