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between the γδ chains and the CD3 compo-nents must differ from those with the corre-sponding C regions of the αβ TCR. In someways this is quite surprising, but suggestssome further adaptation in this interactionthat may reflect formation of a different typeof signaling complex2. The ability of thesame receptor, in this case, to form differentinterfaces with multiple ligands is similar tothe plasticity demonstrated by the hemato-poietic receptors.

Therefore, we can now conclude from thisfirst example of a γδ TCR that it has manyantibody-like features, but also some αβ TCRinheritance. These structural features are notsufficient to evaluate which receptors arosefirst, but nevertheless reflect a remarkablyhigh degree of conservation in the structure

of these antigen-recognition molecules,despite 400–500 million years of evolution.The specific adaptations of the γδ TCR to itspresent day role in cellular immunity stillremains somewhat of a mystery and necessi-tates the structure determinations of specificcomplexes of γδ TCRs with their various lig-ands, as has been done so successfully forantibodies and now for αβ TCRs. Comparedto the process of somatic mutation in anti-bodies or for MHC-restriction combined withpositive or negative selection in αβ TCRs,how specificity for particular ligands devel-ops is not yet clear. The γδ TCR structurealready suggests that individual γ and δgermline sequences may have become adapt-ed for specific antigens, as for receptors inthe innate immune system.

Acknowledgements

We thank the Garboczi lab for providing the coordi-nates before publication.

1. Garcia, K. C.,Teyton, L. & Wilson, I. A. Annu. Rev. Immunol. 17369–397 (1999).

2. Allison,T. J.,Winter, C. C., Fournié, J. -J., Bonneville, M. &Garboczi, D. N. Nature 411, 820–824 (2001).

3. Hayday, A. C. Annu. Rev. Immunol. 18, 975–1026 (2000).4. Parker, C. M. et al. J. Exp. Med. 171, 1597–1612 (1990).5. Morita, C.T. et al. Immunity 3, 495–507 (1995).6. Belmant, C. et al. J. Biol. Chem. 274, 32079–32084 (1999).7. Chien,Y.-H., Jores, R. & Crowley, M. P. Annu. Rev. Immunol. 14,

511–532 (1996).8. Moody, D. B., Besra, G.,Wilson, I. A. & Porcelli, S. A. Immunol.

Rev. 172, 285–296 (1999).9. Li, H. et al. Nature 391, 502–506 (1998).10. Wilson, I.A. & Stanfield, R. L. Curr. Opin. Struct. Biol. 3, 113–118 (1993).11. Johnson, G. & Wu,T.T. Nucleic Acids Res. 29, 205–206 (2001).12. Berman, H. M. et al. Nucleic Acids Res. 28, 235–242 (2000).

Department of Molecular Biology and SkaggsInstitute for Chemical Biology,The Scripps ResearchInstitute, La Jolla, CA 92037, USA(wilson@scripps.edu or robyn@scripps.edu)

A basic tenet of B cell biology is that surface,membrane-bound, IgM (mIgM) is essential forB cell maturation and is a prerequisite for Bcell isotype switching. This view is supportedby studies of µMT mice, which are unable toexpress the membrane form of the immuno-globulin M (IgM) heavy chain (µm) so that Blineage cell development is arrested at the pro-B (pre-B1) cell stage1. In addition, B cells diewhen they are induced to stop expressing Bcell receptor (BCR) complexes2. Thus, BCRsare required for B cell survival. In this issue ofNature Immunology Macpherson et al.3 reportthat production of, and isotype class switchingto, IgA can occur in the absence of mIgM andT cells. Some of this B cell isotype switchingto IgA occurs in gut-associated lymphoid tis-sues and requires commensal bacteria in thegut.

Certain anatomical sites within mammalsare designed to support appropriate immuneresponses to pathogens within a particularmicroenvironment4,5. B cells are sequesteredto several sites, and each site has a set of cellsand elements designed for specialized res-ponses to pathogens (Fig. 1). Small, recirulat-ing mIgMloIgDhi B cells are located principal-ly in B cell follicles (FO). In the presence of aT cell–dependent (TD) antigen, FO B cellsproliferate and germinal centers (GCs) areformed. The principal function of the GC

B cells with the guts to switchEDWARD A. CLARK AND KEVIN L. OTIPOBY

Expression of mIgM was thought to beessential for the differentiation of B cellsexpressing antibodies of other classes. Newevidence suggests isotype class switching toIgA can occur in the absence of mIgM.

microenvironment is to generate memory Bcells in response to TD antigens. GC forma-tion, which requires receptors such as CD40,tumor necrosis factor receptor 1 (TNFR1) andCR2 (also called CD21), supports somaticmutation, antigen-driven selection, B cell sur-vival and isotype class switching. In contrast,nonrecirculating mIgMhiIgDlo/–CR2hi splenic Bcells reside in a distinct area in the spleen, themarginal zone (MZ), which surrounds B cellfollicles. The splenic MZ also contain special-ized macrophages, dendritic cells and endo-thelial cells. Unlike GCs, the major functionof the MZ is to generate a rapid defense toblood-borne pathogens, such as encapsulatedbacteria, that includes T cell–independent(TI) maturation of specific IgM-producing Bcells. Key elements required for MZ B cellsinclude the transmembrane activator and cal-cium modulator and cyclophilin ligand inter-actor (TACI)–B cell activating factor (BAFF,also called BlyS or zTNF4) receptor-ligandsystem, the NF-κB–inducing kinase (NIK)and Pyk-2 kinases and NF-κB itself4–6.

The data that Macpherson et al.3 reportunderscore the specialized nature of the gut,where extrafollicular B cells are the majorproducers of IgA (Fig. 1). Key players in thismicroenvironment include TI antigens, B1cells, transforming growth factor β (TGF-β)and NIK, which is defective in aly/aly mice4.

Although the main function of IgA is todefend the body against mucosal infections,some IgA is generated in response to com-mensal bacterial antigens. A previous studyfound that IgA responses induced by gut florado not require T cells, CD4 or TNFR17, whichare required for GC formation. In addition,production of commensal bacteria-specificIgA required gut lymphoid tissues such asPeyer’s patches (PPs) with B cell–containingintestinal lymphoid structures.

Subsequently, Macpherson et al.3 foundthat a number of µMT mice that they hadplanned to use as negative controls in factexpressed IgA but no other Ig class isotypes.Similar results were obtained with two inde-pendent µMT mouse colonies. Because mem-brane-bound δ (δm) can substitute in theabsence of µm, it was important for theresearchers to test whether µMT B cells wereable to make IgA simply because they couldexpress mIgD. By several criteria, this doesnot appear to be the case, although it is diffi-cult to rule out formally.

How is it that µMT B lineage cells canswitch isotype class? When put into an in vitroculture system to induce the maturation ofearly B cells, pro-B cells from recombina-tion–activating gene 2–deficient (RAG-2–/–)mice switch their isotype, at least to Cε, with-out rearrangement of variable VH segments or

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NEWS & VIEWS

expression of either the pre-BCR complex ormIgM8. However, this switch from Cµ to Cε

required overexpression of Bcl-2 to preventthe RAG-2–/– pro-B cells from dying. A com-bination of Bcl-2 along with appropriate stim-ulation (interleukin 4 + CD40) enabled pre-cursor B cells to undergo isotype switchingwithout expressing mIgM.

RAG-2–/– mice and JH–/– mice have no

mature B cells or detectable serum Ig, even ifBcl-2 is overexpressed in B lineage cells9.This contrasts with µMT mice expressingeither Bcl-29 or an lpr (CD95) mutation10 in Bcells, which do produce IgM, IgG and IgA.Apparently, if they are provided with anappropriate signal, such as Bcl-2, to block celldeath, developing B cells in µMT mice, unlikeB lineage cells from RAG-2–/– and JH

–/– mice,can circumvent a requirement for mIgMexpression by switching to another isotype.

The combination of an ability to transcribeheavy chains, a supportive environment inthe gut and the ability to express mIgA maybe all that is needed for µMT B cells toswitch, survive and express IgA. Becauseunmanipulated µMT B cells do not switch toother Ig classes3, they appear to receive somespecialized signal(s) in the gut. What mightthe signals be that normally help early Bcells to survive so they can class switch toIgA? One gut-associated signal may bedelivered by TGF-β via TGF-βR2, which isrequired for IgA induction11. Antigen isapparently another key element; the IgAsecreted in µMT mice is largely specific forcommensal bacteria in the ileum. Once mIgAis expressed in µMT B cells, specific anti-gens in the gut probably select and maintainmIgA+ cells. The mechanisms of this selec-tion may be similar to autoantigen-selectedB1 cells found in the peritoneum12.

Several questions remain to be addressed.Where does the switch occur in µMT B lin-eage cells? Do the precursor B cells switch tothe IgA constant region (Cα) in the bone mar-row and then home to the PPs? If so, is the Cµ

to Cα switch a stochastic process followed by

antigen selection in the PPs? Alternatively,does a subpopulation of precursor B cellshome to PPs before switching to Cα? The roleof commensal bacteria suggests that theswitch to Cα is an instructive process. But howthis might occur without previous expressionof mIgM and in the absence of T cells remainsa mystery. Another unresolved puzzle is whyµMT mice make antigen-specific IgA to gutbacteria but do not make specific antibodiesafter immunization via several routes withvarious antigens. It remains unclear just whatspecial conditions in the gut enable antigen-specific IgA responses to occur.

Although µMT mice can produce IgA orother isotypes3,9,10, JH

–/– mice cannot. Thus, theJH

–/– mouse is a more suitable model than theµMT mouse for testing whether or not B cellsare required for a particular phenotype. Inaddition, the conclusions of some studies with

µMT mice that ruled out a role for B cellsmay have to be reconsidered given that matureIgA+ B cells are clearly present in µMT mice,albeit in small numbers.

1. Kitamura, D., Roes, J., Kuhn, R. & Rajewsky, K. Nature 350,423–426 (1991).

2. Lam, K. P, Kuhn, R. & Rajewsky, K. Cell 90, 1073–1083 (1997).3. Macpherson,A. J. S. et al. Nature Immunol. 2, 625–631 (2001).4. Fagarasan, S. & Honjo,T. Science 290, 89–92 (2000).5. Martin, F. & Kearney, J. F. Curr. Opin. Immunol. 13, 195–201 (2001).6. Cariappa,A,. Liou, H. C., Horwitz, B. H. & Pillai, S. J. Exp.

Med.192, 1175–1182 (2000).7. Macpherson,A. J. et al. Science 288, 2222–2226 (2000).8. Rolink,A., Melchers, F. & Andersson, J. Immunity 5, 319–330

(1996).9. Tarlinton, D. M., Corcoran, L. M. & Strasser,A. Int. Immunol.

9,1481–1494 (1997).10. Melamed, D., Miri, E., Leider, N. & Nemazee, D. J. Immunol. 165,

4353–4358 (2000).11. Cazac, B. B. & Roes, J. Immunity 13, 443–451 (2000).12. Hayakawa, K. et al. Science 285, 113–116 (1999).

Departments of Immunology and Microbiology,University of Washington, Seattle,WA 98195, USA.(eclark@bart.rprc.washington.edu)

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Figure 1. Peripheral lymphoid tissues are located at strategic anatomical points and provide a sup-portive microenvironment for the development and/or survival of specific B cell subpopulations.These niches are ideally positioned to survey the body for different types of infectious agents and, with the helpof accessory cells located within the niche, promoting appropriate antibody responses.The signals required for thedevelopment of these B cell subpopulations are just now being elucidated.TD,T cell–dependent;Ag, antigen; FDC,follicular dendritic cell; DC, dendritic cell.

Fetalliver

Bone marrow Lymph node

Follicular B (mIgMloIgDhi)recirculatingIgM, IgA, IgE, IgG

MZ B (mIgMhiIgDlo/–)nonrecirculatingIgM

Peritoneal B (mainly B1)recirculatingIgM, IgA

TD, protein Ag,GCs, FDCs, DCs,CD40, CR2, CD19

TI, blood-borne bacterial Ag,MZ with DCs, macrophages,BLyS/BAFF, Pyk-2,NIK, NF-κB

TI, commensalbacterial Ag,macrophages,TGF-β, NIK, CD19

B cell Key elements

Spleen

Peyer'spatches

Bob

Cri

mi

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