IMMUNOLOGY TODAY

53 Cxnpos, L., Posseit, A.M., Deli, B.C. 1-f nl. (19941 Tunrbfrlnrifnti~rr 57,

95&953

54 Shimizu, Y,, Goto, S., Lord, R. rf III. (1996) ~tlJZsphZtIt. Itfl. 9, 5936%

55 Vriesendorp, H.M. (1985) in BOUP Mflt’t’~u* T~‘~@nr!tntio~f

(van Bekkum, D.W.and Lowenberg, B., eds), pp. 73-145, Marcel Dekker

56 Gale, R.P. and Reisner, Y. (1946) Lar~c‘r’f i, 1468-1470

57 Plotnicky, tf. and Touraine, J.L. (1993) BUIZP Mnrroal fimsj~ht. 12,307-314

58 Burlingham, W.J., Grailer, A.P., Fechner, J.H. t*f R!. (1995) P~r~sy/rz~?fntioi~

59 1

‘. 1 147-1155

59 Miller, R.G. (19801 Nrzf~rl~ 287,544+546

60 Inaha, K., Steinman, R.M., Pack, M.W. rf af. (1992) 1. Es/?. M& 175,1157-1167

61 Lu. L., Rudert, W.A., Qian, S. ef nl. (1995) 1, Er~j. Md. 182,379-387

62 Thomson, A.W., Lu, L., Wan, Y., Qian, 5, Larsen, Cl? and Starzl, T.E.

(1495) Tr~lr~~lln~~fnti[~t~ 60, 1555-1559

63 Lu, L., Woo, J., Rao, AS. rf nl. (lY94) 1. E.r/‘. Med. 179, 1823- 1834

64 Lu, L., McCaslin, D., Starzl, T.E. and Thomson, A.W. (1995)

Ttwplnrlh7~iut7 60, 1539-2545

65 Fu, F., Li, Y., Qian, S. (‘f l11. (1996) ~~771s~,k~r1~~clficrrr 62, hS9-665

66 Nossal, G.J.V. and Pike, B.L. (1981) P~K. NRU. .k,rd. Sri. U. S. A. 78,

3844-3847

67 Nossal, G.J.V. (1983) A77111r. Rev. 11117~z177oI. 1, 33-62

68 Miller, J.F. and Morahan, G. (19921 A71777r. &iv. h7771:wd. 10, S3-69

69 Kobayashi, E., Kamada, N., Delriviere, L. cf 111. (199j) Iwrt~r~~ola,yy 84,

333-336

70 Dejbakhsh-Jones, S., Jerabek, L., Weissman, I.L. and Strober, S. (199.5)

Ontmmlopj 155, 3338-3344

71 Starzl, T.E., Porter, K.A., Andres, G. pf ~1. (1970) Clir~. E.v;>. I7771711rrrol. 6, 803-814

72 Salvatierra, Z., Jr. “1 ,*rlcenti, E, Amend, W.J. it RI. (1980) A127r. Sq. 192,

543-552

73 Monaco, Al?, Clark, A.W., Wood, 1M.L.. Sahyoun, A.(., Co&h, S.D.

and Brown, R.W. (1976) Sulgc~!! 79,3&l-392

74 Barber, W.H., Mankin, J.A., Laskow, D.A. (? nl. (1491) I%@nlrfn/iorI 51,

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75 Fontes, I?, Rao, A., Demetris, A.J. ct RI. (1994) Lnrrrt? 344, 751-155

76 Rao, AS., Fontes, I?, Dodson, F. pf nl. nn~s@~f. Pmt. (in press)

77 Garcia-Morales, R., Esquenazi, V., Zucker, K. cut nl. ~ar:sjllillr~ntillrr (in press) 78 Star& T.E., Tndo, ‘5, Tzakis, A. I’! nl. (1991) S177;y. G1~17rcd. Ohtd. 172.

335-344

Chimerism and transplantation tolerance: cause and effect

Kathryn

J~liL’IVi~ril/frT’r;ii iS i?Lli Olf/,jf Microchimerism: observations

esscrrtinl fo graft I;wi~izbd brrf fhnf Solid-organ grafts contain passenger leuko-

dollor nrrn host cells bnfh phy cytes7**, the number and lineage of which

vary considerably among different organs. allograft. Such recipients are described as ncfiw? mles. Hew, Knflzryrz WOOll For example, the liver contains an abundant exhibiting peripheral donor microchimerism l?l?li hiid Srrclrs cafrfioll fhnl fhc supply of passenger cells, whereas rela- and in some cases the donor material is c1e-

jur!y is still olrf 011 zohflzrr- 5lfch tively few leukocytes are present within the

tected for long periods after transpla!?- heart’. In addition, the passenger leukocytes tation*-‘. It has been suggested not only that frzic~ucl~imcrisrIr is tlrc cof~sc of present in the liver of rodents contain a suf- peripheral donor microchimerism is associ- ficient number of haematopoietic stem ated with long-term acceptance of the organ

rizcrdy flrc cmzsrqzr~rlc~ of hrg-tcr~~rl

nllopwffilzg. cells’O to rescue a lethally irradiated recipi-

graft but ihat it plays an active role in the in- ent when a syngeneic liver graft is trans- ductio:. and maintenance or unresponsive- planted; rescue cannot be achieved reliably ness’- S.h. This hypothesis, first proposed by Starzl and colleagues in ..~ by tile tzqglantation of a syngeneic cardiac allograft”. Passenger

1992 (Ref. 11, has stimulated a great deal of interest and activity in leukocytes p&s& ~ =.++n solid organs have been &orated and

the transplant community. However, from the published reports to shown to possess the potential iz +mulate immune responses ill

date, it remains difficult to determine whether such micro- vitro”. However, b efore acquiring full imW30stimulatory poten- chimerism is the cause or the consequence of long-term graft tialJ it appears that such cells must develop into a%i&zc rrrature survival. form. This has been achieved in vitro by addition of growth f&o&,

IMMUNOLOGY TODAY

Irradiation, anti-T-cell therapy

+ Bone marrow

Mixed chimera t

1

\ Organ transplant ~

+ ~ immunosuppressicrl

Microchimerism

Mixed chimera $

Grafts accepted

permanently

wait one year

Q* R Chime&m

maintained indefinitely disappears

Rechatlenge with a fresh graft from the same donor

Subsequent Subsequent

organ graft accepted organ graft rejected

-. Qe w /

such as granu!(,cyte-n~acrorhaRe colony-

stimulating factor (GM-C%) and a period of

CUltUre".". 011 the bask of these observa-

tions it has been sily%ested that some

passenger leukocytes are immature den-

dritic cells’4. Irr z~iw, passei:ger leukocytes

prewli wilhin an organ graft have &en

shown to migratt3 from the graft to recipient

lympnoid tissue after transplantatiunli’“:

A?lor derived, major Ilistocompatibility

complex IMHCI class If- leukocytes could

be detected in the spleen within 38 hours

of transplantation of a heart allograft into

a :inive mouse”‘. Within the spleen, the

donor leukocytes were associated with

CD+ T cells and it was suggested that

this interaction is responsible for initiating

the rejection response iii uon-immuno-

suppressed recipients . ‘I’ These data support

the conclusion drawn in earlier studies by

Lecliler and Batchelor that intragraft passen-

ger leukor.ytes, most probably dendritic

cclk~, pm7,; ., . .dr the major stiniuius fOi graft

immunngenicity’-.

In support of this hypothesis, organs de-

pleted of passenger leukocytes have been re-

ported in son-w casts to enjoy prolonged sur-

vival withour di;ii::i.5t?tiCUi of exogennits

inimunosul7pressi\fe tlIerhpy17~2’. In r,oinc of

these siudics, nrgans were dq ~lt3xl of pas-

senger wits bv ‘parkill;;’ the graft in a pri-

mary rc+ient receiving immunosuppres- sive tllerapp1’*‘9J’. Such ‘parked’ organs

from long-term surviving primary recipi-

ents were then re-transplanted to fresh non-

immunos,uppressed syngeneir secondary

,h+nts and the surviv:J of the graft moni-

tored. In some strain ,:f.)mbinations, the

passenger-cell-Ccpleted kidneys survived

indefinitely and in others they showed

prolcnged survival. Interestingly, in these

studies, induction of chimerism by the ad-

ministration of donor bone marrow to the

secondary host before transplantation led to

graft rejection, as did the admiv.;lstration of

dendrjtic-cell-,~nrichcd leuky. 1~“‘.

lil primary graft recipie ts, donor Ieuko-

cytes mig:athg fro5 transplanted hearts in

a mouse ~x~odc~ were only detwtable for a

feT$r days within the spleen tmlcss the ani-

mals received irnmu~f,!-~lppression, after

which donor cells could be detected for

longer periods following transplantation. 111

some human kidney transplant patients.

I--IFC-FMBER 1996

donnr mat&a! hrls twen deter& in the periphery more thAn 25

years after transplrrntatk&. Impressively, donor-derived cells have

been detected in some patknts with stable graft fun&En whn hove

stopped taking immunmuppres$jvc drug3’. However, the detec- tion of donor-derived material or cells in patients with long-term surviving organ grafts is not a consistent finding. For example, in a

study carried out in Paris, donor microchimerism could only be de-

tected in a third of patients who had exhibited long-term stable kid-

ney graft h;mction for more than 20 years5. Therefore, the mle that

donor leukocytes migrating from an organ graft to the peripheral

tissues of the recipient play in either the induction or maintcnanrr

~Funrespnnsiveness to the organ graft is unclear. No correlation be-

tween the ststc of microchiimcrism and the absence of acute or

rhmnit fejerftion WRS found in heart transplant patient&_ Moreover,

donor-derived material trras detected in a patient undergoing gr&

rejcctkn eight years after liver transplantation”. If extended donor microchimerism plays a rote in the deveIapment of unresponsive-

n=s to the grafti it is surprisiag that it can be detected in this situation.

In contrast to clonar mirrochimerism detectable after organ

transplantation, haematolymphopoietic chimwism is achieved at

reacliIy dekctablc lcvcls following administration nf aHogeneic

bone marrow to appropriately conditioned recipients. In this situ- ation, bonr marrow engraftment generally requires the dual strat- egy 01 ablation of the host’s haematnpoietic syshzm, in order to ‘make mom’ Lx the donor marrow, along with additional immuno-

suppression to prevent rejectim of the allagcncic cells: the former is

usuaOy achiet.4 by irradiation M radiomimetic drug+ the latter by

T-c& depletiibn or by immunosuppressive agents.

When &ne marrow transplantation is performed to treat haem-

atologic ma3gnanties. complctc ablation (e.g. 1ctW irradiation) is

gwwrally used. since 100% chimtzrism is desired to ensure elimi- nation of leukaemiacek Ey c~ntr~t, when chime&m isbeing used to induce transplantation tolerance, complete chimerism is neither

necessary nor desirable. Instead, it is preferable k achtwe A lcw but

persistent Eve1 of &nor lymphohaematopoietic chimerism, SO that

host-tppe immune-cell popuWions are available tQ pmDvi& im- munocolnpctence peripherally, while donor-derived cells (probably

dendriticcells> provide a persistent source ofantigen in thefhymus,

capable of effecting negative selectionz4, A non-my&aMakive PI-+ pxativc regimen, r;sing sublethal irradietioil and anti-T-cell mono- clcmal antibodies, has been demunstrated to achieve long-

lasting mixed Iy~CsII~naenlatopdetic chimerism without the

requirement for immunosuppressive therapy beyond the immedi- ate pust-transplant period’*~“” Data from these studies Aowed that T-cell dcplction and partiih ablntinrr of the rerip’pr-&immune sys-

tern before bone marrow infusion were trquird to chip-m the per-

sbtmt level of chimLtism necessary to induce ti!er;:t\f;e.

PhIkrmcl -en the Gw?w of rhimerism detected in these rjtuationr There is a fundamental difference between the microchimerism de- tected following ailogeneic organ tnnsplantatlon using standard

immunctsuppression. and the chimtrism t&nblishcd by intentiullal

bone marrow transplantation prior to an organ transplant {Fig. 11.

in :he former case. the chronic immunosuppressive regimen must be sufficient to suppress rejection of the graft, and as such it is un- doubtedly also sufficient to suppress the elimination nf danor cells which might escape from the transplanted organ. Therefort, dctec-

tion of such ~4s elsewhere in the recipient might be considered as

evidence of microchimerism, but it does not imply that lhe cells

detected are thernirar of the graft’s acceptance. On the contrary, they

may be the ~41 nF the graft’s acceptance and of the immuno-

suppression requirt?d to maintain that acceptance.

By contrast, when chlmerism is cstabtishcd by deliberate bone marrow infusion following T-ml1 depletion of the mcipjent, and par-

tial or complete abI8tion of the recipient’s lymphnhaematopoietic system, this chime&n is clearly the cause of tderancti527-z9. In this case, subsequent transplants of other tissues or organs From the same donor are uniformly acreptti Winout tPlt: requirement for additional

long-term immunosc,~pression. Moreover, the luss of tolerance fol-

lowing elimination of Jonor haematopoietic cells l&m the recipient

demonskateF that chimerism is responsible for inducing toleran&.

This fundamental mechanistic difference is also evident in the

behaviour of subsequent allogcafts from the same donor after E- mot4 of the original transplant. In the case of long-term graft ac- ceyiar?re induced by immunosuppressive agents, removal of the allografl leads to loss of the t&rant state over A period oi weeks to months following explant 71-3R. Thus, although a second graft is

usually accepted if transplanted imrndiately into such recipients, it

is r+rted if the animal is allowed to remain without it grafP”. By

contrast, when tolerance is induced by establishment of mixed

chime&m, that tolerance is stable after graft removal. A second alI*

graft from the same donor atrain will he accepted without immuno-

suppression at any time thereafter, for the life of the recipient9 This is not to say that detection of chimer-km is not important in

both cases. Indeed, regardless of the mechanism responsible for the

stablishment of mixed chimerism, its detection might serve as a

marker for graft acceptance, and thus might be useful diagnostic-

ally. Indeed, cells from the dnnor which escape to other sites might play a rule in diminishing the immune response to the trans:*lanP.

Hcavevw, since detectable microchimerkm after organ trsnsplan-

tatbn is not a cotisistetit finding in pntients with short- or long-term

stable graft functiorG, and micmchimerism hasbeen shown to per-

sist in patients during graft rejectimP, it might only be us&l as rl marker in conjunction with other parameters-‘. Thedetection or tack of detection of donor micmchim&sm after solid-organ transplan-

tation alone may be misleading.

Clinical implicatians Starzl and calleague Lowe proposed a paradigm in which the states

of immunologic tolerance achiowrd eithm by bone marrow tr:ns-

plantation or by organ transplantation are linked by a common de-

pcndence on the presence of hacmatr@etic chimerism’+. The de- scription of this paradigm ie important both bccausc iL provides a

theoretical construct for understanding the complex ~!leractions

DECEM8ER 1946

bebeen host and g&t that OCCUI fullowing any Itatisplant, and

because it suggests experiment;il m;lnipulntions whirh increase the

likelihood OF tolerance induction and which are therefore worthy of

further testing. Hcwever, given the likely diffewnres in mechanism

by which chime&m is irchieved in these two distinct situations, it would be p~matitie and potentially dangerous to discontinue im-

munosuppression in transplant patients solely on the baasis of the

detection of peripheral micrc~himerism. Imrnunosupprrssion gen-

craEy diminishes T--cell responses by suppressing the activity of

T ce1Iscapableof req+ziag the tritnsplankd tissue rather than by

eliminating them. Thewforc, when immunosuppressbn is stopped,

T-cell reactivity to the transplant ran IX expected to return. un1ess

some additional mechanbm to dcIete or inhibit the xtivity of donor-reactive T cells is arqujrPd in the interim.

Such rn+=chanismu may exist nnJ should & ~+oti. For ex-

ample, it might& possible to inducespecific aneqqy among rcsid~l

T cells during thp per%+ crt immunosuppressiuG. Alternatively, if

cell6 frum the transplant migra& to sites capable of achieving neg.+

tive selection of new T cells, IA&. the thymes, and ii sufficient time

pi~ass~s during the period of immunosuppressbn for existing T celIs

to be repIaced, then a deletional tolerance could resuIt. However,

some T cells areknown to be extremely long-Iived.F, thus the p-iud

neessary for swh a result b be A-tievcd coulc2 b long anJ variabk.

Clearly, additional laboratory studies directed towards elucidating

the mechanisms by which the unresponsive state i5 maintained are

essential bckrc it will besafe to discontinue immun-uppression in

clinical transplantation.

The authors would likp tG thank A. Bushell, I? Morris and M. Sykes- for

helpful &uggesttone and critical Reading nf the manuwripf.

DECEMBER I996