A SPECIFIC MONOCLONAL-ANTIBODY (PG-BG) DETECTS EXPRESSION OF THE BCL-6 PROTEIN IN GERMINAL CENTER B-CELLS

The BCL-6 gene is frequently involved in translocations occurring at t he 3q27 locus and is rearranged in approximately 30% of diffuse large cell lymphomas and in a small fraction of follicular lymphomas, The BC L-6 gene encodes for a Kruppel-type zinc-finger protein, the cell/tiss ue expression and function of which is unknown In this study, we descr ibe a new monoclonal antibody CPG-BG) that is specifically directed ag ainst a fixative-sensitive epitope on the amino-terminal region of the BCL-6 protein. By immunocytochemical analysis, BCL-6 localizes in the nucleus where PG-BG staining gives a microgranular/diffuse pattern wi th exclusion of the nucleoli. The main reactivity of PG-BG in tonsil a nd spleen is with the nuclei of germinal center B cells, whereas B cel ls within the mantle and marginal zones do not express BCL-6. No other lymphoid cells in the tonsil express BCL-6 except for a subset of CD3 (+)/CD4(+) intrafollicular and interfollicular T cells, A few lymphoid cells of unknown phenotype express BCL-6 in the thymus. Extra-lymphoi d BCL-6 expression includes a weak nuclear positivity of epithelia. In non-Hodgkin's lymphomas, BCL-6 expression parallels that observed in normal lymphoid compartments, eg, expression in germinal center-derive d tumors (follicular and diffuse large cell lymphomas), but not in man tle cell and marginal zone lymphomas, In most diffuse large cell lymph omas, the BCL-6 protein is expressed at high levels in cases with or w ithout BCL-6 gene rearrangements. These findings indicate that BCL-6 e xpression is specifically regulated during B lymphocyte development an d suggest that BCL-6 may play a role during B cell differentiation in the germinal center.