NEWS & VIEWS

http://immunol.nature.com • july 2001 • volume 2 no 7 • nature immunology 573

Complete T cell activation requires two dis-tinct signals. Signal one requires engagementof the T cell receptor (TCR); it is followed bya second costimulatory signal. For the pastdecade, it has been thought that CD28 is theprincipal mediator of this second signal.However, in recent years, we have begun toappreciate that many molecules function inconcert to positively and negatively influenceT cell activation and differentiation.Although CD28 is essential for naïve T cellsurvival and differentiation, effector andmemory T cell responses are often CD28 cos-timulation–independent1. These findings ledresearchers to suspect that costimulatorymolecules, other than CD28, may play impor-tant roles in different types of T cell respons-es. In this issue of Nature Immunology,Hancock and colleagues report that theinducible costimulatory molecule (ICOS)plays a critical role in the pathway that regu-lates both chronic and T helper subset 1(TH1)-dependent acute allograft rejection2.Although the investigators do not examinethe basis for allograft blockade in this set-ting, the paper raises important questions asto the role of ICOS in primary and effectorT cell responses.

ICOS, cloned two years ago3, is a homod-imer of 55–60 kD that shares ∼ 20% homol-ogy with CD28. However, unlike CD28,ICOS is not present on naïve T cells; insteadit is up-regulated after T cell activation andretained on many memory T cells.Engagement of ICOS is particularly effec-tive in costimulating interleukin 10 (IL-10)and IL-4, but not IL-2, production. A seriesof studies have supported the concept thatICOS functions as a critical costimulatorypathway for TH2 responses. For example,ICOS blockade inhibits the ability of invitro–generated TH2 cells to mediate TH2-type inflammation in the lungs upon adop-tive transfer4. In a model of allergic airwaydisease, TH2 effector function, but not TH2differentiation, is inhibited by ICOS block-ade5. Another study has shown T cell differ-entiation, in the presence of an ICOS antag-onist, leads to the development of more TH1than TH2 cells6. Finally, ICOS-deficient mice

ICOS costimulation:it’s not just for TH2 cells anymoreANNE I. SPERLING1 AND JEFFREY A. BLUESTONE2

A current paradigm has ICOS participating inTH2 costimulation. New data indicates ICOSregulates not only TH2 cells, but also TH1s.

have reduced amounts of TH2 immunoglobu-lin isotypes and produce little IL-4 after sec-ondary stimulation7–9. Thus, a paradigm hasdeveloped in which ICOS provides a uniquerole in TH2 costimulation.

Hancock and colleagues provide evidencethat challenges this paradigm; they show thatblockade of ICOS signaling leads toincreased heart allograft acceptance andreduced evidence of chronic rejection2. Acuteallograft rejection is not TH2-mediated, but isa TH1-dependent inflammatory response thatleads to the destruction of graft tissue2. Theyshow clearly that anti-ICOS therapy alone, orin combination with blockade of CD40 ligand(CD40L, also known as CD154), has a pro-found effect on the TH1 response. It markedlydecreases graft infiltration by both CD4+ andCD8+ T cell subsets; it also suppresses up-regulation of the proinflammatory cytokineinterferon-γ (IFN-γ) in the intragraft whilemarkedly down-regulating chemokines criti-

cal for allograft responses. In light of theseresults, a re-examination of the role of ICOSas a regulator of T cell differentiation is war-ranted.

Although in the original studies ICOS-mediated costimulation augmented TH2cytokine secretion, the production of IFN-γand tumor necrosis factor-α (TNF-α) by cos-timulated T cells also increased3. Under cer-tain circumstances, ICOS costimulationresulted in increased IL-2 production10. ICOSblockade of T cell receptor (TCR)-transgenicencephalitogenic T cells during in vitroexpansion can prevent the TH1-mediated dis-ease, experimental encephalomyelitis (EAE),in normal recipient mice11; in addition, treat-ment with B7RP-1–Fc protein can augmentTH1, as well as TH2, responses in mice12.Therefore, ICOS almost certainly plays a crit-ical role in the differentiation and effectorfunction of both TH1 and TH2 cell types.

If ICOS regulates all T cell types, how is it

Figure 1. “Strength of signal” models forICOS-mediated costimulation. (a) Differentia-tion phase. In this model T cell differentiation isdirectly related to the strength of TCR signaling. Atlow signal strength the T cells are unable to mounta productive response and become anergic (green).At medium signal strength T cell activation results inTH1 responses (yellow). At maximal signal strengthTH2 responses dominate (purple). Costimulatorysignals provided by ICOS or CD28 alter the slopeand maximum signal strength so that a typicalimmune response in the absence of ICOS engage-ment results in either the absence of any T cellresponse or a TH1 response (lower curve), whereasICOS costimulation promotes TH2 differentiation(upper curve). Exogenous cytokines can alter thebalance (red arrows). (b) Effector phase. ICOS cos-timulation plays essential role in the effector phaseof immunity, including expansion and homing as wellas cytokine production by memory-effector T cells.As ICOS costimulation increases, T cell stimulationthrough the TCR is converted from an ineffectivesignal (green) to a normal signal (purple) and finallyto hyperactivity, which leads to autoimmunity (lightblue). As proposed, negative regulatory pathwaysmediated by CTLA-4 or PD-1 will directly modulateICOS activity in these effector cells (arrow).

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nature immunology • volume 2 no 7 • july 2001 • http://immunol.nature.com

NEWS & VIEWS

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that the majority of studies suggested ICOSfavored TH2 responses? One simple modelwould be that, under many circumstances,ICOS signaling leads to the selective produc-tion and translocation of TH2-promotingtranscription factors such as GATA-3 and c-Maf, or the decreased expression of TH1-pro-moting transcription factors such as T-bet.Cytokine signaling can directly regulate Tcell differentiation and effector functionthrough specific induction of these criticaltranscription factors. However, there is nopublished data to suggest that ICOS, or anyother costimulatory pathway, has a selectiveeffect on the balance of TH1- and TH2-associ-ated transcription factors. In addition, thismodel does not explain the ability of ICOS toregulate TH1 responses, as reported byHancock and colleagues2. Thus, it is morelikely that ICOS costimulation affects theoverall strength of the T cell activation sig-nal, which leads to more effective signaltransduction, a change in the balance of tran-scription factor induction and TH2 differenti-ation (Fig. 1a).

Why does TH2 development require andrespond to stronger signaling? It has beensuggested that as T cells undergo morecycling, the cells differentiate towards a TH2-type phenotype. Thus, stronger signalsinduce more T cell proliferation and TH2development. Alternatively, it is possible thatthe threshold of GATA-3 or c-Maf activationis greater than that of T-bet. However, at pre-sent there is no data to support this. We favora third possibility, recently proposed by A.Abbas (personal communication), that thedifference lies in the fact that TH2 cells aredriven by autocrine growth factors (such asIL-4), whereas TH1 cell replication and dif-ferentiation relies on antigen-presenting cell(APC)-derived factors such as IL-12 and IL-15. Perhaps only the strongest signaling con-ditions will drive enough IL-4 production forTH2 cell division and, under those circum-stances, autocrine growth factor activity mayprovide a selective advantage to the out-growth of this subset. In any case, with fewexceptions, stronger TCR and costimulatorysignals induce more TH2 cells. In this regard,it is impressive how dominant TH2 responsescan be in an unregulated immune setting. In

cytolytic T lymphocyte-associated antigen 4(CTLA-4)–deficient mice, which suffer aprofound lymphoproliferative disorder, the Tcells are largely skewed towards the TH2 lin-eage. Data suggesting that CTLA-4 negative-ly regulates ICOS function could be viewedinstead as a coalescence of signals, both pos-itive and negative, which regulate the TCRrheostat to promote downstream secondarymessengers and differentiation signals.Molecules such as CTLA-4 and PD-1 mayoppose optimal activation, whereas 4-1BB,CD40L and CD30 may promote it. Thus, thetotal signal received by a T cell can determinethe outcome of both the differentiation andeffector phases of the immune response.

The phenotype of ICOS–/– mice has strikingresemblance to the phenotype of CD28–/–

mice: both show dramatic defects in their TH2responses. Induction of a TH2 response inCD28–/– mice requires infection with apathogen that is capable of dramaticallyskewing the immune response toward the TH2phenotype. Thus, achieving sufficient signal-ing to produce TH2 responses in ICOS–/– micemay be difficult with conventional immuniza-tion protocols. Because addition of exoge-nous IL-4 induces a TH2 phenotype in bothCD28–/– and ICOS–/– T cell cultures, themilieu in which differentiation is occurringcan also influence TH cell differentiation.

It is likely that ICOS not only plays animportant role in the differentiation phase ofthe immune response; it is likely that ICOSalso plays a key role in the effector-memoryphase of the immune response when antigen-exposed T cells expand, home to the site ofinflammation, and release effector cytokines(Fig. 1b). It is not clear yet whether theeffects of using the ICOS antagonists in allo-transplantation might include the preventionof differentiation or expansion of TH1 cellsand the ability of these cells to enter the graftand produce cytokines2. A previous studyshowed that treatment of mice with an ICOSagonist increases the number of cytokine-producing cells in the draining lymphnodes12. In addition, when challenged withantigen under the cover of an ICOS antago-nist, TH2 cells induced in vitro fail to migrateinto the lungs and cause inflammation aftertransfer into naïve recipients4. Moreover,

cytokine production by effector lung TH2cells is inhibited by ICOS blockade after invitro restimulation5. Finally, two other studiespublished in this issue of Nature Immunologyhave found that blockade of ICOS only dur-ing the effector phase, leads to dramaticdecreases in TH2-mediated airway inflamma-tion and TH1-mediated EAE13,14. Theseresults suggest that expansion, T cell survivaland function may be independently regulatedby ICOS signaling during the effector phaseof both TH1 and TH2 responses.

In summary, the work reported by Hancockand colleagues represents important newobservations on the nature of T cell costimu-lation pathways2. In addition to the findingthat ICOS can control TH1 responses, anti-ICOS therapy in conjunction with low dosecyclosporin A induced long-term allograftsurvival. However, the study falls short ofdetermining the tolerogenic potential of ICOSblockade, as the authors do not carry on theexperiment beyond 100 days. In addition, theydo not investigate the immune responsivenessof the alloreactive T cells or determinewhether the mice would accept a second graftin the absence of further immunotherapy.However, it is clear that manipulation of theICOS–B7RP-1 pathway has important thera-peutic implications beyond the TH2 field andmay add to the growing arsenal of selectiveimmunotherapeutics.

1. London, C.A., Lodge, M. P. & Abbas,A. K. J. Immunol. 164,265–272 (2000).

2. Ozkaynak, E. et al. Nature Immunol. 2, 591–596 (2001).3. Hutloff,A. et al. Nature 397, 263–266 (1999).4. Coyle,A. J. et al. Immunity 13, 95–105 (2000).5. Tesciuba,A. G. et al. J. Immunol. (in the press, 2001).6. McAdam,A. J. et al. J. Immunol. 165, 5035–5040 (2000).7. Dong, C. et al. Nature 409, 97–101 (2001).8. McAdam,A. J. et al. Nature 409, 102–105 (2001).9. Tafuri,A. et al. Nature 409, 105–109 (2001).10. Riley, J. L. et al. J Immunol 166, 4943–4948 (2001).11. Sporici, R. et al. Clin. Immunol. (in the press, 2001).12. Guo, J. et al. J. Immunol. 166, 5578–5584 (2001).13. Gonzalo, J.A. et al. Nature Immunol. 2, 597–604 (2001).14. Rottman, J. B. et al. Nature Immunol. 2, 605–611 (2001).

1Section of Pulmonary and Critical Care Medicine,Department of Medicine, and the Committee onImmunology, University of Chicago, Chicago, IL 60637,USA. (asperlin@medicine.bsd.uchicago.edu) 2A.W.and Mary Margaret Clausen Professor, UCSFDiabetes Center, Department of Medicine, Universityof California, San Francisco, San Francisco, CA 94143,USA. (jbluest@diabetes.ucsf.edu)

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