Flow Cytometric Expression of bcl-2 by CD10⁺ B-Cells Reliably Distinguishes Follicular Lymphoma from Follicular Hyperplasia.

The diagnosis of follicular lymphoma (FL) is often straightforward in most biopsy specimens. In a minority of cases, however, a differentiation from follicular hyperplasia (FH) can be challenging, especially in small needle core biopsies. The difficulty can be compounded in cases of FL with non-diagnostic immunoglobulin light chain ratios and cases of lymphoma without any significant surface expression of either immunoglobulin kappa or lambda light chain. The expression of bcl-2 by follicle center B-cells is often used to distinguish FL from FH using immunohistochemistry. The interpretation of bcl-2 expression patterns by immunohistochemistry, however, can be difficult because of normal expression of bcl-2 by T-cells present in reactive lymphoid follicles and by normal expression of bcl-2 by mantle zone B-cells. We have utilized multiparameter flow cytometry to evaluate the expression of bcl-2 in cases of FL and FH with successful results. This study includes consecutive biopsy specimens from well-documented cases of FL (n = 12) and FH (n = 16). Other lymphoma types were not included. Cytoplasmic expression of bcl-2 was evaluated on CD10⁺CD19⁺ B-cells using permeabilization techniques on a four-color Beckman-Coulter Epics-XL flow cytometer. Appropriate controls were run in each case. All cases of CD10-expressing FL showed significant expression of bcl-2 by CD10⁺ B-cells in contrast to no significant expression of bcl-2 by CD10⁺ follicle center B-cells of reactive follicles (Positive predictive value, 100%; Negative predictive value, 100%). Although our patient population is small, the results indicate that the expression of cytoplasmic bcl-2 on CD10⁺ B-cells by flow cytometry reliably distingushes FL from FH using CD10 gating. This method is superior to immunohistochemical evaluation of bcl-2 in cases with CD10 expression. A minority of FL, however, do not show bcl-2 expression (not seen in our series) and such cases must be taken into consideration when evaluating a clonal B-cell population with CD10 but no bcl-2 expression.