Distribution pattern of follicular dendritic cells in low grade B-cell lymphomas of the gastrointestinal tract immunostained by Ki-FDC1p: a new paraffin-resistant monoclonal antibody.

This study surveyed 97 cases of low grade B-cell lymphoma (LGBL) of the gastrointestinal tract (GIT) by conventional morphology and immunohistochemistry, focusing on the most frequent subtype: the so-called LGBL of the mucosa-associated lymphoid tissue (MALT). Special reference was made to the follicular dendritic cells (FDCs) selectively visualized by a new paraffin-resistant monoclonal antibody Ki-FDC1p. LGBL of the MALT accounted for 83 cases. The diagnosis was based on (a) a characteristic cytology, which included centrocytoid cells, a varying degree of plasma cell differentiation, and some blasts; (b) the presence of lymphoepithelial lesions; and (c) the occurrence of two types of follicles easily detectable with Ki-FDC1p. Some were restricted to the mucosa, contained normal germinal center cells, and were indistinguishable from reactive follicles. Others consisted of small clusters of FDCs, were randomly distributed throughout the tumor, and escaped detection in conventional stainings. Such small clusters of FDCs were found to be restricted to LGBL of the MALT, not occurring in other types of LGBL, and were interpreted as tumor-associated abortive follicles discernable from residues of reactive follicles due to their cellular constituents, localization, and distribution pattern. Eight cases showed closed sheets of blasts and were classified as high grade malignant lymphoma secondary to LGBL of the MALT. In two cases the LGBL of the MALT were restricted to the mucosa, in 31 cases the submucosa was also infiltrated, and in the remaining 50 cases the infiltration also involved deeper wall layers of the GIT.(ABSTRACT TRUNCATED AT 250 WORDS)