NATURE MEDICINE • VOLUME 8 • NUMBER 2 • FEBRUARY 2002 107

NEWS & VIEWS

Stemming the tide of rejectionTransplant recipients require chronic administration of drugs that suppress their immune system to prevent

rejection of foreign tissues. The finding in rats that grafting embryonic stem-like cells to a transplant recipient caninduce tolerance to transplants has important clinical implications. (pages 171–178)

As proven by the morbidity of vari-ous inherited and acquired im-

mune deficiencies, humans need anintact immune system to be protectedagainst pathogens ubiquitously present inthe environment. But the cellular arm ofthe immune system, representedby T lymphocytes, also ‘protects’people from transplanted tissuesand organs. T-cell–directed re-sponses against foreign histo-compatibility antigens expressedon transplanted organs almostinvariably lead to acute rejection.With the exception of organs ortissues donated by an identicaltwin, virtually all transplant re-cipients require lifelong im-munosuppressive drugs toprevent rejection. The drugs havesubstantial side effects, includingsusceptibility to opportunistic in-fection, cardiovascular diseaseand malignancy.

For the past five decades, trans-plant immunologists havesought to develop safe and effec-tive approaches to induce trans-plantation tolerance, a state inwhich the recipient ‘accepts’ theforeign transplanted tissue as‘self’, without a need for chronicimmunosuppression. In the cur-rent issue of Nature Medicine,Fändrich et al.1 describe the useof embryonic stem (ES) cells as atool to induce tolerance, provid-ing an interesting and timelynew twist to an old clinical chal-lenge.

Whereas most approaches toinduce transplantation tolerancein rodent models fail when trans-lated into large animals, twostrategies have shown particularpromise in non-human primatemodels: costimulatory blockade2

and mixed hematopoieticchimerism3. Of the two, mixedchimerism is the older and morereliable method. Pioneered byIldstad and Sachs4, mixedchimerism involves ablation ofthe host immune system fol-lowed by its reconstitution with

a mixture of host and donor T cell–de-pleted bone marrow (T-cell depletion is

needed to prevent donor T-cellsfrom attacking host tissues). As thehost’s immune system is reconsti-tuted, antigens on the donor bone

marrow cells are perceived as ‘self’. As aconsequence, newly developing donor-re-

active T cells are physicallyeliminated in the thymus, inthe same way as self-reactive Tcells are eliminated during de-velopment to prevent autoim-mune disease. The resultanthematopoietic chimera hasdurable tolerance to donor-specific organs and tissues,which can then be trans-planted without any immuno-suppression.

Although mixed chimerismis clearly the most effective ap-proach to achieve transplanta-tion tolerance, its inherenttoxicity precludes it at presentfrom widespread clinical appli-cation3. Toxicity arises frompre-conditioning regimens thatconsist of combined irradiationand chemotherapeutic agentstraditionally used to disable thehost’s immune system, so as toprevent rejection of donorbone marrow cells. As a result,application of the mixedchimerism approach has beenlimited to those rare patientswho require bone-marrowtransplantation for treatmentof hematologic malignancyand simultaneous kidney trans-plantation for treatment of co-existing renal failure5. Morerecent work using mouse mod-els suggests that a much moregentle conditioning regimencombined with high doses ofdonor bone marrow may be ef-fective in inducing tolerance,but such approaches have notbeen studied in large animals todate.

Enter ES cells. Due to theirpluripotent nature, ES cellshave generated intense interestin a variety of areas of biomed-ical research, in hopes they

SCOTT H. ADLER, STEVEN J. BENSINGER &LAURENCE A. TURKA

Fig. 1 Two potential paths to mixed chimerism and transplantationtolerance. Transplantation tolerance can be achieved through thedevelopment of mixed chimerism, a state in which allogeneichematopoietic cells coexist with recipient cells. In the traditional ap-proach (left), the host is pre-conditioned with irradiation and/orchemotherapy. The preconditioning regimen serves to transientlyinactivate the host immune system—a necessary step to prevent thedonor cells from being rejected. Subsequent engraftment of donorcells gives rise to a state of mixed hematopoietic chimerism in whichdonor and recipient bone marrow–derived cells coexist. As new Tcells are produced, they encounter antigen-presenting cells derivedfrom donor bone-marrow elements. Donor antigens are thus per-ceived as ‘self’ and the newly developing T cells are eliminatedand/or inactivated, allowing for the acceptance of a transplantedorgan (such as a heart, as shown here) of identical donor origin. Inthe approach outlined in by Fändrich et al. (right panel), long-termtransplantation tolerance can be achieved in the absence of pre-con-ditioning by substituting ES cells for bone-marrow cells. ES cells areintroduced into the portal vein of the recipient followed later byheart transplantation. The ES cells contribute to the recipienthematopoietic compartment, leading to mixed chimerism. T cells re-active to donor antigens on the ES cells are either eliminated or inac-tivated. It is not known how long ES cells will survive in the recipienthematopoietic compartment, or whether such survival is necessaryto maintain tolerance.

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108 NATURE MEDICINE • VOLUME 8 • NUMBER 2 • FEBRUARY 2002

NEWS & VIEWS

might eventually provide ‘replacementparts’ for diseased tissues and organs.However, as recently pointed out in aneditorial in Nature Immunology6, the cellsthemselves, or the differentiated tissues towhich they give rise, may provoke an allo-geneic immune response in geneticallynon-identical individuals, similar to othertransplanted tissues.

In the current issue of Nature Medicine,Fändrich et al.1 describe the use of ES cellsas a tool to induce tolerance. They derivedrat embryonic stem cell–like (RESC) celllines from blastocysts. (While the RESClines appear by all accounts to resemblemurine ES cell lines, the phenotypic andpluripotent nature of the RESC have notbeen fully characterized, and so the au-thors cautiously refer to them as ‘ES-like’cells.) To study the ability of the RESCcells to provoke an immune response, theinvestigators injected them into the por-tal vein of non-immunosuppressed genet-ically disparate rats. Seven days later,donor RESC cells were detectable in liver,lymph nodes, spleen and thymus of therecipients. More remarkably, the persis-tence of RESC cells in the allogeneic hostwas associated with the development ofpartial hematopoietic chimerism, with5–8% of B cells and monocytes being de-rived from the RESC cells. Finally, accep-tance of the allogeneic RESC graft and thesubsequent development of hematopoi-etic chimerism led to long-term accep-tance of heart transplants from the samedonor, but not from third party donors.

How do RESC cells escape immune sur-veillance and engraft in the absence ofhost pre-conditioning or immunosup-pression? Normally, allogeneic cellswould be readily eliminated by T cells ofthe host’s immune system. The author’sargue that RESC escape this fate throughexpression of a molecule called Fas ligand.This molecule is normally expressed in se-lected anatomic sites (such as the testis orthe anterior chamber of the eye) and

helps confer to them the property of im-mune privilege, wherein activated im-mune cells (including T cells) are killed bythe binding and activation of the Fasdeath receptor on the immune cell7.Fändrich et al. suggest that RESC cells es-cape rejection through expression of Fasligand, which enables them to kill re-sponding host T cells. Although their datasupports this contention, it is far from de-finitive. Moreover, Fas ligand expressionis not unique to RESC cells. Bone-marrowcells can also express Fas ligand under cer-tain circumstances, which may be linkedto their ability to establish chimerism fol-lowing pre-conditioning8. Therefore, theunique properties of RESC cells describedby Fändrich et al. cannot be attributed toFas ligand alone.

But even without a clear mechanism forthe survival of RESC cells in an immunecompetent host, the data have importantclinical implications. If ES cells can beused to create hematopoietic chimerism,and by extension transplantation toler-ance, without a need for a toxic condi-tioning regimen, it would be a majorbreakthrough. Researchers have long un-derstood the effectiveness of hematopoi-etic chimerism as an approach totransplantation tolerance, but the devilhas been in the details of the condition-ing therapies needed to create a chimera.Although it seems highly unlikely that EScells will be available from all potentialtransplant donors, understanding whichproperties of ES cells permit engraftmentwithout host pre-conditioning will openthe door to more general application ofthis approach.

To be sure, there are caveats. It is possi-ble that the mechanism by which ES cellsengraft without a need for recipient con-ditioning is unique to rats or rodents, andmay not hold true in other species, in-cluding large animals (for preclinical test-ing), and of course humans. Furthermore,we do not know whether or not RESC

cells (or ES cells) will spontaneously dif-ferentiate into nonhematopoietic lin-eages, as this issue was not exhaustivelyanalyzed by the authors. It is possible thatthe route of administration they chose—the portal vein—favored hematopoieticdifferentiation, which is a primary func-tion of the liver during embryonic devel-opment. An additional question iswhether or not differentiated products ofES cells would provoke an immune re-sponse. If so, this also could potentially beprevented by the establishment ofhematopoietic chimerism.

While these questions remain unan-swered, the paper by Fändrich et al. ap-pears to be an exciting advance, both fortransplantation tolerance and use of EScells in general, as treatment for a multi-tude of disorders including juvenile-onsetdiabetes, Parkinson disease and cardiovas-cular disease.

1. Fändrich, F. et al. Preimplantation-stage stem cellsinduce allogeneic graft tolerance without supple-mentary host conditioning. Nature Med. 8, 171–178(2002).

2. Sayegh, M.H. & Turka, L.A. The role of T cell co-stim-ulatory activation pathways in transplant rejection.N. Engl. J. Med. 338, 1813–1821 (1998).

3. Sykes, M. Mixed chimerism and transplant toler-ance. Immunity 14, 417–424 (2001).

4. Ildstad, S.T. & Sachs, D.H. Reconstitution with syn-geneic plus allogeneic or xenogeneic bone marrowleads to specific acceptance of allografts orxenografts. Nature 307, 168–170 (1984).

5. Spitzer, T.R. et al. Combined histocompatibilityleukocyte antigen-matched donor bone marrow andrenal transplantation for multiple myeloma with endstage renal disease: the induction of allograft toler-ance through mixed lymphohematopoieticchimerism. Transplantation 68, 480–484 (1999).

6. Editorial. Accepting stem cells. Nature Immunol. 2,1085 (2001).

7. Griffith, T.S., Brunner, T., Fletcher, S.M., Green, D.R.& Ferguson, T.A. Fas ligand-induced apoptosis as amechanism of immune privilege. Science 270,1189–1192 (1995).

8. George, J.F. et al. An essential role for Fas ligand intransplantation tolerance induced by donor bonemarrow. Nature Med 4, 333–335 (1998).

Department of MedicineUniversity of PennsylvaniaPhiladelphia, Pennsylvania, USAEmail: turka@mail.med.upenn.edu

An essential link to mammary cancer?The α isoform of the IκB protein kinase has been identified as the missing link in a signaling pathway that controlsmammary epithelial proliferation via the cell-cycle regulator cyclin D1. This new player may provide a promising

target for inhibiting cyclin D1 expression, which is elevated in 50% of breast malignancies.

Breast cancer, the most common ma-lignancy in women, is a genetically

heterogeneous disease that is difficultto treat. In most cases, cancer cells orig-inate from the normal mammary glandepithelium and they frequently retain

and subvert tissue-specific pathways re-quired for the normal growth of the

gland during puberty and pregnancy. Afocus of current research is to identifyand functionally characterize the mole-cular components of these pathways toreveal new targets for breast cancer-spe-cific therapy. A report in the 14

SIBYLLE TONKO-GEYMAYER &WOLFGANG DOPPLER

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