Demonstration of a Germinal Center Immunophenotype in Lymphomas by Immunocytochemistry and Flow Cytometry

The germinal center plays an important role in the pathogenesis of B-cell lymphomas, and evidence exists to suggest that most cases are germinal center or postgerminal center derived. Burkitt lymphoma and follicular lymphoma are derived from the germinal center stage of differentiation. It has been shown that diffuse large B-cell lymphomas with a germinal center-type pattern of RNA expression or a germinal center cell phenotype using immunocytochemistry have a more favorable outcome compared with those with a postgerminal center/activated profile. Microarray technology may not be available in many diagnostic laboratories, and antibody-based methods are much simpler and cheaper and are therefore more applicable to the routine setting. Immunocytochemistry has the advantage that the cells of interest are identified morphologically, and it is applicable retrospectively to fixed tissue. The main disadvantage is that only single-color staining is currently used in the routine setting. Flow cytometry allows one to obtain a more precise definition of individual cell types. The cells of interest are identified by a combination of physical characteristics and by the use of multiple antibodies labeled with different fluorochromes. Flow cytometry has the major advantage of being able to analyze very large numbers of cells, and results can be obtained within a few hours of the specimen being taken. The methods described allow B-cell lymphomas to be crudely divided into two groups, those with a germinal center phenotype and those that are mainly postgerminal center tumors. As knowledge of the normal biology of the germinal center develops, it will become possible to use immunophenotypic methods to more precisely classify all types of mature B-cell malignancies.