REVIEW J Am Soc Nephrol 14: 2402–2410, 2003

Memory T Cells: A Hurdle to Immunologic Tolerance

FADI G. LAKKIS* and MOHAMED H. SAYEGH† *Section of Nephrology, Department of Internal Medicine, and Section of Immunobiology, Yale University School of Medicine, New Haven, Connecticut; and †Renal Division, Brigham and Women’s Hospital, and Nephrology Division, Children’s Hospital, Harvard Medical School, Boston, Massachusetts.

The induction of immunologic tolerance is an important Immunobiology of Memory T Cells clinical goal in transplantation and autoimmunity. Immuno- Upon exposure to a foreign antigen, antigen-specific T logic tolerance is traditionally defined as specific unrespon- cells proliferate (the expansion phase) and differentiate into siveness to a self- or foreign antigen while maintaining reac- effectors that eliminate the foreign intruder (Figure 1). The tivity to other (third party) antigens (1,2). In the context of vast majority of effector T cells, however, undergo apopto- transplantation, a tolerant patient is someone who is capable of sis as the immune response progresses (the death phase), mounting an effective immune response to vaccines and mi- and the few lymphocytes that survive become long-lived crobial pathogens but is incapable of rejecting the transplanted memory T cells (the memory phase) (8,9). Memory T cells organ. Similarly, induction of tolerance to an antigen that that recognize microbial antigens provide the organism with incites self-reactivity would ensure that the patient is free of long-lasting protection against potentially fatal infections. autoimmune manifestations in the absence of global and harm- Conversely, memory T cells that recognize donor alloanti- ful immunosuppression. Although several immunomodulatory gens jeopardize the survival of life-saving organ transplants strategies have been used successfully to induce immunologic by mediating rejection (10). Therefore, the central question in the pursuit of transplantation tolerance is how to coerce tolerance in rodents, the same strategies have failed in larger an immune response determined to generate T cell memory animals and in humans. Examples of induced tolerance in into a state of antigen-specific unresponsiveness. To begin organ transplant recipients or in patients with autoimmune to answer this question, we first highlight the general char- disease have been rare and often unintentional (3–6). acteristics of memory T cells that distinguish them from Why, then, has clinical tolerance been such an elusive goal? naïve T cells. Second, we summarize what is known about The answer to this question most likely lies in the immunologic the generation, maintenance, and activation (recall) of mem- barriers that preclude the induction of antigen-specific unre- ory T cells. sponsiveness in the adult animal (1,2). These barriers include the limitations of peripheral (extrathymic) immunoregulatory Memory Advantage mechanisms that are commonly exploited to induce tolerance Memory T cells have over their naïve counterparts several (T cell deletion, suppression, deviation, and anergy), the large inherent advantages that endow them with the ability to clear a repertoire of alloreactive T cells in the case of transplantation, foreign antigen, whether a microbial pathogen or an allograft, the uncertain nature of the pathogenic antigen in many auto- in a vigorous manner (Table 1). The first advantage that immune diseases, and the unavoidable fact that the adaptive memory T cells have is that their response to a foreign antigen immune response, by virtue of evolutionary design, is destined (recall response) is greater in magnitude and faster than the to generate immunologic memory. It is the last barrier that is naïve T cell response. Memory T cells generate a considerable perhaps the most important obstacle to immunologic tolerance. number of effector T cells, capable of cytokine secretion and/or Here, we address this thesis by reviewing the immunobiology cytolytic activity, within hours of antigenic restimulation, of memory T cells and their impact on solid organ transplan- whereas naïve T cells generate a smaller number of effectors tation. The same concepts that apply to understanding and and at a much slower pace (days) (11–14). Second, memory T controlling memory T cells in the context of transplantation are cells have a survival advantage over their naïve counterparts also relevant to autoimmunity (7). (15,16). Antigen-specific memory T cell populations persist for years to a lifetime in humans, and their survival seems to be antigen and MHC independent (17–19). Mature naïve T cell populations also persist for a relatively long period of time (months to a few years in humans), but their survival is Correspondence to Dr. Fadi G. Lakkis, Section of Nephrology, 333 Cedar Street, P.O. Box 208029, New Haven, CT 06520-8029. Phone: 203-737-2619; dependent on constant, low-grade stimulation with MHC–self- Fax: 203-737-1810; E-mail: [email protected] peptide complexes (20–22). Third, the circulation of naïve T 1046-6673/1409-2402 cells is restricted to secondary lymphoid tissues, the site where Journal of the American Society of Nephrology they are activated by foreign antigens presented by antigen- Copyright © 2003 by the American Society of Nephrology presenting cells (APC), whereas memory T cells circulate DOI: 10.1097/01.ASN.0000085020.78117.70 through both secondary lymphoid tissues and peripheral non- J Am Soc Nephrol 14: 2402Ð2410, 2003 Memory T Cells 2403

Figure 1. Foreign antigen triggers the proliferation of antigen-specific T cells (the expansion phase) and their differentiation into effectors. The vast majority of effector T cells undergo apoptosis (the death phase) as the foreign antigen is eliminated, and the few lymphocytes that survive become long-lived memory T cells (the memory phase). Immunologic tolerance can perhaps be viewed as the demise of antigen-specific memory lymphocytes or their precursors (red arrows).

Table 1. Advantages of memory T cells over naõ¬ve and effector T cellsa

Characteristic Naõ¬ve T Cell Effector T Cell Memory T Cell

Survival Months to years Hours to days Years to lifetime Dependent on TCR Ag persistence leads Not dependent on TCR interaction with MHC/Ag to AICD interaction with MHC/Ag Migration Secondary lymphoid tissues Nonlymphoid tissues Both lymphoid and nonlymphoid tissues Response and effector Requires stimulation with Immediate Requires restimulation with function Ag Ag but is faster and larger in magnitude than naõ¬ve T cells In vivo protection against Long-lived but relatively Short-lived but very Long-lived and efficient foreign Ag inefficient efficient

a AICD, activation-induced cell death; Ag, antigen; TCR, T cell receptor for antigen. lymphoid tissues (23–25). Unlike naïve T cells, memory T pated. An antigen-experienced animal will eliminate the for- cells can directly encounter foreign antigen and mount a pro- eign antigen, whether a virus or an allograft, more efficiently ductive immune response within nonlymphoid tissues (25). than a naïve animal. The migratory advantage of memory T cells, therefore, allows them to detect and eliminate a foreign intruder long before it Generation of Memory T Cells reaches secondary lymphoid tissues. In summary, the activa- Antigenic stimulation of naïve T cells is a prerequisite for tion, survival, and migration advantages of memory T cells memory generation (26,27). It is also established that the confer the organism with enhanced protection against foreign magnitude of the expansion phase (burst size) of the primary antigens long after the primary immune response has dissi- immune response determines the size of the memory T cell 2404 Journal of the American Society of Nephrology J Am Soc Nephrol 14: 2402Ð2410, 2003 pool (28,29). Vaccination strategies that enhance T cell acti- naïve T cells that received suboptimal antigenic stimulation vation and proliferation by using strong adjuvants lead to larger and failed to proliferate. Importantly, it is not known how memory T cell populations. Because adjuvants directly influ- activated/effector T cells transition to the memory phenotype ence APC maturation and function, it is generally believed that (which is relatively quiescent and resembles that of a naïve T factors that stimulate the innate immune response, for example cell) and whether this transition is a stochastic process or an by inducing IFN-1 production or engaging Toll-like receptors instructional one (51). on APC (30,31), result in stronger adaptive immune responses Finally, recent studies propose the existence of two subsets and, subsequently, more memory T cells. Allografts are potent of CD4ϩ and CD8ϩ memory T cells based on the expression adjuvants as they contain their own APC and are subjected to of the homing receptors CC-chemokine receptor 7 (CCR7) and ischemia-reperfusion injury at the time of transplantation, L-selectin (CD62L) (56). The CCR7ϩ CD62Lhi “central” Ϫ lo which leads to activation of the innate immune system (32). In memory T cells (TCM) and CCR7 CD62L “effector” mem- fact, signaling via Toll-like receptors on graft dendritic cells ory T cells (TEM) have distinct migratory and functional char- plays an important role in initiating the alloimmune response acteristics (56Ð58). TCM circulate through secondary lymphoid ␥ (33). Therefore, it should not come as a surprise that transplant tissues, produce IL-2 but little IFN- and no perforin. TEM recipients harbor significant numbers of alloreactive memory T circulate through nonlymphoid peripheral tissues and produce and B cells. IFN-␥, perforin, and IL-4 but little IL-2. It has been proposed

Because optimal T cell activation and proliferation require that the secondary lymphoid tissue resident TCM are responsi- the presence of adequate T cell co-stimulation, several co- ble for replenishing the memory T cell pool because of their stimulatory molecule pairs contribute to the generation of increased proliferative capacity, whereas the nonlymphoid tis- ϩ memory T cells. CD4 T cell proliferation and memory gen- sue resident TEM mediate effector functions and rapid elimi- eration are significantly impaired but not completely absent in nation of antigen (23,24,56). The origin of these memory mice deficient in CD28, CD40L (CD154), or OX40 (34Ð36). subpopulations and whether they are interrelated are not com- ϩ ϩ However, generation of CD8 memory T cells is reduced in pletely understood. One study suggested that CD8 TEM de- mice that lack 4-1BBL, CD40L, CD27 (CD70L), and, to a scend from fully differentiated effectors, whereas TCM arise lesser extent, CD28 (37Ð41). Therefore, redundancy in T cell from intermediates that have not yet acquired effector function co-stimulation ensures that memory generation is not abolished (54). In contrast, Ahmed and colleagues (59) showed in a in the absence of a single co-stimulatory pathway. Likewise, no murine viral infection model that both subsets in the CD8ϩ single cytokine deficiency blocks memory T cell generation. compartment arise from fully differentiated effector T cells and ϩ This is particularly true of the CD4 population (42), whereas that TEM slowly convert over time to the TCM phenotype. the absence of IL-2 or IL-15 reduces the number of CD8ϩ T cells generated during the primary immune response (43Ð45). Maintenance of Memory T Cells Recent studies provide evidence that generation of CD8ϩ Memory T cell populations persist for a very long period of memory T cells is dependent on signals provided by CD4ϩ T time, often for the lifetime of an individual, in the absence of cells at an early time point in the primary immune response repeated antigenic exposure. At least two mechanisms are (46Ð48). Although the exact nature of these signals is still responsible for the long-term maintenance of memory T cell unclear, it is possible that CD4ϩ T cells condition the APC via populations: constant turnover (antigen-independent homeo- CD40LÐCD40 interactions to provide optimal CD8ϩ T cell static proliferation) and the intrinsic ability of memory T cells triggering (49). to survive in a relatively resting state for an extended duration Which activated T cells constitute the precursors of memory (19,43,60Ð63). The homeostatic proliferation of memory T T cells remains a subject of controversy. Some investigators cells is much slower than antigen-driven lymphocyte prolifer- have provided evidence that memory T cells are direct descen- ation but is considerably faster than the homeostatic prolifer- dants of effector T cells, whereas others contend that they arise ation of naïve T cells (43,60,62,63). Homeostatic proliferation from a second lineage of activated T cells, perhaps an “inter- of naïve T cells is what maintains the mature T cell pools in the mediate” cell population (8,50,51). Recent studies of CD8ϩ periphery even in the absence of thymic function (22). In a virus-specific memory T cells suggest a linear model of mem- viral infection mouse model, the CD8ϩ virus-specific memory ory T cell differentiation in which a naïve T cell is committed population turns over approximately once every 35 d to become an effector and, later, a memory T cell, shortly after (18,64,65). IL-15 is the central driver of memory T cell turn- antigenic stimulation (26,52,53). However, others have argued over in the CD8ϩ compartment (66,67), whereas IL-2 inhibits that effector differentiation is not required for the generation of homeostatic proliferation (43,63). Conversely, IL-7, seems to CD8ϩ memory T cells (54). Similarly, evidence has been be responsible for conferring a survival advantage to CD8ϩ presented to support either the linear differentiation of CD4ϩ memory T cells by upregulating antiapoptotic molecules such memory T cells from polarized effectors or their derivation as Bcl-2 and Bcl-XL (68). It is unclear what maintains CD4ϩ from a separate lineage of activated T cells with a nonpolar- T cell memory populations. Because long-lived CD4ϩ mem- ized, intermediate phenotype (27,55). Irrespective of which ory T cells can be generated from common cytokine receptor differentiation pathway is operational, memory T cells arise ␥-chain (␥c) knockout precursors, it seems that ␥c-binding from antigen-activated lymphocytes that proliferated during cytokines (IL-2, -4, -7, -9, and -15) are not required for either the expansion phase of the immune response and not from the generation or the maintenance of this memory T cell J Am Soc Nephrol 14: 2402Ð2410, 2003 Memory T Cells 2405 population (42). Finally, it is not known whether the rules that lose the ability to migrate to secondary lymphoid tissues as apply to the maintenance of the overall memory T cell popu- they shed the CCR7 and CD62L (also known as L-selectin) and lation also apply to the maintenance of the “effector” and acquire adhesion molecules and tissue-specific chemokine re- “central” memory subsets. ceptors that direct them to nonlymphoid tissue microenviron- ␣ ments (e.g., to skin: CCR8, CCR10, CLA; to gut mucosa: 4␤7 Recall of Memory T Cells integrins; and to sites of inflammation: CCR5) (23,24,81Ð83). When restimulated with antigen, memory T cells multiply Homing receptor expression depends on the route of antigen and differentiate into effector T cells much more rapidly than exposure during the primary immune response with preferen- their naïve counterparts. It is estimated that the average time tial migration of memory T cells back to the site of initial that it takes a virus-specific CD8ϩ memory T cell to differen- antigen entry (84,85). Both CD4ϩ and CD8ϩ memory T cells tiate into a cytotoxic T lymphocyte is 12 h as opposed to can be isolated from nonlymphoid tissues even in the absence several days for a naïve CD8ϩ T cell (Rafi Ahmed, Emory of inflammation (23,24), suggesting that memory T cells cir- University, personal communication, June 2001). In addition, culate through these tissues. More recently, it has been dem- the number of effector T cells generated during a recall re- onstrated that, unlike naïve T cells, memory T cells mount a sponse exceeds the number generated during a primary im- productive immune response independent of secondary lym- mune response by approximately fivefold (53). The vigor of phoid tissues. In a murine transplantation model, as little as a the recall response can be attributed to the increased frequency few thousand alloreactive memory T cells migrated directly to of antigen-specific CD4ϩ and CD8ϩ T cells in the memory the allograft and mediated its rejection in an immunodeficient population (28,69Ð71) and to the intrinsic antigen hyperre- host that lacks the spleen, lymph nodes, and mucosal lymphoid sponsiveness of memory T cells (11,12,71Ð73). The antigen structures (25). This observation applies to both CD4ϩ and hyperresponsiveness of memory T cells is best exemplified by CD8ϩ memory T cell recall. Therefore, the migratory advan- the short lag time to cytokine production, entering cell cycle, tage of memory T cells allows them to detect and eliminate a and acquisition of effector function after antigen stimulation. foreign antigen long before it reaches secondary lymphoid ϩ In fact, CD8 effector memory T cells (TEM) may have tissues. In contrast, naïve T cells are activated only after constitutive cytolytic function (56), effectively reducing lag antigen-bearing dendritic cells have reached the draining time between antigen encounter and effector function to zero. lymph nodes or the spleen, a process that could take at least The mechanisms responsible for the antigen-hyperrespon- 2d. siveness of memory T cells are not completely understood but may include enhanced signaling via the T cell receptor (TCR) Memory T Cells and Allograft Rejection complex. For example, plasma membrane-associated Lck, a The same characteristics of memory T cells that make them protein kinase that phosphorylates CD3 and the CD3-associ- very efficient at eliminating microbial pathogens also enable ated ␨ chain leading to enhanced TCR signaling, has been them to rapidly reject foreign tissues. Unlike inbred mice, observed in memory T cells (74,75). Gene expression profiling outbred animals including humans harbor a significant number experiments confirmed that several gene products associated of memory T cells that are alloreactive. These memory T cells with TCR signaling are elevated in memory T cells compared generally arise if an individual is exposed to alloantigens via with naïve T cells (76). In addition, memory T cells may be pregnancy, blood transfusion, or a previous organ transplant. hyperresponsive to antigen because of affinity maturation, a However, alloreactive memory T cells also exist in individuals process in which T cells bearing TCR with high affinity to who have never been exposed to foreign tissues before (10). It antigen are selected during the immune response (9,77). Re- is proposed that such alloreactivity in the T cell memory pool duced activation threshold of memory T cells via the TCR is caused largely by cross-reactivity; in other words, exposure complex is consistent with mounting evidence that the activa- to viral and bacterial antigens over time leads to the develop- tion of memory T cells is less dependent on co-stimulation than ment of memory T cells that also recognize alloantigens. This naïve T cells (74,78Ð80). Normal cytokine production is ob- phenomenon, commonly referred to as heterologous immunity, served when ovalbumin-specific TCR-transgenic CD4ϩ mem- has been demonstrated in murine infection models (86,87) and ory T cells are stimulated with antigen-loaded APC that lack is supported by experimental findings in humans (88). either B7 or CD40 (80). Moreover, the proliferation of these Several investigators have provided evidence that alloreac- CD4ϩ memory T cells is normal in the absence of CD40 and tive memory T cells indeed contribute to both acute and is only slightly impaired in the absence of B7 (80). Antigen- chronic allograft rejection. Heeger et al. (10) found that the specific CD8ϩ memory T cells confer normal protective im- pretransplant frequency of donor-specific memory T cells, munity in the absence of CD28 (41). Similarly, CD40ÐCD40L identified by their ability to produce IFN-␥ within 24 h of co-stimulation is dispensable for the recall of alloreactive allostimulation, correlates with the risk of rejection after trans- CD8ϩ memory T cells (35). Recent work suggests that CD27Ð plantation. Others have observed that the presence of memory- CD70 interaction may be important for the recall of CD8ϩ phenotype T cells (CD45ROϩ) in heart and kidney allograft memory T cells (39). biopsies and in the peripheral blood of transplant recipients Another important advantage of memory T cells over naïve correlates with the incidence and severity of rejection (89Ð92). T cells is their ability to seek antigen in nonlymphoid tissues. This was also true in patients experiencing acute rejection Memory T cells, particularly those of the “effector” phenotype, episodes 2 to 18 yr after transplantation (93). In addition, the 2406 Journal of the American Society of Nephrology J Am Soc Nephrol 14: 2402Ð2410, 2003 frequency of IFN-␥Ðproducing peripheral blood lymphocytes, of cardiac allograft survival induced through a combination of possibly memory T cells, responding to donor-derived peptides anti-CD40L monoclonal antibody treatment and donor spleen via the indirect antigen presentation pathway are more fre- cell transfusion. The same therapeutic strategy also failed to quently found in renal transplant recipients who previously prolong allograft survival or to induce tolerance in mice har- experienced acute rejection episodes than in those without boring cross-reactive memory T cells (e.g., mice previously rejection (94). Although these studies do not establish a causeÐ infected with lymphocytic choriomeningitis virus or Leishma- effect relationship between T cell memory and rejection, they nia major) (86,87). A recent manuscript provided direct evi- suggest that the persistence of memory T cells hinders long- dence that virally induced alloreactive memory T cells interfere term allograft survival in humans. Moreover, donor-reactive T with the most robust form of transplantation tolerance, that cells with memory phenotype have been detected in organ induced by mixed allogeneic chimerism (104). In this study transplant recipients receiving high levels of immunosuppres- (104), CD8ϩ “central” memory T cells were responsible for sion (10,94), indicating that current immunosuppressive ther- preventing tolerance, and they did so in a dose-dependent apies do not inhibit the generation or maintenance of T cell manner. Therefore, alloreactive T cells, whether generated by memory. previous encounter to alloantigens or via heterologous immu- The existence of allospecific memory T cells in experimen- nity, constitute a formidable barrier to achieving transplanta- tal models of alloimmunity was first suggested by the finding tion tolerance. Alloreactive memory T cells generated as a that spleen cells from allosensitized mice mount a stronger result of heterologous immunity may very well offer an expla- mixed leukocyte response and generate more cytotoxic effec- nation for why transplantation tolerance has been such an tors than naïve splenocytes (95). More recently, TCR-trans- elusive goal in human and nonhuman primates that have a rich genic models have been used to identify allospecific CD8ϩ immunologic history of exposure to environmental and micro- memory T cells generated after cardiac allograft rejection in bial antigens. mice (96). Memory T cells mediate an accelerated form of rejection as indicated by earlier demonstration that mice that Repressing Undesirable Memories have previously rejected an allograft reject a second graft from The success of experimental tolerance strategies has been the same donor with accelerated kinetics, a phenomenon re- largely restricted to naïve hosts that do not harbor significant ferred to as second set rejection (97). Furthermore, purified numbers of memory T cells, for example, mice housed in a memory T cell populations, unlike their naïve counterparts, pathogen-free environment (1). These strategies induce immu- cause allograft rejection without the need to home first to nologic tolerance to a foreign antigen by exploiting the same secondary lymphoid tissues (25), and the rejection response is principles that underlie tolerance to self-antigens: deletion, not inhibited by agents that block CD28 or CD40L (CD154) T anergy, suppression, and immune deviation (105). Deletion is cell co-stimulation (Sayegh et al., unpublished data). These mediated by the apoptosis of antigen-specific naïve or recently findings underscore the formidable challenge to prevent suc- activated T cells leading to antigen-specific unresponsiveness. cessfully the recall of alloreactive memory T cells. Alternatively, a tolerance-inducing strategy could inactivate T cells without causing their death (anergy), generate regulatory Memory T Cells and Transplantation Tolerance cells that block T cell activation and function (suppression), or Memory T cells not only endanger allograft survival by induce the differentiation of naïve T cells into a nonharmful causing both acute and chronic rejection but also impede the phenotype (immune deviation). Therefore, for tolerance to induction of transplantation tolerance. Several rodent studies work in immunologically experienced hosts, such as humans, provide indirect evidence in support of this observation. Re- one first has to ask whether memory T cells can be “tolerized.” sistance to neonatal tolerance in certain mouse strains is asso- In other words, are memory T cells subject to deletion, anergy, ciated with the persistence of memory-type T cells (98), suppression, or immune deviation in the same way that naïve T whereas successfully tolerized mice have much lower frequen- cells are? cies of these cells (99,100). Moreover, immunomodulatory The answers to these questions are beginning to emerge, and agents that do not consistently induce donor-specific tolerance the initial data are encouraging. First, CD8ϩ memory T cells invariably fail to suppress immunologic memory in experimen- seem to be as susceptible to tolerance as their naïve counter- tal animals (100,101). parts. Kreuwell et al. (106) demonstrated that influenza hem- Direct evidence that memory T cells impede tolerance in- agglutinin A (HA)-specific CD8ϩ memory T cells undergo duction derives from studies using co-stimulation blockade peripheral tolerance after either exogenous administration of and/or mixed allogeneic chimerism strategies. Zhai et al. (102) soluble HA peptide or cross-presentation of the HA antigen by found that anti-CD40L (CD154) monoclonal antibody therapy, dendritic cells in transgenic mice that produce HA in the which induces transplantation tolerance in some rodent strain pancreatic ␤ cells (the latter phenomenon is referred to as combinations, fails to do so in mice presensitized with skin cross-tolerance). Tolerance of CD8ϩ memory T cells to the grafts from the same donor. They further demonstrated that, HA antigen was mediated by peripheral clonal deletion. Sec- unlike naïve T cells, T cells from presensitized mice can be ond, although memory T cells are less susceptible to apoptosis activated in vitro in the absence of CD40ÐCD40L co-stimula- than naïve T cells, they can still be coerced to die. Memory tion. Valusjkikh et al. (103) found that the adoptive transfer of CD8ϩ T cells that migrate to immune-privileged sites undergo primed donor-reactive T cells interferes with the prolongation apoptosis mediated by the TNF receptor (TNFR) family (Dai, J Am Soc Nephrol 14: 2402Ð2410, 2003 Memory T Cells 2407

Lakkis, et al., unpublished observations). Third, memory T immunologic tolerance would perhaps become a reality if we cells are subject to suppression by regulatory T cells. were to master the art of repressing undesirable memories CD4ϩCD25ϩ T cells suppress in vivo CD8ϩ memory T cell while keeping the sweet and desirable ones alive. responses to Listeria monocytogenes (107), and, in an adoptive transfer model, they significantly delay allograft rejection me- diated by CD8ϩ memory T cells (Dai and Lakkis, unpublished References observations). Fourth, memory T cells are not rigidly differen- 1. Salama A, Remuzzi G, Harmon W, Sayegh M: Challenges to achieving clinical transplantation tolerance. J Clin Invest 108: tiated and may be amenable to immune deviation. Ahmadza- ϩ 943Ð948, 2001 deh and Farber (108) observed that antigen-specific CD4 2. Chalasani G, Lakkis, FG: Immunologic ignorance of organ al- memory T cells produce either T helper 1 or T helper 2 effector lografts. 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