FL is the most common of the indolent non-Hodgkin’s lymphomas, and the second-most-common form of non-Hodgkin’s lymphomas overall. Chinese FL had its unique epidemiological and clinicalopathological features. This study demonstrated a high proportion of FL3 (46.1%), which was very close to a previous report (45.7%) on Chinese cohort , but higher than that in western countries . According to a nationwide multicenter study of 10002 cases , FL accounts for about 5.5% of all lymphomas in China. The incidence was close to those in Asia and in developing countries, but much lower than those in USA and Western Europe [13, 14]. In published studies [15, 16], areas of DLBCL are present in 60-80% of FL3B cases and less frequently in FL3A. Our results are very close to those data.
FL is a neoplasm composed of germinal center (GC) B-cells (typically both ceotrocytes and centroblasts cells). FL cells typically express B-cell markers (CD19, CD20, CD22, and CD79a), together with GC-markers (CD10 and BCL6) and BCL2 [11, 17]. Some cases, especially FL3B, may lack CD10, but retain BCL6 expression [2–4]. In our FL study CD10 positive rate significantly reduced in high-grade FL compared with low-grade FL, Whereas BCL6 seems to be a more reliable GC-marker than CD10, as it is conserved in higher grade .
MUM1 is a marker for late germinal center B cells (GCBs) and early post-GCBs . In our FL series GC-marker CD10 decreased with the increase of FL grade, while post-GC marker MUM1 increased with the increase of FL grade. There were eleven CD10-/MUM1+ cases, of which ten were high grade and one was low grade. The only one low-grade case was morphologically composed of large central cells, and was t(14;18) translocation negative. This was consisted with the previous report that FL composed of large central cells is more aggressive than other low-grade FL, and had lower CD10-positive rate and t(14;18) translocation rate . The proportion of CD10-/MUM1+ cases in CD10- FL rose from 55.0% to 80.0% if the cut-off value for MUM1 positive decreased from 30% to 10%. Hans’ algorithm  has been widely used as standard to sub-classify DLBCL into (GCB) and non-GCB origins . If classified according to Hans’s algorithm, 80% (16/20) of CD10- FL were NONnon-GCB subtype and the remaining 20% (4/20) were GCB subtype, indicating possible different origins of CD10-/MUM1+ and CD10-/MUM1- FLs.
BCL2 is a well-known anti-apoptotic protein. Bcl-2 expressed in 91.0% of our FL cases, which falls at the higher end of the reported range of 75-98% [20–22]. Besides, BCL2 protein is expressed by a variable proportion of the tumor cells in 85-90% of cases of low-grade FL, but only 50% of high-grade FL using standard antibodies . Therefore, in clinical work it must be recognized that absence of BCL2 protein dose not exclude FL. In this study, we found that BCL2 was consistently strong in the tumor cells in low-grade FLs, but was inconsistently medium to negative in grade 3 FLs. Whether this is conducive to the diagnosis of high-grade FL needs to be verified by more cases.
The t(14;18) translocation rate in FL1-2 in this study was 73.5%, which was close to Japanese data [23, 24], and slightly lower than most of the western reports (80-90%) . This difference may be caused by regional and ethnical differences. The t(14;18) translocation rate in FL3A (51.9%) was close to that in low-grade FL (73.5%). While the rate in FL3B (11.1%) was close to those reported in DLBCL  and our previous study . It was reported  that FLs without t(14;18) are usually histological FL3B. This may suggest that FLs without the t(14;18) are pathophysiologically distinct from those cases with BCL2 translocation.
The t(14;18)(q32;q21) translocation results in juxtaposition of the IgH and bcl-2 genes, and causes overexpression of BCL2, which prevents cells in follicular center from undergoing apoptosis . However, this study demonstrated no correlation between t(14;18) translocation and BCL2 expression. Absence of BCL2 protein in t(14;18)+/BCL2- cases is partly due to mutations in the BCL2 gene that eliminate the epitopes recognized by the most commonly used antibody . For t(14;18)-/BCL2+ cases, other molecular mechanisms, for example other types of chromosomal alterations involving the BCL2 gene on chromosome 18, activation of NF-κB signaling pathway, viral infections, cytokine effects and interactions with cell cycle regulators [6, 29]. In this study, the consistent rate of t(14;18) translocation and BCL2 expression decreased with the increase of grading. 83.3% of FL3B were t(14;18) translocation-/BCL2+, suggesting a potential different pathogenesis compared with FL3A (34.6%) and FL1-2 (21.7%).
The value of grading FL has been debated since the 1980s. The 2008 WHO grades classification maintains a histological grading system (grades 1–3) for FL. It has been suggested that the number of grades could be reduced, since FL1 and FL2 are clinically indolent and pathologically similar. Our study revealed that low-grade FL had a high homogeneity in morphology, immune phenotype and t(14;18) translocation. FL3A and FL3B showed significant heterogeneity in t(14;18) translocation and MUM1 expression, although both types showed certain homogeneity in morphology and immunophenotype (CD10, BCL2 and BCL6). Consistent with previous studied [16, 30], FL3A was genetically similar to low-grade FL, while FL3B was more similar to DLBCL in t(14;18) translocation.
In conclusion, our study revealed that FL1 and FL2 were immunophenotypically and genomically similar, while FL3A and FL3B were partly immunophenotypically similar but morphologically, genomically distinct. FL3A was genomically closer to FL1-2, whereas FL3A was genomically closer DLBCL. CD10- FL was correlated with MUM1+ and t(14;18)-,suggesting its potentially different origin or pathogenesis from other subgroups. Thus we hypothesize that FL may in fact be a heterogeneous indolent lymphoma encompassing entities with distinct molecular pathogenesis and genetic characteristics. Immunohistochemical and genetic characterization helps to distinguish subgroups of FLs.