Effectors and memories: Bcl-6 and Blimp-1 in T and B lymphocyte differentiation

Abstract

Bcl-6 and Blimp-1 have recently been identified as key transcriptional regulators of effector and memory differentiation in CD4⁺ T cells and CD8⁺ T cells. Bcl-6 and Blimp-1 were previously known to be critical regulators of effector and memory differentiation of B lymphocytes. The new findings unexpectedly point to the Bcl-6 and Blimp-1 regulatory axis as a ubiquitous mechanism for controlling effector and memory lymphocyte differentiation and function. Bcl-6 and Blimp-1 are antagonistic transcription factors and can function as a self-reinforcing genetic switch for cell-fate decisions. However, their influences in different lymphocytes are complex. Here we review and examine the commonalities and differences in the functions of these transcription factors in CD4⁺ follicular helper T_FH lymphocytes, effector CD8⁺ T lymphocytes and B lymphocytes.

CD4⁺ T cells, CD8⁺ T cells and B cells are distinct lymphocyte populations with unique, critical roles in adaptive immunity. Each population has been thought to be controlled by quite different transcriptional regulation for its individual effector and memory differentiation programs¹. Bcl-6 and Blimp-1 (encoded by Prdm1) are transcriptional repressors with the ability to block each other's expression (Fig. 1). The recent identification of central roles for Bcl-6 and Blimp-1 in the differentiation of follicular helper CD4⁺ T cells^2–4 and CD8⁺ T cell effector and memory differentiation processes^5–7, combined with their well documented roles in B cells, has revealed an unexpected common system used in key fate decisions made by mature CD4⁺ T cells, CD8⁺ T cells and B cells.

Figure 1.

Bcl-6 and Blimp-1 are reciprocally antagonistic transcription factors. Mechanisms of Bcl-6 and Blimp-1 antagonism and their broad influences on lymphocyte differentiation and function. CTL, cytotoxic T lymphocyte.

Bcl-6 and Blimp-1 in B cell differentiation

Bcl6 was first identified as a proto-oncogene frequently expressed in non-Hodgkin's lymphoma as a result of chromosomal translocations^8–12. Bcl-6 protein was found to be highly expressed in germinal center B cells, which led to the conclusion that many B cell lymphomas begin as germinal center B cells that then acquire dysregulated Bcl-6 expression¹¹. The germinal center reaction is the process by which antigen-specific B cells mature into the long-lived plasma cells (antibody-secreting cells) and memory B cells that constitute the cellular components of immunological memory for the B cell lineage^1,13 (Fig. 2). After activation and costimulation, B cells differentiate into either short-lived plasma cells or germinal center B cells^1,14 (Fig. 2). Short-lived plasma cells stop proliferating and elaborate a large secretory apparatus for producing antibodies to quickly combat infection^1,14. In contrast, germinal center B cells have a minimal secretory apparatus and are instead specialized for proliferation and affinity maturation, the exquisitely complex process of rapidly evolving a B cell receptor of higher affinity for the production of high-affinity, isotype-switched antibody^11,15. This process involves rapid proliferation and substantial amounts of DNA damage (both class-switch recombination and somatic hypermutation, controlled by activation-induced cytidine deaminase (AID)¹⁶). Bcl-6 is a crucial inducer of germinal center B cell proliferation and a crucial inhibitor of the DNA-damage response¹¹ (Figs. 1 and 2). Bcl-6-deficient mice have an absence of germinal center B cells and affinity maturation^17–19. Bcl-6-deficient B cells are nevertheless able to differentiate into plasma cells and secrete antibodies^18,20, although long-term antibody responses in Bcl6^−/− mice are considerably diminished, as the germinal center reaction is required to produce the majority of long-lived plasma cells¹⁹ (Fig. 2). Constitutive expression of Bcl-6 in B cells in vivo results in large germinal centers, which again confirms the central role of Bcl-6 in germinal center B cell differentiation²¹.

Figure 2.

Roles of Bcl-6 and Blimp-1 in B cell differentiation. Bcl-6 and Blimp-1 are required at various steps of B cell differentiation (as indicated by arrow labels).

Plasma cell differentiation, in contrast, critically depends on Blimp-1 (refs. 22,23) and the absence of Bcl-6 (refs. 11,23). The role of Blimp-1 in B cell development was first demonstrated by the ability of ectopic Blimp-1 expression to induce human B lymphoma cells to differentiate into cells with plasma cell features²⁴. Blimp-1 is highly expressed in plasma cells and controls many genes important for plasma cell differentiation^23,25–27, including Xbp1, which induces formation of the secretory apparatus necessary for the production of large amounts of antibody^28,29. Blimp-1 also inhibits genes involved in cellular proliferation, such as Myc and Bcl6, which thereby allows the terminal differentiation of plasma cells into a post-mitotic state^23,30.

Genetic ablation of Blimp-1 in B cells prevents mature B cells from differentiating into either short-lived or long-lived plasma cells, which results in dramatically reduced antibody titers²². Interestingly, germinal centers are enlarged in these mice, and memory B cells are generated²². Together with experiments using ectopic expression of Blimp-1, these data suggest that Blimp-1 is a master regulator of plasma cell differentiation with limited or no roles in germinal center B cell and memory B cell differentiation (Fig. 2).

Bcl-6 protein can bind to Prdm1 and directly repress Blimp-1 expression^20,31,32 (Fig. 1) and thereby induce germinal center differentiation while inhibiting plasma cell differentiation¹¹. Conversely, Blimp-1 protein can bind to Bcl6 and directly repress Bcl-6 production³³ (Fig. 1) and thereby induce plasma cell differentiation while inhibiting germinal center B cell differentiation^23,25. Although details about the kinetics of the expression of Bcl-6 and Blimp-1 during B cell activation and differentiation in vivo still remain to be fully elucidated^25,34, the reciprocal antagonism between Bcl-6 and Blimp-1 clearly provides a powerful mechanism for inducing and then reinforcing bimodal fate ‘decisions’ in B cell effector and memory differentiation.

Bcl-6 and Blimp-1 in CD4⁺ T cell differentiation

Although Bcl-6 and Blimp-1 have been studied intensively in B cells for some 15 years, the functions of Bcl-6 and Blimp-1 in T lymphocytes remained relatively unexplored until recently²⁵. A series of new data has now demonstrated that Bcl-6 is the master regulator of CD4⁺ follicular helper T cells (T_FH cells)^2–4 and that Blimp-1 is a critical antagonist of T_FH differentiation².

CD4⁺ T cells can differentiate into at least four different effector lineages (T helper type 1 (T_H1) cells, T_H2 cells, T_H17 cells and regulatory T cells (T_reg cells)), which allows them to powerfully tailor the larger immune response to best clear a given infection or control inflammation³⁵. CD4⁺ T cell help to B cells is essential for the generation of germinal centers, memory B cells and long-lived plasma cells¹. CD4⁺ T_H2, T_H1 and T_H17 cells are not required for germinal center formation or B cell help^36–38. Therefore, it had been proposed, mainly on the basis of gene-expression profiling and in vitro T cell help–B cell help assays, that T_FH may represent a distinct CD4⁺ T cell effector lineage specialized for B cell help^39–41. However, unlike the other effector lineages, until recently T_FH cells had no lineage-specifying master regulator transcription factor (such as T-bet, GATA-3, RORγt or Foxp3)⁴².

Three new studies have now shown that Bcl-6 is a master regulator of T_FH differentiation^2–4. Constitutive expression of Bcl-6 in CD4⁺ T cells drives nearly absolute T_FH differentiation in vivo². These T_FH cells induce larger germinal centers and higher antigen-specific antibody titers². In contrast, Bcl6^−/− CD4⁺ T cells are unable to differentiate into T_FH cells in vivo, which demonstrates that Bcl-6 is necessary for T_FH differentiation^2–4. Bcl6^−/− CD4⁺ T cells become activated and proliferate in response to protein immunization but are unable to drive germinal center formation^2–4. These results confirm that T_FH cells are a distinct subset of helper CD4⁺ T cells, reveal that Bcl-6 is a T_FH master regulator both necessary and sufficient for T_FH differentiation, and demonstrate that T_FH cells are uniquely able to drive the germinal center reaction.

Interestingly, although T_FH cells have high Bcl-6 expression⁴², the remaining antigen-specific CD4⁺ T cells (non-T_FH cells) have high Blimp-1 expression ^2,43,44, which indicates that Bcl-6 versus Blimp-1 expression may be a central cell fate decision of differentiating CD4⁺ T cells. Constitutive expression of Blimp-1 blocks Bcl-6 expression and T_FH differentiation but does not block the proliferation and differentiation of non-T_FH CD4⁺ cells². Like Bcl6^−/− CD4⁺ T cells, Blimp-1-expressing CD4⁺ T cells cannot induce germinal center formation, which results in considerably reduced antibody titers². Conversely, Blimp-1-deficient CD4⁺ T cells preferentially differentiate into T_FH cells in vivo². Therefore, in CD4⁺ T cells, Blimp-1 is a physiological regulator of Bcl-6 expression, and vice versa².

There are two models for interpreting the Bcl-6 and Blimp-1 results outlined above in the context of CD4⁺ T cell lineage differentiation (Fig. 3a,b). The simplest model proposes that T_FH differentiation occurs via a fully independent differentiation pathway controlled by Bcl-6, analogous to the differentiation pathways for T_H1, T_H2, T_H17 and inducible T_reg (iT_reg) cells³⁷ (model 1; Fig. 3a). A second model posits that T_FH cells are a distinct type of effector CD4⁺ T cell but that the T_FH differentiation pathway is not fully independent of T_H1, T_H2, T_H17 or iT_reg differentiation^2,45 (model 2; Fig. 3b). In this second model, a CD4⁺ T cell is primed by a dendritic cell and acquires early T_H1, T_H2, T_H17, iT_reg or unbiased T_H0 cell characteristics (Fig. 3b). The primed CD4⁺ T cell (early effector) can then further polarize into a Blimp-1^hi full effector T_H1 cell², T_H2 cell^2,43, T_H17 cell or iT_reg cell⁴⁶ (Fig. 3b). Alternatively, the primed CD4⁺ T cell may encounter a B cell—which induces high Bcl-6 expression—and differentiate into a T_FH cell (Fig. 3b). In support of model 1, there is evidence that Bcl-6 can inhibit T_H1, T_H2 and/or T_H17 gene expression^3,4,17,47, and this model fits well with conventional helper T cell schematics. However, several lines of evidence indicate that the second model is probably a more accurate representation of the in vivo biology. First, T_FH cells produce T_H1 cytokines^2,48–50, T_H2 cytokines,^{43,48,50–52} or T_H17 cytokines⁵³. The production of T_H1, T_H2 or T_H17 cytokines probably enables T_FH cells to specify the antibody isotypes produced during the B cell response⁵⁰. Nevertheless, T_FH cells produce lower amounts of T_H1, T_H2 or T_H17 cytokines than do non-T_FH cells^2,4,43, which express Blimp-1 (refs. 2,43). Therefore, Bcl-6-expressing T_FH cells seem to have some T_H1, T_H2 or T_H17 characteristics acquired at the initial dendritic cell priming (Fig. 3b), but those characteristics are partially subdued in the presence of Bcl-6. Second, dendritic cell priming is insufficient for T_FH differentiation in vivo, as T_FH cells are not observed in the absence of cognate B cells^2,54, which indicates that the interaction of a primed CD4⁺ T cell with a cognate B cell is a key second signal for the induction of T_FH differentiation^2,45,54. T_FH differentiation can be restored in B cell–deficient mice by the expression of Bcl-6 in antigen-specific CD4⁺ T cells, which suggests that cognate B cells specifically induce Bcl-6 expression in CD4⁺ T cells² (Fig. 3b).

Figure 3.

Roles of Bcl-6 and Blimp-1 in T cell differentiation. (a,b) Two proposed models of effector CD4⁺ T cell differentiation: model 1 (a) and model 2 (b). in b, the partial T_H1, T_H2, T_reg and T_H17 characteristics of T_FH cells are not shown, for simplicity. (c) A model of CD8⁺ T cell effector and memory differentiation highlighting the similarities to CD4⁺ T cells. Dotted lines and question marks indicate uncertainty. +Ag, antigen stimulation; SLeC, short-lived effector cell; MPeC, memory precursor effector cell.

Blimp-1 is expressed late in CD4⁺ T cell differentiation in vitro^25,55. Blimp-1 expression is therefore associated with highly polarized effector CD4⁺ T cells, late in differentiation, with a high cytokine-secretion capacity (Fig. 3b). This is intriguingly similar to the role of Blimp-1 in effector CD8⁺ T cell differentiation (Fig. 3c; discussed below) and similar to (although less extreme than) its role in plasma cells, which have high expression of Blimp-1 and an enormous secretion capacity (Fig. 4). Furthermore, the highly polarized Blimp-1-expressing effector CD8⁺ T cells or plasma cells have limited or no proliferative capacity, respectively, which suggests that Blimp-1^hi effector CD4⁺ T cells may have similar properties (Fig. 4). Blimp-1-deficient T cells exhibit hyperproliferation in vivo, which results in autoimmunity^46,56. Early CD4⁺ T cell effector differentiation depends on neither Bcl-6 nor Blimp-1 (refs. 3,46,56). Those findings are consistent with our model, in which Blimp-1 is not required for the priming and development of CD4⁺ T cell effector functions (cytokine secretion) but is critical for full effector cell differentiation and inhibition of the proliferation of those effector cells (Fig. 3b).

Figure 4.

Functional effects of different amounts of Bcl-6 and Blimp-1 expression. The amount of Bcl-6 and Blimp-1 expression is balanced differently in different lymphocytes at distinct differentiation stages. Lymphocytes with higher expression of Bcl-6 exhibit greater proliferative capacity but less secretory capacity. Lymphocytes with higher expression of Blimp-1 exhibit lower proliferative capacity and greater secretory capacity. Although exhausted CD8⁺ T cells have large amounts of Blimp-1 and lower proliferative capacity, they do not have more secreted effectors. T_CM, central memory T cell; T_EM, effector memory T cell.

Although Bcl-6 and Blimp-1 are clearly central to effector CD4⁺ T cell differentiation (and possibly memory, as discussed below), many aspects of their involvement remain unclear. In particular, how Bcl-6 controls T_FH differentiation is an area of intensive study. Initial reports have indicated that Bcl-6 can regulate key cytokine receptors (such as the interleukin 6 receptor (IL-6R) or IL-21R)³ and suppress an important cluster of microRNAs⁴, as well as suppress T_H1, T_H2 and T_H17 gene expression^3,4,44. The specific roles of Blimp-1 in CD4⁺ T cell effector differentiation remain to be extensively explored.

Bcl-6 and Blimp-1 in CD8⁺ T cell differentiation

Naive CD8⁺ T cells differentiate into cytotoxic effectors (cytotoxic T lymphocytes) that are critical for the control and clearance of intracellular pathogens. Recent studies have shown that Blimp-1 is expressed in effector and memory CD8⁺ T cells^46,56,57 and that the absence of Blimp-1 can result in excessive proliferation and increased numbers of memory CD8⁺ T cells⁵⁶. Recent data have now convincingly demonstrated that Blimp-1 is an important regulator of effector CD8⁺ T cell function, proliferation and conversion into memory cells^5–7. Virus-specific Blimp-1-deficient CD8⁺ T cells fail to differentiate into KLRG1^hiIL-7R^lo cells^6,7, also known as short-lived effector cells because of their effector functions and limited ability to survive and convert into memory cells⁵⁸. Instead, Blimp-1-deficient CD8⁺ T cells preferentially differentiate into KLRG1^loIL-7R^hi memory precursor effector cells^6,7, which exhibit better survival than short-lived effector cells and have the potential to convert into memory CD8⁺ T cells⁵⁸ (Fig. 3c). Blimp-1-deficient CD8⁺ T cells have lower expression of effector molecules important for cytotoxicity, such as granzyme B^5–7. Nonetheless, Blimp-1-deficient CD8⁺ T cells have sufficient effector functions to control and clear acute infection with lymphocytic choriomeningitis virus or influenza virus^6,7. Together these studies elegantly demonstrate that Blimp-1 expression is required for the terminal differentiation of effector CD8⁺ T cells^6,7.

Blimp-1-deficient CD8⁺ T cells express more Bcl-6 than do wild-type CD8⁺ T cells^6,7. Constitutive Bcl-6 expression suppresses granzyme B expression⁵⁹. Conversely, Bcl6^−/− CD8⁺ T cells express more granzyme B⁵⁹. Bcl-6 and Blimp-1 are therefore probably reciprocal regulators of CD8⁺ T cell effector differentiation.

Certain commonalities can be gleaned from the CD8⁺ and CD4⁺ T cell studies about how Blimp-1 and Bcl-6 regulate these cell types. In both CD8⁺ and CD4⁺ T cells, Blimp-1 is critical for most terminal effector cell differentiation (Fig. 3b,c). Terminal differentiation is characterized by low proliferative potential and high effector molecule secretion (Fig. 4). Blimp-1-deficient CD8⁺ T cells are diverted to a memory-precursor differentiation pathway with enhanced proliferative potential and have lower effector molecule production (Figs. 3 and 4). Blimp-1-deficient CD4⁺ T cells are diverted away from terminal T_H1 or T_H2 differentiation and toward T_FH differentiation² (Fig. 3) and exhibit greater proliferative potential^46,56. These proliferation and secretion commonalities of Blimp-1-expressing T cells are again shared with the B cell lineage, in which B cells with high Blimp-1 expression are terminally differentiated effectors (plasma cells) with low proliferative potential and high effector molecule production (Fig. 4).

Regulation of T and B cell memory by Bcl-6 and Blimp-1

Blimp-1-deficient CD8⁺ T cells preferentially differentiate into memory CD8⁺ T cells^6,7,56, and Bcl-6 is upregulated in memory CD8⁺ T cells⁶⁰. Blimp-1 is also expressed differently by the two main memory CD8⁺ T cell subsets^6,7. Effector memory T cells, which are functionally potent memory CD8⁺ T cells that patrol the periphery but proliferate poorly^58,61, have relatively high Blimp-1 expression. In contrast, central memory T cells have relatively low Blimp-1 expression^6,7 (Fig. 4) and are characterized by lower immediate effector functions and high proliferative potential^58,61.

Bcl-6-deficient CD8⁺ T cells proliferate poorly and are less able to acquire a memory-like phenotype⁶². Conversely, CD8⁺ T cells overexpressing Bcl-6 generate a larger memory cell population⁶³. There is also some limited evidence that Bcl-6 has a role in memory CD4⁺ T cells. Bcl-6-deficient CD4⁺ T cells have an impaired ability to persist as memory cells after immunization⁶⁴. Consistent with that idea, Blimp-1 expression is lower in memory CD4⁺ T cells than in effector CD4⁺ T cells⁴⁶. Given that Bcl-6 promotes proliferation, Bcl-6 expression may be a key attribute of CD8⁺ and CD4⁺ T cell memory differentiation that Blimp-1 inhibits to direct cells toward terminal effector differentiation.

A simple model is that expression of Bcl-6 and Blimp-1 is a bimodal, self-reinforcing switch between effector and memory T cell differentiation, which is conserved between CD8⁺ and CD4⁺ T cells. However, this is an oversimplification, because among effector CD4⁺ T cells, only T_FH cells depend on Bcl-6 expression^2–4 and the absence of Blimp-1 (ref. 2). Furthermore, effector CD8⁺ T cells have a substantial amount of functionality in the absence of Blimp-1 (refs. 5–7,56). A somewhat more sophisticated model (Fig. 3b,c) incorporates the multiple stages of these CD4⁺ and CD8⁺ T cell differentiation pathways and highlights possible parallels in the activities of Bcl-6 and Blimp-1 in each lineage. Of course, many other proteins are also important participants in CD4⁺ and CD8⁺ T cell differentiation and functionality, but here we are focusing only on the contributions of Bcl-6 and Blimp-1.

The parallels between the functions of Blimp-1 and Bcl-6 in CD4⁺ and CD8⁺ T cells also raise a challenging question about the relationship between memory CD4⁺ T cells and the T_FH and non-T_FH effector CD4⁺ T cell lineages (Fig. 3b,c). Do Bcl-6^hiBlimp-1^lo T_FH cells convert more readily into memory cells than do Bcl-6^loBlimp-1^hi effector CD4⁺ T cells? Additional studies of the mechanistic role of Bcl-6 in CD8⁺ and CD4⁺ T cell memory differentiation are much needed.

The role of Bcl-6 in memory B cell differentiation is controversial^13,65,66. Bcl-6 is generally considered crucial for the generation of B cell memory, as long-lived plasma cells and memory B cells are mainly products of germinal centers (Fig. 2). It has been proposed that Bcl-6 expression is necessary to prevent germinal center B cells from differentiating into plasma cells, allowing them to differentiate into memory B cells instead^65,67. Human memory B cells do not express Blimp-1 (refs. 66,68). It has also been shown that Bcl-6 expression can enhance B cell survival and thus may regulate memory B cell self-renewal^19,69. However, Bcl-6 mRNA expression is not correlated with memory B cell differentiation in mice⁷⁰ or humans⁶⁶, and some memory B cells can develop in Bcl-6-deficient mice in the absence of germinal centers¹⁹. High Bcl-6 expression sustains a germinal center B cell phenotype and inhibits human memory B cell differentiation in vitro⁶⁶. It remains to be determined what transcription factor induces memory B cell differentiation from Bcl-6-expressing germinal center B cells.

Blimp-1 in exhausted CD8⁺ T cells

Effector CD8⁺ T cells can become exhausted by persistent antigen presentation during chronic viral infection^71,72. Chronic viral infections associated with T cell exhaustion, such as infection with human immunodeficiency virus or hepatitis C virus, are the cause of enormous human suffering and loss of life⁷³. Exhausted CD8⁺ T cells are functionally impotent and proliferate poorly⁷², but when exhaustion is reversed in mice by therapeutic treatment, proliferation and effector functions are impressively restored^73–77. Exhausted CD8⁺ T cells in mice chronically infected with lymphocytic choriomeningitis virus have abnormally high Blimp-1 expression⁵. In contrast, Blimp-1-deficient CD8⁺ T cells resist exhaustion and exhibit improved cell numbers⁵. Despite this less exhausted state, mice with conditional knockout of Blimp-1 are unable to efficiently clear the virus⁵. This is probably because Blimp-1 is necessary for effector functions and terminal differentiation, as has been shown in mice with CD8⁺ T cells heterozygous for Prdm1 deletion, which clear virus more rapidly than do wild-type mice⁵.

Mechanisms of action

Although Bcl-6 and Blimp-1 are clearly powerful regulators of effector and memory cell fate decisions by CD4⁺ T cells, CD8⁺ T cells and B cells, it is less clear how Bcl-6 and Blimp-1 accomplish these functions at the level of transcriptional repression. It is also unclear to what extent the gene targets of Bcl-6 and Blimp-1 remain constant across different types of lymphocytes.

Bcl-6 targets are best understood in B cells. Bcl-6 suppresses a collection of DNA-damage–response genes and inducers of apoptosis and cell cycle arrest (p53, cyclin D2 and ATR) while facilitating AID expression in germinal center B cells¹¹ (Fig. 1). AID induces class-switch recombination and somatic hypermutation¹⁶, which would result in cell cycle arrest and apoptosis if these normal DNA-damage responses were not suppressed by Bcl-6 (ref. 11). These Bcl-6 targets are B cell specific and are not shared by CD4⁺ or CD8⁺ T cells (although, curiously, some AID expression is observed in T_FH CD4⁺ T cells² and Blimp-deficient CD8⁺ T cells⁶).

A key aspect of Bcl-6 biology is that Bcl-6 dimers repress transcription only in combination with corepressors, and there are many corepressors that Bcl-6 can partner with. Most corepressors bind to or near the Bcl-6 bric-a-bric, tramtrack, broad complex–poxvirus zinc-finger domain, including BCoR^78,79, N-CoR^80,81, SMRT^80,81, CtBP³¹, BAZF⁸², PLZF⁸³, MIZ1 (encoded by Zbtb17)⁸⁴ and others. At least some of these partners can also form mixed complexes^31,83. The corepressor MTA3 binds to a second domain of the Bcl-6 protein called RDII (refs. 32,85). The corepressor ETO (encoded by Runx1t1) binds to the zinc-finger domain of Bcl-6⁸⁶. The use of these many different corepressors allows combinatorial targeting of Bcl-6 to different collections of genes in different cell types at different times. For example, Bcl-6 inhibits Blimp-1 via interactions with MTA3 (refs. 32,87; Fig. 1) and AP-1 (ref. 88). Bcl-6 protein inhibition of the Bcl6 gene instead depends on CtBP³¹ (Fig. 1). Whole-genome chromatin immunoprecipitation plus micrarray analysis has revealed that Bcl-6 regulates a very different set of genes in primary germinal center B cells than in a diffuse large B cell lymphoma³³. Of 3,345 genes bound by Bcl-6, less than half of those targets are bound in both cell types³³, which indicates that differences in corepressor availability probably have considerable effects on Bcl-6 gene targeting in different lymphocyte types at different differentiation stages.

There are only a smattering of obvious gene-expression changes conserved between Bcl-6-expressing T_FH CD4⁺ T cells and Bcl-6-expressing germinal center B cells, beyond Blimp-1 inhibition^2,89. This indicates that different Bcl-6 corepressors are present in T_FH cells and germinal center B cells, which act together to suppress different constellations of genes in the two cell types. There are also only limited similarities in the gene-expression changes in Bcl-6-expressing T_FH CD4⁺ T cells and Blimp-1-deficient CD8⁺ T cells (which have higher expression of Bcl-6)^2,6, which again suggests that the specific gene-expression changes controlled by the Bcl-6–Blimp1 regulatory axis are more different than similar in B cells, CD4⁺ T cells and CD8⁺ T cells and are therefore probably highly influenced by the available corepressors. The present understanding of the genes and gene networks controlled by Bcl-6 remains limited, particularly in T cells, and this is an area that now needs extensive examination, given the importance of Bcl-6 in T cell functions.

It is also important to note that Bcl-6 mRNA quantities are a poor indicator of Bcl-6 protein expression⁹⁰ and a poor indicator of Bcl-6 function, as Bcl-6 is controlled by a wide array of post-transcriptional regulatory mechanisms, including absence of translation at the mRNA level⁹⁰, as well as protein phosphorylation⁹¹, acetylation⁹² and cofactor-mediated degradation⁹³. This allows intense signal integration by Bcl-6, as the functional amount of Bcl-6 protein in a cell can be heavily influenced by many transcriptional and post-transcriptional activities, in combination with changes in the availability of corepressors.

The inhibition of Blimp-1 by Bcl-6 is a common feature in germinal center B cells and CD4⁺ T_FH cells^2,20,31 and probably in differentiating CD8⁺ T cells^6,7. Because Bcl-6 and Blimp-1 are reciprocal antagonists, it is challenging to disentangle their direct effects versus their effects achieved via inhibition of each other. How much of the effect of Bcl-6 expression in a given cell type is via inhibition of Blimp-1 and vice versa? These and related questions need to be addressed.

Like Bcl-6 target genes, Blimp-1 target genes are best characterized in B cells. Blimp-1 targets in B cells can be placed into three functional categories: inhibition of proliferation (c-Myc^30,94 and E2F1 (ref. 27)), induction of secretory machinery (XBP-1 (ref. 27)), and inhibition of the germinal center B cell program (Bcl-6 (ref. 27), Pax5 (ref. 95) and CIITA^96,97). Inhibition of proliferation is a common feature of B cells, CD4⁺ T cells and CD8⁺ T cells with high expression of Blimp-1 (it is important to note that the antiproliferative effects of Blimp-1 are dose dependent^2,26,55). The gene encoding Id3, a pro-proliferation transcription factor⁹⁸ expressed in T_FH cells², germinal center B cells²⁷ and Blimp-1-deficient CD8⁺ T cells⁶, seems to be one common Blimp-1 target in both B cells and T cells and may be a key component of the antiproliferative effects of Blimp-1.

Induction of the secretory apparatus by Blimp-1 (via XBP-1 and possibly other mechanisms) is most dramatic for plasma cells²³ but also occurs in T cells⁹⁹, as terminally differentiated effector T cells are specialized producers of cytokines and other secreted products. In both plasma cells and terminally differentiated effector T cells, the cellular metabolism is optimized for protein production and secretion instead of DNA synthesis and proliferation (Fig. 4).

One of the most interesting targets of Blimp-1 that has been characterized in T cells is the gene encoding IL-2 (refs. 25,55,100), a cytokine critical for T cell and B cell proliferation and differentiation. T_FH cells have been observed to produce more IL-2 than do non-T_FH cells^2,4, consistent with the smaller amount of Blimp-1 in T_FH cells². Blimp-1-deficient CD8⁺ T cells also exhibit greater IL-2 production^6,7. Memory CD8⁺ T cells are also frequently characterized by their ability to produce large amounts of IL-2. Given the importance of IL-2 for T cell function and memory differentiation, Il2 is a key Blimp-1 target gene.

Concluding remarks

Transcription factors drive B lymphocytes and T lymphocytes to differentiate into a variety of different effector and memory cells. Bcl-6 and Blimp-1 are unusual in that they are not unique to one lineage but are common to B cells, CD4⁺ T cells and CD8⁺ T cells. Although the roles of Bcl-6 and Blimp-1 in these cell lineage programs share general features—Bcl-6 frequently sustains proliferative potential but not terminal effector function, Blimp-1 enables secretion and effector functions but inhibits proliferation, and the two genes serve as a bimodal self-reinforcing genetic switch—the genes regulated by Bcl-6 and Blimp-1 to produce those phenotypes seem to be predominantly distinct in B cells, CD4⁺ T cells and CD8⁺ T cells, which highlights the complexity of the gene programs involved and the importance of additional layers of gene-expression regulation.