Reduced Tet2 Function Contributes to Development of Peripheral T-Cell Lymphoma with Follicular Helper T-Cell-Like Features in Mice

Loss-of-function mutations in TET2 are frequent in human myeloid and lymphoid malignancies. Especially, TET2 mutations were found in 30–47% of angioimmunoblastic T-cell lymphoma (AITL) and 10–38% of peripheral T-cell lymphoma, not otherwise specified (PTCL, NOS). The tumor origin of AITL is thought to be follicular helper T cells (Tfh). PTCL, NOS is a group of heterogenous T-cell-derived lymphomas, whereas some cases of PTCL, NOS also appear to be derived from Tfh. Several recent papers reported that Tet2 knockout mice demonstrated premalignant status of myeloid lineages, implicating that TET2 mutations have a driver's role in myeloid malignancies; however, there are no clues to how impaired Tet2 function provokes T-cell lymphomas. To address this issue, we analyzed Tet2 gene trap (Tet2^gt) mice.

Tet2^gt mice (Transgenic Res 17:599, 2008) have a trapping vector inserted into the second intron of Tet2 locus. Lineage-, Sca1+, and c-kit+ (LSK) cells were sorted from Tet2^gt E15.5 fetal liver (FL) cells and transplanted into lethally irradiated Ly5.1 wild-type (WT) mice. Bone marrow, spleen, and tumors when developed were analyzed in the recipient mice as well as adult Tet2^gtmice by flow cytometory and immunohistochemical staining.

Adult Tet2^gt mice were obtained under the following ratio; Tet2^+/+:Tet2^+/gt:Tet2^gt/gt=48:52:25. TET2 mRNA expression level in Tet2^gt/gt Lin- FL cells was reduced to 20% of that in WT Lin- FL cells, while the 5-hydroxymethyl cytosine level was reduced to around a half of that in WT cells. At 40 weeks of age, the proportion of CD4+/PD1+/Cxcr5+ cells, immunophenotypically similar to Tfh cells, significantly increased in Tet2^gt/gt mice compared to WT mice (P=0.022). Two out of 8 Tet2^gt/gt mice developed marked splenomegaly (more than 300 mg) at 40 weeks, and 4 out of 6 developed marked splenomegaly with swollen lymph nodes and multi-nodular tumors in liver and lungs at 60 weeks or older (median, 67 weeks). Histological examination of the enlarged spleen and swollen lymph nodes in these 4 mice demonstrated completely destroyed follicular structures. The spleen, lymph nodes, and nodular tumors showed infiltration of morphologically abnormal lymphocytes, which were proven to be CD4+/PD1+ T cells. These cells were also weakly positive for Cxcr5. CD4+ cells purified from the tumors revealed restricted rearrangement patterns of T-cell receptor genes, implying T-cell lymphoma. There were no increased vascular structures representing AITL. CD4+/PD1+ cells purified from the tumors expressed Bcl6 and cMaf, two key transcription factors of Tfh, at significantly higher levels compared to CD4+/PD1+ cells purified from spleen of WT mice. Meanwhile, one of 6 mice transplanted with Tet2^gt/gt LSK cells also developed lymphoma with similar pattern of CD4+/PD1+/CXCR5 expression at 40 weeks after transplantation.

Our observations indicate that reduced expression of TET2 may cause skewed differentiation into Tfh, which eventually develop tumors that pathologically recapitulate PTCL, NOS in human, while the tumor-comprising cells show characteristics of Tfh. The long latency period required for tumor development indicates secondary genetic hits in addition to the downregulation of TET2. These secondary events might determine the specificity of the hematologic malignancies, particularly AITL and PCTL, NOS with propensity for AITL.

No relevant conflicts of interest to declare.