PLZF Staining Identifies Peripheral T-Cell Lymphomas Derived From Non-Conventional T-Cells With Limited α-Chain Diversity

Peripheral T-cell lymphomas are uncommon and account for 10-15% of all non-Hodgkin lymphomas (NHL). The current classification and treatment strategy of peripheral T-cell lymphomas relies on integrating morphology with immunophenotype, genetics and clinical presentation. However, the most common category of peripheral T-cell lymphomas remains peripheral T-cell lymphoma, not otherwise specified (PTCL-NOS) reflecting the lack of specific parameters to better define these lymphomas in a biologically relevant way. As our understanding of the biology of peripheral T-cell development continues to improve, several immunophenotypic markers have become available that can delineate peripheral T-cells into functional subsets. It is now recognized that peripheral T-cell lymphomas can arise from both conventional and innate-like T-cells. Classically, peripheral lymphomas with the γδ T-cell receptor (TCR) as well as lymphomas derived from true natural killer (NK) cells are considered to be arising from the innate T-cells whereas T-cell lymphomas with an αβ TCR are assumed to be derived from the adaptive, conventional T-cells. However, several recent studies have identified relatively rare populations of αβ T cells with extremely limited α chain diversity. These T-cells are characterized by the ability to mount immune responses by interacting with non-classical MHC class I antigen presenting molecules even in the absence of intentional priming. The cells within these populations express markers characteristic of NK cells and/or memory T-cells and include cells frequently labeled as “NKT” cells and mucosal-associated invariant T-cells (MAIT). Whether these cells contribute to peripheral T-cell lymphomas is not known. The transcription factor promyelocytic leukemia zinc finger (PLZF) is indispensable for development and maturation of these T-cells. We therefore asked the question whether PLZF expression could be used to identify peripheral T-cell lymphomas derived from these innate like non-conventional T-cells. To answer this question, we generated a tissue microarray that included biopsies from 26 PTCL-NOS, 11 anaplastic large cell lymphomas (ALCL), ALK-, and 13 ALCL, ALK+. Histologically normal tonsil, lymph node, thymus and gastrointestinal biopsies were used as controls. Immunohistochemistry with the PLZF antibody was performed in the clinical immunohistochemistry laboratory. Only rare PLZF positive cells were observed in uninvolved tonsils, lymph nodes, and thymus. In contrast, the intestinal mucosa, which is normally enriched in PLZF positive innate type T-cells showed a relative abundance with expression observed in 8-10% lymphocytes. Lymphomas were scored as positive when 20% or more of tumor cells showed expression with nuclear localization. Within the lymphomas, PLZF expression was observed in 2/26 PTCL-NOS and 2/13 ALCL, ALK+. PLZF expression was not observed in any of the ALCL, ALK- lymphomas included in the current study. PCR amplification followed by sequencing identified the Vα7.2-Jα33 TCR rearrangement characteristic of the MAIT cells in the two PLZF positive PTCL-NOS lymphomas confirming the origin of these lymphomas from bona fide innate-like T-cells. Sequencing of the TCR in the remaining three PLZF positive lymphomas is currently in progress. Cytogenetic analysis was available in three of the 4 cases. While t(2;5) was the sole cytogenetic abnormality in one ALCL, ALK+ lymphoma, the remaining two cases, including one ALCL, ALK+ had a complex karyotype that included t(2;5). In view of the relatively small number of patients available for analysis and the heterogeneous therapy administered to patients included retrospectively in the study, an outcome analysis was not performed. In conclusion, we demonstrate that PLZF expression identifies lymphomas derived from non-conventional innate-like T-cells and likely represent a biologically unique group of peripheral T-cell lymphoma. It is well known that innate T-cells are highly resistant to xenobiotics due to high expression of the transporter ATP binding cassette B1 (MDR). Prospective evaluation for PLZF expression may therefore be useful in identifying patients who will benefit from therapy that specifically targets this pathway of drug resistance.