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possibility remains that humans evolved a more stringent control system for ALR inflammasomes than that required in mice. Disease states such as systemic lupus erythematous (an autoimmune disease that most commonly targets host dsDNA) and Aicardi-Goutieres (a congenital neurological disease that stems from hyperactivation of inflam-matory DNA-response pathways)2 might exem-plify why a dedicated system of DNA-response safeguards is indeed needed in humans.

COMPETING FINANCIAL INTERESTSThe authors declare no competing financial interests.

Despite having AIM2 and other NLRs that form inflammasomes, mice do not have POPs or COPs. However, mice also do not demon-strate evidence of uncontrolled and persistent inflammasome activity. That puzzling observa-tion indicates that either mice have alternative strategies to restrain excessive activity of the ALR inflammasome or they do not need such regu-latory mechanisms. The authors1 point out that mice (but not humans) have a potential alterna-tive AIM2 antagonist, p202, which has a HIN-200 domain11. Other potential molecules that could fulfill the function of POP3 are proposed, but the

1. Khare, s. et al. Nat. Immunol. 15, 343–353 (2014).2. Hornung, v. & Latz, e. Nat. Rev. Immunol. 10,

123–130 (2010).3. Jin, T. et al. Immunity 36, 561–571 (2012).4. Kerur, N. et al. Cell Host Microbe 9, 363–375 (2011).5. Atianand, M.K., Rathinam, v.A. & Fitzgerald, K.A. Cell

153, 272–272.e1 (2013). 6. Le, H.T. & Harton, J.A. Front. Immunol. 4, 275 (2013). 7. Johnston, J.B. et al. Immunity 23, 587–598

(2005).8. stehlik, C. et al. Biochem. J. 373, 101–113

(2003).9. Dorfleutner, A. et al. Infect. Immun. 75, 1484–1492

(2007).10. Rathinam, v.A.K. et al. Nat. Immunol. 11, 395–402

(2010).11. Roberts, T.L. et al. Science 323, 1057–1060 (2009).

ILCs in the zoneGabriel D Victora

Innate lymphoid cells, marginal reticular cells and B cell–helper neutrophils interact to promote antibody secretion by B cells in the marginal zone of the spleen in humans and mice.

Gabriel D. victora is with the whitehead

Institute for Biomedical Research, Cambridge,

Massachusetts, USA.

e-mail: victora@wi.mit.edu

Innate lymphoid cells (ILCs) are a diverse population of cells that have lymphoid

characteristics but lack rearranged antigen receptors. ILCs have been ‘distilled’ into three groups: ILC1 cells, which include classical natural killer cells, express the transcription factor T-bet and produce the T helper type 1 cytokine interferon-γ; ILC2 cells, which include nuocytes and natural helper cells, express the transcription factor GATA-3 and produce typical T helper type 2 cytokines; and ILC3 cells, which include lymphoid tissue–inducer cells, express the transcription factor RORγt and are associated with the gastroin-testinal mucosa1. Reflective of their pheno-typic variability, ILCs have important roles in several aspects of the immune system, from killing transformed cells to inducing the for-mation of lymphoid tissue during embryo-genesis. In this issue of Nature Immunology, Magri et al. propose yet another function of ILCs in the immune response: enhanc-ing the production of T cell–independent (TI) antibodies by B cells in the marginal zone (MZ) of the spleen2.

MZ B cells are a subset of splenic B lym-phocytes defined mainly by their anatomical location, peripheral to the B cell follicle where traditional follicular B cells reside. The phenotypes and features of these cells differ in mice versus humans. In mice, MZ B cells are a well-defined population of non-recirculating

IgM+IgDloCD21hiCD23lo B cells that are thought to be a lineage separate from follicular B cells. Although they share certain phenotypic characteristics with their mouse counterparts, human MZ B cells recirculate freely and are somatically hypermutated, which suggests a memory B cell origin3. Despite such differ-ences, MZ B cells are believed to have a con-served role in both species. Their localization in the MZ, the macrophage-rich border of the splenic white pulp through which blood drains from fenestrated arterioles into venous sinuses, ensures that these B cells are promptly exposed to any antigens entering the bloodstream. MZ B cells thus function as first responders to blood-borne pathogens, producing most of

the low-affinity TI type 2 antibody response that bridges the gap between infection and the production of T cell–dependent antibodies of higher affinity.

Unlike follicular B cells, which after receiv-ing the first signal delivered by the recognition of antigen by the B cell antigen receptor require a second signal from T cells to become fully activated, MZ B cells are thought to rely on other sources of additional stimulation. Various infectious non-self signatures can serve that role. Those include Toll-like receptor (TLR) ligands and the periodic spacing of epitopes characteristic of certain pathogen-derived molecules and of the prototypical TI type 2 model antigen, haptenated Ficoll. Likewise,

Figure 1 ILCs stimulate antibody production by MZ B cells. A three-way interaction among ILC3 cells, MRCs and marginal zone B cells (BMZ cell) enhances TI antibody production in the spleen, a process further aided by ILC-driven activation of B cell–helper neutrophils (NBH). BAFF, B cell–activation factor; APRIL, proliferation-inducing ligand; DLL1, Notch ligand; TNF, tumor-necrosis factor; Lt, lymphotoxin; AsC, antibody-secreting cell.

BAFF APRIL DLL1

APRILTNFLT

Marginal zone

Red pulp

Follicle

IL-7

IL-23IL-1β

GM-CSF

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ILC3 cell

MRC

NBH cell

BMZ cell ASC

TLR ligands

IgMIgGIgA

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metabolism. In DCs or macrophages acti-vated by the Toll-like receptor 4 (TLR4) ago-nist lipopolysaccharide (LPS) over the course of 24 hours, profound metabolic repro-gramming occurs. Notable events include enhanced glycolysis, the accumulation of succinate and the biosynthesis of fatty acids from citrate1–3. The enhanced glycolysis occurs in part because of the generation of nitric oxide, which disables oxidative phos-phorylation in the mitochondria2. Glycolysis is needed to generate sufficient ATP to keep the DC functioning4. The accumulation of succinate has been shown to lead to activa-

experiments with genetic and antibody-mediated depletion show that ILC-derived cytokine GM-CSF increases number of neu-trophils, which in turn increases the number of antibody-secreting cells and serum concentra-tion of IgG3. Together the mouse data largely confirm the authors’ findings obtained with the human system, which supports the notion that ILCs have a role in enhancing the responsiveness of MZ B cells.

Relying on an effective strategy for immu-nological investigation—identifying cellular populations and functions in human and testing their hypotheses by depleting mice of those populations—Magri et al. reveal a com-plex web of intercellular interactions, centered on splenic ILC3-like ILCs, that facilitates anti-body production by MZ B cells in both TI type 1 settings and TI type 2 settings2. Such multi-tered control may be responsible for lowering the threshold of the activation of MZ B cells to TI antigens in the presence of pathogen-related stimuli, increasing responsiveness to infectious non-self while maintaining toler-ance to self antigens.

COMPETING FINANCIAL INTERESTSThe author declares no competing financial interests.

1. spits, H. et al. Nat. Rev. Immunol. 13, 145–149 (2013).

2. Magri, G. et al. Nat. Immunol. 15, 354–364 (2014).3. Pillai, s., Cariappa, A. & Moran, s.T. Annu. Rev.

Immunol. 3, 161–196 (2005).4. Puga, I. et al. Nat. Immunol. 13, 170–180 (2012).5. sun, Z. et al. Science 288, 2369–2373 (2000).6. sonnenberg, G.F., Monticelli, L.A., elloso, M.M., Fouser, L.A. &

Artis, D. Immunity 34, 122–134 (2011).

receptor, these experiments more closely model a polyclonal TI type 1 response.

Magri et al. then embark in a series of mouse experiments that use genetic and antibody-mediated depletion of ILCs to determine the role of those cells in antibody production by MZ B cells in vivo2. Depleting mice of ILCs without affecting other cells is a complex task because genes expressed exclusively in this population are yet to be discovered. To circumvent that, the authors use two comple-mentary approaches. Given the dependence of ILC3 cells on RORγt, their first strategy uses RORγt-deficient (Rorc–/–) mutant mice as a strategy for depletion of this cell popu-lation. However, in addition to lacking ILC3 cells, Rorc–/– mice also lack peripheral lymph nodes, Peyer’s patches and the TH17 subset of helper T cells5, issues that the authors address only partly by the reconstitution of irradiated Rorc–/– hosts with a mixture of recombination- activating gene 1–deficient (Rag1–/–) bone marrow and wild-type or Rorc–/– bone marrow or by comparison of Rorc–/– mice with Rorc–/–Cd3e–/– double-deficient mice. To circumvent those caveats, they use a second model in which they reconstitute ILCs in Rag1–/– mice with Thy-1.1+ lymphocytes, followed by depletion with a Thy-1.2-specific antibody6. In both models, the absence of ILCs negatively affects the ability of mice to pro-duce immunoglobulin G3 (IgG3), an isotype commonly associated with TI responses, both spontaneously and in response to nitrophenyl-Ficoll (the last a TI type 2 response). Further

input from other cells, such as macrophages and dendritic cells, can serve a role in enhancing the antibody responses of MZ B cells, lifting some of the burden of antigen recognition from the MZ B cell alone. Magri et al. now add ILCs to that roster of accessory cells2.

The authors begin by describing a small sub-set of ILCs that reside mainly in the MZ of the human spleen2. These cells express CD127 and CD117 (the receptors for interleukin 7 (IL-7) and stem-cell factor, respectively), as well as the transcription factor RORγt, and produce IL-22 in response to IL-1β and IL-23, all of which places them in the mucosal cell–like ILC3 category. MZ ILCs are in close apposi-tion to stromal cells that express the integ-rin ligand MAdCAM-1, which the authors propose are the human equivalent of mouse marginal reticular cells (MRCs). Human MRCs express TLR3, TLR4 and TLR9, which makes them good candidates for the role of sentinel cells that help engage ILCs in the immune response. Through the use of an elegant com-bination of histological staining and ex vivo coculture of human splenic cells, the authors identify an intriguing three-way interaction in which MRCs and ILCs synergistically amplify a signal from TLR ligands to enhance the acti-vation of MZ B cells and their differentiation into antibody-secreting cells (Fig. 1). That signal is boosted by ILC-driven activation of MZ-resident neutrophils, which further pro-mote antibody production by MZ B cells4. Because antibody production happens in the absence of stimulation via the B cell antigen

tion of the transcription factor HIF-1α and the induction of HIF-1α-dependent genes2. Citrate is withdrawn from the Krebs cycle for the biosynthesis of fatty acids, with one out-put of this diversion being the production of prostaglandins3, key proinflammatory lipids. In this issue of Nature Immunology, Everts et al. provide the most detailed description yet of the rapid metabolic changes that occur in LPS-activated DCs5. They demonstrate that glycolysis is activated within minutes and ultimately generates citrate, which, as shown before, leads to the synthesis of fatty acids and prostaglandins but is also needed

Glycolytic reprogramming by TLRs in dendritic cellsLuke A J O’Neill

The activation of dendritic cells by Toll-like receptors leads to a rapid enhancement in glycolysis. Glucose is metabolized to pyruvate and from there to citrate in the mitochondria, which leads ultimately to membrane biosynthesis in the endoplasmic reticulum and Golgi to support the activation of dendritic cells.

Luke A.J. O’Neill is with the Trinity Biomedical

Sciences Institute, School of Biochemistry and

Immunology, Trinity College Dublin, Dublin, Ireland.

e-mail: laoneill@tcd.ie

To immunologists, the activation of den-dritic cells (DCs) means the capture and

processing of antigens, followed by presenta-tion of antigens by major histocompatibility complex molecules, as well as the induc-tion of various cytokines, chemokines and costimulatory molecules. To biochemists, however, the activated DC is turning out to be a veritable treasure trove of altered

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