Follicular lymphomas contain a clonally linked but phenotypically distinct neoplastic B-cell population in the interfollicular zone.

Follicular lymphomas are thought to arise from the follicle center B cells and are characterized by follicular structures that recapitulate many features of normal secondary lymphoid follicles. The neoplastic B cells of follicular lymphoma reside not only in follicles but also in the interfollicular zone in which they form a diffuse infiltrate. We have investigated the frequency, extent, and biological characteristics of this interfollicular component in 30 cases of follicular lymphoma. An interfollicular B-cell infiltrate of variable extent (minimal, moderate, or prominent) was present in all cases. Morphologically interfollicular neoplastic B cells were small centrocyte-like cells with lower grade cytology and lower proliferation fraction compared with the neoplastic follicles. The neoplastic phenotype of these cells (CD20+, light chain restricted) was confirmed in 18 cases. Clonal identity between the follicular and interfollicular components was shown in five cases using microdissection and PCR amplification of immunoglobulin heavy chain genes. Analysis of Ig heavy chain gene sequences showed identical variants of tumor subclones in both follicular and interfollicular compartments, indicating active tumor cell traffic between the two. In six cases in which frozen tissue was available, the immunophenotype of follicular and interfollicular tumor cells were compared using immunohistochemistry. Activation markers such as CD10, CD38, and CD95 and T-cell costimulatory molecules CD80 and CD86, which were expressed by neoplastic follicles, were either downregulated or absent in the interfollicular component in most of the cases. The low-grade cytological features, low proliferation fraction, and downregulation of activation markers in the interfollicular neoplastic B cells suggests that these are resting cells analogous to memory B cells of normal lymphoid tissues. The presence of such a resting tumor cell subpopulation in the majority of follicular lymphomas may partly account for the remarkable resistance to therapy of this disease.