Tanaka S, Ise W, Baba Y, Kurosaki T. The Role of TET Proteins in B Cell Biology. J Immunological Sci. (2021); 5(1): 1-5 Journal of Immunological Sciences Mini review Article Open Access The Role of TET Proteins in B Cell Biology Shinya Tanaka1*, Wataru Ise2, Yoshihiro Baba1, Tomohiro Kurosaki2,3# 1Division of Immunology and Genome Biology, Medical Institute of Bioregulation, Kyushu University, Higashi-ku, Fukuoka, 812-0054, Japan 2Laboratory of Lymphocyte Differentiation, WPI Immunology Frontier Research Center, Osaka University, Osaka, Suita, 565-0871,Japan 3Laboratory of Lymphocyte Differentiation, RIKEN Center for Integrative Medical Sciences, Yokohama, Kanagawa, 230-0045, Japan Article Info ABSTRACT Article Notes Gene expression must be strictly controlled during cell differentiation Received: December 23, 2020 and function in mammalian systems. DNA methylation plays an important Accepted: February 01, 2021 role in this process, and its pattern is shaped by balancing the activity of *Correspondence: methyltransferases and demethylases. Ten-eleven translocation (TET) was *Dr. Shinya Tanaka, Division of Immunology and Genome identified as a demethylase that catalyzes the oxidation reaction of the methyl Biology, Medical Institute of Bioregulation, Kyushu University, group of 5-methylcytosine (5mC), converting it to 5-hydroxymethylcytosine Higashi-ku, Fukuoka, 812-0054, Japan; TEL: (+81)-92-642-6839; (5hmC). Recently, indispensable roles of TET proteins in the regulation of Email: [email protected]. immune cells have been identified. Here, we review recent studies on the #Dr. Tomohiro Kurosaki, Laboratory of Lymphocyte biological consequences of dysregulation of TET proteins in the immune Differentiation, WPI Immunology Frontier Research Center, system, with a particular focus on B cell biology. Finally, we discuss future Osaka University, Osaka, Suita, 565-0871, Japan; TEL: (+81)-6- perspectives in this research field. 6879-4456; Email: [email protected]. © 2021 Tanaka S, Kurosaki T. This article is distributed under the terms of the Creative Commons Attribution 4.0 International Introduction License. The methylation pattern of eukaryotic DNA, which is critical Keywords: for appropriate cell differentiation and function, is dynamically B cell differentiation regulated by the activity of DNA methyltransferase and demethylases. Ten-eleven translocation DNA methylation Approximately ten years ago, Ten-eleven translocation 1 (TET1) was reported to act as a DNA demethylase in acute myeloid and lymphocytic leukemia1 and named after a t(10;11) (q22;q23) translocation. After this discovery, other TET family proteins TET2 and TET3 were identified by sequence homology. These TET proteins2. In mammalianhave a CpG cells,DNA bindingthese TET motif proteins CXXC catalyzeat the N-terminus the oxidation (TET2 of 5-methylcytosine does not contain CXXC.) and a catalytic domain at the C-terminus the(5mC), generation sequentially of an generating unmethylated 5-hydroxymethylcytosine cytosine. Recent publications (5hmC), have5-formylcytosine reported roles (5fC) of TETand proteins5-carboxylcytosine other than (5caC),their role resulting as tumor in suppressors3 in immune cells4-13. In this mini review, we discuss the role of TET proteins, especially in B cell biology. Role of TET Proteins in Early B Cell Development Tet2 and Tet3 andB defective cell-specific Ig kappa (κ) gene rearrangement, gene-deficient which mice is ausing somatic an rearrangementMb1Cre driver showed process an of impaired Vκ and Jtransitionκ gene segments of Pro B toto Preproduce B cells a functional Igκ light chain gene, during early B cell development5, 6. An analysis at the molecular level revealed that TET2/TET3 double- κ locus germline transcripts and interferon regulatory factor (IRF)4/IRF8 expression5, 6, leading todeficiency speculation resulted that in TET-regulated decreased Ig IRF4/IRF8 might promote the expression of the germline transcripts, an activity that precedes Page 1 of 5 Tanaka S, Ise W, Baba Y, Kurosaki T. The Role of TET Proteins in B Cell Biology. J Journal of Immunological Sciences Immunological Sci. (2021); 5(1): 1-5 and may be required for successful Ig light chain gene rearrangement. The Igκ region cis-regulatory elements, reduced at the TetE1, where BATF bound. In addition, TET2 3’ enhancer (3’Eκ) and distal 3’ enhancer (dEκ), contain andthat, BATFin the wereabsence physically of TET2/TET3, associated 5hmC in modification primary B cells was stimulated in vitro, suggesting that BATF and TET proteins development progresses5 coordinately induce AID expression11. CpG nucleotides, which are demethylated as early B Tet2cell As mentioned above, TET2 and TET3 play a critical role and Tet3 genes enhanced. DNAIn Pro methylation B cells differentiated at the 3’Eκ, in plasma cell differentiation11, 13, 14. We found that Tet2/ by co-culture with OP9 cells, disruption κof transcripts the Tet3 and Igκ germline transcripts, accompanied by impaired differentiation in vivo chromatinwhich in turnaccessibility reduced of the the amountIgκ locus. of Therefore,C DNA by the deletion observation caused of impaired defective antigen-specific plasma cell differentiation plasma cell demethylation at 3’Eκ is a crucial epigenetic event for Igκ in vitro resulting from. This Tet2finding/Tet3 was further supported13 gene rearrangement. Although overexpression of IRF4 did closer examination, we could show that high level IRF4 not restore the defective Igκ rearrangement caused by Tet expression, which is required for plasma deficiency cell differentiation,. Upon was drastically diminished in Tet2/Tet3 κ cis- deficiency, knock down of the E-protein E2A and the ETS- cells, although low to medium level expression was still regulatory elements, with enhanced DNA methylation in -deficient B family protein PU.1 attenuated TET2-binding to Ig maintained. Mechanistically, DNA demethylation occurred at the 5’ region of the Irf4 gene during the transition from regulate Igκ rearrangement and expression during early B naive B cells to plasma cells. In contrast, in the absence of cellPre Bdevelopment cells, suggesting5. that E2A, PU.1 and TET coordinately TET2/TET3, this 5’ region was kept methylated. Therefore, Role of TET Proteins in Peripheral B Cell the epigenetic changes, including DNA demethylation Differentiation at these sites, may be required to induce high level IRF4 expression14. Notably, although IRF4 is necessary for early It has been reported that the TET proteins play a critical 16, 17 cell differentiation was observed in the Tet2/Tet plasma cells as well11, 13, 14. Tet2/Tet3 BGC cells. B cell This differentiation may be because, no the remarkable low to medium defect levelin GC ofB role in the differentiation of geminal center+ (GC) B and cells in vitro. 3-deficient A similar defect was observed in vivo after deficiency immunization did not 11,affect13 cell proliferation, but formation of IgG1 IRF4Role expressionof TET Proteins is sufficient in B forCell GC Tolerance B cell development. was rather increased in the absence of TET2/TET3 in B cells, even11. Thereby,though theTET2/TET3 number of were antigen-specific suggested toB cellsplay as a break of self-tolerance is a direct cause of autoimmune Self-tolerance is a vital biological event for homeostasis, which is an irreversible gene rearrangement process to an important role in class switch recombination (CSR), of self-reactive B cells, which is induced by discontinuous activationdisease. Peripheral due to lack B cell of tolerance T cell help. is established In other words,by death B with this idea, the expression of activation-induced cell peripheral tolerance is established by an intrinsic cytidinegenerate deaminase different classes/isotypes (AID), which is anof antibody.essential Consistentmutagenic suppressive mechanism of self-reactive B cell activation enzyme that catalyses the deamination of deoxycytidine and an extrinsic mechanism that prevents interaction of self-reactive B cells with self-reactive T cells. Although and gene conversion, was substantially reduced in several endogenous factors that dampen self-reactive B cell to deoxyuracil/ to initiate 11CSR,. In fact,somatic we havehypermutation previously Tet2 Tet3 18, little is known about the Tet2/Tet3 deletion upon expression -deficient B cells latter mechanism. In our current study, we discovered one activation have been identified confirmed that acute of the mechanisms by which self-reactive T-B interaction is inhibited by TET2/TET3 to induce/maintain peripheral of a Tamoxifen-inducible Cre, decreasedTet2 AID/Tet3 expression, accompanied by impairment of IgG1 CSR (unpublished tolerance . B cells by enforced AID expression. On the other hand, the 19 expressiondata). IgG1 expressionlevel of µ and was γ 1restored germline in transcripts-deficient in Tet2/ In the above-cited publications5, 6, splenomegaly Tet3 Tet2/ Tet3 due -deficientto decreased B cellsAID expression.was equivalent Regarding to those the ofdetailed wild- and lymphadenopathy were evident in Mb1 CreTet2 x /Tet3 moleculartype B cells. mechanism, Therefore, theit defectivehas already IgG1 beenCSR wasreported solely conditional double knockfloxed outmice. mice We (hereafteralso found calledsimilar Tet lymphoid bDKO), that the transcription factor basic leucine zipper ATF- buttissue no abnormalitiesconspicuous defects in CD19 in earlyCre-mediated B cell development. Like transcription factor (BATF) can induce AID expression As discussed in our publication , this difference