a localized IgG2c extrafollicular B cell antibody response is de­ tectable within a few days in mediastinal lymph nodes after intranasal infection (Coro et al., 2006). Similarly, vesicular stomatitis virus has been demonstrated to have a highly or­ ganized antigenic capsid capable of eliciting TI antibody responses (Bachmann et al., 1995) while simultaneously inducing an early isotype­ switched antibody response (Bachmann et al., 1996). These early class­switched responses are not exclusive to antiviral responses, as Salmonella typhimurium likewise promotes an extrafollicular IgG2c switched plasma cell response detectable 4 d after infection (Cunningham et al., 2007). Thus, the common feature of this rapid and highly efficient IgG subclass elicited to both major anti­ pathogen responses further highlights the physiological and biological relevance of our findings. Mature naive murine B cells constitutively express tran­ scripts for the majority of TLRs (Barr et al., 2007; Genestier et al., 2007; Gururajan et al., 2007), and mature B cells can respond to TLR3 signaling in vitro (Alexopoulou et al., 2001). The requirement for direct TLR signaling by B cells during antibody responses has been controversial. In particu­ lar, previous studies have demonstrated a B cell–intrinsic requirement for TLR signaling in the generation of B cell antibody responses (Pasare and Medzhitov, 2005; Jegerlehner et al., 2007). Conversely, studies have also provided evidence that disregards TLR signaling for initiating antibody responses and supporting an instructive role for TLRs in augmenting antibody isotypes (Gavin et al., 2006). Our results demon­ strate a minor role for TLR3 signaling in optimal induction of IgG2c by poly(I:C), supporting previous studies showing that TLR signaling is largely unnecessary for efficient anti­ body responses (Gavin et al., 2006). However, the ability of Tlr3/ B cells to mount normal antibody responses in re­ constituted hosts clearly negates a role for B cell–intrinsic TLR3 signaling in IgG2c induction by poly(I:C). Discrepan­ cies observed in the role of B cell–intrinsic TLR signaling may be explained by a lack of direct conjugation of the TLR agonist with antigen, whereby conjugation would allow for the TLR agonist to be directed through BCR­ mediated internalization to responding B cells for concomitant TLR signaling (Eckl­Dorna and Batista, 2009). Our data do not exclude whether autonomous B cell expression of other PRRs, such as MDA5, is required for the IgG2c TI­2 anti ­ body response. Whether B cell–intrinsic RIG­I–like helicase signaling may affect antibody responses remains unclear but would be interesting to address with a TLR3/ MDA5/ B cell chimeric model similar to that used here. Our findings indicate that poly(I:C) is recognized pre­ dominantly by TLR3 and MDA5 for subsequent indirect in­ duction of the TI­2 IgG2c antibody response through type I IFN. However, we note that in Tlr3/ Mda5/ double knockout mice, poly(I:C) remains able to induce NP­specific IgG2c but is 80% reduced compared with wild type (Fig. 4 A), suggesting residual type I IFN induction. In ac­ cordance with this, it has recently been reported that poly(I:C) induction of CD4 T cell responses is incompletely abolished 1494 in TLR3/ MDA5/ double knockout chimeric mice (Longhi et al., 2009), which is similar to our findings with the B cell antibody response. As RIG­I is also a cytosolic PRR capable of recognizing low­molecular­wt poly(I:C) to signal type I IFN production (Kato et al., 2008), we envision that this dsRNA sensor is likely responsible for the type I IFN that promotes a modest increase in NP­specific IgG2c in the double mutants. Ultimately, our data demonstrate that non– TLR­innate signals, such as type I IFN, can substitute for TLR sensing of non­self to augment antibody responses. This redundancy in the recognition of poly(I:C) by multiple dsRNA­ sensing receptors highlights the importance of IFN­ / as a global non­self warning signal for enhancing the efficiency of anti­pathogen antibody responses. Indeed, it has been noted that all vertebrates examined have immune systems com­ prised by T and B lymphocytes and genes encoding IFN­  and IFN­  (Stetson and Medzhitov, 2006), suggesting that the type I IFN­mediated innate response is closely associated with adaptive immunity. For humoral immunity, type I IFN elaborated during antiviral responses promotes IgG2a/c antibody production in vivo (Finkelman et al., 1991; Heer et al., 2007). However, the role for B cell–intrinsic signaling by type I IFN to directly instruct IgG2c class switching in vivo is unclear. Specifically, signaling through the IFN­ R expressed by B and T cells has been shown to enhance IFN­ –mediated IgM and IgG re­ sponses to soluble proteins (Le Bon et al., 2006). In opposi­ tion, direct IFN­ R signaling by B cells has been found to be unnecessary for the antiviral IgG2c response during acute in­ fluenza infection (Coro et al., 2006). Using a chimeric B cell competition model, we demonstrate that B cell–autonomous IFN­ R signaling does induce IgG2a/c switching in vivo during systemic type I IFN responses, such as that elicited by poly(I:C). The discordance between our findings and other studies likely illustrates the complexity of additional inflam­ matory cytokines, beyond type I IFN, that are elicited during viral infections and that also regulate the instruction to switch between Ig subclasses (Coro et al., 2006). A potential mechanism to account for the type I IFN­ induced response from FO B cells is suggested by the ability of type I IFN to modulate CD69 expression on FO B cells. It has been shown that IFN­ induced CD69 surface expression inhibits lymphocyte egress from lymphoid organs by the concomitant loss of sphingosine­1­phosphate responsiveness (Shiow et al., 2006). In accord with this, virus­induced IFN­ / also causes accumulation of circulating FO B cells to re­ gional lymph nodes early during antiviral responses (Chang et al., 2007). The enhanced response by FO B cells to up­ regulate CD69 in response to IFN­  and BCR­ mediated activation would presumably ensure that these normally recirculating B cells would be retained within secondary lymphoid organs where they can receive further noncognate secondary signals necessary for TI­2 antibody responses (Chang et al., 2007). In addition, as TI­2 antigens would likely be associated with TD antigens within a pathogen, the IFN- drives FO B cell TI-2 responses | Swanson et al.