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http://immunol.nature.com • march 2001 • volume 2 no 3 • nature immunology 193

In 1933 Thomas Rivers, a world-renowned virol-ogist, performed a seminal immunological exper-iment. In this experiment he set out to establishwhether an inflammatory reaction in the brainthat occurred after rabies vaccination (or certainviral infections) was related to the virus or to anallergic reaction against the brain tissue in whichthe virus was grown. He showed that monkeysthat had received up to 85 injections of a rabbitbrain extract over 1 year developed an inflamma-tory response in the brain: experimental allergicencephalo-myelitis (EAE) was born1. In 1947Kabat showed that EAE could be induced by asingle injection of brain tissue in Freund’sadjuvant. Since then EAE has become theprimary animal model for the human dis-ease multiple sclerosis (MS) and for inves-tigation of basic mechanisms of cellularimmune reactions against brain tissue. Atlast count there were over 4,500 articles onEAE in the scientific literature. In the1980s, as more was learned about cellularimmunology and the T helper 1–T helper 2(TH1-TH2) paradigm, EAE became experi-mental “autoimmune” encephalomyelitisand the prototypic TH1 cell–mediatedautoimmune model. EAE can be inducedby a number of well defined myelin anti-gens—including myelin basic protein(MBP), proteolipid protein (PLP) andmyelin oligodendrocyte glycoprotein(MOG)—and has gone beyond a simplelaboratory model. Making the leap fromEAE to MS, several investigators, includingJonas Salk, have attempted to treat MS bydesensitization or by tolerization to myelin anti-gens such as MBP, peptide fragments or synthet-ic polymers. In this issue of Nature Immunology,Pedotti et al. show that, under certain conditions,injection of myelin antigens evoke more than aTH1 autoimmune reaction against self: fatal ana-phylaxis can be induced2. In addition, they pre-sent provocative data that the definition of self-protein that can be targeted by autoimmune orallergic reactions is not only defined by their pres-ence in host tissue but by whether they areexpressed in the thymus.

In the process of investigating peptide-basedtherapy for EAE, the investigators injectedSJL/J mice multiple times with the peptide

PLPp(139–151) in complete Freund’s adjuvant.As expected, the majority of mice developedEAE approximately10–14 days after the initialinjection. However, when animals were rechal-lenged 3 or 4 weeks after the initial sensitiza-tion, during the process of recovery, over two-thirds developed a fatal anaphylactic reaction. Aclassic pathological picture of anaphylaxisaccompanied this process but did not occur ifanimals were injected 2 weeks after initial sen-sitization at the height of clinical disease. Theeffect was not unique to PLP and was alsoobserved with the peptide MOG(33–55) in

C57Bl/6 mice. However, anaphylaxis was notobserved when PLPp(178–191) was injectedinto SJL/J mice or MBPAc(1–11) was injectedin PL/J mice.

If EAE has been studied for so long, why has-n’t this effect been reported previously? Theanswer most probably relates to the timing of thesecond immunological challenge. Most studies ofEAE have focused on the characterization ofmechanisms involved in the induction of EAE oreffector mechanisms associated with diseasemanifestations. Therapeutic approaches haveinvolved treatment before induction or after dis-ease but seldom with the same peptide or proteinused to induce the illness administered in

Freund’s adjuvant at the time as animals wererecovering. During recovery from EAE, twoimmunological events occur: apoptosis ofencephalitogenic cells and a pronounced immunedeviation that can include increased secretion ofinterleukin 4 (IL-4), IL-10 and transforminggrowth factor β (TGF-β)3,4. Pedotti et al. showthat the physiological TH2-type response thatoccurs during recovery can be boosted if the ani-mals are challenged at the same time as the phys-iological response is at its zenith.

Why didn’t injection of all self-peptides lead toanaphylaxis? The answer appears to be related to

whether the self-antigen is expressed in thethymus (Fig. 1). Recent evidence suggeststhat antigens, previously thought to besequestered behind the blood-brain barrieror in other privileged sites or tissues, arealso expressed in thymus. The thymicexpression of myelin antigens leads to aform of central tolerance, reducing both thesize and affinity of the self-reactive reper-toire. This was shown to be particularlyimportant in response to PLPp(139–151) inthe SJL/J mice where the PLPp(139–151)-reactive cells are present, even in naïvemice5. The expanded PLPp(139–151)-reac-tive repertoire in the SJL/J mice is attrib-uted to lack of central tolerance to this pep-tide. This is because an isoform of PLP,called DM20, which lacks resides 116–150and essentially lacks the encephalitogenicPLPp(139–151) epitope, is expressed in thethymus during ontogeny. As is shown in thePedotti paper2, the SJL/J mice not only

develop severe autoimmunity after immunizationwith the PLPp(139–151) peptide but also developan allergic and/or anaphylactic response toPLPp(139–151). Thus, besides shaping the reper-toire and reducing the chances of autoimmunity,this paper shows a critical role of thymic expres-sion of self-antigen for limiting or completelydeleting the cells that play a role in inducing ana-phylaxis to self-antigens.

Whether the induction of anaphylaxis isdirectly related to the size and/or the affinity ofthe self-reactive T cell repertoire, or whetherthymic expression of self-antigen deletes the Tcells that are uniquely capable of inducing ana-phylaxis to self, remains to be determined. Thus

The fine line betweenautoimmune and allergicencephalomyelitisHOWARD L.WEINER

What determines whether immune responsesagainst a self-peptide can evoke ananaphylactic response? New evidencedescribes anaphylaxis that appears to begoverned by thymic expression of the self-antigen.

Figure 1.Thymic expression of self-antigens and the devel-opment of allergic responses.

Self-antigen

Thymicexpression

Centraltolerance

No thymicexpression

No centraltolerance

Autoimmunity(EAE)

Autoimmunity(EAE)

Anaphylaxis

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Phagocytes destroy foreign pathogens with anoxidative burst that originates from the NADPHoxidase, a multisubunit enzyme comprised ofp22phox, p40phox, p47phox, p67phox, gp91phox (phox forphagocyte oxidase) and the GTP-bound form ofthe Rho family GTPase Rac21. To prevent dam-age to surrounding healthy tissue, the phagocytemust tightly control deployment of the oxidaseweaponry. Genetic defects that compromiseNADPH oxidase function manifest clinically aschronic granulomatous disease (CGD), in whichpatients suffer persistent infections as a result ofthe inability of their phagocytes to destroy for-eign pathogens. The phagocyte achieves a highdegree of control over the oxidase by partition-ing components of the system in distinct cellularlocations, only assembling the subunits into anactive enzyme in response to the internalization

Rac inserts its way intothe immune responseGREGORY R. HOFFMAN1 AND RICHARD A. CERIONE1,2

New data shows that NADPH oxidaseactivation by Rac2 involves an insert-dependent interaction between Rac2 andcyt b.This unique activation mechanism hasfar-reaching implications for the regulationof related signaling systems.

of a foreign particle. The electron-transfermachinery is a flavocytochrome known as cyto-chrome b-558 (cyt b), which exists in the mem-brane as a dormant heterodimer of gp91phox andp22phox. Upon a phagocytic event, p67phox and theactivated GTP-bound form of Rac2 indepen-dently translocate to the membrane from thecytosol. Once armed by the binding of thesecytosolic components, the active NADPH oxi-dase transfers electrons from NADPH to molec-ular oxygen, producing superoxide anions (O2

–),which are used to destroy the internalized target.Although stimulation of NADPH oxidase activ-ity by the interaction of Rac2 and p67phox withcyt b has been clearly shown in reconstitutedcell-free systems2, the contribution of theseinteractions to the underlying electron-transfermechanism is less clear. In this issue of Nature

Immunology, Diebold and Bokoch show thatRac2 can independently undergo GTP-depen-dent interactions with p67phox and cyt b. Theyalso show that each of these interactions con-tributes to distinct steps in the electron-transferreaction3, which suggests a unique mechanismfor Rac2 activation of the NADPH oxidase thatmay also provide clues to the regulation of relat-ed signaling systems.

To delineate the specific protein-protein inter-actions that occur between Rac2 and the oxidasesystem, Diebold and Bokoch have taken advan-tage of the fluorescent GTP analog mant-GppNHp. Mant-GppNHp is used extensively toinvestigate the interactions of low molecularweight GTP-binding proteins with their specificeffector proteins. Repeat-ing previously report-ed results4, the authors show that this fluorescent

nature immunology • volume 2 no 3 • march 2001 • http://immunol.nature.com

NEWS & VIEWS

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the definition of a “self-antigen”, as it relates toautoimmunity, involves two components. Theseare whether the antigen is present in tissue andthe degree to which it is expressed in the thymus.Thymic expression of self-antigens thus plays amajor role in organ-specific autoimmune dis-eases both by shaping the repertoire and deter-mining what type of self-immune responsesoccur.

What are the implications of these findings forthe application of antigen-specific therapy tohuman diseases such as MS? It is worth notingthat a TH2-type “allergic” EAE can be created inmice by the adoptive transfer of TH2 cells intosevere-combined immunodeficient (SCID) recip-ients. However, this does not appear to be relatedto MS, where the pathological picture is one of aTH1-type6 delayed hypersensitivity response withexpression of IL-12 and TH1-type chemokines inthe brain7,8. Current therapies for MS basicallywork according the TH1-TH2 paradigm. Interferonβ (IFN-β) decreases IFN-γ and IL-12 expressionand is associated with an increase in IL-4 and IL-109,10. Glatiramer acetate, a random copolymergiven by injection that appears to function as analtered peptide ligand, induces immune deviationtowards TH2 and TH3 (TGF-β) responses11. Cyclo-phosphamide, an effective immune-suppressantagent for nervous system inflammatory responsesin MS12, decreases IL-12, increases IL-4, IL-10

and TGF-β and is associated with eosinophilia inthe peripheral blood9,13.

The findings of Pedotti, et al.2 suggest that theapplication of antigen-specific therapy may notbe as straightforward as one would have hoped.Thus, repeated injections of myelin antigens orpeptides could lead to “allergic”-type reactions.This was observed in a recent trial of an alteredpeptide ligand of MBP for MS14. In addition, pru-ritis has been reported in patients injected withglatiramer acetate, although such mild side-effects have not precluded the widespread use ofglatiramer acetate in MS with some patients ontherapy for over 5 years15. The mucosal adminis-tration of antigen may obviate “allergic” reactionsseen when self-peptides are given by injection16

and, if a peptide is given that is expressed in thethymus, there may be no risk of allergic respons-es. Glatiramer acetate is now being tested in MSpatients by the oral route17 and, although an initialtrial of oral myelin was not successful in MS, noallergic responses were observed16.

Recent advances in our understanding ofautoreactivity have also shown that, under somecircumstances, autoimmunity can be beneficial18.By showing that “self-peptides” can cause ana-phylaxis, the authors have expanded further ourview of autoimmunity and have highlighted therole of the thymus in shaping a repertoire thatmay predispose not only to autoimmune disease

but also to allergic reactions. Thus, the basic con-cepts of EAE as originally described by Rivershave been expanded so that EAE can be viewedas having both an “autoimmune” and “allergic”component. Expression of the different pheno-types depends on the fine balance of the differentlimbs of the immune response.

1. Rivers,T. M., Sprunt, D. H. & Berry, G. P. J. Exp. Med. 58, 39–53(1933).

2. Pedotti, R. Nature Immunol. 2, 216–222 (2001).3. Khoury, S. J., Hancock,W.W. & Weiner, H. L. J. Exp. Med. 176,

1355–1364 (1992).4. Chen,Y., Hancock,W.W., Marks, R. P., Gonnella,A. & Weiner, H. L. J.

Neuroimmunol. 82,149–159 (1998).5. Anderson,A. C. et al. J. Exp. Med. 191, 761–770 (2000).6. Lafaille, J. J. et al. J. Exp. Med. 186, 307–312 (1997).7. Windhagen,A. et al. J. Exp. Med. 182, 1985–1996 (1995)8. Balashov, K. E., Rottman, J. B.,Weiner, H. L. & Hancock,W.W. Proc.

Natl Acad. Sci. USA 96, 6873–6878 (1999).9. Smith, D. R., Balashov, K. E., Hafler, D.A. Khoury, S. J. & Weiner, H. L.

Ann. Neurol. 42, 313–318 (1997).10. Wang, X., Chen, M.,Wandinger, K. P.,Williams, G. & Dhib-Jalbut, S. J.

Immunol. 165, 548–557 (2000).11. Miller,A. S. et al. J. Neuroimmunol. 92, 113–121 (1998).12. Gobbini, M. I., Smith, M. E., Richert, N. D., Frank, J.A. & McFarland, H.

F. J. Neuroimmunol. 99, 142–149 (1999).13. Comabella, M. et al. J. Clin. Invest. 102, 671–678 (1998).14. Kappos, L. et al. Nature Med. 6, 1176–1182 (2000).15. Johnson, K. P. et al. Mult. Scler. 6, 255–266 (2000).16. Faria,A. M. C. & Weiner, H. L. Adv. Immunol. 73, 153–264 (1999).17. Weiner, H. L. Proc. Natl Acad. Sci. USA 96, 3333–3335 (1999).18. Schwartz, M. & Cohen, I. R. Immunol.Today 21, 265–268 (2000).

Harvard Medical School, Center for Neurologic Diseases,Brigham and Women’s Hospital, 77 Ave Louis Pasteur, HIM720 Boston, MA 02115, USA.(weiner@cnd.bwh.harvard.edu)

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