We report four cases of PAX5-positive T-cell anaplastic large cell lymphoma. Extra copies of the PAX5
gene locus were demonstrated in all three cases evaluable by FISH. PAX5 is a transcription factor in the paired-box-containing family, which is involved in control of organ development and tissue differentiation.41
PAX5 plays an essential role in B-lymphoid lineage commitment,26-28
and is widely used as a B-cell marker in immunohistochemical evaluation of lymphoid tissues.30
Anaplastic large cell lymphomas may share morphologic and phenotypic features with B-lineage neoplasms, particularly classical Hodgkin lymphoma. Therefore, our findings have important implications for interpreting PAX5 immunohistochemistry in lymphoma classification.
Our PAX5-positive anaplastic large cell lymphomas had clinical presentations, histologic features, and phenotypes (other than PAX5 expression) characteristic of anaplastic large cell lymphoma, allowing definitive classification despite the unusual positivity for PAX5. Consistent with previously published data,42
the three ALK-negative cases lacked clonal immunoglobulin gene rearrangements, and two of three had clonal TCR gene rearrangements. Case 3 demonstrated coexpression of CD15, a finding typical of classical Hodgkin lymphoma but which also may be seen in anaplastic large cell lymphoma.12-14
The other features did not support a diagnosis of classical Hodgkin lymphoma. There were characteristic hallmark cells with only occasional inflammatory cells seen in the background. In addition to the expression of T-cell antigens and cytotoxic markers, the tumor cells expressed BOB1 and (focally) OCT2, transcription factors typically absent in classical Hodgkin lymphoma.43
Finally, the presence of a clonal TCR gene rearrangement and absence of clonal immunoglobulin gene rearrangement support the diagnosis of anaplastic large cell lymphoma in this case. Case 4 was a decalcified specimen and molecular studies were unsuccessful, but positivity for ALK assisted in confirming the diagnosis of anaplastic large cell lymphoma.
In a study of cases with overlapping features of anaplastic large cell lymphoma and classical Hodgkin lymphoma, Tamaru et al found weak PAX5 expression in 3 of 17 ALK-negative anaplastic large cell lymphomas and 0 of 11 ALK-positive anaplastic large cell lymphomas.44
Though gene rearrangement studies were not performed to confirm T-cell origin, the three PAX5-positive tumors expressed both CD45 and BOB1, and two expressed EMA. These immunophenotypic features support the diagnosis of ALK-negative anaplastic large cell lymphoma rather than classical Hodgkin lymphoma. The tumors lacked T-cell antigen expression, except for CD45RO in one case and TIA-1 in another, and were negative for OCT2. The phenotypes of our cases were similar in the intensity of PAX5 staining and variable staining for EMA. We found more consistent positivity for T-cell antigens and observed OCT2 expression in two cases; conversely, BOB1 was seen focally in only one of our cases and CD45 expression was more variable. In addition, one of our cases was ALK-positive.
A single previous case of peripheral T-cell lymphoma, NOS expressing PAX5 was reported by Tzankov et al.45
No PAX5-positive cases were identified in additional peripheral T-cell lymphomas studied by Tzankov et al (n=43),45
Krenacs et al (n=20),31
Foss, et al (n=40),34
or Torlakovic et al (n=26).32
We did not identify any additional PAX5-positive cases in 198 peripheral T-cell lymphomas, including 66 additional anaplastic large cell lymphomas. Thus, the overall incidence of PAX5 positivity in peripheral T-cell lymphomas appears low. Nevertheless, PAX5 expression is not entirely specific for B-cell lineage in lymphomas. Furthermore, occasional non-lymphoid neoplasms express PAX5, including t(8;21)-positive acute myelogenous leukemias, small cell carcinomas, and other neuroendocrine tumors.30
Translocations between PAX5
and the immunoglobulin heavy chain gene (IGH@
) drive PAX5 expression in mature B-cell lymphomas,46,47
In addition, PAX5 is oncogenic in T cells, since a reconstructed PAX5/IGH@
translocation induces T-cell lymphoblastic lymphomas in mice.48
Therefore, to investigate the mechanism for PAX5 expression in anaplastic large cell lymphoma, we performed FISH using a PAX5
breakapart probe. We did not identify PAX5
translocations. Unexpectedly, however, all (100%) PAX5-positive anaplastic large cell lymphomas with informative FISH studies had extra copies of the PAX5
gene locus. In contrast, only 4% of PAX5-negative T-cell lymphomas had extra copies of PAX5
. No PAX5-negative anaplastic large cell lymphoma had extra copies of PAX5
, and previous genomic studies of anaplastic large cell lymphoma have not identified recurrent gains of 9p, on which PAX5
These findings suggest a possible association between extra copies of PAX5
and PAX5 protein expression in anaplastic large cell lymphomas. The finding of rare PAX5-negative T-cell lymphomas with extra copies of PAX5
(all peripheral T-cell lymphomas, NOS) indicates that factors besides gene dosage influence PAX5 protein expression in T-cell lymphomas. PAX5
methylation is associated with PAX5 negativity in human tumors52,53
and might represent a mechanism by which T-cell lymphomas with extra copies of PAX5
do not express PAX5 protein. However, we did not have adequate material to assess gene methylation in our cases.
In conclusion, recognizing the existence of PAX5-positive anaplastic large cell lymphomas is important to avoid incorrectly assigning B-cell lineage to these rare tumors. Specifically, PAX5 can not always differentiate anaplastic large cell lymphoma from classical Hodgkin lymphoma, particularly since the intensity of staining in PAX5-positive anaplastic large cell lymphomas is similar to that typically seen in classical Hodgkin lymphoma. Diagnostic errors can be avoided by interpreting PAX5 immunohistochemistry in the context of clinical features, morphology (including both cytologic features of the tumor cells and cellular background), and a panel of B- and T-lineage-associated antibodies. Molecular studies are recommended in cases with ambiguous lineage. Extra copies of the PAX5
gene may contribute to PAX5 expression in anaplastic large cell lymphomas. Finally, since PAX5 is oncogenic in T cells,48
PAX5 expression may have contributed to lymphomagenesis in our cases.