selected GC B cells will then differentiate into high-affinity plasma cells or memory cells. Disturbances in these tightly regulated processes can have profound pathogenic conse- quences, and dysregulation of follicular and extrafollicular antibody production is commonly encountered in autoim- mune disorders, particularly in systemic lupus erythematosus (SLE; Wardemann and Nussenzweig, 2007; Shlomchik, 2008; Vinuesa et al., 2009). Among the signals provided to B cells by T helper cells to drive humoral responses, the production of IL-21 has recently emerged as a critical element in this process (Ettinger et al., 2008; Spolski and Leonard, 2008). Production of high-levels of IL-21 is the hallmark of a novel class of effector T helper cells termed follicular helper T cells (Tfh), which are special- ized in providing help to B cells in GCs (Crotty, 2011). Syn- thesis of IL-21 is, however, not exclusive to Tfh cells, as IL-21 can also be produced by other T helper subsets including extra- follicular T helper cells and Th17 cells (Korn et al., 2007; Nurieva et al., 2007; Wei et al., 2007; Zhou et al., 2007; Odegard et al., 2008). IL-21 plays a multifaceted role in T cell–dependent humoral responses. In addition to helping support the main- tenance of Tfh cells (Nurieva et al., 2008; Vogelzang et al., 2008), IL-21 acts directly on B cells to promote GC formation, somatic hypermutation, follicular and extrafollicular plasma cell differentiation, and memory B cell responses (Linterman et al., 2010; Zotos et al., 2010; Rankin et al., 2011).The critical effects of IL-21 on B cell responses are related to its capacity to drive the expression of major regulators of the B cell differen- tiation program including activation-induced cytidine deami- nase (AID; also known as AICDA), Bcl-6, and Blimp-1 (Ozaki et al., 2004; Pène et al., 2004; Kobayashi et al., 2009). Given that the presence of these factors marks distinct stages of B cell differentiation, the ability of IL-21 to induce the expression of these molecules must be selectively controlled as B cells pro- ceed along their differentiation program. The mechanisms by which exposure to IL-21 leads to different functional outcomes in B cells as they proceed through different stages of differenti- ation are, however, unknown. The molecular pathways regulating the production of IL-21 have recently been investigated. Interferon regulatory factor 4 (IRF4), a transcription factor induced upon stimula- tion of T and B cells, has emerged as an essential controller of IL-21 production because its absence prevents IL-21 produc- tion by multiple T helper subsets (Chen et al., 2008; Huber et al., 2008). The role of IRF4 in T cell activation is not restricted to the control of IL-21 production, as the lack of IRF4 also results in defects in the function of Th0, Th2, Th9, and Th17 cells (Rengarajan et al., 2002; Brüstle et al., 2007; Chen et al., 2008; Honma et al., 2008; Staudt et al., 2010). IRF4 also exerts a fundamental role in B cell differentiation. IRF4 regulates class-switch recombination via its ability to control AID expression and is absolutely required for the gener- ation of plasma cells (Klein et al., 2006; Sciammas et al., 2006). This latter effect has been ascribed to the capacity of IRF4 to control the expression of Blimp-1 by directly targeting the Prdm1 regulatory regions (Sciammas et al., 2006). Interestingly, recent 582 studies have demonstrated that IRF4 regulates the respon- siveness of both T and B cells to IL-21 and that the expres- sion of IL-21–regulated genes is broadly controlled by the functional cooperation of IRF4 with STAT3 (Kwon et al., 2009). Given the essential and complex role of IRF4 in T and B cell responses (Nutt and Tarlinton, 2011), its activity in both of these compartments needs to be tightly regulated to ensure that the execution of T and B cell differentiation programs is strictly controlled in time and space. During a search for regulatory proteins interacting with IRF4, our laboratory previously isolated a protein termed DEF6 (also known as IBP or SLAT; Hotfilder et al., 1999; Gupta et al., 2003b; Tanaka et al., 2003). DEF6 exerts an im- portant immunoregulatory role in vivo, as demonstrated by the finding that DEF6-deficient (Def6tr/tr) mice can spontane- ously develop either a lupus-like syndrome (on a 129/B6 back- ground) or RA-like arthritis (when crossed to a TCR transgenic mouse, DO11.10, on a BALB/c background; Fanzo et al., 2006; Chen et al., 2008). The autoimmune disorders that develop in the absence of DEF6 are characterized by deregu- lated production of IL-17 and IL-21, which is caused by the unrestrained ability of IRF4 to target the regulatory regions of IL-17 and IL-21 (Chen et al., 2008; Biswas et al., 2010). DEF6 shares a high degree of homology with only one other protein, SWAP-70 (Hotfilder et al., 1999; Gupta et al., 2003b; Tanaka et al., 2003). Unlike DEF6, which is highly expressed in naive T helper cells and B cells, SWAP-70 is expressed in B cells but not in naive T helper cells (Borggrefe et al., 1999). Consistent with this expression pattern, the lack of SWAP-70 results in defects within the B cell compartment. SWAP-70–deficient B cells are less efficiently recruited into the secondary lym- phoid organs during the initial phases of an inflammatory re- sponse (Pearce et al., 2006). Furthermore, SWAP-70 deficiency uncouples GC formation from long-lived plasma cell produc- tion leading to the extrafollicular generation of high-affinity plasma cells after T cell–dependent immunization (Quemeneur et al., 2008). The molecular mechanisms by which SWAP-70 regulates T cell–driven plasma cell differentiation are, how- ever, unknown. Given the emerging role of this unique family of proteins in the control of immune responses, in this study we investigated the effects of the simultaneous lack of DEF6 and SWAP-70 on the immune system. RESULTS Spontaneous development of a lymphoproliferative disorder in double KO (DKO) female mice DEF6 and SWAP-70 are the only two members of a unique family of proteins (Borggrefe et al., 1998; Hotfilder et al., 1999; Gupta et al., 2003b; Tanaka et al., 2003). To elucidate the broad role of these two molecules in the immune system, Def6tr/tr mice on a C57BL/6 background were bred with Swap-70/ mice (also on a C57BL/6 background) to gener- ate Def6tr/trSwap-70/ (DKO) mice. No significant differ- ences were observed in the cellularity and surface phenotypic markers of the lymphoid organs in 6–8-wk-old DKO mice except for a slight decrease in the number of splenic CD8+ IRF4 in autoimmunity | Biswas et al.