Aberrant BCL10 nuclear expression in nasal NK/T-cell lymphoma

The BCL10 gene, originally identified through its recurrent involvement in t(1;14)(p22;q32) of the mucosa-associated lymphoid tissue (MALT) lymphoma, encodes a cytosolic protein composed of 233 amino acid residues with proapoptotic and proinflammatory activities.¹  B-cell chronic lymphocytic leukemia/lymphoma 10 (BCL10) is widely expressed in the cytoplasm of normal lymphoid tissues, with the expression level depending on the developmental stage of lymphocytes.²  Many B-cell lymphomas also express BCL10, including MALT lymphoma.²  In contrast to normal B cells that express BCL10 in the cytoplasm, MALT lymphoma cells with t(1;14)(p22;q32) or t(11;18)(q21;q21) express the protein predominantly in the nucleus.^2-5  Since there are no available data on BCL10 expression in natural killer (NK) cells, we examined BCL10 expression in normal NK cells and nasal NK/T-cell lymphoma (NL) in this study.

BCL10 immunostaining was performed on the cytospin preparations of CD3^–/(CD16+CD56)⁺ normal NK cells isolated from the peripheral blood mononuclear cells (PBMCs) of 2 healthy volunteers by cell sorting and paraffin sections of 40 NL tumor specimens using the monoclonal antibody clone 151 (1:50; Zymed, South San Francisco, CA).²  Because the NK cells were isolated from PBMCs by cell sorting, anti-(CD16+CD56) monoclonal antibodies were still bound to the antigens on the cell surface. Blocking them by a goat antimouse antibody (DAKO, Glostrup, Denmark) was performed to reduce the nonspecific binding. Immunostaining results showed BCL10 expression in NK cells and all 40 NL cases. BCL10 expression was detected only in the cytoplasm of NK cells (Figure 1A); by comparison, in addition to cytoplasmic expression, 28 (70%) of 40 NL cases showed aberrant nuclear staining of BCL10 in a variable percentage of NL tumor cells (Figure 1B). Paraffin sections of reactive tonsils were stained as controls, and only cytoplasmic staining of BCL10 was present among these tissues.

It has been previously shown that BCL10 is involved in nuclear factor–κB (NF-κB) activation by in vitro overexpression assay¹  and by BCL10 knock-out in mice.⁶  Because NF-κB transfer from cytoplasm to nucleus is the hallmark of its activation, we investigated NF-κB localization in the same 40 NL cases by immunostaining using polyclonal antibody C-20 (1:2000; Santa Cruz Biotechnology, Santa Cruz, CA) against NF-κB p65 and compared the results with the BCL10 expression pattern in NL (Figure 1C-F). The statistical analysis of the data by the chi-square test in Table 1 showed a strong association between BCL10 nuclear localization and NF-κB activation in NL (P = .001). The positive cases contained more than 10% of the tumor cells with nuclear staining.

In conclusion, our results show that BCL10 is widely expressed among normal and neoplastic NK cells and may exert the same regulatory role in NF-κB signaling in NK cells as in T and B cells. Many NL tumors have the same nuclear staining of BCL10 as the MALT lymphomas, but the chromosomal abnormalities, t(1;14)(p22;q32) and t(11;18)(q21;q21), which are the hallmarks of MALT lymphoma,^1-5  are not present in NL.^7,8  The unexpected finding of BCL10 nuclear localization and its association with NF-κB activation indicated a novel mechanism leading to aberrant BCL10 nuclear localization during tumorigenesis of NL.