The Thymic Way to Transplantation Tolerance1 Giuseppe Remuzzi,2 Norberto Perico, Charles B. Carpenter, and Mohamed H. Sayegh the thymus and the possible mechanism(s) of ac- G. Remuzzi, N. Perico, Mario Negri Institute for Phar- quired thymic tolerance are reviewed. macological Research and Division of Nephrology, Key Words: Thymus. self-tolerance, acquired tolerance, pan- Ospedali Riuniti di Bergamo, Bergamo, Italy creatic islets. kidney, heart, liver, small bowel, skin, MHC C.B, Carpenter. M,H. Sayegh, Laboratory of Immuno- allopeptides. transplantation genetics and Transplantation, Renal Division, Depart- ment of Medicine, Brigham and Women’s Hospital, T he major goal in transplantation research is the Harvard Medical School, Boston, MA development of strategies that lead to specific immunologic unresponsiveness or tolerance to the (J, Am, Soc. Nephrol. 1995; 5:1639-1646) allograft. The thymus plays the major role in devel- opment of self-tolerance (1 ,2), and initial work by ABSTRACT Waksman and colleagues more than 20 yr ago (3,4) indicated that the thymus may play a role in the Within the pastthree decades, extensive research has induction of acquired tolerance to exogenous anti- been carried out with the aim to prevent graft rejec- gens. Recently, there has been a renewed interest in tion by minimizing the side effects related to the use of studying the role of the thymus in acquired tolerance, immunosuppressants. The major goal in transplanta- and several investigators have shown that the intra- tion research remains the development of strategies thymic injection of antigen induces a state of specific that would allow one to achieve a state of donor- systemic unresponsiveness in experimental autoim- specific unresponsiveness in order to promote a con- mune (5-7) as well as transplantation models (see dition of true tolerance without the need of immuno- below). The purpose of this article is to provide the suppressants. Recent evidence has been provided reader with an overview of the role of the thymus in that this Is a pursuing goal, at least in experimental acquired tolerance with special emphasis on trans- plantation tolerance, review the mechanisms of ac- animals. The thymus plays the major role in the devel- quired thymic tolerance, and discuss the potential opment of self-tolerance, and initial work in the late applications of acquired thymic tolerance in humans. 96Os indicated that the thymus also plays a critical role in the induction of acquired tolerance to exoge- THE ROLE OF THE THYMUS IN “SELF”-TOLERANCE nous antigens. Recently, the interest in acquired thy- Studies of the crystal structure and function of the mic tolerance has been renewed by the observation major histocompatibility complex (MHC) molecules that, in the rat, the thymus is an immunologically and the sequencing of T cell receptors (TCR) have privileged site in which isolated pancreatic islets can yielded important information on the mechanisms of T be engrafted and survive indefinitely. Moreover, intra- cell recognition of antigen. Foreign antigens are rec- thymic injection of the islets induced donor-specific ognized after they are processed and presented as unresponsiveness, which allowed survival of a second peptides in the binding groove of cell surface mole- donor-strain islet cell allograft transplanted into an cules encoded by the MHC genes to the cs//3 TCR extrathymic site. These findings on cellular allografts (8-12). The TCR on each T cell are identical and specific for a given self-MHC plus antigenic peptide have been extended to vascularized organ al- complex (9,10,13). TCR are selected to react with lografts. Studies have documented that, in rodents, foreign (nonseif-) antigens: those that recognize autol- the intrathymic injection of donor cells induces a state ogous (self-) antigens are anergized or eliminated. of tolerance and prolongs the survival of allografts, Failure of these mechanisms may result in autoim- including kidney, heart, liver, and small bowel. Unre- mune diseases. Self- /nonself-discrimination is estab- sponsiveness to organ graft is donor but not tissue lished, early in life (14), and is dependent on a func- specific, and evidence is presented here that the tionally intact thymus, as suggested by early thymus has a central role in such a phenomenon. experiments on thymus transplanted mice (15). TCR, The nature of the alloantigen(s) being recognized in unlike the antigen receptors of B cells, are not se- creted after antigen stimulation but remain on the 1 Received August 5, 1994. Accepted October 19, 1994. surface of T lymphocytes in two heterodimeric forms 2correspondence to Dr. G. Remuzzi. Mario Negri Institute for Pharmacological (ie., a and 13 or y and proteins) (10). Throughout life, Research, Via Gavazzeni 11, 24125 Bergamo, Italy. antigen receptors of the immune system are generated 1046-6673/0509-1 639$03.00/0 Journal of the American society of Nephrology by the random rearrangement of DNA segments en- copyright © 1995 by the American society of Nephrology coding the variable parts of the a and f3 chains of TCR, Journal of the American Society of Nephrology 1639 Transplantation Tolerance with specificity for self- and nonseif-antigens (16). promote cell deletion (18,24). Whatever the mecha- This implies that the capacity to distinguish self from nisms are, only 1 to 5% of the total thymocyte popu- nonself is not genetically determined but is acquired lation completes maturation; the remaining 95 to 99% during the process ofT cell development. Although the of T cells are deleted in the thymus (25) (Figure 1). random rearrangement of the gene gives rise, by dif- Apoptosis is a form of cell death more subtle than ferent combinations of the variable segments of the necrosis, which requires new gene expression (26,27). two chains, to a different T cell repertoire that has As cells undergo apoptosis, the surface membrane more than iO possible specificities (1 7), mature T becomes ruffled and produces blebs (apoptotic bod- cells have in fact much fewer TCR combinations, les), which are usually phagocytosed by other cells. because of a number of selection events occurring in Coincident with these changes, nuclear DNA is di- the thymus (18). gested by an endogenous nuclease that cleaves chro- Precursor T lymphocytes reach the thymus early in matin between individual nucleosomes (27). Genes fetal life, although there is a continuous input of and mechanisms that control programmed cell death prothymocytes throughout life (1 9). Immature T cells are the subject of intense investigation. One such gene reside in the cortex and lack the TCR, CD4, and CD8 has been identified, bcl-2, and functions as a repres- molecules. These cells undergo population growth, sor of programmed cell death in many, but not all, rearrangement of TCR genes, and surface expression types of cells (28). In the thymus, bcl-2 is present in of CD4 and CD8 molecules (“double-positive” thymo- the mature surviving thymocytes of the medulla, but cytes) (14, 18-22). TCRCD4CD8 T cells interact is not expressed in most cortical thymocytes, the with thymic epithellal cells or bone marrow-derived majority of which die by apoptosis (28). Thus, bcl-2 macrophages/dendritic antigen-presenting cells appears to be involved in the salvation of maturing T (APC) that express self-MHC molecules and undergo cells, as shown by data that the introduction of bct-2 selection processes that ultimately shape the T cell into the normally vulnerable cortical thymocytes pro- repertoire toward self-MHC restriction and self-toler- ance. Self-tolerance is mediated by “negative selec- tion” by the deletion of autoreactive T cell clones (14). Precursor Surviving TCR T cells move into the medulla, in- crease the density ofTCR expression to that of periph- eral T cells, and before leaving the thymus, lose one of Thymus the costimulatory accessory molecules, thus becom- if Double-positive CD4 ing “single-positive” (CD4CD8 T cells or CD4CD8 thymocyte T cells) T cells. At the same time, “positively selected” medullary thymocytes acquire MHC Class Il-re- stricted helper (CD4) or MHC Class I-restricted cy- CD8 totoxic (CD8) functions and migrate to the periphery. The way these two processes occur is still debated, and two working hypothesis have been proposed. von Boehmer and Kisielow (23) provided a fundamental contribution to the current understanding of thymic selection, using transgenic SCID mice bred to express N.gotlv. SIctIon Posittv#{149}51#{149}ction one type of TCR only, specific for the male (H-Y) antigen presented by Class I MHC molecules. Their 11’ APOPTOS1S data suggest that double-positive thymocytes form a MATURATiON CD4 heterogeneous population of potentially “useful,” “useless,” and “harmful” cells. They elaborated the theory that cells with a TCR complementary to self- MHC are selected for further maturation, but not all * MHC-restrlcted T cells reach full maturity. Those with To prlphry a TCR complementary fitting MHC molecule plus self- Figure 1. T cell development In the thymus. Bone marrow- antigen are eliminated by apoptosis (programmed cell derived T cell precursors migrate to the thymus, where TCR death of “harmful” T cells in the thymus). The remain- genes are randomly expressed in parallel to the concomi- ing T lymphocytes, MHC restricted but not autoreac- tant expression of the CD4 and CD8 molecules on their tive, down-regulate one of the two costimulatory mol- surface (“double-positive” thymocytes). At this stage, cells ecules (CD4 or CD8) and colonize the peripheral with TCR complementary to self-MHC and also to self-antigen lymphoid compartments as mature T cells. As a pos- are destroyed by a process of programmed cell death sible alternative, the “affinity-avidity” hypothesis of T (apoptosis); cells that recognize self-MHC molecule but not cell development implies a low-affinity interaction be- the self-antigen are rescued from death and mature to CD4 tween the TCR and the thymic ligand as the factor or CD8 cells, which leave the thymus and colonize the underlying positive selection and survival of the thy- peripheral lymphold tissues.