Th eJournal of Brief Reviews Immunology

T Cell Fates Zipped Up: How the Bach2 Basic Transcriptional Repressor Directs T Cell Differentiation and Function x x Martin J. Richer,*,†,1 Mark L. Lang,‡, and Noah S. Butler‡, ,1 Recent data illustrate a key role for the transcriptional and T cells (4). In this review, we discuss our present un- regulator bric-a-brac, tramtrack, and broad complex derstanding of the role of Bach2 in regulating T cell devel- and cap’n’collar homology (Bach)2 in orchestrating opment and homeostasis, as well as the emerging role of Bach2 T cell differentiation and function. Although Bach2 in regulating the differentiation and function of effector and has a well-described role in B cell differentiation, emerg- memory T cells. The potential for Bach2 to regulate various ing data show that Bach2 is a prototypical member of a states of T cell activation, including quiescence and exhaustion, novel class of transcription factors that regulates tran- is also discussed. scriptional activity in T cells at super-enhancers, or regions of high transcriptional activity. Accumulating data dem- Bach2 basics onstrate specific roles for Bach2 in favoring regulatory Bach2 is a member of the Bach family of basic leucine zipper T cell generation, restraining effector T cell differentia- transcription factors (Fig. 1A). The Bach2 is located on tion, and potentiating memory T cell development. Ev- human 6, 6q15 (chromosome 4, 4A4 in mouse), idence suggests that Bach2 regulates various facets of and encodes a 741-aa whose functional domains are T cell function by repressing other key transcriptional highly conserved (.94%) in mice and humans (5, 6). Bach2 regulators such as B lymphocyte–induced maturation expression was originally described as being confined to the protein 1. In this review, we examine our present un- B cell lineage and to some neuronal cells that expressed a derstanding of the role of Bach2 in T cell function and neuron-specific splice variant (7). However, Bach2 expression highlight the growing evidence that this transcriptional was later identified in the T cell lineage where it was reported to bind the IL-2 promoter and was required for maintenance repressor functions as a key regulator involved in main- + tenance of T cell quiescence, T cell subset differentia- of IL-2 production by human cord blood CD4 T cells (8). tion, and memory T cell generation. The Journal of The basic leucine zipper transcription factors characteristi- cally form heterodimers through their leucine zippers with the Immunology, 2016, 197: 1009–1015. musculoaponeurotic fibrosarcoma (Maf) family of yielding NF-E2 transcription factors (reviewed in Ref. 9). ignificant progress has been made in identifying and Bach2 forms heterodimers with proteins, including, delineating the effects of key transcriptional regulators MafF, MafG, and MafK, allowing binding to Maf recog- S that govern the diverse fates of lymphocytic effector nition elements (MAREs) with the consensus sequence subsets. One such regulator is the transcriptional repressor TGCTGA(G/C)TCA(T/C) (7) (Fig. 1B). Bach2 contains a bric-a-brac, tramtrack, and broad complex and cap’n’collar nuclear localization signal in its Zip domain and a nuclear homology (Bach)2. In addition to its well-defined role in export signal at its C terminus. Several factors regulate Bach2 B cell and plasma cell differentiation (recently reviewed in activity and localization (reviewed in Ref. 2), including PI3K Refs. 1–3), Bach2 is emerging as a functionally important signaling in B cells, which leads to phosphorylation of Ser512 regulator of other immune cell types, including macrophages and cytosolic accumulation. Oxidative stress inhibits the activity

*Department of Microbiology and Immunology, McGill University, Montreal, Quebec of Allergy and Infectious Diseases Grant AI078993 and by funds from the Presbyterian H3A 2B4, Canada; †Microbiome and Disease Tolerance Centre, McGill University, Health Foundation of Oklahoma City. Montreal, Quebec H3A 2B4, Canada; ‡Department of Microbiology and Immunology, Address correspondence and reprint requests to Dr. Martin J. Richer or Dr. Noah S. Butler, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104; and x McGill University, Department of Microbiology and Immunology, 3775 University Graduate Program in Biosciences, University of Oklahoma Health Sciences Center, Street, Room 406, Montreal, QC H3A 2B4, Canada (M.J.R.) or University of Okla- Oklahoma City, OK 73104 homa Health Sciences Center, Department of Microbiology and Immunology, 940 1M.J.R. and N.S.B. contributed equally to this work. Stanton L. Young Boulevard, Oklahoma City, OK 73104 (N.S.B.). E-mail addresses: [email protected] (M.J.R.) or [email protected] (N.S.B.) ORCIDs: 0000-0002-5737-3217 (M.J.R.); 0000-0002-5137-0576 (M.L.L.); 0000- 0002-1429-0796 (N.S.B.). Abbreviations used in this article: Bach, bric-a-brac, tramtrack, and broad complex and cap’n’collar homology; Blimp-1, B lymphocyte–induced maturation protein 1; iTreg, Received for publication May 12, 2016. Accepted for publication June 8, 2016. peripherally induced regulatory T cell; Maf, musculoaponeurotic fibrosarcoma; MARE, M.J.R. is supported by funds from the Canada Foundation for Innovation (John R. Maf recognition element; miRNA, microRNA; PRDM1, PR domain zinc finger protein 1; Evans Leaders Fund) and by National Sciences and Engineering Research Council of ROR, retinoic acid–related orphan ; SE, super-enhancer; Treg, regulatory T cell; Canada Grant RGPIN-2016-04713. N.S.B. is supported by National Institutes of Tfh, T follicular helper. Health/National Institute of Allergy and Infectious Diseases Grant AI125446 and Na- tional Institutes of Health/National Institute of General Medical Sciences Grant Copyright Ó 2016 by The American Association of Immunologists, Inc. 0022-1767/16/$30.00 GM103447. M.L.L. is supported by National Institutes of Health/National Institute

www.jimmunol.org/cgi/doi/10.4049/jimmunol.1600847 1010 BRIEF REVIEWS: REGULATION OF T CELLS BY Bach2

to susceptibility to different diseases, but they may have varying importance depending on the immune mechanisms driving pathology.

Bach2 regulation of T cell development and homeostasis Originally described as a transcriptional regulator of B cells, it is now well appreciated that a/b T cells also express Bach2 FIGURE 1. Bach2 basics. (A) Schematic representation of Bach2 protein (Fig. 2A). Bach2-encoding mRNA has been detected in thymic structure. Broad complex–tamtrack–bric-a-brac (BTB) region, basic region, and peripheral T-lineage cells (21). Bach2 transcription grad- and leucine zipper are depicted. (B) DNA binding motif for Bach2. ually increases as T cells undergo differentiation and selection in the thymus, with more pronounced increases as T cells reach of the nuclear export signal and thus leads to nuclear the single-positive developmental stage (21). Expression is ad- accumulation. ditionally regulated upon thymic egress, at least in CD8 T cells, Bach2 function has been most extensively investigated in as Bach2 mRNA expression further increases in naive CD8 B cells where it is known to repress expression of B lymphocyte– T cells as they seed the periphery (21). The factors regulating induced maturation protein 1 (Blimp-1), also known as PR the expression of Bach2 in developing and mature T cells re- domain zinc finger protein 1 (PRDM1), by binding to the main poorly defined, but TCR crosslinking via Ag encounter MARE 59 of the (Blimp-1) gene (10). Conditional may lead to a reduction in Bach2 expression. Consistent with ablation of Bach2 in the B cell lineage has revealed that Bach2 this, Ag-experienced effector and memory CD4 and CD8 downregulation is essential not only for Blimp-1 de-repression T cells recovered from the periphery express lower levels of and differentiation of B cells into plasma cells, but also for class Bach2 transcripts, compared with their naive counterparts (21). switch recombination leading to IgG1 secretion (11). However, As detailed below, cytokine signaling and other extrinsic factors the fates of B cells are not governed simply by Bach2 re- are likely important for modulating Bach2 expression or ac- pression of Blimp-1. Rather, a complex tivity in T cells. Furthermore, the expression of Bach2 is likely network controls memory B cell and plasma cell differenti- to be posttranscriptionally regulated in lymphocytes, as CD4 ation and key molecular events concomitant with differentiation, T cells express lower amounts of Bach2 protein compared including class switch recombination, somatic hypermutation, with B cells, despite similar abundance of Bach2 transcripts and Ig secretion (1, 2). (21). Recent data also established that the epigenetic regulator MENIN promotes Bach2 expression in CD4 T cells via Bach2 in disease maintenance of histone acetylation at the bach2 locus. This The bach2 gene locus is susceptible to modifications that prevents T cell acquisition of a potentially damaging senescence- impact health. Aberrations in the long arm of associated secretory phenotype, characterized in part by spon- are often associated with B cell malignancies. This includes a taneous proinflammatory cytokine production associated with Bach2–Bcl2LI fusion product detected in a lymphoma line loss of Bach2 expression (22), and further links Bach2 activity to (12). This resulted in enhanced expression of the antiapoptotic the maintenance of T cell homeostasis. Delineating the mech- protein Bcl2LI (also known as Bcl-xL). In another study, anisms that govern Bach2 transcription and posttranscriptional chromosomal rearrangements in an IgH-–positive lym- regulation in T cells as they develop, mature, undergo ho- phoma resulted in fusion of exon 1 of IgHCd on 14q32 to meostatic proliferation, and respond to infection will be critical exon 2 of bach2 (13). The fusion transcript spanned the entire to further elucidate the biological role and function of Bach2 in coding region (exons 2–9) of bach2 and was highly expressed. scenarios of both health and disease. Thus, aberrant expression of Bach2 and/or its fusion partners is T cell expression of Bach2 is critical for the generation of 2 2 concordant with B cell malignancies, reinforcing the notion regulatory T cells (Tregs). bach2 / mice develop a progressive that Bach2 critically governs homeostasis in B lymphocytes. wasting disease, marked by an increased presence of antinuclear The bach2 locus was shown to be a site of HIV-1 integration Abs associated with severely decreased numbers of Foxp3+ in resting CD4+ T cells in patients undergoing retroviral Tregs (23, 24). Expression of Bach2 may also be important for therapy (14). The bach2 locus may therefore constitute part of regulating the function of Tregs, as the limited number of 2 2 the mechanism by which patients with HIV infection maintain Tregs recovered from bach2 / mice exhibited a terminally a reservoir of latently infected T cells. Bach2 expression changes differentiated phenotype and were unable to prevent disease in also correlate with some diseases where aberrant T cell function a T cell adoptive transfer model of colitis (23, 24). Bach2 also is evident. For example, in patients with celiac disease, IFN-g regulates the formation of peripherally induced Tregs (iTregs). expression is strongly upregulated whereas Bach2 expression is The absence of Bach2 severely abrogated the generation of strongly downregulated (15). Genome-wide association studies iTregs following activation in the presence of TGF-b,instead have revealed single nucleotide polymorphisms in the bach2 yielding activated T cells expressing a number of asso- gene that are associated with a greater risk for developing ciated with other functionally distinct CD4 T cell subsets (23, rheumatoid arthritis (16), Crohn’s disease (17), multiple scle- 24). This includes a number of genes associated with T helper rosis (18), type I diabetes (19), and asthma (20). The bach2 lineage commitment such as GATA-3, T-bet, and retinoic polymorphisms observed were common to more than one acid–related orphan receptor (ROR)gt. Thus, Bach2 may serve disease, but often not equally associated. For example, the to allow for iTreg differentiation by preventing the expression rs1847472 single-nucleotide polymorphism was strongly associ- of other key regulators of effector T cell differentiation. In- ated with celiac disease, but weakly associated with asthma. triguingly, Bach2 may also function differentially depending on Therefore, bach2 polymorphisms are likely important contributors the T cell developmental stage or maturation checkpoint, as The Journal of Immunology 1011

FIGURE 2. Bach2 expression in T cells and its role in CD4 T cell subset differentiation. (A) Bach2 mRNA expression increases as T cells differentiate and mature into single-positive T cells in the thymus. Once in the periphery, Bach2 mRNA expression declines when T cells are activated and develop into effector T cells. Expres- sion profiles in phenotypically and functionally distinct memory T cell subsets are unknown. (B) Bach2 expres- sion promotes Treg differentiation, whereas Bach2 sup- presses factors involved in Th1, Th2, and Th17 subset formation. The role of Bach2 in Tfh and Th9 develop- ment and differentiation remains to be elucidated.

Bach2 serves as a transcriptional inducer of IL-2 in human cord pathogenic challenge (21). Importantly, although these data blood CD4 T cells (8). The factors that determine reported strongly support the idea that Bach2 functions to maintain 2 2 differential functions of Bach2 in CD4 T cells, and whether the T cell quiescence in the periphery, note that bach2 / , 2 functional impacts of Bach2 depend on the developmental CD62L T cells did not develop into bona fide, functional stage of T cells, currently remain unknown and warrant further Ag-specific effector memory T cells, as evidenced by their low investigation. expression of CD44. Additionally, despite their activated 2 2 phenotype, bach2 / T cells were less efficient than wild-type Bach2 regulation of effector T cell differentiation and activity cells in protecting the host from Listeria monocytogenes in- 2 2 Functionally distinct subsets of T cells regulate various aspects fection or inducing colitis following transfer into rag1 / 2 2 of immunologic protection against microbial infections, hu- recipient mice (21). Thus, bach2 / T cells may not acquire moral immunity, and autoimmune pathogenesis. In addition full effector function, despite their activated phenotype. to Tregs, emerging data support the concept that Bach2 plays a Rather, T cells lacking Bach2 appear to spontaneously de- critical role in regulating the differentiation of functionally velop an activated phenotype independently of antigenic distinct effector T cells (Fig. 2B). A recent study identified encounter, suggesting that Bach2 actively restrains T cell Bach2 as the potential first member of a novel class of tran- activation at steady-state or during homeostatic, cytokine- scriptional repressors that limit transcriptional activity at ge- driven turnover and maintenance. Thus, although Bach2 re- nomic loci referred to as super-enhancers (SEs) (25). SEs are strains T cell activation at steady-state, its absence is insufficient regions of enhanced transcriptional activity that are postulated for T cells to develop the functional advantages and heightened to play essential roles in establishment of the functional protective capacity associated with effector or memory T cells. identity of T cell lineages and subsets (26). For example, in In further support of the role of Bach2 as a regulator of 2 2 peripheral T cells, SE regions are typically associated with T cell subset differentiation, bach2 / CD4 T cells exhibited genes that regulate lineage-specific cytokine responses (25). enhanced Th2 differentiation and enhanced Th2-associated These include cytokine or cytokine receptor genes such as cytokine expression (21). Notably, enhanced Th2 differenti- 2 2 Ifng, Il17a,f,andIl4ra as well as lineage-determining tran- ation among bach2 / CD4 T cells was apparent even in the scription factor genes such as (T-bet), (GATA-3), presence of Th1-promoting cytokines that normally restrain and rorc (RORgt) (25). Thus, as a repressor of transcription at Th2 differentiation, including IL-12 (21). One explanation SE regions, Bach2 can profoundly suppress the development for these observations is the capacity of Bach2 to suppress and polarization of functional T cell responses. In support of multiple genes that are linked to Th2 differentiation and this, whereas Bach2 is not required for T cell development in function, including the transcription factor Blimp-1, which the thymus, it appears required to maintain quiescence of can potently suppress Th1 profiles (21, 27, naive T cells in the periphery (21). Indeed, specific deletion 28). Thus, Bach2 can act as an important transcriptional of Bach2 in T cells leads to a spontaneous decrease in the regulator that facilitates CD4 T cell differentiation toward a number of phenotypically naive CD4 T cells (based on Th1 phenotype and function via suppression of Th2 tran- CD62Lhi and CD44lo expression patterns) and an increase of scriptional programs. Similarly, Bach2 can restrain Th17 2 CD62L cells that exhibit transcriptional profiles resembl- differentiation, potentially through the repression of the aryl ing those of effector memory T cells, in the absence of any hydrocarbon receptor (24), as aryl hydrocarbon receptor ligands 1012 BRIEF REVIEWS: REGULATION OF T CELLS BY Bach2 strongly promote Th17 differentiation (29). Whereas the fac- phase to establish memory populations (33). Similarly, Restifo 2 2 tors that regulate the expression of Bach2 to favor differentia- and colleagues (34) recently demonstrated that bach2 / CD8 tion toward the various T helper subsets remain undefined, the T cells fail to accumulate or form numerically stable memory key role for cytokines in dictating CD4 T cell subset differ- pools after acute viral infection. Additionally, they demon- entiation strongly suggests that cytokines may also be involved strated that Bach2 restrains CD8 T cell effector differentiation in Bach2 regulation. It will be of tremendous interest to de- by interfering with the function of a subset of AP-1 family lineate how Bach2 expression is regulated during the course of transcription factors (34). Taken together, these genetic studies T cell activation and to determine potential context-specific confirm that Bach2 is functionally linked to the establishment roles of Bach2 in the differentiation of functionally distinct of memory potential in T cells. CD4 T cells subsets, including other key CD4 subsets such as Although the transcriptional networks governing the in- T follicular helper (Tfh) and Th9 cells. duction and effects of Bach2 on memory T cell development The Tfh cell subset is essential for orchestrating the germinal have not been fully elucidated, repression of Blimp-1 expression center reaction, affinity maturation, Ig class switching, and the by Bach2 is likely a critical component of the mechanism by generation of memory B cells. Bcl-6 is identified as a key which Bach2 promotes memory T cell formation. Blimp-1 regulator of Tfh development and function, and similar to expression is required for the differentiation of short-lived ef- Bach2, Bcl-6 can function to suppress Blimp-1, an antagonist fector cells, and Blimp-1 deficiency favors the development of of both Th1 and Tfh development. Given the parallels between memory T cells (35, 36). Indeed, Blimp-1 deficiency in T cells Bcl-6 and Bach2 in suppressing Blimp-1 expression, and the promotes the acquisition of memory characteristics among ef- key role of Blimp-1 in repressing Tfh differentiation (30), it is fector T cell populations (36). Thus, Bach2-mediated en- interesting to speculate that Bach2 and Bcl-6 independently, hancement of memory CD8 T cell generation is likely linked simultaneously, or coordinately function to govern Tfh cell to its capacity to downregulate the expression of genes associ- development. Indeed, at least in B cells, there is precedent for ated with effector differentiation, thereby tipping the balance in complex formation between Bach2 and Bcl-6 during repres- favor of generating memory cells earlier in the response. This is sion of the promoter of prdm-1, the gene encoding Blimp-1 supported by recent data demonstrating that Bach 2 can (31). Further work is required to dissect the specific contri- compete with AP-1 family transcription factors to prevent the butions of Bach2 in regulating Tfh development, commit- induction of the effector transcriptional program in CD8 ment, and activity. T cells (34). Notably, XBP-1, which is classically regarded as a Accumulating data support the idea that Bach2 also exerts critical regulator of Ab secretion by B cells (37, 38), has also profound effects on the function of fully differentiated, mature 2 2 been linked to promoting the terminal differentiation of T cells T cells. As noted above, studies show that bach2 / mature via activation of endoplasmic reticulum stress (39). In B cells, T cells express significantly elevated levels of the Th2- the transcriptional control circuit of XBP-1 splicing and acti- associated cytokine IL-4 following activation compared with vation is in turn regulated by Blimp-1, whose activity is further wild-type counterparts (21). These data are consistent with a repressed by Bach2. It is therefore likely that many of the same role for Bach2 in limiting T cell activation and function. transcriptional networks operating in B cells also impact the 2/2 Alternatively, studies of bach2 Tregs reveal markedly fate, function, and survival of effector and memory T cells. impaired chemokine receptor expression, which may explain Given the intimate functional links between the activity of why Bach2-deficient Tregs can still suppress effector T cells in Bach2 and the expression and function of Blimp-1, it is 2/2 vitro (23), but bach2 Tregs fail to limit colitis in the naive plausible that Bach2 functions upstream of pathways that CD4 T cell transfer models of colitis (23) or infiltrate tumors promote the formation and maintenance of long-lived T cell 2/2 (32). Thus, although bach2 T cells generally exhibit en- immunity following infection or vaccination. hanced effector function, loss or downregulation of Bach2 in The development of memory characteristics in CD8 T cells T cells may limit their capacity to efficiently traffic to sites of has been postulated to represent a default pathway that gets inflammation or infection. More studies are required to fully diverted toward effector CD8 T cell development and ac- elucidate the role of Bach2 in regulating T cell homing, quisition of effector function by inflammatory cytokines (40) trafficking, and functional activities that extend beyond (Fig. 3). Recent data show that specific inflammatory cyto- elaboration of cytokines during health and disease. kines and innate signals, including IL-12/IL-18 or IL-15, as well as TLR signaling, promote microRNA (miRNA) miR- Bach2 regulation of memory T cell development and function 181a and miR-148a expression in human and murine leuko- Recent studies are establishing an important role for Bach2 cytes (41, 42), and miR-148a has been shown to directly limit in shaping the development and biology of memory T cells Bach2 expression (43). Altered regulation of Bach2 may (Fig. 3). Using bioinformatics approaches, Hu and Chen function as a mechanistic link by which specific inflammatory (33) mined 386 publicly deposited gene expression databases cytokines and miRNA regulate T cell differentiation, survival, detailing the transcriptional profiles of CD8 T cells in different and memory formation. Consistent with this hypothesis, states of activation. This analysis led to the identification of a overexpression of miR-181a in T cells enhances expression of number of transcription factors not previously associated with costimulatory molecules and potentiates calcium flux fol- memory CD8 T cell memory generation, including Bach2 lowing TCR crosslinking (44). Both the quality and duration (33). The biological and functional relevance of Bach2 in of TCR signaling regulate memory T cell progression (45–48). memory T cell progression was demonstrated by retroviral- Thus, miRNA-mediated gene regulation represents a critical mediated transduction of Bach2 in CD8 T cells, which resul- additional level of posttranscriptional genetic control with ted in increases in the number of CD8 T cells at the peak of the profound effects on gene expression in immune cells. Moreover, response as well as increased survival during the contraction the cytokine-induced activity of specific miRNAs targeting The Journal of Immunology 1013

FIGURE 3. Postulated role of Bach2 in T cell memory progression. In the absence of inflammatory cytokine signaling, Ag-activated T cells default toward memory T cell progression. Low inflammatory conditions also bias T cells toward a memory phenotype, whereas high sys- temic inflammatory events divert T cells toward an ef- fector phenotype. The role of Bach2 in these processes and its modulation by inflammation-induced miRNA ex- pression remain to be elucidated. MPEC, memory pre- cursor effector cell; SLEC, short-lived effector cell.

Bach2 would also be consistent with data demonstrating that studies of exhausted CD4 and CD8 T cells responding to Bach2 expression is posttranscriptionally regulated in distinct chronic viral infection showed that Bach2 expression is reduced lymphocyte subsets (21). It is possible that Bach2 actively 2- to 4-fold compared with conventional (i.e., functional) suppresses T cell effector functions (favoring the default memory CD4 and CD8 T cell subsets (57). Repeated TCR memory pathway) and that T cell exposure to inflammatory stimulation may also be important for potential reductions in cytokines is required to reduce its expression allowing for the Bach2 expression in T cells responding to chronic infection or establishment of T cell transcriptional effector programs and persistent Ag. As noted above, effector and memory T cells enhanced effector function. Collectively, these studies further express relatively reduced Bach2 compared with naive T cells support the hypothesis that Bach2 functions at the nexus of a (33), and as T cells are driven through sequentially progressive critical transcriptional network that governs effector and recall responses to generate secondary, tertiary, and quaternary memory T cell differentiation, activity, and survival. memory cells, Bach2 expression is proportionately reduced (58). Interestingly, these multiply stimulated memory T cells Bach2 as a potential regulator of T cell exhaustion? maintain a more effector-like profile, suggesting that this may Following acute infection or vaccination, Ag-specific T cell be controlled by progressive loss of Bach2 following repeated populations undergo proliferative expansion, acquire and ex- TCR triggering. As a composite, these data are consistent with ert effector functions, and subsequently undergo numerical an important mechanistic role for Bach2 in limiting terminal contraction leaving pools of memory T cells that afford the differentiation of T cells, preventing T cell exhaustion, and host enhanced protection against reinfection (49). Similarly, promoting long-lived numerically and functionally stable pop- T cells responding to chronic infections in which the patho- ulations of memory T cells. gen is not efficiently eliminated also undergo similar phases of expansion and contraction. However, unlike scenarios of Conclusions acute infection or vaccination, T cells responding to persistent T cells are essential for providing protection against intra- infection exhibit specific alterations in gene expression profiles cellular pathogens and orchestrating humoral immunity. that are associated with functional impairments, including Conversely, T cells can also be pathologic, driving autoim- loss of cytokine expression, reduced T helper and cytolytic mune disease, allergy, or atopy. Thus, understanding the potential, and increased rates of apoptosis (50, 51). These molecular mechanisms that govern the development, differ- biological processes are collectively known as T cell exhaus- entiation, and function of Ag-specific T cells remains a crit- tion, which is phenotypically, functionally, and molecularly ically important goal. Bach2 is emerging as a fundamentally distinct from T cell anergy or senescence (reviewed in Ref. important molecular switch that can limit terminal differen- 52). The molecular mechanisms responsible for T exhaustion tiation of T cells and constrain their functional activity, po- are multifactorial and include perturbations in the transcrip- tentially promoting the generation of long-lived, highly tional programming of T cells. functional memory T cell populations. Despite this, little is Regarding altered expression and function of transcription known about the pathways that regulate Bach2 expression in factor networks, Eomes (53), T-bet (54), and Blimp-1 (55) T cells. It will be of significant interest to determine whether promote T cell exhaustion, whereas Id3 (56) limits T cell Bach2 is mechanistically linked to limiting or preventing terminal differentiation and exhaustion. Given the capacity T cell exhaustion or senescence. Discovering new circuits of for Bach2 to regulate these transcription factor networks, it is molecular control of T cell differentiation, function, and tempting to speculate that Bach2 may critically govern T cell memory formation will facilitate the development of new exhaustion. In support of this hypothesis, recent profiling approaches to maintain immune homeostasis, limit patho- 1014 BRIEF REVIEWS: REGULATION OF T CELLS BY Bach2 logical T cells responses, and enhance T cell vaccine-induced 20. Ferreira, M. A., M. C. Matheson, D. L. Duffy, G. B. Marks, J. Hui, P. Le Soue¨f, P. Danoy, S. Baltic, D. R. Nyholt, M. Jenkins, et al; Australian Asthma Genetics protective immunity. Consortium. 2011. Identification of IL6R and chromosome 11q13.5 as risk loci for asthma. Lancet 378: 1006–1014. 21. Tsukumo, S., M. Unno, A. Muto, A. Takeuchi, K. Kometani, T. Kurosaki, Acknowledgments K. Igarashi, and T. Saito. 2013. Bach2 maintains T cells in a naive state by sup- We thank Dr. S. A. Condotta (McGill University) for generating the figures pressing effector memory-related genes. Proc. Natl. Acad. Sci. U S A 110: 10735– and Dr. L. Thompson (Oklahoma Medical Research Foundation, Oklahoma 10740. City, OK), Dr. C. Webb (University of Oklahoma Health Sciences Center), 22. Kuwahara, M., J. Suzuki, S. Tofukuji, T. Yamada, M. Kanoh, A. Matsumoto, and Dr. S. A. Condotta for critical reading of the manuscript. S. Maruyama, K. Kometani, T. Kurosaki, O. Ohara, et al. 2014. The Menin-Bach2 axis is critical for regulating CD4 T-cell senescence and cytokine homeostasis. Nat. Commun. 5: 3555. 23. Kim, E. H., D. J. Gasper, S. H. Lee, E. H. Plisch, J. Svaren, and M. Suresh. 2014. Disclosures Bach2 regulates homeostasis of Foxp3+ regulatory T cells and protects against fatal The authors have no financial conflicts of interest. lung disease in mice. J. Immunol. 192: 985–995. 24. Roychoudhuri, R., K. Hirahara, K. Mousavi, D. Clever, C. A. Klebanoff, M. Bonelli, G. Sciume`, H. Zare, G. Vahedi, B. Dema, et al. 2013. BACH2 re- presses effector programs to stabilize Treg-mediated immune homeostasis. Nature References 498: 506–510. 1. Igarashi, K., K. Ochiai, A. Itoh-Nakadai, and A. Muto. 2014. Orchestration of 25. Vahedi, G., Y. Kanno, Y. Furumoto, K. Jiang, S. C. Parker, M. R. Erdos, S. R. Davis, plasma cell differentiation by Bach2 and its gene regulatory network. Immunol. Rev. R. Roychoudhuri, N. P. Restifo, M. Gadina, et al. 2015. Super-enhancers delineate 261: 116–125. disease-associated regulatory nodes in T cells. Nature 520: 558–562. 2. Igarashi, K., K. Ochiai, and A. Muto. 2007. Architecture and dynamics of the 26. Witte, S., J. J. O’Shea, and G. Vahedi. 2015. Super-enhancers: asset management in transcription factor network that regulates B-to-plasma cell differentiation. J. Bio- immune cell genomes. Trends Immunol. 36: 519–526. chem. 141: 783–789. 27. Kallies, A., E. D. Hawkins, G. T. Belz, D. Metcalf, M. Hommel, L. M. Corcoran, 3. Nutt, S. L., N. Taubenheim, J. Hasbold, L. M. Corcoran, and P. D. Hodgkin. P. D. Hodgkin, and S. L. Nutt. 2006. Transcriptional repressor Blimp-1 is essential 2011. The genetic network controlling plasma cell differentiation. Semin. Immunol. for T cell homeostasis and self-tolerance. Nat. Immunol. 7: 466–474. 23: 341–349. 28. Martins, G. A., L. Cimmino, M. Shapiro-Shelef, M. Szabolcs, A. Herron, 4. Zhou, Y., H. Wu, M. Zhao, C. Chang, and Q. Lu. 2016. The Bach family of E. Magnusdottir, and K. Calame. 2006. Transcriptional repressor Blimp-1 regulates transcription factors: a comprehensive review. Clin. Rev. Allergy Immunol. 50: 345– T cell homeostasis and function. Nat. Immunol. 7: 457–465. 356. 29. Veldhoen, M., K. Hirota, A. M. Westendorf, J. Buer, L. Dumoutier, J. C. Renauld, 5. Sykiotis, G. P., and D. Bohmann. 2010. Stress-activated cap’n’collar transcription and B. Stockinger. 2008. The aryl hydrocarbon receptor links T 17-cell-mediated factors in aging and human disease. Sci. Signal. 3: re3. H autoimmunity to environmental toxins. Nature 453: 106–109. 6. Sasaki, S., E. Ito, T. Toki, T. Maekawa, R. Kanezaki, T. Umenai, A. Muto, 30. Rai, D., N. L. Pham, J. T. Harty, and V. P. Badovinac. 2009. Tracking the total H. Nagai, T. Kinoshita, M. Yamamoto, et al. 2000. Cloning and expression of CD8 T cell response to infection reveals substantial discordance in magnitude and human B cell-specific transcription factor BACH2 mapped to chromosome 6q15. kinetics between inbred and outbred hosts. J. Immunol. 183: 7672–7681. Oncogene 19: 3739–3749. 31. Ochiai, K., A. Muto, H. Tanaka, S. Takahashi, and K. Igarashi. 2008. Regulation of 7. Oyake, T., K. Itoh, H. Motohashi, N. Hayashi, H. Hoshino, M. Nishizawa, the plasma cell transcription factor Blimp-1 gene by Bach2 and Bcl6. Int. Immunol. M. Yamamoto, and K. Igarashi. 1996. Bach proteins belong to a novel family of 20: 453–460. BTB-basic leucine zipper transcription factors that interact with MafK and regulate 32. Roychoudhuri, R., R. L. Eil, D. Clever, C. A. Klebanoff, M. Sukumar, F. M. Grant, transcription through the NF-E2 site. Mol. Cell. Biol. 16: 6083–6095. 8. Lesniewski, M. L., P. Haviernik, R. P. Weitzel, S. Kadereit, M. M. Kozik, Z. Yu, G. Mehta, H. Liu, P. Jin, et al. 2016. The transcription factor BACH2 L. R. Fanning, Y. C. Yang, Y. Hegerfeldt, M. R. Finney, M. Z. Ratajczak, et al. promotes tumor immunosuppression. J. Clin. Invest. 126: 599–604. 33. Hu, G., and J. Chen. 2013. A genome-wide regulatory network identifies key 2008. Regulation of IL-2 expression by transcription factor BACH2 in umbilical + cord blood CD4+ T cells. Leukemia 22: 2201–2207. transcription factors for memory CD8 T-cell development. Nat. Commun. 4: 9. Blank, V. 2008. Small Maf proteins in mammalian gene control: mere dimerization 2830. partners or dynamic transcriptional regulators? J. Mol. Biol. 376: 913–925. 34. Roychoudhuri, R., D. Clever, P. Li, Y. Wakabayashi, K. M. Quinn, C. A. Klebanoff, Y. Ji, M. Sukumar, R. L. Eil, Z. Yu, et al. 2016. BACH2 regulates 10. Ochiai, K., Y. Katoh, T. Ikura, Y. Hoshikawa, T. Noda, H. Karasuyama, S. Tashiro, + A. Muto, and K. Igarashi. 2006. Plasmacytic transcription factor Blimp-1 is re- CD8 T cell differentiation by controlling access of AP-1 factors to enhancers. Nat. pressed by Bach2 in B cells. J. Biol. Chem. 281: 38226–38234. Immunol. 17: 851–860. 35. Kallies, A., A. Xin, G. T. Belz, and S. L. Nutt. 2009. Blimp-1 transcription factor is 11. Kometani, K., R. Nakagawa, R. Shinnakasu, T. Kaji, A. Rybouchkin, S. Moriyama, + K. Furukawa, H. Koseki, T. Takemori, and T. Kurosaki. 2013. Repression of the required for the differentiation of effector CD8 T cells and memory responses. transcription factor Bach2 contributes to predisposition of IgG1 memory B cells Immunity 31: 283–295. toward plasma cell differentiation. Immunity 39: 136–147. 36. Rutishauser, R. L., G. A. Martins, S. Kalachikov, A. Chandele, I. A. Parish, € E. Meffre, J. Jacob, K. Calame, and S. M. Kaech. 2009. Transcriptional repressor 12. Turkmen, S., M. Riehn, E. Klopocki, M. Molkentin, R. Reinhardt, and + T. Burmeister. 2011. A BACH2-BCL2L1 fusion gene resulting from a t(6;20)(q15; Blimp-1 promotes CD8 T cell terminal differentiation and represses the acquisition q11.2) chromosomal translocation in the lymphoma cell line BLUE-1. Genes of central memory T cell properties. Immunity 31: 296–308. Cancer 50: 389–396. 37. Calfon, M., H. Zeng, F. Urano, J. H. Till, S. R. Hubbard, H. P. Harding, 13. Kobayashi, S., T. Taki, Y. Chinen, Y. Tsutsumi, M. Ohshiro, T. Kobayashi, S. G. Clark, and D. Ron. 2002. IRE1 couples endoplasmic reticulum load to se- Y. Matsumoto, J. Kuroda, S. Horiike, K. Nishida, and M. Taniwaki. 2011. Iden- cretory capacity by processing the XBP-1 mRNA. Nature 415: 92–96. tification of IGHCd-BACH2 fusion transcripts resulting from cryptic chromosomal 38. Reimold, A. M., N. N. Iwakoshi, J. Manis, P. Vallabhajosyula, E. Szomolanyi- rearrangements of 14q32 with 6q15 in aggressive B-cell lymphoma/leukemia. Genes Tsuda, E. M. Gravallese, D. Friend, M. J. Grusby, F. Alt, and L. H. Glimcher. Chromosomes Cancer 50: 207–216. 2001. Plasma cell differentiation requires the transcription factor XBP-1. Nature 14. Ikeda, T., J. Shibata, K. Yoshimura, A. Koito, and S. Matsushita. 2007. Recurrent 412: 300–307. HIV-1 integration at the BACH2 locus in resting CD4+ T cell populations during 39. Kamimura, D., and M. J. Bevan. 2008. Endoplasmic reticulum stress regulator + effective highly active antiretroviral therapy. J. Infect. Dis. 195: 716–725. XBP-1 contributes to effector CD8 T cell differentiation during acute infection. 15. Quinn, E. M., C. Coleman, B. Molloy, P. Dominguez Castro, P. Cormican, J. Immunol. 181: 5433–5441. V. Trimble, N. Mahmud, and R. McManus. 2015. Transcriptome analysis of CD4+ 40. Pham, N. L., V. P. Badovinac, and J. T. Harty. 2009. A default pathway of memory T cells in coeliac disease reveals imprint of BACH2 and IFNg regulation. PLoS One CD8 T cell differentiation after dendritic cell immunization is deflected by en- 10: e0140049. counter with inflammatory cytokines during antigen-driven proliferation. 16. McAllister, K., A. Yarwood, J. Bowes, G. Orozco, S. Viatte, D. Diogo, J. Immunol. 183: 2337–2348. L. J. Hocking, S. Steer, P. Wordsworth, A. G. Wilson, et al; UK Rheumatoid 41. Presnell, S. R., A. Al-Attar, F. Cichocki, J. S. Miller, and C. T. Lutz. 2015. Human Arthritis Genetics Consortium; Rheumatoid Arthritis Consortium International. natural killer cell microRNA: differential expression of MIR181A1B1 and 2013. Identification of BACH2 and RAD51B as rheumatoid arthritis susceptibility MIR181A2B2 genes encoding identical mature microRNAs. Genes Immun. 16: 89– loci in a meta-analysis of genome-wide data. Arthritis Rheum. 65: 3058–3062. 98. 17. Franke, A., D. P. McGovern, J. C. Barrett, K. Wang, G. L. Radford-Smith, 42. Liu, X., Z. Zhan, L. Xu, F. Ma, D. Li, Z. Guo, N. Li, and X. Cao. 2010. T. Ahmad, C. W. Lees, T. Balschun, J. Lee, R. Roberts, et al. 2010. Genome-wide MicroRNA-148/152 impair innate response and antigen presentation of TLR- meta-analysis increases to 71 the number of confirmed Crohn’s disease susceptibility triggered dendritic cells by targeting CaMKIIa. J. Immunol. 185: 7244–7251. loci. Nat. Genet. 42: 1118–1125. 43. Porstner, M., R. Winkelmann, P. Daum, J. Schmid, K. Pracht, J. Coˆrte-Real, 18. Sawcer, S., G. Hellenthal, M. Pirinen, C. C. Spencer, N. A. Patsopoulos, S. Schreiber, C. Haftmann, A. Brandl, M. F. Mashreghi, et al. 2015. miR-148a L. Moutsianas, A. Dilthey, Z. Su, C. Freeman, S. E. Hunt, et al; International promotes plasma cell differentiation and targets the germinal center transcription Multiple Sclerosis Genetics Consortium; Wellcome Trust Case Control Consortium factors Mitf and Bach2. Eur. J. Immunol. 45: 1206–1215. 2. 2011. Genetic risk and a primary role for cell-mediated immune mechanisms in 44. Li, Q. J., J. Chau, P. J. Ebert, G. Sylvester, H. Min, G. Liu, R. Braich, multiple sclerosis. Nature 476: 214–219. M. Manoharan, J. Soutschek, P. Skare, et al. 2007. miR-181a is an intrinsic 19. Cooper, J. D., D. J. Smyth, A. M. Smiles, V. Plagnol, N. M. Walker, J. E. Allen, modulator of T cell sensitivity and selection. Cell 129: 147–161. K. Downes, J. C. Barrett, B. C. Healy, J. C. Mychaleckyj, et al. 2008. Meta-analysis 45. Smith-Garvin, J. E., J. C. Burns, M. Gohil, T. Zou, J. S. Kim, J. S. Maltzman, of genome-wide association study data identifies additional type 1 diabetes risk loci. E. J. Wherry, G. A. Koretzky, and M. S. Jordan. 2010. T-cell receptor signals direct the Nat. Genet. 40: 1399–1401. composition and function of the memory CD8+ T-cell pool. Blood 116: 5548–5559. The Journal of Immunology 1015

46. Teixeiro, E., M. A. Daniels, S. E. Hamilton, A. G. Schrum, R. Bragado, 53. Paley, M. A., D. C. Kroy, P. M. Odorizzi, J. B. Johnnidis, D. V. Dolfi, S. C. Jameson, and E. Palmer. 2009. Different T cell receptor signals determine B. E. Barnett, E. K. Bikoff, E. J. Robertson, G. M. Lauer, S. L. Reiner, and CD8+ memory versus effector development. Science 323: 502–505. E. J. Wherry. 2012. Progenitor and terminal subsets of CD8+ T cells cooperate to 47. Caserta, S., J. Kleczkowska, A. Mondino, and R. Zamoyska. 2010. Reduced contain chronic viral infection. Science 338: 1220–1225. functional avidity promotes central and effector memory CD4 T cell responses to 54. Kao, C., K. J. Oestreich, M. A. Paley, A. Crawford, J. M. Angelosanto, M. A. Ali, tumor-associated antigens. J. Immunol. 185: 6545–6554. A. M. Intlekofer, J. M. Boss, S. L. Reiner, A. S. Weinmann, and E. J. Wherry. 2011. 48. Catron, D. M., L. K. Rusch, J. Hataye, A. A. Itano, and M. K. Jenkins. 2006. CD4+ Transcription factor T-bet represses expression of the inhibitory receptor PD-1 and T cells that enter the draining lymph nodes after antigen injection participate in the sustains virus-specific CD8+ T cell responses during chronic infection. Nat. primary response and become central-memory cells. J. Exp. Med. 203: 1045–1054. Immunol. 12: 663–671. 49. Youngblood, B., J. S. Hale, and R. Ahmed. 2013. T-cell memory differentiation: 55. Shin, H., S. D. Blackburn, A. M. Intlekofer, C. Kao, J. M. Angelosanto, insights from transcriptional signatures and epigenetics. Immunology 139: 277– S. L. Reiner, and E. J. Wherry. 2009. A role for the transcriptional repressor Blimp- 284. 1 in CD8+ T cell exhaustion during chronic viral infection. Immunity 31: 309–320. 50. Moskophidis, D., F. Lechner, H. Pircher, and R. M. Zinkernagel. 1993. Virus 56. Menner, A. J., K. S. Rauch, P. Aichele, H. Pircher, C. Schachtrup, and persistence in acutely infected immunocompetent mice by exhaustion of antiviral K. Schachtrup. 2015. Id3 controls cell death of 2B4+ virus-specific CD8+ T cells in cytotoxic effector T cells. Nature 362: 758–761. chronic viral infection. J. Immunol. 195: 2103–2114. 51. Zajac, A. J., J. N. Blattman, K. Murali-Krishna, D. J. Sourdive, M. Suresh, 57. Crawford, A., J. M. Angelosanto, C. Kao, T. A. Doering, P. M. Odorizzi, J. D. Altman, and R. Ahmed. 1998. Viral immune evasion due to persistence of B. E. Barnett, and E. J. Wherry. 2014. Molecular and transcriptional basis of CD4+ activated T cells without effector function. J. Exp. Med. 188: 2205–2213. T cell dysfunction during chronic infection. Immunity 40: 289–302. 52. Crespo, J., H. Sun, T. H. Welling, Z. Tian, and W. Zou. 2013. T cell anergy, 58.Wirth,T.C.,H.H.Xue,D.Rai,J.T.Sabel,T.Bair,J.T.Harty,andV.P.Badovinac. exhaustion, senescence, and stemness in the tumor microenvironment. Curr. Opin. 2010. Repetitive antigen stimulation induces stepwise transcriptome diversification but Immunol. 25: 214–221. preserves a core signature of memory CD8+ T cell differentiation. Immunity 33: 128–140.