BCL7A was cloned from a chromosomal translocation in a Burkitt lymphoma cell line (Zani et al, 1996). BCL7A protein shares a conserved amino terminal domain with BCL7B and BCL7C (Jadayel et al, 1998). All BCL7 family members interact with SWI/SNF components, implicating these proteins in chromatin remodelling (Kaeser et al, 2008), although their specific functions remain unknown.
Several lines of evidence indicate that BCL7A may play important roles in B cell malignancy. BCL7A over-expression has been associated with the germinal centre (GC) phenotype in diffuse large B cell lymphoma (DLBCL) (Blenk et al, 2007). Gene expression studies indicated that IGHV unmutated chronic lymphocytic leukaemia (CLL) cases have higher BCL7A expression than IGHV mutated cases (Rosenwald et al, 2001). Whole genome sequencing of multiple myeloma (MM) samples reported BCL7A mutations within coding and non-coding regions (Chapman et al, 2011). BCL7A is down-regulated in mycosis fungoides (MF) and in peripheral T cell lymphoma (PTCL) (Tracey et al, 2003; Martinez-Delgado et al, 2004). These results prompted us to develop a BCL7A monoclonal antibody (mAb). This report describes BCL7A protein expression in normal lymphoid tissues and lymphomas using immunohistochemistry.
The anti-BCL7A mAb (clone 15C, isotype IgG1/κ), generated in BALB/c mice, was produced using bacterially-expressed human BCL7A protein as immunogen and validated as described (Data S1; Fig S1A). BCL7A expression was assessed using immunohistochemistry. Tissue samples and additional antibodies used in this study are listed in the Data S1 and Table SI. A Bond automated system (Leica, Milton Keynes, UK) was used for immunostaining of tissue microarray sections. Double labelling studies were performed as described (Kanellis et al, 2009). The specificity of mAb 15C for BCL7A was tested using Western Blotting, confirmed with 15C that recognizes an epitope within the carboxy-terminal region of BCL7A with no cross-reactivity with human BCL7B or BCL7C (see Data S1, Fig S1).
BCL7A protein was highly expressed in the nuclei of GC B cells of lymph node and tonsil (Fig 1A,D respectively) and some nuclei of lymphocytes in the mantle zone (MZ) and interfollicular areas (Fig 1A,D). In the spleen, strong nuclear staining for BCL7A was observed in the GC within secondary lymphoid follicles of the white pulp and mantle B cells (Fig 1B) while weaker labelling was observed in the marginal zone (Fig 1D). Little to no staining was found in the red pulp (Fig 1B). BCL7A staining was mostly confined to T cells in the thymic cortex (Fig 1C). Using double immunoenzymatic staining, BCL7A was confined to tonsillar CD20-positive GC and MZ B cells (Fig 1E) but absent in CD3-positive T cells both within and outside the GC (Fig 1F). The majority of CD20-positive B lymphocytes in the interfollicular region were also BCL7A-positive. CD10-positive GC B cells showed co-expression of BCL7A (Fig 1G). Plasma cells in the GC were positive for both CD138 and BCL7A (Fig 1H1), while interfollicular plasma cells were CD138-positive only (Fig 1H2). This finding suggests that BCL7A is expressed in early CD138-positive pre-B cells (pre-plasma cells) present in the GC light zone but lost in mature plasma cells. Double immunoenzymatic labelling highlighted the presence of BCL7A protein in CD30-positive activated B cells (Fig 1I). Moderate BCL7A expression was detected in CD23-positive follicular dendritic cells as well as in some CD23-positive mantle zone B cells (Fig 1J). Although CD68-positive tingible body macrophages lacked BCL7A (Fig 1K), strong BCL7A expression was detected in the CD123-positive plasmacytoid dendritic cells (Fig 1L).
BCL7A protein expression in lymphomas was heterogeneous (Table SII). All cases derived from lymphoid precursors including B and T cell lymphoblastic lymphoma/leukaemia (LBL) were BCL7A-positive (Fig 2A,B) (B-LBL; 3/6 weak and 3/6 strong; T-LBL; 1/7 weak and 6/7 strong). Homogeneous, intense staining for BCL7A was found in all cases of follicular lymphoma (FL) (56/56) and Burkitt lymphoma (57/60) (Fig 2C,D). Similarly, most DLBCL cases were BCL7A positive (5/212 weak and 199/212 strong) (Fig 2E). 15/16 plasmablastic lymphomas, defined by morphology and Blimp1 and/or XBP1s positivity (Montes-Moreno et al, 2010), were BCL7A-positive (4/16 weak and 11/16 strong). BCL7A expression was not detected in any of the 29 cases of MM. All 16 cases of nodular lymphocyte predominant Hodgkin lymphoma were BCL7A-positive, (13/16 strong and 3/16 cases weak) (Fig 2F). Weak or heterogeneous BCL7A protein expression was found in classical Hodgkin lymphomas including the mixed cellularity (12/31 weak only), nodular sclerosis, (16/77 weak and 2/77 strong) and lymphocyte rich (19/25 weak only) cases. Heterogeneous staining for BCL7A was observed in CLL (6/13 negative, 3/13 weak and 4/13 strong) (Fig 2G,H,I). Strong BCL7A staining was seen in proliferation centres (Fig 2G). As previous studies indicated higher BCL7A RNA expression in CLL lacking IGHV mutations (Rosenwald et al, 2001) we investigated BCL7A expression with IGHV mutations in 35 cases. There was no clear relationship between BCL7A protein expression and IGHV mutation (see Data S1 and Fig S2) but further studies are required to confirm this. Variations in staining were also detected in mantle cell lymphoma (MCL) (55/78 weak and 8/78 strong) and marginal zone lymphomas (MZL), including splenic marginal zone lymphoma, mucosa-associated lymphoid tissue lymphoma and nodal marginal zone lymphoma types (59/97 weak and 26/97 strong).
In contrast, most mature T cell malignancies, such as PTCL (1/51 weak and 3/51 strong) (Fig 2J), angioimmunoblastic T cell lymphoma (AITL) (2/63 strong) (Fig 1K), ALK-positive anaplastic large cell lymphoma (0/6) and MF (0/9) were BCL7A-negative. The expression of BCL7A in the tumour cells in two cases of AITL and four cases of PTCL was confirmed by the combined immunofluorescence and immunohistochemical staining technique (Fig 1L).
In conclusion, BCL7A was expressed at the pro B and T cell stage and persisted through the pre-B and mature B cell stages. BCL7A expression was maintained in late B cell differentiation but down-regulated in late plasma cells and mature T cells. In haematological malignancies, BCL7A protein was present in the majority of precursor and mature B cell lymphomas with increased expression amongst GC-related lymphomas. Mature T cell malignancies did not express BCL7A. Additional studies are essential to clarify the role of this protein in normal lymphocyte biology and lymphomagenesis.