Follicular lymphoma grade 3B is a distinct neoplasm according to cytogenetic and immunohistochemical profiles

Abstract

Background: According to the current World Health Organization Classification of Lymphoid Tumours, follicular lymphoma is subclassified into three grades according to the number of centroblasts. Follicular lymphoma grade 3 can be further divided into types A and B. Almost all available genetic data on grade 3B follicular lymphomas have been generated from tumors with an additional diffuse large B-cell lymphoma component. The purely follicular type of follicular lymphoma grade 3B is a rare neoplasm.

Design and methods: We performed a detailed immunohistochemical (CD10, IRF4/MUM1, BCL2, Ig light chains) and genetic (translocations of BCL2, BCL6, MYC, IRF4) characterization of the largest series of purely follicular cases of grade 3B follicular lymphoma available to date, comprising 23 tumor samples. We also included 25 typical grade 1 or 2 follicular lymphomas, 9 follicular lymphomas with large centrocytes and/or high proliferation indices (FL/LCC), 12 cases of follicular lymphoma grade 3A, 16 cases of diffuse large B-cell lymphoma/follicular lymphoma grade 3B and 15 follicular lymphomas in which a straightforward distinction between grades 3A and 3B was not possible.

Results: Translocations affecting BCL2 and BCL6 genes are rare in grade 3B follicular lymphomas (2/23, 9% and 4/23, 17%) when compared with grade 1 or 2 follicular lymphomas (22/25, 88%, P<0.001 and 0/25, P<0.05), FL/LCC (7/9, 78%, P<0.001 and 2/9, 22%), grade 3A follicular lymphomas (7/12, 58%, P<0.01 and 2/12, 17%), unclassified grade 3 follicular lymphomas (6/15, 40% and 2/15, 13%) and diffuse large B-cell lymphoma/follicular lymphoma grade 3B (2/16, 13% and 8/16, 50%, P<0.05). MYC translocations were detected occasionally in FL/LCC, follicular lymphoma grade 3B, and diffuse large B-cell lymphoma/follicular lymphoma grade 3B (13%-22%), but not in grade 1, 2 or 3A follicular lymphomas (P<0.05 when compared with follicular lymphoma grade 3B). Both follicular lymphoma grade 3B and diffuse large B-cell lymphoma/follicular lymphoma grade 3B were enriched in samples with a CD10(-)IRF4/MUM1(+) immunophenotype (8/19, 42% and 7/16, 44%), with the vast majority of them lacking BCL2 translocations. In contrast, 42/46 grade 1 or 2 follicular lymphomas, FL/LCC and grade 3A follicular lymphomas were CD10(+) (91%) while 0/46 expressed IRF4/MUM1. None of the tumor samples tested with increased IRF4/MUM1-expression harbored a translocation of the IRF4 gene locus.

Conclusions: Our results show that grade 3B follicular lymphomas form a distinct category of follicular lymphomas with infrequent BCL2 and BCL6 translocations, while grades 1, 2 and 3A follicular lymphomas and FL/LCC display homogeneous features with frequent BCL2 translocations and a CD10(+)IRF4/MUM1(-) immunophenotype.

Figure 1.

Examples of cytological details in FL3A (A), FL3B (B), FL3U (C) and FL/LCC (D). In FL3A, the centroblasts and cento-cytes differ markedly in size and overall appearance (A). In FL3B, the blastic cells are homogeneous in size and shape and, in this case, many display immunoblast-like features (B). In FL3U, next to easily discernible blasts, some medium-sized cells are present that display characteristics of both centroblasts and centrocytes (C). FL/LCC harbors many cells with the overall appearance of centrocytes, but that are large and have open nuclear chromatin, giving them blastoid features (D).

Figure 2.

Distribution of genetic alterations in FL3B in comparison to FL subtypes FL1/2, FL/LCC, FL3A, FL3B, FL3U, and DLBCL with a FL3B component (DLBCL/FL3B). P values <0.05 were considered statistically significant (*P<0.05; **P<0.01; ***P<0.001). (A) Genetic alterations affecting the BCL2 gene locus. The frequency of BCL2 gene translocations was significantly higher in FL1/2, FL/LCC and FL3A when compared to FL3B (P<0.001, P<0.001 and P<0.01, respectively). FL1/2 and FL3A were different as regards BCL2 translocation status (P<0.05). No significant differences were observed comparing FL3B with FL3U or DLBCL/FL3B. (B) Translocations of the BCL6 and the MYC gene loci. A significantly higher number of MYC rearrangements was found in FL3B than in FL1/2 or FL3A (P<0.05). DLBCL/FL3B harbored a significantly higher proportion of BCL6 rearrangements when compared to FL3B (P<0.05).

Figure 3.

Results of immunohistochemical stainings for CD10, IRF4/MUM1, BCL2 and cytoplasmic Ig light chains (cIg⁺) in FL3B compared to FL subtypes FL1/2, FL/LCC, FL3A, FL3B, FL3U and DLBCL with a FL3B component (DLBCL/FL3B). P values <0.05 were considered statistically significant (*P<0.05; **P<0.01; ***P<0.001). (A) CD10 and IRF4/MUM1 expression levels. The number of cases expressing CD10 was significantly higher in FL1/2, FL/LCC and FL3A than in FL3B (P<0.001, P<0.05 and P<0.05, respectively), while FL3B expressed IRF4/MUM1 unlike FL1/2, FL/LCC and FL3A (P<0.001, P<0.05 and P<0.05, respectively). (B) BCL2 staining patterns. FL1/2 harbored distinctly more cases expressing BCL2 than did FL3B (P<0.001). In contrast, BCL2 expression levels were comparable in FL/LCC, FL3A, FL3U and DLBCL/FL3B and FL3B. (C) Distribution of CD10⁻IRF4/MUM1⁺ immunophenotype. Significantly more FL3B showed a CD10⁻IRF4/MUM1⁺ staining pattern in contrast with FL1/2, FL/LCC and FL3A (P<0.001). (D) Immunohistochemical expression levels of cIg light chains. FL3B were frequently cIg⁺, unlike FL1/2 (P<0.001).