Feeling a Little SYK After Mixing BAFF with BCR

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nor a speciﬁc treatment to permanently Cachero, T.G., Batten, M., Wheway, J., et al. et al. (2010). Proc. Natl. Acad. Sci. USA 107, restore glandular secretion, so these (2002). J. Clin. Invest. 109, 59–68. 15193–15198. exciting experimental data may open Guo, Z., Li, H., Han, M., Xu, T., Wu, X., and Zhuang, new avenues for the treatment of this Y. (2011). Immunol. Lett. 135, 34–42. Raasch, J., Zeller, N., van Loo, G., Merkler, D., Mildner, A., Erny, D., Knobeloch, K.P., Bethea, devastating disorder. Thus, future studies Motoyama, M., Yamazaki, S., Eto-Kimura, A., J.R., Waisman, A., Knust, M., et al. (2011). Brain should investigate the signiﬁcance of Takeshige, K., and Muta, T. (2005). J. Biol. Chem. 134, 1184–1198. STAT3-mediated IkB-z-dependent in- 280, 7444–7451. Ramos-Casals, M., Brito-Zero´ n, P., Siso´ -Almirall, creased apoptosis in human SS dis- Okamoto, K., Iwai, Y., Oh-Hora, M., Yamamoto, A., and Bosch, X. (2012). BMJ 344, e3821. ease and other autoimmune pathologies M., Morio, T., Aoki, K., Ohya, K., Jetten, A.M., in which aberrant apoptosis has been Akira, S., Muta, T., and Takayanagi, H. (2010). Nature 464, 1381–1385. van Loo, G., De Lorenzi, R., Schmidt, H., Huth, M., noted. Mildner, A., Schmidt-Supprian, M., Lassmann, H., Okuma, A., Hoshino, K., Ohba, T., Fukushi, S., Prinz, M.R., and Pasparakis, M. (2006). Nat. Aiba, S., Akira, S., Ono, M., Kaisho, T., and Muta, REFERENCES Immunol. 7, 954–961. T. (2013). Immunity 38, this issue, 450–460.

Groom, J., Kalled, S.L., Cutler, A.H., Olson, C., Peng, B., Ling, J., Lee, A.J., Wang, Z., Chang, Z., Yamazaki, S., Muta, T., and Takeshige, K. (2001). Woodcock, S.A., Schneider, P., Tschopp, J., Jin, W., Kang, Y., Zhang, R., Shim, D., Wang, H., J. Biol. Chem. 276, 27657–27662.

Feeling a Little SYK after Mixing BAFF with BCR

Simona Infantino1,* and David M. Tarlinton1,* 1The Walter and Eliza Hall Institute of Medical Research, 1G Royal Parade, Parkville 3052, Australia *Correspondence: [email protected] (S.I.), [email protected] (D.M.T.) http://dx.doi.org/10.1016/j.immuni.2013.03.001

In this issue of Immunity, Schweighoffer et al. (2013) report that full BAFF responsiveness in B cells requires the tyrosine kinase Syk and that BAFF may co-opt components of the B cell receptor in transmitting its survival signal.

It is axiomatic to mammalian immune make a dramatic addition to this topic PI3K, depends on the noncanonical system function that adequate popula- by providing compelling evidence that (or alternate) pathway of NF-kB (re- tions of recirculating B and T lympho- BAFF signaling impinges on components viewed (Rickert et al., 2011)). Although cytes bearing unique antigen-binding of the BCR itself and requires the the intersections of these two path- receptors are maintained in peripheral kinase Syk, a key enzyme downstream ways were reviewed recently (Mackay lymphoid organs. The maintenance of of the BCR (Schweighoffer et al., 2013). et al., 2010; Rickert et al., 2011), it mature B cells in this steady state Thus our understanding of BAFF-driven is sufficient here to note the following. requires the continuous receipt and B cell survival appears to be incomplete First, activation of PI3K is necessary appropriate processing of environmental and the signaling pathways emanat- and sufficient for BCR-mediated survival cues. There are two receptors expressed ing from the BAFFR and BCR might and necessary but insufficient for by B cells that are crucial for maintaining share common, crucial, ligand-proximal BAFFR-mediated survival. Second, acti- B cell survival in the periphery; the clon- elements. vation of PI3K and ERK downstream of ally restricted B cell receptor (BCR) for BAFFR and BCR signaling are BAFF is biphasic with early and late antigen and the general receptor for complex and multifaceted, not all of peaks, the latter requiring noncanonical BAFF (B cell activating factor; BLyS), which are required for B cell survival NF-kB signaling (Otipoby et al., 2008; a member of the TNF superfamily of (Figure 1). Elegant experiments showed Patke et al., 2006). Third, withdrawal of receptors. Although originally considered conclusively that the survival com- BAFF or the BCR has different conse- independent, the redundancy, intersec- ponent of BCR signaling could be quences; BCR deficiency leads to rapid tion, and crosstalk between the signaling substituted completely and exclusively loss of all B cells while loss of BAFF pathways triggered by these two media- by a constitutively active form of the leaves B1 cells in the peritoneal cavity tors of B cell viability have been matters lipid-metabolizing enzyme, phosphoi- unaffected. Fourth, BCR stimulation is of debate over many years (Mackay nositide-3-kinase (PI3K) (Srinivasan required to upregulate BAFFR during et al., 2010; Rickert et al., 2011). In this et al., 2009). Survival signaling through development, and fifth, the canonical issue of Immunity, Schweighoffer et al. BAFFR, however, despite activating NF-kB pathway downstream of the

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These data support the conclusion that BAFF sustains B cell survival in vivo by mandatorily activating Syk. Is this so and if so, how, given that Syk activation requires phosphorylated ITAMs as dock- ing sites for its two tandem SH2 domains? Schweighoffer et al. found that BAFF stimulation of B cells did indeed induce the rapid phosphorylation of Syk (Fig- ure 1). As for the ITAM, Schweighoffer et al. provide data that Ig-a, a BCR signaling subunit, is tyrosine phosphorylated in response to BAFF (Figure 1). Although the mechanics behind BAFF- dependent Ig-a phosphorylation remain to be determined, Schweighoffer et al. show that inhibition of Src-family kinases Figure 1. BAFFR Signaling Incorporating the Activation of Syk and Leading to B Cell Survival impairs BAFF-induced Syk activation BAFF binding to BAFFR leads to the early (min; blue) activation of Syk through the activation of Src family and consequently in vitro B cell survival kinases (Src) and by binding to the phosphorylated ITAM of Ig-a. Active Syk leads to the late activation of PI3K and Akt (hr; orange), which promotes survival (green) by inhibiting proapoptotic proteins. Syk is also in the presence of BAFF (Figure 1). involved in BAFF-mediated activation of Erk1/2 (early) and the canonical NF-kB pathway (not shown). By showing a requirement for Syk, BAFF also induces Syk-independent activation of the noncanonical NF-kB pathway, leading to prosurvival Schweighoffer et al. have discovered, gene expression (green) and late PI3K activation (orange). at a minimum, exciting and unexpected new steps in BAFF signaling involving Src family kinases, phosphorylated BCR produces NF-kB p100, a sub- offer et al. show that Syk is required in ITAMs, and Syk, finishing in PI3K and strate for the noncanonical pathway B cells for BAFF to increase baseline MAPK. As the authors note, other ITAMs activated by BAFF. Whereas most of phosphorylation of Akt, a known target to which Syk can bind could contribute the debate on the interrelationship of PI3K. The conditional deletion of to BAFF signaling, but if one accepts the between BAFF and BCR has focused PTEN or PDK1 provided additional implication that following exposure to on the endlessly fascinating topic of the evidence placing PI3K downstream of BAFF, phosphorylated Ig-a is a point of canonical and noncanonical NF-kB path- BAFF and Syk. recruitment and activation of Syk, then ways (Mackay et al., 2010; Rickert et al., Several ‘‘simple’’ explanations for the this study puts a new complexion on 2011), the activation of the lipid-metabo- requirement for Syk in BAFF signaling the concept of crosstalk between BCR lizing enzyme PI3K by BAFF remains were excluded by Schweighoffer et al. and BAFFR. It suggests that BAFF unexplained. with varying degrees of certainty. BAFFR might activate both its own well-defined Schweighoffer et al. induced the loss expression and activation of the nonca- pathway and several facets of the path- of Syk from mature B cells and found nonical pathway of NF-kB were shown ways arising from the BCR, possibly that it was required to transmit survival not to be limiting in the absence of Syk activating both survival components. signals in follicular and marginal zone for B cell survival and BAFF responsive- There remain important questions in B cells but not B1 cells. The similarity ness. Because stimulation along the addition to the mechanical ones already with BAFF deficiency prompted exami- BCR and BAFFR pathways induces mentioned. Is there any role in this nation of the involvement of Syk in BAFF phosphorylation of ERK, the authors process for TACI, an alternate receptor responsiveness in vitro and this, unex- examined whether constitutively active for BAFF expressed by B cells? Is pectedly, was found to be deficient. MEK1 (caMEK1) could counteract the the PI3K activity induced by Syk by Continuing with unexpected outcomes, loss of Syk. Somewhat curiously, they BAFF sufficient for survival in the absence Schweighoffer et al. also discovered found that the ectopic expression of of a BCR signal or are there different, that BAFF induced the rapid phospho- caMEK1 enhanced the in vivo survival early and late, pools of PI3K activated rylation of both Syk and the BCR signal- of Syk-deficient B cells but had no by these two receptors that combined ing subunit, Ig-a (Figure 1), establishing effect on BAFF responsiveness in vitro, mediate survival? Does the biphasic a very proximal connection between suggesting a partial involvement in activation of PI3K by BAFF reflect Syk BAFF and the BCR. Previous identifi- modulating B cell survival. Lastly, the acting early and noncanonical NF-kB cation of PI3K as being activated by requirement for Syk activity, as opposed late (Figure 1)? Although unraveling both BCR and BAFF and as mediating to Syk structure, in mediating BAFF- these two apparently intimately linked the BCR survival signal (Srinivasan dependent B cell survival was demon- pathways is a technically challenging et al., 2009) suggest that PI3K could strated by ectopically expressing Syk or proposal, it remains crucial in deter- represent the outcome of some crucial Syk kinase dead (KD) in both control mining whether BCR tonic signaling is crosstalk in these two survival pathways and Syk-deficient B cells (Schweighoffer independent of, or commandeered by, (Figure 1). Consistent with this, Schweigh- et al., 2013). BAFF.

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REFERENCES sky, A., and Rajewsky, K. (2008). Proc. Natl. Schweighoffer, E., Vanes, L., Nys, J., Cantrell, D., Acad. Sci. USA 105, 12435–12438. McCleary, S., Smithers, N., and Tybulewicz, V.L.J. (2013). Immunity 38, this issue, 475–488. Mackay, F., Figgett, W.A., Saulep, D., Lepage, M., Patke, A., Mecklenbra¨ uker, I., Erdjument-Brom- and Hibbs, M.L. (2010). Immunol. Rev. 237, age, H., Tempst, P., and Tarakhovsky, A. (2006). 205–225. J. Exp. Med. 203, 2551–2562. Srinivasan, L., Sasaki, Y., Calado, D.P., Zhang, B., Paik, J.H., DePinho, R.A., Kutok, J.L., Kearney, Otipoby, K.L., Sasaki, Y., Schmidt-Supprian, M., Rickert, R.C., Jellusova, J., and Miletic, A.V. (2011). J.F., Otipoby, K.L., and Rajewsky, K. (2009). Cell Patke, A., Gareus, R., Pasparakis, M., Tarakhov- Immunol. Rev. 244, 115–133. 139, 573–586.

Basophil-Macrophage Dialog in Allergic Inﬂammation

Yu Hui Kang1 and Subhra K. Biswas1,* 1Singapore Immunology Network (SIgN), Agency for Science, Technology & Research (A*STAR), #04-01 Immunos, 8A Biomedical Grove, Singapore 138648, Singapore *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2013.02.015

Monocyte-macrophage differentiation under pathological conditions is poorly understood. In the present issue of Immunity, Egawa et al. (2013) report how basophils drive the differentiation of inﬂammatory monocytes into M2 macrophages, thereby regulating allergic skin inﬂammation.

Monocytes and macrophages can per- IL-10) trigger the M2 state, which and CCR2+ basophils and expression form a diverse range of functions under dampens excessive inflammation and of CCR2 ligands CCL8 and CCL12. homeostatic and pathogenic conditions. promotes Th2 cell responses, tissue re- Based on these observations, a key role This is facilitated by the existence of modeling, protumor functions, and clear- for CCR2 in the recruitment of these distinct subsets with specific functions ance of parasites (Biswas and Mantovani, cells and the development of IgE-CAI and the ability to switch between dif- 2010). However, M1 and M2 represent was suggested. Contrary to this expecta- ferent functional phenotypes in response two extremes of a spectrum of macro- tion, Ccr2À/À mice showed exacerbated to microenvironmental cues. Two main phage functional states that may exist IgE-CAI with abolished monocyte-macro- monocyte subsets have been recognized, in vivo. phage infiltration in the skin lesion but namely the Ly6ChiCCR2+ and Ly6Clo A key question arising from the above enhanced infiltration of basophils (as CX3CR1+ monocytes in mice and the context is the relationship between the well as neutrophils), suggesting CCR2 to corresponding CD14+CD16À and CD16+ monocyte subsets and the macrophage be dispensable for basophil recruitment. CD14dim monocytes in humans (Geiss- activation states. In addition, the in vivo Further investigation profiling monocyte- mann et al., 2010). The Ly6ChiCCR2+ microenvironmental cues and interact- macrophages from IgE-CAI skin lesions ‘‘inflammatory’’ monocytes infiltrate in- ing cells that induce monocyte-macro- of wild-type mice revealed that the flamed tissue, produce inflammatory phage differentiation to distinct functional majority of these cells expressed the cytokines, and are involved in host re- phenotypes, especially under patho- inhibitory molecule and M2 marker, Pro- sponse to infection. The Ly6CloCX3CR1+ logical settings, is poorly understood. grammed Death 1 ligand 2 (PD-L2), monocytes ‘‘patrol’’ blood vessels under Addressing these areas, Egawa et al. together with other M2 genes like Arg1, steady state, extravasate during tissue (2013) in the present issue of Immunity Chi3l3, and Fizz1, indicating an M2 injury, and promote tissue repair and investigate monocyte-macrophage differ- phenotype (Figure 1). Experiments via healing. Similarly in macrophages, at least entiation in immunoglobulin E (IgE)-medi- adoptive transfer of CD115+ bone marrow two main types of functional activation ated skin allergic inflammation (IgE-CAI), monocytes or inflammatory monocytes states exist: the M1 (or classically acti- uncovering a crucial role for basophils confirmed that inflammatory monocytes vated) and the M2 (or alternatively acti- therein (Figure 1). infiltrate skin lesions and differentiate vated) macrophages. Microbial stimuli In mice sensitized with allergen- into M2 macrophages, which in turn and T helper 1 (Th1) cell-related cytokines specific IgE, IgE-CAI was characterized dampened the allergic inflammation, as (such as IFN-g) trigger the M1 phenotype, by late-phase ear swelling after intra- demonstrated by their ability to attenuate which promotes inflammation, Th1 cell dermal allergen challenge and was the exacerbated IgE-CAI in Ccr2À/À responses, and microbicidal-tumoricidal dependent on basophils. The skin animals. functions. Th2 cell-related cytokines lesions showed increased infiltration of Several scenarios have emerged on the (such as interleukin-4 [IL-4], IL-13, and Ly6C+CCR2+ monocyte-macrophages relationship between monocyte subsets

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