Low-grade B-cell lymphomas of the splenic marginal zone: a clinicopathological and immunohistochemical study of 14 cases.

We have studied 14 cases of low-grade, splenic marginal zone, B-cell non-Hodgkin's lymphoma. The clinical presentation in all cases was with splenomegaly and, in 10 cases, circulating neoplastic lymphoid cells in the peripheral blood with involvement of bone marrow. In all cases the splenic white pulp was hyperplastic with expansion of marginal zones and varying degrees of infiltration of germinal centres by neoplastic cells. The cells were a mixture of medium sized lymphocytes with moderate amounts of cytoplasm and finely dispersed nuclear chromatin, together with occasional blast cells with small nucleoli. Satellite red pulp aggregates of tumour cells centred on small epithelioid cell clusters were seen in all cases. These appear to be a characteristic and diagnostically important feature of splenic marginal zone lymphoma. The tumour cells expressed CD20, CD45RA, bcl-2 and the antigens detected by MB2. All cases expressed IgM with light chain restriction. In addition, IgD was expressed in four cases. The follicular dendritic cell network was disrupted in those follicles which were infiltrated by tumour cells. A network of stromal myoid cells, at the periphery of the marginal zone, identified by expression of alpha-smooth muscle actin, was preserved. Alpha-smooth muscle actin positive dendritic cells were also seen within and around satellite tumour nodules in the red pulp.