CD73 and adenosine generation in the creation of regulatory microenvironments

Extracellular adenosine 5'-triphosphate (ATP) acts on many immune cells to promote inflammation. Conversely, the ATP metabolite adenosine is mainly an anti-inflammatory molecule. The ecto-enzymes CD39 and CD73 can dephosphorylate extracellular ATP to adenosine, thereby controlling this important pathway of immune modulation. Despite their established roles in the immune system, little is known of how CD39 and CD73 are themselves regulated. Recent data have shown that CD73 expression and adenosine generation are up-regulated by transforming growth factor-β, depending on the cytokine content of the local microenvironment. We review here these recent findings and discuss their implications in disease.

Th17 cells induce a distinct graft rejection response that does not require IL-17A

IL-17A-producing helper T (Th17) cells have been implicated in the pathogenesis of autoimmune disease, inflammatory bowel disease and graft rejection, however the mechanisms by which they cause tissue damage remain ill-defined. We examined what damage Th17 cell lines could inflict on allogeneic skin grafts in the absence of other adaptive lymphocytes. CD4(+) Th17 cell lines were generated from two TCR transgenic mouse strains, A1(M).RAG1(-/-) and Marilyn, each monospecific for the male antigen Dby. After prolonged in vitro culture in polarizing conditions, Th17 lines produced high levels of IL-17A with inherently variable levels of interferon gamma (IFNγ) and these cells were able to maintain IL-17A expression following adoptive transfer into lymphopenic mice. When transferred into lymphopenic recipients of male skin grafts, Th17 lines elicited a damaging reaction within the graft associated with pathological findings of epidermal hyperplasia and neutrophil infiltration. Th17 cells could be found in the grafted skins and spleens of recipients and maintained their polarized phenotype both in vivo and after ex vivo restimulation. Antibody-mediated neutralization of IL-17A or IFNγ did not interfere with Th17-induced pathology, nor did it prevent neutrophil infiltration. In conclusion, tissue damage by Th17 cells does not require IL-17A.

CD8+ T-Cell depletion and rapamycin synergize with combined coreceptor/stimulation blockade to induce robust limb allograft tolerance in mice

The growing development of composite tissue allografts (CTA) highlights the need for tolerance induction protocols. Herein, we developed a mouse model of heterotopic limb allograft in a stringent strain combination in which potentially tolerogenic strategies were tested taking advantage of donor stem cells in the grafted limb. BALB/c allografts were transplanted into C57BL/6 mice treated with anti-CD154 mAb, nondepleting anti-CD4 combined to either depleting or nondepleting anti-CD8 mAbs. Some groups received additional rapamycin. Both depleting and nondepleting mAb combinations without rapamycin only delayed limb allograft rejection, whereas the addition of rapamycin induced long-term allograft survival in both combinations. Nevertheless, robust donor-specific tolerance, defined by the acceptance of a fresh donor-type skin allograft and simultaneous rejection of third-party grafts, required initial CD8(+) T-cell depletion. Mixed donor-recipient chimerism was observed in lymphoid organs and recipient bone marrow of tolerant but not rejecting animals. Tolerance specificity was confirmed by the inability to produce IL-2, IFN-gamma and TNF-alpha in MLC with donor antigen while significant alloreactivity persisted against third- party alloantigens. Collectively, these results show that robust CTA tolerance and mixed donor-recipient chimerism can be achieved in response to the synergizing combination of rapamycin, transient CD8(+) T-cell depletion and costimulation/coreceptor blockade.

Fc-disabled anti-mouse CD40L antibodies retain efficacy in promoting transplantation tolerance

CD40L antibodies have proven to be powerful immunosuppressive agents in nonhuman primates but unfortunately perturb blood coagulation. Neither the therapeutic nor the prothrombotic mechanism of anti-CD40L is defined sufficiently to determine whether these effects can be uncoupled. Recent evidence suggests that the Fc region of anti-CD40L antibodies interacting with Fc receptors plays an important role in stabilizing platelet aggregates. An Fc-disabled, aglycosylated anti-CD40L heavy chain variant was therefore created to determine whether it might still be useful in promoting transplantation tolerance. In a number of mouse models an engineered aglycosyl anti-CD40L recapitulated the effects of the intact anti-CD40L antibody in tolerance protocols involving transplantation of allogeneic bone marrow and skin. In contrast, another anti-CD40L variant with a conventional rat gamma2b heavy chain was less effective in ensuring long-term skin graft survival, possibly associated with its faster clearance from the circulation. These results show that short pulses of anti-CD40L antibody therapy may still be useful in tolerance protocols even when the Fc region is disabled.

Regulatory T cells and transplantation tolerance

Rodent models of transplantation have demonstrated that it is possible to induce specific immunological tolerance of donor antigens and indefinite graft survival in the absence of any continued immunosuppression. If this situation could be achieved clinically it would avoid many of the longer term complications of organ grafting, such as the increased risk of infection and cancer and the nephrotoxicity of many immunosuppressive agents. In this review we shall consider the interplay between regulatory T cells, dendritic cells and the graft itself and the resulting local protective mechanisms that are coordinated to maintain the tolerant state. We will discuss how both anti-inflammatory cytokines and negative costimulatory interactions can elicit a number of interrelated mechanisms to regulate both T cell and antigen-presenting cell activity. The induction and maintenance of tolerance via acquired local immune privilege has implications for the design of therapeutic regimens and the monitoring of the tolerant status of patients being weaned off immunosuppression.

Therapeutic aspects of tolerance

The immune system is naturally unresponsive to 'self' antigens. Improved knowledge of mechanisms underlying self tolerance is giving rise to a new generation of immunosuppressive agents, that can exploit these mechanisms and so reduce the nature and level of medication that needs to be given long-term to control diseases where the immune system does harm.

The role of CD4+ T-cell subsets in determining transplantation rejection or tolerance

Peripheral tolerance to allogeneic organ grafts can be induced in rodents by treating with non-depleting CD4 and CD8 monoclonal antibodies. This tolerance is maintained by CD4+ T cells with a potent capacity to induce tolerance in further cohorts of T cells (i.e. infectious tolerance). We have cloned CD4+ T-cell subsets against the male transplantation antigen in vitro and find, in contrast to Th1 or Th2 clones that elicit rejection, that there is a distinct population of CD4+ T cells that suppress rejection by adoptive transfer (here called Treg). In order to identify molecular markers associated with tolerance and gain insights into the mechanisms of action of Treg cells, we carried out serial analysis of gene expression. We identified genes overexpressed in Treg compared to Th1 and Th2 cultures and found that some of these correlated in vivo with CD4-induced transplantation tolerance rather than rejection. The genes overexpressed in Treg cultures and within tolerated skin grafts were primarily expressed by mast cells (e.g. tryptophan hydroxylase and FcepsilonR1alpha), suggesting that regulatory cell activity and this form of tolerance may be associated with a localised but non-destructive form of Th2-like activation and a recruitment of mast cells.

CD4 and CD8 monoclonal antibody therapy in canine renal allografts

Therapy with CD4 and CD8 monoclonal antibodies was evaluated in dogs which received double-haplotype MHC-mismatched renal allografts. Neither CD4 nor CD8 monoclonal antibodies given alone prolonged allografts survival (creatinine > or = 300 micromol/l) beyond 7 days. However, combined therapy with CD4 and CD8 antibodies given up to day 10 did prolong allograft survival to a median of 14 days. A longer (21 day) course of CD4 and CD8 antibodies did not extend allograft survival further. The effect of prolonged antibody therapy was restricted by the occurrence of both an antiglobulin response and an anaphylactoid reaction to the monoclonal antibody preparation. When the CD4 and CD8 antibodies were combined with a pan-T-cell-depleting Thy-1 antibody, the survival of double-haplotype mismatched allografts was further prolonged (median 16 days). The median survival of single-haplotype mismatched renal allografts on this triple therapy was 21 days, with one surviving to day 36.

Cutting edge: anti-CD154 therapeutic antibodies induce infectious transplantation tolerance

Nondepleting anti-CD154 (CD40 ligand) mAbs have proven effective in inducing transplantation tolerance in rodents and primates. In the induction phase, anti-CD154 Ab therapy is known to enhance apoptosis of Ag reactive T cells. However, this may not be the sole explanation for tolerance, as we show in this study that tolerance is maintained through a dominant regulatory mechanism which, like tolerance induced with CD4 Abs, manifests as infectious tolerance. Therefore, tolerance induced with anti-CD154 Abs involves not only the deletion of potentially aggressive T cells, but also a contagious spread of tolerance to new cohorts of graft-reactive T cells as they arise.

High dose bone marrow transplantation induces deletion of antigen-specific T cells in a Fas-independent manner

BACKGROUND

Monoclonal antibody induced tolerance to high doses of multiple lymphocyte stimulating (MLS)+minor mismatched bone marrow has recently been associated with clonal deletion, as reported in fully allogeneic models of bone marrow transplantation. FasL-induced apoptosis has been shown to mediate antigen-specific T cell deletion after antigenic stimulation in wild-type and T cell receptor transgenic mice. Therefore, we investigate a role for the Fas pathway in deletional tolerance to high dose bone marrow.

METHODS

Fas mutant and control mice (H-2k, MLS-1b) were tolerized under the cover of monoclonal antibodies to high dose (5 x 10(7) cells) AKR (H-2k, MLS-1a) bone marrow. Tolerance was confirmed by AKR skin grafting after antibody clearance. Antigen-reactive cell deletion was monitored by Vbeta6+ T cell elimination, measured by flow cytometry of peripheral blood throughout the experiment. Donor T cell (Thy1.1+) chimerism was assessed in a similar manner.

RESULTS

Fas mutant mice infused with high dose AKR bone marrow under the cover of antibody were tolerant, as demonstrated by indefinite survival of AKR skin grafts. When high levels of donor cell chimerism were established in Fas mutant mice, peripheral deletion of antigen-reactive cells was observed to be independent of signaling through Fas.

CONCLUSIONS

Apoptosis mediated by Fas receptor signaling is not the mechanism of clonal deletion of antigen-reactive cells after antibody facilitated high dose marrow transplantation. However, the Fas mutation does impair the development of adequate donor chimerism.

Dominant tolerance and linked suppression induced by therapeutic antibodies do not depend on Fas-FasL interactions

BACKGROUND

Nonlytic anti-CD4 monoclonal antibody therapy can be used to induce transplantation tolerance in rodent models. Such tolerance is often associated with dominant regulation, mediated by CD4+ cells, and characterized by infectious tolerance and linked suppression. Understanding the mechanisms by which CD4+ regulatory cells function may improve the manner in which current immunosuppressants are applied and may lead to the development of new tolerance-inducing therapeutics. Fas-mediated apoptosis has been characterized as an important mechanism of peripheral self-tolerance and we here examine whether it has any role in anti-CD4 monoclonal antibody-induced dominant tolerance.

METHODS

Tolerance to transplanted skin and bone marrow, mismatched for multiple minor histocompatibility antigens, was induced in Fas mutant and control mice using anti-CD4 and anti-CD8 monoclonal antibodies. To test for linked suppression, animals were transplanted with a second graft-bearing tolerated and third party antigens. The ability of splenocytes from tolerant animals to suppress graft rejection was assessed by transfer into partially immunocompromised recipients.

RESULTS

Monoclonal antibody therapy rendered Fas mutant mice tolerant of minor disparate skin and bone marrow. Splenocytes from these and control tolerant animals when transferred into partially immunocompromised Fas mutant or control recipients, induced antigen-specific suppression of graft rejection. Additionally, tolerant Fas mutant mice accepted grafts bearing tolerated and third party antigens.

CONCLUSIONS

Signal transduction through the Fas receptor plays no essential role in the induction of tolerance using anti-CD4 and anti-CD8 monoclonal antibodies or its maintenance by active regulation.

Anti-globulin responses to rat and humanized CAMPATH-1 monoclonal antibody used to treat transplant rejection

BACKGROUND

Antiglobulin responses are a significant limitation to the repeated use of murine monoclonal antibodies for treatment of transplant rejection. It is hoped that these might be largely overcome by using antibodies genetically engineered to resemble human antibodies.

METHODS

We have compared the responses in patients treated with the CD52 monoclonal antibodies CAMPATH-1G (rat IgG2b) or its humanized derivative, CAMPATH-1H (human immunoglobulin G1).

RESULTS

A majority of patients (15 of 17) made responses to the rat antibody, but there were no detectable responses to the humanized antibody (0 of 12).

CONCLUSIONS

Although anti-idiotype responses are theoretically possible against humanized therapeutic antibodies and are especially likely to be provoked by cell-binding antibodies, these data show that humanization offers a significant reduction in immunogenicity, potentially allowing repeat courses of treatment.

CD40 ligand blockade induces CD4+ T cell tolerance and linked suppression

The CD40-CD40 ligand (CD40L) interaction is a key event in the initiation of an adaptive immune response, and as such the therapeutic value of CD40L blockade has been studied in many experimental models of tissue transplantation and autoimmune disease. In rodents, transplantation of allogeneic tissues under the cover of anti-CD40L Abs has resulted in prolonged graft survival but not tolerance. In this report, we show that failure to induce tolerance probably results from the inability of anti-CD40L Abs to prevent graft rejection elicited by the CD8+ T cell subset. When the CD8+ T cell population is controlled independently, using anti-CD8 Abs, then tolerance is possible. Transplantation tolerance induced by anti-CD4 mAbs can often be associated with dominant regulation, manifested as infectious tolerance and linked suppression, both of which are mediated by CD4+ T cells. We show here that CD4+ T cells rendered tolerant using anti-CD40L therapy exhibit the same regulatory property of linked suppression, as demonstrated by their ability to accept grafts expressing third party Ags only if they are expressed in conjunction with the tolerated Ags. This observation of linked suppression reveals a hitherto undocumented consequence of CD40L blockade that suggests the tolerant state is maintained by a dominant regulatory mechanism. Our results suggest that, although anti-CD40L Abs are attractive clinical immunotherapeutic agents, additional therapies to control aggressive CD8+ T cell responses may be required.

Dominant regulation: a common mechanism of monoclonal antibody induced tolerance?

Transplantation tolerance can be induced by a range of agents that block T cell/antigen-presenting cell (APC) interactions known to be important for initiation of the adaptive immune response. Tolerance so induced has been shown to have a regulatory phenotype dependent on CD4+ cells. This was first observed with nonlytic anti-CD4 antibodies, and was recently demonstrated following other therapeutic approaches. Dominant tolerance also plays a role in natural regulation of the immune response, functioning to prevent autoaggressive cells mediating self-destruction. The mechanism by which dominant tolerance is established and maintained remains unclear, and the reported characteristics of regulatory cells in different experimental models vary widely. Here we review the evidence for potential mechanisms involved and propose that there is a common pathway by which dominant tolerance is mediated.

Linked suppression of skin graft rejection can operate through indirect recognition

Adult mice can be rendered immunologically tolerant of allogeneic tissues if transplanted under cover of mAbs to CD4 and CD8. Tolerance generated in this manner is characterized by the presence of regulatory CD4+ T cells that can recruit naive T cells to become tolerant also through "infectious tolerance." Regulatory CD4+ T cells can also suppress rejection of third party transplant Ags provided they are expressed on the same graft as the tolerated Ags. This process of linked suppression can act across whole MHC barriers and represents a powerful mechanism with therapeutic potential. Tolerance can also be induced to reprocessed minor transplantation Ags presented through host APCs (indirect recognition). We here demonstrate that linked suppression can also be induced through the indirect pathway. This finding may be important in the development of transplantation tolerance in the clinic.

Rejection of H-Y disparate skin grafts by monospecific CD4+ Th1 and Th2 cells: no requirement for CD8+ T cells or B cells

We wished to determine whether CD4+ T cells could reject a skin graft that was discordant for a single minor transplantation Ag in the absence of CD8+ T cells or Ab. Transgenic A1(M) mice were constructed that express the rearranged V beta 8.2 and V alpha 10 TCR genes from a T cell clone that is specific for the male Ag (H-Y) in the context of H2-Ek. In addition, the RAG-1(-/-) background was bred onto these mice to eliminate any endogenous TCR rearrangements. As expected, clonal deletion was found to be complete in the thymus of male A1(M) x RAG-1(-/-) mice, while only CD4+ T cells were positively selected and found in the periphery of females. Female A1(M) x RAG-1(-/-) mice were able to rapidly reject (in <14 days) male (but not female) skin grafts in a CD4-dependent fashion. After multiple grafts, it was confirmed that no CD8+ T cells or surface Ig+ B cells were present. An immunofluorescent analysis of spleen cells after grafting showed that the majority of T cells expressed activation markers (CD44, CD25, and intracytoplasmic IL-2) and a significant proportion were making IFN-gamma and IL-4. Surprisingly, the transfer of either Th1 or Th2 CD4+ T cell lines from these mice into T cell-depleted recipients was sufficient to cause a specific rejection of male skin.

A role for Th2 cytokines in the suppression of CD8+ T cell-mediated graft rejection

A major histocompatibility complex (MHC) class I-specific T cell receptor (TCR)-transgenic mouse was used to study classical-type transplantation tolerance in the adult. Engraftment of MHC class I-incompatible bone marrow and tolerance to donor-type skin grafts were obtained using dimethylmyeleran (DMM) as a myeloablative agent and a non-depleting anti-CD8 monoclonal antibody (mAb) as the sole immunosuppressant. Surprisingly, bone marrow engraftment was facilitated by host CD4+ T cells, a subset normally considered unable to reject class I MHC-incompatible grafts. A combination of mAb to interleukins (IL)-4 and -10 antagonized the "permissive" effects of host CD4+ T cells, indicating a possible role for Th2-type immunoregulation that can act on CD8+ T cells in this form of transplantation tolerance. The fate of graft-reactive T cells was monitored using anti-clonotypic antibodies. It was observed that bone marrow engraftment then led to peripheral deletion of mAb-blockaded, clonotype+ CD8+ T cells.

Strain variation in susceptibility to monoclonal antibody-induced transplantation tolerance

BACKGROUND

We have reproducibly induced specific tolerance to multiple minor histocompatibility antigens with nondepleting anti-CD4 and -CD8 monoclonal antibodies. The tolerance induced is effective for the lifetime of the host. We have tested this therapy in a number of mouse strain combinations to further understand the mechanisms.

METHODS

Various mouse strains were grafted with allogeneic tail skin with and without nondepleting CD4- and CD8-specific monoclonal antibody therapy. The grafts were monitored daily for signs of rejection.

RESULTS

Whereas the CBA/Ca (H2k) strain can be made tolerant to skin grafts that are mismatched at multiple minor histocompatibility antigens indefinitely, using the same protocol, long-term survival of similarly mismatched grafts on the HW80 (B6 congenic for BALB H1) mouse strain is limited to around 8 weeks. Interestingly, the B10.BR strain, which is also of the H2k haplotype, is also not readily tolerized. In addition, an F1 between the CBA/Ca and the resistant B10.BR strains is B10.BR-like in its susceptibility to tolerance induction. Susceptibility to such antibody-dependent tolerance induction is not related to immunogenicity because grafts mismatched at only a single minor antigen also do not reproducibly survive beyond 8 weeks when grafted onto HW80 mice in the presence of the antibody therapy.

CONCLUSIONS

The data strongly suggest that the B6/B10 genetic background confers a level of resistance to CD4- and CD8-specific monoclonal antibody-dependent tolerance induction.

Tolerance and suppression in a primed immune system

The induction of tolerance in a primed immune system would be valuable therapeutically, but has been difficult to achieve. Mice primed to multiple minor histoincompatible antigens (minors) are able to rapidly reject secondary grafts using either their CD4+ or CD8+ T-cell subpopulations. Short courses of treatment with nonlytic anti-CD4 and anti-CD8 antibodies targeted at both T-cell subsets can induce long-term peripheral T-cell tolerance in primed mice. We examine the mechanisms by which peripheral tolerance is maintained, and show that tolerant mice harbor CD4+ T cells capable of specifically suppressing rejection mediated by either subset of primed T cells. Remarkably, elimination of CD4+ T cells from tolerant mice resulted in graft rejection, suggesting that graft-reactive CD8+ T cells had not been eliminated, but had been under continuous regulation by "tolerant" CD4+ T cells. This result demonstrates that it may be possible to establish therapeutic operational tolerance without permanently inactivating all antigen-reactive cells.

Amplification of natural regulatory immune mechanisms for transplantation tolerance

There is a need to derive donor-specific tolerance in clinical organ transplantation, where potential benefits remain overshadowed by chronic rejection and side effects of continual immunosuppressive therapy. It is known that the mature immune system in mice can be reprogrammed to accept a foreign graft as if it were "self." Here we show that, once generated, this state of operational tolerance becomes self-sustaining, imposing itself on new cohorts of lymphocytes as they arise. These new cohorts retain specificity for the tolerizing antigen and can be selectively amplified to tolerate new antigens that have linked expression with the original tolerogen. Regulation is critically dependent upon the continuous presence of tolerizing antigen and is mediated by the CD4+ lymphocyte population. We propose that such natural mechanisms of immune regulation may eventually be exploited for transplantation tolerance, even in fully immune-competent recipients.

T cell regulation in adult transplantation tolerance

An encounter of the mature immune system with Ag usually leads to an immune response. If Ag is administered with CD4- and CD8-specific mAbs, the outcome of the response can be tolerance. This form of tolerance is peripheral, Ag specific, and maintained lifelong, and is associated with the suppression of nontolerant cells by CD4 cells of the tolerant host. Here we demonstrate that the degree of suppression is dependent on the number of suppressor cells. A neutralizing anti-IL-4 Ab was partially able to inhibit suppression, indicating a role for IL-4 in the regulation of Th1 rejection responses.

T cell regulation in adult transplantation tolerance

An encounter of the mature immune system with Ag usually leads to an immune response. If Ag is administered with CD4- and CD8-specific mAbs, the outcome of the response can be tolerance. This form of tolerance is peripheral, Ag specific, and maintained lifelong, and is associated with the suppression of nontolerant cells by CD4 cells of the tolerant host. Here we demonstrate that the degree of suppression is dependent on the number of suppressor cells. A neutralizing anti-IL-4 Ab was partially able to inhibit suppression, indicating a role for IL-4 in the regulation of Th1 rejection responses.

T cell suppression in transplantation tolerance through linked recognition

Allogeneic tissues transplanted to mice treated with CD4- and CD8-specific Abs are often accepted indefinitely due to the induction of immunologic tolerance. When transplantation tolerance was induced to grafts mismatched at multiple minor histocompatibility loci, Ag specificity was inferred because third party grafts, mismatched at the MHC, were rejected normally. However, some "third party" grafts were either accepted, or rejected more slowly. Tolerant mice possess CD4+ cells, which suppress rejection by T cells reacting to the same grafts. Therefore, we hypothesized that tolerated third party grafts might share Ags with the original tolerizing graft, and that these Ags are a target for such suppression. To test this idea, we tolerized mice to a set of minor Ags (B10 minors) and challenged them with third party grafts that carried those minors, as well as an additional strong transplantation Ag, the class I MHC molecule, H-2Kb. This class I molecule acts as a good target for rejection in both naive mice and in mice tolerized to B10 minors. However, when this third party class I molecule is provided "linked" to those B10 minors on an F1 graft, rejection was significantly impaired. The data suggest that suppression within tolerant animals operates locally (perhaps on the same APC) via linked recognition. In addition, our preliminary findings suggest that suppression via linked recognition can also lead to tolerance to the third party Ag.

T cell suppression in transplantation tolerance through linked recognition

Allogeneic tissues transplanted to mice treated with CD4- and CD8-specific Abs are often accepted indefinitely due to the induction of immunologic tolerance. When transplantation tolerance was induced to grafts mismatched at multiple minor histocompatibility loci, Ag specificity was inferred because third party grafts, mismatched at the MHC, were rejected normally. However, some "third party" grafts were either accepted, or rejected more slowly. Tolerant mice possess CD4+ cells, which suppress rejection by T cells reacting to the same grafts. Therefore, we hypothesized that tolerated third party grafts might share Ags with the original tolerizing graft, and that these Ags are a target for such suppression. To test this idea, we tolerized mice to a set of minor Ags (B10 minors) and challenged them with third party grafts that carried those minors, as well as an additional strong transplantation Ag, the class I MHC molecule, H-2Kb. This class I molecule acts as a good target for rejection in both naive mice and in mice tolerized to B10 minors. However, when this third party class I molecule is provided "linked" to those B10 minors on an F1 graft, rejection was significantly impaired. The data suggest that suppression within tolerant animals operates locally (perhaps on the same APC) via linked recognition. In addition, our preliminary findings suggest that suppression via linked recognition can also lead to tolerance to the third party Ag.

Mechanisms of peripheral tolerance and suppression induced by monoclonal antibodies to CD4 and CD8

Over the last five years it has become increasingly clear that the peripheral immune system can maintain tolerance to both self and non-self antigens through a variety of mechanisms. Although clonal deletion may play an important part in limiting rapidly expanding responses, there are many examples where antigen reactive T cells remain. It has been proposed that tolerance is maintained in this situation either by the induction of anergy or by ongoing suppression. The phenomenon known as immune deviation, where non-inflammatory Th2 responses could suppress Th1 and positively reinforce themselves provided an attractive explanation for infectious tolerance, where tolerant T cells could guide further naive T cells also to tolerance. However, experiments to test this hypothesis in the models of CD4 and CD8 antibody-induced tolerance have given conflicting data, with no clear evidence of Th2 responses in tolerant mice. In this paper we review recent data that IL-4 plays a role in suppression, but that the source of IL-4 may not be the tolerant/suppressor T cell. We also discuss how infectious tolerance can operate on third party antigens if they are linked on the same antigen presenting cell and how CD4+ T cells can suppress CD8+ T-cell responses. Finally, we suggest a model of infectious anergy that is compatible with the available data.

Immunosuppression of canine renal allograft recipients by CD4 and CD8 monoclonal antibodies

A state of tolerance to MHC mismatched allografts can be generated in rodents by treatment with CD4 and CD8 monoclonal antibodies (mAb). In order to transpose this type of therapy to large animals and ultimately to the clinic, a suitable model is required. To this end we have generated a series of mAb to the canine CD4, CD8, and Thy-1 antigens and have tested their ability to prevent rejection of renal allografts. Donor-recipient pairs were selected from a colony of mongrel dogs in which untreated rejection of two haplotype-mismatched kidneys occurred by day 7 (defined as a serum creatinine > 300 mumol/l). Therapy with either the CD4 or the CD8 mAb, using no other immunosuppression, did not prolong graft survival. Depletion of T cells by a Thy-1 mAb prior to surgery only extended graft survival to day 9. However, treating with combinations of mAb up to day 10 (CD4 plus Thy-1; CD4 plus CD8; or CD4 plus CD8 plus Thy-1) prolonged renal allograft function up to 25 days. Combination of the triple mAb therapy with a sub-therapeutic immunosuppressive drug regimen (cyclosporin A plus azathioprine that alone gave a median survival of 15 days) favored survival to a median of 38 days. This protocol also inhibited the antiglobulin response that had curtailed the effects of mAb treatment, opening the way to more extended, and potentially tolerizing, mAb plus drug regimens.

Depletion of CD4+ and CD8+ cells eliminates immunologic memory of thyroiditogenicity in murine experimental autoimmune thyroiditis

Experimental autoimmune thyroiditis (EAT) develops in genetically susceptible mice after immunization with mouse thyroglobulin (MTg), and is mediated by T cells, both CD4+ and CD8+, infiltrating the thyroid. Previous work showed that depletion of CD4+, but not CD8+, cells with rat monoclonal antibodies (mAbs) interfered with EAT induction. To test if concomitant CD4+ cell depletion and immunization led to EAT resistance, mice were reimmunized at an interval of 15 or 43 days after injection of CD4 mAbs. No resistance had been established; disease severity and anti-MTg titers were comparable to mice with primary immunization. Previous work also showed that treatment during advancing EAT with only CD4 mAbs on days 21, 25 led to long-lasting, reduced severity in EAT, whereas administration of CD8 mAbs alone reduced the smaller CD8+ subset only. However, therapy with both mAbs was most efficacious; > 50% of thyroids were purged of all cellular infiltrate after only two doses. Moreover, T cells emerging subsequent to depletion were not retained in the thyroid, despite ongoing antibody production. To test if nondepleting CD4 and CD8 mAbs were similarly effective for therapy, mAbs of the IgG2a isotype were administered during advancing EAT. No effect on thyroidal infiltration was observed, indicating that modulation of the CD4 and CD8 antigen without depletion was insufficient for efficacious therapy. To determine if combined therapy with depleting mAbs reestablished self tolerance, treated mice were reimmunized on days 70, 77, when T cell recovery was nearly complete. Thyroiditis was comparable to controls given primary immunization, despite high antibody levels.(ABSTRACT TRUNCATED AT 250 WORDS)

CD4 and CD8 monoclonal antibody therapy: strategies to prolong renal allograft survival in the dog

The value of CD4 and CD8 monoclonal antibody therapy in tolerance induction has been demonstrated in rodent transplant models. In this paper the immunosuppressive potential of CD4 and CD8 monoclonal antibodies for dog renal allografts was evaluated as a preliminary to tolerogenic studies in this large animal model. Monoclonal antibodies were given for a maximum of 10 days after transplantation. Therapy was stopped prematurely following adverse reactions associated with the recipient developing an antibody response against the foreign (rat) therapeutic monoclonal antibody. Blood trough levels of CD4 and CD8 antibodies indicated that saturating doses were achieved. Although neither CD4 nor CD8 alone prolonged allograft survival (rejection by day 7), combination of CD4 and CD8 antibodies resulted in good graft function for a median of 14 days. The effect of removing circulating T lymphocytes was also assessed using a lytic Thy-1 monoclonal antibody. Alone Thy-1 had little effect but, when combined with CD4, the median allograft survival time was increased to 15.5 days. Reduction of the number of circulating T lymphocytes appears complementary to blockade of CD4 for immunosuppression, while blockade of CD4 combined with removal of CD8 also favours allograft survival.

"Infectious" transplantation tolerance

The maintenance of transplantation tolerance induced in adult mice after short-term treatment with nonlytic monoclonal antibodies to CD4 and CD8 was investigated. CD4+ T cells from tolerant mice disabled naïve lymphocytes so that they too could not reject the graft. The naïve lymphocytes that had been so disabled also became tolerant and, in turn, developed the capacity to specifically disable other naïve lymphocytes. This process of "infectious" tolerance explains why no further immunosuppression was needed to maintain long-term transplantation tolerance.

Therapeutic immunosuppression of T cells

Current immunosuppressive therapy carries a range of unwanted side effects, and tends to penalize the whole immune system. It is desirable to develop therapies that are more selective for antigen-reactive cells. As T lymphocytes use a wide range of surface receptors to interact with antigen-bearing cells and with each other, much interest is devoted to trying to develop agents that selectively block the interaction of these receptors with their ligands, and others that could be used to reprogram the immune system so that it might become tolerant to the antigens rather than attack them.

Classical transplantation tolerance in the adult: the interaction between myeloablation and immunosuppression

Allogeneic bone marrow transplantation in the neonate is an effective way of inducing permanent tolerance to donor tissue. To do the same in the immunocompetent adult requires immunosuppression to counter host-versus-graft alloreactivity. Conditioning with monoclonal antibodies (mAb) to CD4 and CD8 has been sufficient where donor and recipient are mismatched at only multiple "minor" histocompatibility loci, or at major histocompatibility complex (MHC) class I plus "minor" loci, but not where the mismatch involves the entire MHC. Tolerance across the MHC barrier requires extra conditioning with agents that happen to be both immunosuppressive and myeloablative, so obscuring the assessment of which effect is important. By using dimethylmyleran as a selective "space"-creating myeloablative agent, and CD4 plus CD8 mAb as sole immunosuppressive agents, we have been able to dissect the relative requirements for immunosuppression and myeloablation. We show here that transplantation tolerance could only be achieved when both types of agent were combined together so as to guarantee sufficient donor-type hemopoietic chimerism. We argue that the donor marrow, given sufficient space, will engraft and provide a sustained source of tolerogen overriding any host resistance that antibodies cannot control.

Humanised monoclonal antibody therapy for rheumatoid arthritis

Monoclonal antibodies that target T cells have shown some benefit in rheumatoid arthritis although responses have not been long lasting. This is partly due to insufficient therapy consequent upon antibody immunogenicity. Use of humanised antibodies, which are expected to be less foreign to man than conventional rodent antibodies, might overcome this problem. We therefore assessed in a phase 1 open study the potential of a "lymphocyte depleting" regimen of the humanised monoclonal antibody CAMPATH-1H in 8 patients with refractory rheumatoid arthritis. Apart from symptoms associated with first infusions of antibody, adverse effects were negligible. Significant clinical benefit was seen in 7 patients, lasting for eight months in 1. After one course of therapy, there was no measurable antiglobulin response, although 3 out of 4 patients have become sensitised on retreatment. Humanisation reduces the immunogenicity of rodent antibodies but anti-idiotype responses may still be seen on repeated therapy, even in patients sharing immunoglobulin allotype with the humanised antibody.

Immunomodulation of dog islets using a cocktail of monoclonal antibodies

Islet allografts are particularly vulnerable to rejection, and current immunosuppressive agents are deleterious to their function. They are, however, highly suitable for 'immunomodulation', i.e., the removal or inactivation of passenger leukocytes to reduce their immunogenicity. For this purpose we have used 3 rat anti-dog monoclonal antibodies (Mabs) which are synergistic for leukocytolysis in the presence of autologous dog serum. Spleen cells or purified islets treated with these Mabs together with autologous serum were tested in mixed leukocyte and islet co-culture assays. The stimulatory properties of the Mab-pretreated splenocytes or islets were markedly reduced; moreover, the Mab cytolytic activity was shown to be confined to the leukocyte target cells and did not affect islet secretory function upon glucose stimulation. We conclude that this method of modifying the immunogenicity of dog islets could lead to successful islet grafting in vivo, allowing the reduction of conventional immunosuppression. Successful in vivo studies in this model, which are currently in progress, could have implications for clinical islet transplantation.

Suppression in murine experimental autoimmune thyroiditis: in vivo inhibition of CD4+ T cell-mediated resistance by a nondepleting rat CD4 monoclonal antibody

Genetically susceptible mice become resistant to experimental autoimmune thyroiditis (EAT) induction with mouse thyroglobulin (MTg) and lipopolysaccharide after pretreatment with deaggregated MTg (dMTg). Recent work showed this suppression to be mediated by CD4+ suppressor T cells (Ts). To study Ts action in vivo, we used a rat IgG2a monoclonal antibody (mAb), YTS 177.9, which modulates CD4 antigen in vivo without depleting CD4+ cells. Initial studies showed that after two 1-mg doses of mAb 7 days apart, extensive CD4 antigen modulation of peripheral blood leukocytes occurred within 4 days. Mice given CD4 mAb 24 hr before dMTg (2 doses, 7 days apart) were resistant to EAT induction when immunized with MTg and LPS 20 days later. Also, anti-rat IgG2a titers were reduced following challenge with heat-aggregated rat IgG2a compared to controls. Subsequent analysis of serum in CD4 mAb-treated animals revealed that mAb was present in the circulation for 14 days. Moreover, mice given CD4 mAb and dMTg, then challenged after only 10 days, when CD4 mAb was still circulating, developed a significantly higher incidence of thyroid damage than controls. These findings suggest that modulation of CD4 antigen does not interfere with Ts activation, but the presence of CD4 mAb, at the time of autoantigenic challenge, can interfere with tolerance to EAT induction. Thus, the direct relationship between the presence of CD4 mAb and inhibition of EAT suppression implicates a role for CD4 molecules in the mediation of suppression.

Monoclonal antibody therapy for the induction of transplantation tolerance

There are three ways in which monoclonal antibodies could be used to facilitate the induction of tolerance to foreign tissues after organ transplantation. First, depleting monoclonal antibodies could be directed against the T cells responsible, thereby reducing their number and acting to non-specifically immunosuppress the patient. This is generally not sufficient to allow tolerance induction in the T cells which repopulate the periphery. Second, depleting monoclonal antibodies could be used to remove donor passenger leukocytes and antigen-presenting cells from the donor organ, which may both reduce immunogenicity and increase the chance of tolerance induction. Third, non-depleting, but functionally blocking, monoclonal antibodies to T cell molecules such as CD4 and CD8 can allow the specific induction of transplantation tolerance in mouse models, an approach which might be applicable to man, not only for organ transplantation, but also in the treatment of autoimmune diseases. These three approaches are, in time, likely to complement each other in clinical practice. Monoclonal antibodies can be tailored to each approach by choosing appropriate specificities and isotypes, and further refinements can be made where necessary by making monovalent or humanised antibodies. The application of each of these approaches to clinical therapy is described.

Reshaping a therapeutic CD4 antibody

An immunosuppressive rat antibody (Campath-9) against human CD4 has been reshaped for use in the management of autoimmunity and the prevention of graft rejection. Two different forms of the reshaped antibody were produced that derive their heavy chain variable region framework sequences from the human myeloma proteins KOL or NEW. When compared to a chimeric form of the CD4 antibody, the avidity of the KOL-based reshaped antibody was only slightly reduced, whereas that of the NEW-based reshaped antibody was very poor. The successful reshaping to the KOL-based framework was by a procedure involving the grafting of human framework sequences onto the cloned rodent variable region by in vitro mutagenesis.

The induction of skin graft tolerance in major histocompatibility complex-mismatched or primed recipients: primed T cells can be tolerized in the periphery with anti-CD4 and anti-CD8 antibodies

Mice given short courses of anti-CD4 and anti-CD8 monoclonal antibodies became tolerant of allogeneic skin grafted at the same time. Tolerance could be obtained without T cell depletion across multiple minor antigen mismatches, both in naive and primed animals, demonstrating that peripheral T cells could be tolerized, even if they had been previously activated. Where donor and recipient were incompatible across the whole major histocompatibility complex, specific tolerance could be achieved by using a combination of depleting followed by non-depleting antibodies, where each alone was unsuccessful. Although mice clearly tolerated their original skin grafts, we observed in some strain combinations that a second fresh, but genotypically identical graft, was slowly rejected. Such mice also possessed T cells which could proliferate to donor-type stimulator cells in vitro. Whatever the mechanisms, we have demonstrated that operational transplantation tolerance can be achieved with simple, non-toxic antibody therapy. The introduction of comparable tolerance-inducing regimens in clinical organ transplantation could obviate the need for long-term immunosuppression and its unfortunate side effects.

Induction of tolerance in peripheral T cells with monoclonal antibodies

Our goal has been to develop ways to tolerize the mature immune system to any defined antigen. In this report we show that peripheral (post-thymic) T cells of mice can become tolerant to a range of antigens (human and rat immunoglobulins, and bone marrow and skin grafts that differ at multiple minor transplantation antigens). In the case of human gamma globulin (HGG), this required that the antigen be given under the cover of a short course of non-depleting anti-CD4 antibody, while for tolerance to skin and marrow grafts anti-CD8 antibody was also required. Tolerance to HGG could be reinforced by repeated injections of HGG, but was lost in the absence of any further exposure to antigen. This reversal of tolerance with time was due to new T cells being exported from the thymus, as it was not observed in tolerized, adult thymectomized mice. In contrast, tolerance to marrow and skin grafts was permanent, presumably because the established grafts acted as a continuous source of antigen to reinforce the tolerant state. Tolerance could not be broken by the infusion of unprimed spleen cells and in one example (tolerance to Mls-1a) there was clear evidence that specific peripheral T cells were anergic. We propose that anergic cells may themselves participate in reinforcing the tolerant state by competing at sites of antigen presentation.