Primary mediastinal B-cell lymphoma: high frequency of BCL-6 mutations and consistent expression of the transcription factors OCT-2, BOB.1, and PU.1 in the absence of immunoglobulins

Abstract

Although primary mediastinal (thymic) large B-cell lymphoma has been primarily studied, its precise phenotype, molecular characteristics, and histogenesis are still a matter of debate. The International Extranodal Lymphoma Study Group collected 137 such cases for extensive pathological review. Histologically, the lymphomatous growth was predominantly diffuse with fibrosis that induced compartmentalized cell aggregation. It consisted of large cells with varying degrees of nuclear polymorphism and clear to basophilic cytoplasm. On immunohistochemistry, the following phenotype was observed: CD45(+), CD20(+), CD79a(+), PAX5/BSAP(+), BOB.1(+), Oct-2(+), PU.1(+), Bcl-2(+), CD30(+), HLA-DR(+), MAL protein(+/-), Bcl-6(+/-), MUM1/IRF4(+/-), CD10(-/+), CD21(-), CD15(-), CD138(-), CD68(-), and CD3(-). Immunoglobulins were negative both at immunohistochemistry and in situ hybridization. Molecular analysis, performed in 45 cases, showed novel findings. More than half of the cases displayed BCL-6 gene mutations, which usually occurred along with functioning somatic IgV(H) gene mutations and Bcl-6 and/or MUM1/IRF4 expression. The present study supports the concept that a sizable fraction of cases of this lymphoma are from activated germinal center or postgerminal center cells. However, it differs from other aggressive B-cell lymphomas in that it shows defective immunoglobulin production despite the expression of OCT-2, BOB.1, and PU.1 transcription factors and the lack of IgV(H) gene crippling mutations.

Figure 1.

Histopathology of PMBL. The tumoral population homogeneously consists of large cells with clear cytoplasm (H&E, ×400).

Figure 2.

Cytological details of PMBL. Neoplasm cells with clear cytoplasm show some variability both in size and nuclear contours, the latter being at times multilobated (H&E, ×600).

Figure 3.

Fibrotic component of PMBL. Groups of neoplastic cells are surrounded by a delicate meshwork of reticulin fibers (Gomori silver impregnation, ×50).

Figure 4.

Immunohistochemical features of PMBL. CD79a staining in a case characterized by almost complete positivity of neoplastic cells (monoclonal antibody JCB117; APAAP technique; Gill’s hematoxylin nuclear counterstaining; ×600).

Figure 5.

Immunohistochemical features of PMBL. CD30 expression: the staining occurs in the majority of neoplastic cells and is weaker than in classical Hodgkin’s lymphoma and anaplastic large cell lymphoma (monoclonal antibody Ber-H2; APAAP technique; Gill’s hematoxylin nuclear counterstaining; ×400).

Figure 6.

Immunohistochemical features of PMBL. All neoplastic cells carry HLA-DR (monoclonal antibody DK22; APAAP technique; Gill’s hematoxylin nuclear counterstaining; ×500).

Figure 7.

Immunohistochemical features of PMBL. CD10 expression by neoplastic cells (monoclonal antibody 56C6; APAAP technique; Gill’s hematoxylin nuclear counterstaining; ×700).

Figure 8.

Immunohistochemical features of PMBL. Expression of the BCL-6 gene product by neoplastic cells (monoclonal antibody PG-B6p; APAAP technique; Gill’s hematoxylin nuclear counterstaining; ×600).

Figure 9.

Immunohistochemical features of PMBL. The search for the MUM1/IRF4 molecule produces a staining pattern that corresponds to the one observed with the PG-B6p antibody (monoclonal antibody mum-1p; APAAP technique; Gill’s hematoxylin nuclear counterstaining; ×600).

Figure 10.

Immunohistochemical features of PMBL. Most neoplastic cells express the MAL protein at the cytoplasmic level: some positivities confined to the Golgi area can also be seen (monoclonal antibody; LSAB technique; hematoxylin nuclear counterstaining; ×500).

Figure 11.

In situ hybridization features of PMBL. One single positive plasma cell is comprised among negative neoplastic elements (in situ hybridization; mRNA κ Ig light chain complementary PNA probe; development in New Fuchsin; Gill’s hematoxylin nuclear counterstaining; ×700).

Figure 12.

Immunohistochemical features of PMBL. All lymphomatous cells react with anti-Oct-2 antibody (polyclonal antibody; APAAP technique; Gill’s hematoxylin nuclear counterstaining; ×500).

Figure 13.

Immunohistochemical features of PMBL. The same pattern is observed at the determination of BOB.1, which is expressed both at the nuclear and cytoplasmic level (polyclonal antibody; APAAP technique; Gill’s hematoxylin nuclear counterstaining; ×400).

Figure 14.

Immunohistochemical features of PMBL. Most neoplastic cells, intermingled with some granulocytes, do express PU.1 (monoclonal antibody; Envision technique; Gill’s hematoxylin nuclear counterstaining; ×500).