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Immunologists gather at Asilomar each January in a pleasant and informal setting to discuss recentfindings on the development and regulation of immune responses.

A season for midwinter immunologyJonathan Sprent1, Christel H. Uittenbogaart2 and Pamela J. Fink3

1The Scripps Research Institute, Department of Immunology, 10550 N.Torrey Pines Road, La Jolla, CA 92037, USA. 2University of California, Los Angeles School of Medicine,Departments of Pediatrics, Microbiology, Immunology and Molecular Genetics, Los Angeles, CA 90095, USA. 3University of Washington, Department of Immunology, Box357650, Seattle,WA 98195, USA. (jsprent@scripps.edu and pfink@u.washington.edu).

The Midwinter Conference of Immunologists was founded in 1962by a small group of immunologists, among them Dan Campbell andRay Owen. For the last 39 years, immunologists from all over theUSA and from abroad have gathered annually in the (usually) mildclimate of Monterey, CA, on the historic Asilomar grounds. The 41st

meeting, held in January 2002, was broadly centered on the devel-opment and function of the immune system. Outlined below areonly a few of the many topics covered by the meeting.

Shaping the T and B cell repertoires by selfThe capacity to mount vigorous responses to foreign antigens whiletolerating self-components is one of the hallmarks of the adaptiveimmune system. Until recently, it has generally been assumed thatT cells with overt autoreactivityare destroyed in the thymus; deathis presumed to reflect the fact that,unlike mature T cells, immature T cells are unable to withstandstrong T cell receptor (TCR) sig-naling. Despite the wealth of evi-dence for such intrathymic nega-tive selection, it is now apparentthat some autoreactive thymocytescan avoid death by changing theirspecificity through secondary TCRgene rearrangement. As discussedby David Nemazee (The ScrippsResearch Institute), this process ofreceptor editing is well document-ed for B cells1. For T cells, KristinHogquist (University of Minnesota) presented evidence that expres-sion of a new TCRα chain via receptor editing occurs when ovalbu-min (OVA)-specific OT-I TCR–transgenic (Tg) thymocytesencounter OVA in the thymus2. However, in two other TCR-Tglines, 2C and HY, intrathymic contact with antigen led to clonaldeletion rather than receptor editing. This finding was confirmed byKlaus Rajewsky (Harvard Medical School) using HY-specificTCRα knock-in TCRβ-Tg mice, whose thymocytes are also unableto edit their TCRs in the face of antigen. For B cells, Nemazee sug-gested that the developmental stage at which cells encounter anti-gen is important: late contact with antigen induces deletion andearly contact causes receptor editing1. It is therefore surprising thatthe mode of tolerance (receptor editing versus deletion) in the threeTCR-Tg lines examined by Hogquist occurred independently ofwhether antigen was expressed ubiquitously in a membrane-boundform (actin promoter) or selectively as a peptide in thymic epithe-lial cells (K14 promoter). One possibility is that the disparateresults reflect differences in the time at which TCRα expression

first occurs: early expression (HY, 2C) favors deletion and laterexpression (OT-I) permits receptor editing. Because TCRα expres-sion in OT-I Tg and normal mice is similar, the data on OT-I Tgmice could well be applicable to the normal thymus.

A second and likely related nondeletional mechanism of thymocytetolerance has emerged from studies on the origin of CD25+CD4+ Tregulatory (TR) cells that, in the periphery, play an important role inpreventing autoimmune disease3. Andrew Caton (The Wistar Institute)has shown that intrathymic contact with antigen causes a proportionof autoreactive T cells, including high-affinity cells, to evade deletionand differentiate into TR cells. TCR stimulation of TR cells exportedfrom the thymus does not induce proliferation, perhaps because ofchronic antigenic exposure in vivo, but instead enables the cells to

inhibit the function of other T cells4,5.Unresolved is the key issue of how TR

cells evade intrathymic clonal deletiondespite their high affinity for self-anti-gens, although receptor editing may beinvolved. If so, there is a striking par-allel between thymic selection ofCD4+ TR cells and OT-I CD8+ T cells(see above). However, further infor-mation will be required to define therules governing thymic deletion versusreceptor editing.

Peripheral toleranceAs the result of clonal deletion andreceptor editing, most thymocyteswith strong self-reactivity for

intrathymic antigens are destroyed locally or undergo a change intheir specificity and function. However, low-affinity T cells survivethymic selection and are exported to the periphery. Here, the cellsmake continuous contact with a range of self-antigens expressed onantigen-presenting cells (APCs). Because most of these antigens areexpressed throughout the body, including the thymus, the post-thymic repertoire is largely depleted of T cells with overt reactivityfor these ubiquitously expressed antigens. As the result of self-pep-tide–mediated positive selection in the thymus, however, mature T cells do retain covert responsiveness for these ligands. Indeed,low TCR recognition of self-ligands on APCs plays a vital role inkeeping mature post-thymic T cells alive6. Such recognition is nor-mally too weak to cause entry into the cell cycle—thus ensuringself-tolerance—but is presumed to be sufficient to induce continu-ous synthesis of life-sustaining anti-apoptotic molecules. In supportof this idea, Rajewsky used Cre-assisted gene targeting to show thatinhibiting TCR expression on naïve T cells curtails the survival ofthese cells, especially CD8+ T cells7. Similar findings apply to

The Asilomar beach on a January day.

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http://immunol.nature.com • may 2002 • volume 3 no 5 • nature immunology 415

B cells, although here it is uncertain whether BCR signaling reflectscontact with self-antigens or is simply a byproduct of intact receptorexpression per se.

For T cells, a corollary of the view that peripheral T cells engage incovert interaction with self-ligands is that increasing the density ofthese ligands on APCs might enhance TCR signaling and drive matureT cells to proliferate and differentiate, thus breaking self-tolerance. Formost self-antigens, this scenario is unlikely because the density ofthese ligands on APCs is presumed to be relatively stable. However,for certain self-antigens, namely endogenous superantigens encodedby defective mammary tumor viruses (Mtv), their expression tends tobe low at birth and then increases with age. Yet delayed expression ofMtv antigens does not break self-tolerance to these antigens. Why?

In examining this question, Pamela Fink (University ofWashington) studied post-thymic tolerance to a weak Mtv antigen,Mtv-8, which is recognized by Vβ5+ T cells. Her surprising observa-tion is that mature peripheral CD4+ T cells from Vβ5-Tg mice under-go an age-dependent process of TCR revision (editing). This editingis characterized by down-regulation of Vβ5 surface expression, re-expression of recombination-activating genes and rearrangement andexpression of a diverse Vβ repertoire8. By this process, chronicrecognition of self-antigens can induce responsive T cells to losereactivity to these antigens and form a diverse pool of cells reactiveto other antigens. Whether TCR revision can target T cells specificfor non-Mtv self-antigens is still unclear9.

Recently, it has become apparent that T cell tolerance to ubiqui-tous self-antigens is abrogated when total T cell numbers arereduced below a certain threshold10. Under these conditions, typi-cal self-ligands on APCs become overtly immunogenic and causeresidual naïve T cells to proliferate and differentiate into effector T cells. This process of “homeostatic” expansion does not seem toreflect an increase in self-ligand expression, but instead may resultfrom increased concentrations of a stimulatory cytokine, inter-leukin 7 (IL-7)11,12. Charles Surh (The Scripps Research Institute)suggests that IL-7 concentrations are normally kept low throughabsorption by T cells, but rise appreciably when T cell numbersare reduced. Under these conditions, an increase in IL-7 potenti-ates (costimulates) T cell reactivity to self-ligands and causes T cells to mount overt proliferative responses to these ligands. Insupport of this model, Surh showed that contact with self-ligandsin IL-7–Tg mice causes wide-scale differentiation of T cells with amemory phenotype.

As discussed above, tolerance to ubiquitous self-ligands involvesmultiple mechanisms, including the culling of high-affinity T cellsin the thymus, revision of TCRs on T cells that make late post-thymic contact with antigen and the maintenance of stimulatorycytokines in low concentrations. The situation for sequestered self-antigens—such as those expressed selectively in the brain, skin andpancreatic islets—is quite different. Thus, unlike ubiquitous ligands,sequestered self-antigens are prime targets for autoimmune disease.In work described by Hogquist, selective expression of OVA peptidein thymic epithelium and skin not only leads to intrathymic receptorediting of OT-I TCR–Tg thymocytes, but causes peripheral T cellsto mount autoimmune disease directed to OVA expressed in theskin2. This syndrome is not seen in mice in which membrane-boundOVA is expressed in all tissues.

Despite these unexpected findings with OT-I mice, unresponsive-ness to sequestered self-antigens may, for the most part, simplyreflect that being confined to the recirculating lymphocyte pool,nontolerant naïve T cells do not make contact with these antigens.

In support of this idea—termed T cell ignorance—Diane Mathis(Harvard Medical School) found no evidence for diabetes inductionin a mouse model in which a neoantigen, β-galactosidase, wasexpressed continuously or conditionally in β cells; insulitis wasundetectable, yet there was no tolerance of CD4+ or CD8+ T cells orB cells. In light of this finding, how is organ-specific autoimmunityinitiated? An attractive possibility is that autoimmune responsesbegin when parenchymal cells in the relevant organ die, thus releas-ing their sequestered self-antigens into the draining lymph node(LN); T cells then recognize the antigens and initiate an autoim-mune response. This scenario could explain why, in other models,inserting neoantigens in pancreatic β cells sometimes induces T pro-liferative responses in the draining LN13. However, these responsesare often abortive and lead to rapid T cell death, perhaps reflectingthat the APCs in resting LNs are not activated. Hence, as for foreignantigens (see below), breaking tolerance to self-antigens may hingenot only on T cell recognition of these antigens on APCs (overcom-ing ignorance) but also on APC activation, for example by a virus.

Fate of activated lymphocytes and memory cellsReflecting the need to rapidly eliminate pathogens, the primaryimmune response to foreign peptide present on adjuvant-activatedAPCs is intense and involves marked clonal expansion and differen-tiation of antigen-specific T and B cells. At the end of the response,most effector cells are destroyed, but some of these cells survive tobecome long-lived memory cells. For T cells, Philippa Marrack(National Jewish Medical and Research Center) showed that the fate(death versus survival) of effector cells hinges on the balance ofintracellular expression of anti-apoptotic (Bcl-2, Bcl-xL and Bcl-3)and pro-apoptotic (Bim and reactive oxygen species) molecules14;based on studies with Bim–/– mice, Bim seems to be crucial foreffector cell elimination.

For B cells, differentiation of antigen-stimulated cells into long-lived memory B cells occurs in germinal centers and involves anti-gen-driven affinity maturation. This process requires somatic hyper-mutation and leads to preferential survival of high-affinity B cellsand their differentiation into memory cells. As discussed by TasukuHonjo (Kyoto University), somatic hypermutation, immunoglobulin(Ig) class–switching and gene conversion in B cells are now knownto be controlled by activation-induced cytidine deaminase (AID), anenzyme that may edit the RNA encoding a nickase essential to theseprocesses15,16. In a surprising development, Honjo’s lab has succeed-ed in inducing efficient class switch recombination of a construct infibroblasts expressing AID17. This system has clear potential for iso-lating AID cofactors and defining the precise role of AID in lym-phocyte-specific DNA rearrangement events.

At the end of the immune response, a small proportion of specificT and B cells survives to form long-lived memory cells18. Studies bySurh, Rajewsky and others have shown that, unlike naïve cells, thesurvival of memory T cells is largely major histocompatibility com-plex–independent. For CD4+ memory T cells, the factors maintain-ing their long-term survival are obscure. However, for CD8+ memo-ry T cells, Jonathan Sprent (The Scripps Research Institute)described how the survival of these cells requires contact with acytokine, IL-15. According to Leo LeFrancois (University ofConnecticut Health Center), IL-15 may also promote clonal expan-sion of CD8+ T cells during the primary response. Surh showed thatthe dependency of memory CD8+ T cells on IL-15 does not apply inIL-7–Tg mice. Thus, memory CD8+ T cells are rare in IL-15–/– micebut are markedly enriched in IL-7–Tg mice, including IL-15–/–

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IL-7–Tg mice. These findings indicate that, as for naïve T cells,numbers of memory CD8+ T cells are determined by the relativeconcentration of stimulatory cytokines. Whether the baseline synthe-sis of these cytokines is constitutive or reflects chronic weak stimu-lation of the innate immune system is unclear.

As for naïve T cells, cytokines presumably mantain the survivalof T memory cells by inducing up-regulation of anti-apoptotic mole-cules. However, direct data on this important issue are lacking.Likewise, the role of cytokines and other ligands in maintainingnaïve and memory B cell survival remains obscure.AcknowledgmentsWe thank all of the speakers at the 41st Midwinter Conference of Immunologists, andoffer our apologies to those who were not mentioned in this Commentary.

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427–438 (2001).6. Freitas,A.A. & Rocha, B. Annu. Rev. Immunol. 18, 83–111 (2000).7. Polic, B., Kunkel, D., Scheffold,A. & Rajewsky, K. Proc. Natl. Acad. Sci. USA 98, 8744–8749 (2001).8. Fink, P. J. & McMahan, C. J. Immunol.Today 21, 561–566 (2000).9. Huang, C.Y., Golub, R.,Wu, G. E. & Kanagawa, O. J. Immunol. 168, 3259–3265 (2002).10. Goldrath,A.W. & Bevan, M. J. Nature 402, 255–262 (1999).11. Schluns, K. S., Kieper,W. C., Jameson, S. C. & Lefrancois, L. Nature Immunol. 1, 426–432 (2000).12. Tan, J.T., et al. Proc. Natl. Acad. Sci. USA 98, 8732–8737 (2001).13. Heath,W. R. & Carbone, F. R. Nature Rev. Immunol. 1, 126–134 (2001).14. Mitchell,T. C., et al. Nature Immunol. 2, 397–402 (2001).15. Honjo,T., Kinoshita, K. & Muramatsu, M. Annu. Rev. Immunol. 20, 165–196 (2002).16. Arakawa, H., Hauschild, J. & Buerstedde, J. M. Science 295, 1301–1306 (2002).17. Okazaki, I., Kinoshita, K., Muramatsu, M.,Yoshikawa, K. & Honjo,T. Nature 416, 340–345 (2002).18. Sprent, J. & Surh, C. D. Annu. Rev. Immunol. 20, 551–579 (2002).

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