Reprogramming the immune system for peripheral tolerance with CD4 and CD8 monoclonal antibodies

CD4+ T Cell Fate Decisions Are Stochastic, Precede Cell Division, Depend on GITR Co-Stimulation, and Are Associated With Uropodium Development

During an immune response, naïve CD4⁺ T cells proliferate and generate a range of effector, memory, and regulatory T cell subsets, but how these processes are co-ordinated remains unclear. A traditional model suggests that memory cells use mitochondrial respiration and are survivors from a pool of previously proliferating and glycolytic, but short-lived effector cells. A more recent model proposes a binary commitment to either a memory or effector cell lineage during a first, asymmetric cell division, with each lineage able to undergo subsequent proliferation and differentiation. We used improved fixation and staining methods with imaging flow cytometry in an optimized in vitro system that indicates a third model. We found that cell fates result from stochastic decisions that depend on GITR co-stimulation and which take place before any cell division. Effector cell commitment is associated with mTORC2 signaling leading to uropodium development, while developing memory cells lose mitochondria, have a nuclear localization of NFκB and depend on TGFβ for their survival. Induced, T helper subsets and foxp3⁺ regulatory T cells were found in both the effector and memory cell lineages. This in vitro model of T cell differentiation is well suited to testing how manipulation of cytokine, nutrient, and other components of the microenvironment might be exploited for therapeutic purposes.

Revisiting the phenotypic and genetic profiling of anergic T cells mediating long-term transplant tolerance

PURPOSE OF REVIEW

Herein our focus will be to revisit peripheral tolerance mechanisms and in particular 'active' or 'dominant' tolerance as originally defined and mediated by regulatory CD4FoxP3 T lymphocytes (Treg) and also T-cell anergy that appears as a major mainstay to support long-term allograft survival.

RECENT FINDINGS

It is at the same time interesting and rewarding that the tool that recently guided our efforts along this path is the in-vivo use of CD3 antibody, the first monoclonal introduced in the clinic (Orthoclone OKT3) about 35 years ago to treat and prevent rejection of renal allografts. Beyond their immunosuppressive activity, whenever administered judiciously, CD3 antibodies promote robust allograft tolerance through selective purging of alloreactive effectors, resetting Treg-mediated active tolerance and promoting a unique subset of anergic CD8 T cells.

SUMMARY

The new findings discussed open up new perspectives from both a fundamental and a clinical point of view. In basic research, concrete molecular signaling paths are now spotted to finely dissect the conditions that lead to the establishment and maintenance of robust T-lymphocyte anergy mediating allograft tolerance. In the clinic, this may rapidly translate into novel biomarkers to be used in parallel to the ones already available, to better adapt posttransplant immunotherapy and monitor for long-term allograft acceptance.

Induction of Immunological Tolerance as a Therapeutic Procedure

A major goal of immunosuppressive therapies is to harness immune tolerance mechanisms so as to minimize unwanted side effects associated with protracted immunosuppressive therapy. Antibody blockade of lymphocyte coreceptor and costimulatory pathways in mice has demonstrated the principle that both naive and primed immune systems can be reprogrammed toward immunological tolerance. Such tolerance can involve the amplification of activity of regulatory T cells, and is maintained through continuous recruitment of such cells through processes of infectious tolerance. We propose that regulatory T cells create around them microenvironments that are anti-inflammatory and endowed with enhanced protection against destructive damage. This acquired immune privilege involves the decommissioning of cells of the innate as well as adaptive immune systems. Evidence is presented that nutrient sensing by immune cells acting through the mammalian target of rapamycin (mTOR) pathway provides one route by which the immune system can be directed toward noninflammatory and regulatory behavior at the expense of destructive functions. Therapeutic control of immune cells so as to harness metabolic routes favoring dominant regulatory mechanisms has offered a new direction for immunosuppressive therapy, whereby short-term treatment may be sufficient for long-term benefit or even cure.

Medawar's legacy to cellular immunology and clinical transplantation: a commentary on Billingham, Brent and Medawar (1956) 'Quantitative studies on tissue transplantation immunity. III. Actively acquired tolerance'

‘Quantitative studies on tissue transplantation immunity. III. Actively acquired tolerance’, published in Philosophical Transactions B in 1956 by Peter Medawar and his colleagues, PhD graduate Leslie Brent and postdoctoral fellow Rupert Billingham, is a full description of the concept of acquired transplantation tolerance. Their 1953 Nature paper (Billingham RE et al. 1953 Nature172, 603–606. (doi:10.1038/172603a0)) had provided initial evidence with experimental results from a small number of neonatal mice, with mention of similar findings in chicks. The Philosophical Transactions B 1956 paper is clothed with an astonishing amount of further experimental detail. It is written in Peter Medawar's landmark style: witty, perceptive and full of images that can be recalled even when details of the supporting information have faded. Those images are provided not just by a series of 20 colour plates showing skin graft recipient mice, rats, rabbits, chickens and duck, bearing fur or plumage of donor origin, but by his choice of metaphor, simile and analogy to express the questions being addressed and the interpretation of their results, along with those of relevant published data and his prescient ideas of what the results might portend. This work influenced both immunology researchers and clinicians and helped to lay the foundations for successful transplantation programmes. It led to the award of a Nobel prize in 1960 to Medawar, and subsequently to several scientists who advanced these areas. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.

Celiac disease: the search for adjunctive or alternative therapies

Celiac disease is a widespread disorder caused by intolerance to gluten, a common protein in food. Currently, a life-long gluten-free diet is the only available treatment for patients with celiac disease. However, adherence to gluten-free diet is difficult due to the widespread use of wheat-derived gluten in the food industry. Therefore, there is a pressing need for the development of novel non-dietary therapies. In this article, we will review several promising strategies focusing on reducing gluten immunogenicity or sequestering to gluten prevent its uptake by the intestinal epithelium. Other possible treatment strategies that will be reviewed include the suppression of the adaptive immune response, permeability modulation and the use of systemic T-cell or cytokine blockers.

Regulatory T cells and transplantation tolerance

The success of clinical organ transplantation relies on life-long use of immunosuppressive drugs that target immune responses associated with graft rejection. Preclinical studies in mice have convincingly demonstrated that robust, long-term transplantation tolerance can be achieved after a short-term treatment with T-cell coreceptor and costimulation blockade even for a fully mismatched graft. Such therapeutically induced tolerance requires the induction of Foxp3+ Tregs, which are essential for both the development and maintenance of the tolerant state. Recent advances in understanding the molecular and epigenetic mechanisms underlying the induction and stabilization of Foxp3 expression, thus guiding Foxp3+ Treg differentiation, have revealed novel therapeutic targets in animal models that can be translated to harness Foxp3+ Tregs from within the patient. Such in vivo induced Foxp3+ Tregs can also induce the tolerant state. Pharmacological compounds are available to exploit these targets and their further development holds great promise for clinical translation.

Regulatory cells and transplantation tolerance

Transplantation tolerance is a continuing therapeutic goal, and it is now clear that a subpopulation of T cells with regulatory activity (Treg) that express the transcription factor foxp3 are crucial to this aspiration. Although reprogramming of the immune system to donor-specific transplantation tolerance can be readily achieved in adult mouse models, it has yet to be successfully translated in human clinical practice. This requires that we understand the fundamental mechanisms by which donor antigen-specific Treg are induced and function to maintain tolerance, so that we can target therapies to enhance rather than impede these regulatory processes. Our current understanding is that Treg act via numerous molecular mechanisms, and critical underlying components such as mTOR inhibition, are only now emerging.

Tolerogenicity of donor major histocompatibility complex-matched skin grafts in previously tolerant Massachusetts general hospital miniature swine

BACKGROUND

Long-term tolerance of class I disparate renal allografts in miniature swine can be induced by a short course of cyclosporine and persists for 3 to 4 months after grafts are removed. Donor class I peptide immunization 6 weeks after graftectomy of tolerated kidneys leads to sensitization, but donor skin grafts do not. Here, we tested the hypothesis that skin grafts prevent rejection after simultaneous peptide administration and skin grafting.

METHODS

Miniature swine underwent bilateral nephrectomy and class I-mismatched renal transplantation with a 12-day course of cyclosporine A to induce long-term tolerance. Tolerated allografts were then replaced with recipient-matched kidneys, and animals were challenged with simultaneous donor-type skin grafts and peptide. Six weeks later, second donor-matched kidneys were transplanted without immunosuppression, and immune responses were characterized.

RESULTS

Animals treated only with peptide (n=2) rejected subsequent renal transplants in 3 to 5 days with strong in vitro antidonor responses. Of five recipients of skin-plus-peptide regimen, two accepted kidneys long term, one demonstrated a modestly prolonged survival (11 days), and two rejected rapidly (5-7 days). The two long-term acceptors maintained donor-specific hyporesponsiveness in vitro.

CONCLUSIONS

Sensitization by class I peptide in previously tolerant swine could be prevented by simultaneous class I skin grafts. These data suggest that skin grafts may actually augment rather than abrogate downregulation in some cases. A mechanistic hypothesis for this surprising result is that recognition of class I antigens through the direct rather than the indirect pathway of antigen presentation promotes tolerance by expanding regulatory T cells.

Immune therapy for type 1 diabetes mellitus-what is unique about anti-CD3 antibodies?

Type 1 diabetes mellitus (T1DM) is a prototypic organ-specific autoimmune disease that results from selective destruction of insulin-secreting beta-cells by immune-mediated inflammation (insulitis), that is, the infiltration of pancreatic islets by autoreactive CD4(+) and CD8(+) T lymphocytes. Current treatment is substitutive-chronic use of exogenous insulin-which, in spite of considerable advances, is still associated with constraints and lack of effectiveness over the long-term in relation to the prevention of vascular and neurological complications. Finding a cure for T1DM is an important medical health challenge, as the disease's incidence is steadily increasing in industrialized countries and projections of future prevalence are alarming. Crucially, as T1DM mainly affects children and young adults, any candidate immune therapy must be safe and avoid chronic use of immunosuppressants that promote sustained depression of immune responses. The ideal approach would, therefore, involve induction or, in the case of established T1DM, restoration of immune tolerance to target autoantigens. This Review presents, in particular, two strategies that are still in clinical development but hold great promise. These strategies are focused on the use of candidate autoantigens and anti-CD3 monoclonal antibodies.

Therapeutic T-cell manipulation in rheumatoid arthritis: past, present and future

Accumulating evidence suggests that RA is a T-cell-mediated autoimmune disease. Early attempts at disease modulation using strategies such as CD4 mAbs were severely hampered by a lack of biomarkers of autoreactivity. Recently, however, co-stimulation blockade has emerged as an effective treatment for RA. Alongside a greatly improved mechanistic understanding of immune regulation, this has rekindled hopes for authentic and robust immune programming. The final pieces of the jigsaw are not yet in place for RA but, in other disciplines, emerging treatment paradigms such as non-mitogenic anti-CD3 mAbs, autoantigenic peptides and even cellular therapies are providing hope for a future in which immunopathology can be specifically and vigorously curtailed.

The use of CD3-specific antibodies in autoimmune diabetes: a step toward the induction of immune tolerance in the clinic

CD3-specific monoclonal antibodies were the first rodent monoclonals introduced in clinical practice in the mid 1980s as approved immunosuppressants to prevent and treat organ allograft rejection. Since then compelling evidence has been accumulated to suggest that in addition to their immunosuppressive properties, CD3-specific antibodies can also afford inducing immune tolerance especially in the context of ongoing immune responses. Thus, they are highly effective at restoring self-tolerance in overt autoimmunity, a capacity first demonstrated in the experimental setting, which was recently transferred to the clinic with success.

T cell immunomodulation--the Holy Grail of therapeutic tolerance

The concept and practice of therapeutic tolerance has successfully been applied to animal models of autoimmunity and transplantation for more than 2 decades. Finally, there are encouraging signs of its translation to clinical practice. Short courses of anti-CD3 monoclonal antibody therapy have provided lasting benefits in recent-onset type 1 diabetes in association with evidence for the induction of immunoregulatory mechanisms. Co-stimulation blockade with abatacept (CTLA4-Ig) will soon be licensed for the treatment of rheumatoid arthritis - over the past year phase III studies have demonstrated impressive improvement in subjective and objective signs of the disease. T cell depletion is in development for several conditions, again with recent studies demonstrating evidence of immune regulation in some instances. More specific antigen-directed peptide therapies have also been applied to atopic asthma, type 1 diabetes, and adult and juvenile arthritis. The tragic sequelae of the phase I trial of TGN1412 at Northwick Park demonstrated the delicate, but unpredictable, therapeutic ratio of some T-cell-directed treatments and, in the UK, have led to new guidelines for early-phase clinical trials of immune-directed therapies.

Biologic therapies in rheumatology: lessons learned, future directions

During the past decade biologic therapies such as monoclonal antibodies and fusion proteins have revolutionized the management of rheumatic disease. By targeting key cytokines and immune cells biologics have provided more specific therapeutic interventions with less immunosuppression. Clinical use, however, has revealed that their theoretical simplicity hides a more complex reality. Efficacy, toxicity and even pharmacodynamic effects can deviate from those predicted, as poignantly illustrated by the catastrophic effects witnessed during the first-into-human administration of TGN1412. This review summarizes lessons gleaned from practical experience and discusses how these can inform future discovery and development of new biologic therapies for rheumatology.

Transforming growth factor-beta and T-cell-mediated immunoregulation in the control of autoimmune diabetes

It is now well-established that CD4+ regulatory T cells are instrumental in controlling immune responses both to self-antigens and to non-self-antigens. However, the precise modalities involved in their differentiation and survival, their mode of action and their antigen specificity are only partially understood. We have been particularly interested in the study of regulatory T cells controlling autoimmune insulin-dependent diabetes. Here, we provide evidence to support the phenotypic and functional diversity of regulatory T cells mediating transferable 'active' or 'dominant' peripheral tolerance in the non-obese diabetic mouse model (NOD). They include natural and adaptive regulatory T cells that are operational both in unmanipulated NOD mice and in animals undergoing treatments aimed at inducing/restoring tolerance to self-beta-cell antigens. At least in our hands, the differential cytokine-dependency appears as a major distinctive feature of regulatory T cells subsets. Among immunoregulatory cytokines, transforming growth factor-beta(TGF-beta) appeared to play a key role. Herein we discuss these results and the working hypothesis they evoke in the context of the present literature, where the role of TGF-beta-dependent T-cell-mediated immunoregulation is still debated.

Immune privilege induced by regulatory T cells in transplantation tolerance

Immune privilege was originally believed to be associated with particular organs, such as the testes, brain, the anterior chamber of the eye, and the placenta, which need to be protected from any excessive inflammatory activity. It is now becoming clear, however, that immune privilege can be acquired locally in many different tissues in response to inflammation, but particularly due to the action of regulatory T cells (Tregs) induced by the deliberate therapeutic manipulation of the immune system toward tolerance. In this review, we consider the interplay between Tregs, dendritic cells, and the graft itself and the resulting local protective mechanisms that are coordinated to maintain the tolerant state. We 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, for example, by catabolism of the amino acids tryptophan and arginine and the induction of hemoxygenase and carbon monoxide. The induction of local immune privilege has implications for the design of therapeutic regimens and the monitoring of the tolerant status of patients being weaned off immunosuppression.

CD3-specific antibodies restore self-tolerance: mechanisms and clinical applications

The treatment of autoimmune diseases using conventional chemical immunosuppressants has short-term effects, imposing the need for chronic treatment with its risks of over-immunosuppression. CD3-specific monoclonal antibodies can restore self-tolerance in a durable fashion after a single short-term treatment, as demonstrated in several experimental models and clinically in recent-onset insulin-dependent diabetes. Disease remission involves first an immediate 'freezing' of the autoimmune response, which is linked to CD3-specific antibody-induced antigenic modulation of CD3-TCR complex at the T lymphocyte surface, followed by 'resetting' of TGF-beta-dependent T-cell mediated immunoregulation. Tolerance induction is demonstrated by persisting disease protection in spite of recovery of full immunocompetence to unrelated antigens.

Induction of antigen-specific immunologic tolerance by in vivo and in vitro antigen-specific expansion of naturally arising Foxp3+CD25+CD4+ regulatory T cells

Naturally arising CD25(+)CD4(+) regulatory T (T(R)) cells can be exploited to establish immunologic tolerance to non-self antigens. In vivo exposure of CD25(+)CD4(+) T cells from normal naive mice to alloantigen in a T cell-deficient environment elicited spontaneous expansion of alloantigen-specific CD25(+)CD4(+) T(R) cells, which suppressed allograft rejection mediated by subsequently transferred naive T cells, leading to long-term graft tolerance. The expanded T(R) cells, which became CD25(low) in the absence of other T cells, stably sustained suppressive activity, maintained expression levels of other T(R) cell-associated molecules, including Foxp3, CTLA-4 and GITR, and could adoptively transfer tolerance to normal mice. Furthermore, specific removal of the T(R) cells derived from originally transferred CD25(+)CD4(+) T(R) cells evoked graft rejection in the long-term tolerant mice, indicating that any T(R) cells deriving from CD25(-)CD4(+) naive T cells minimally contribute to graft tolerance and that natural T(R) cells are unable to infectiously confer significant suppressive activity to other T cells. Similar antigen-specific expansion of T(R) cells can also be achieved in vitro by stimulating naturally present CD25(+)CD4(+) T cells with alloantigen in the presence of IL-2. The expanded CD25(+)CD4(+) T cells potently suppressed even secondary MLR in vitro and, by in vivo transfer, established antigen-specific long-term graft tolerance. Thus, in vivo or in vitro, direct or indirect ways of antigen-specific expansion of naturally arising Foxp3(+)CD25(+)CD4(+) T(R) cells can establish antigen-specific dominant tolerance to non-self antigens, and would also be instrumental in re-establishing self-tolerance in autoimmune disease and antigen-specific negative control of pathological immune responses.

Evaluating the validity of animal models for research into therapies for immune-based disorders

The last few decades of the 20th century have shown an intensified search for safer and more effective medications against chronic diseases that burden ageing societies of the western world. The impressive development of biotechnological production techniques has greatly facilitated the pharmaceutical development of relatively non-toxic biological molecules. However, despite the huge investments, only a few effective therapies for immune-based diseases have reached the clinic. In this article we use examples from monoclonal antibody trials to discuss the validity and predictive strength of the animal models currently used for the development of effective therapies.

Approaches to transplantation tolerance in humans

Although transplantation tolerance to organ allografts has been achieved using a wide variety of immunologic interventions in laboratory animals, few tolerance induction protocols with complete immunosuppressive drug withdrawal have been tested in humans. Preclinical and clinical studies of the use of total lymphoid irradiation for the induction of chimeric and nonchimeric tolerance are summarized here.

Regulatory T cells under scrutiny

Having been long debated, the notion of suppressor T cells--renamed regulatory T cells--is back on the map, but many questions remain regarding the nature of these regulatory cells. Are they specialized cells? What are their phenotype, antigen specificity, mode of action and, above all, biological (and immunopathological) relevance? The predominant role of naturally occurring CD4+CD25+ T cells has been emphasized recently. Other cell types, however, contribute to immunoregulation also, whether they arise spontaneously during ontogeny or during the course of an adaptive immune response.

Reprogramming the immune system

The immune system is organized so as to react to pathogens without risking damage to self. Harnessing those processes that prevent self-reactivity will have enormous potential in clinical medicine. This review outlines the efforts of this laboratory over the last 25 years to exploit tolerance so as to reprogram the immune system for therapeutic purposes.

The use of monoclonal antibodies to restore self-tolerance in established autoimmunity

The author hopes to convince the reader that the data presented argue for a stage during the development of IDDM when beta-cell destruction can be counteracted and tolerance to beta cells restored, provided the immune aggression is arrested. This argument constitutes a solid rationale for immunointervention in established IDDM, especially by using potent agents such as CD3. The future for the application of monoclonal antibodies not only in autoimmunity but also in transplantation is exiting. With the development of humanized monoclonal antibodies, therapeutic uses for them are likely to expand. Enormous progress has been made in the last 15 years, and it is likely that before a similar time period has elapsed, monoclonal antibodies will have become standard tools that will dispense the need for long-term immunosuppression and its inherent dangers in various clinical arenas.

Perturbation of naive TCR transgenic T cell functional responses and upstream activation events by anti-CD4 monoclonal antibodies

It is well established that non-depleting anti-CD4 monoclonal antibodies (mAb) have potent immunomodulatory properties in vivo and as such can induce a profound state of tolerance. Receptor blockade, CD4 modulation, or the transmission of a negative signal have all been proposed to explain their effects, however their precise mechanism of action, particularly at the level of cellular signaling, remains obscure. Experiments were thus carried out to examine the underlying mechanisms of action of two non-depleting anti-mouse CD4 mAb, YTS177 and KT6, which differ in their ability to modulate CD4 expression. The effects of the mAb were examined on CD4(+) T cells derived from D0.11.10 TCR transgenic mice. Functional studies revealed that both mAb could effectively block antigen-driven proliferation and IL-2 production but had only modest effects at higher peptide doses. Importantly, mAb pre-treatment of T cells stimulated by sub-optimal peptide seemed to induce an anergy-like state making them unresponsive to subsequent re-stimulation. Analysis of intracellular signaling demonstrated that certain key upstream events such as the phosphorylation of Zap-70 and LAT were also blocked by mAb pretreatment which may be due to interference with stable T cell-APC conjugate formation.

From bench to bedside: discovering rules for antibody design, and improving serotherapy with monoclonal antibodies

Anti-T-cell monoclonal antibodies (mAbs) form a unique class of therapeutic agent. Their precise specificity offers tremendous potential for the treatment of autoimmune and inflammatory diseases but also prevents meaningful preclinical animal studies. In particular, adverse reactions to therapy may be unanticipated, and the first administration of a novel T-cell mAb to a patient thus marks the beginning of a unique experiment. By comparing clinical parameters and laboratory measurements, small-scale pilot studies can provide detailed information about mAb biology that both predicts and suggests solutions to the complications of therapy. In this essay I illustrate this concept with reference to three specific areas: lymphocyte depletion, mAb immunogenicity and cytokine-release syndromes. In each case, systematic clinical and laboratory science has improved our understanding of the problem and suggested solutions; most of these solutions have been or are being adopted. Thus, small, open studies are an essential step in the development of novel mAbs, provide an ideal platform for the study of mAb biology, and serve as an early warning system for potential adverse effects.

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.

Induction of lymphocyte apoptosis in rat liver allograft with ongoing rejection by FTY720

The action mechanism of FTY720, a novel immunosuppressant, is completely different from conventional immunosuppressants. The drug, which triggers apoptosis in murine and human lymphocytes, has a potent immunosuppressive activity to prevent allograft rejection without any severe side-effect. The present study was designed to determine whether FTY720 induces apoptotic cell death in activated lymphocytes infiltrated into liver grafts with ongoing rejection. FTY720 was orally administered at 5 mg/kg to the recipients on day 3 and day 4 after grafting, when the graft rejection was histologically confirmed. The intragraft patterns of IL-2, interferon-gamma (IFN-gamma), perforin, and granzyme B gene expression were detected by reverse transcriptase-polymerase chain reaction. The treatment reversed ongoing rejection and significantly prolonged recipient survival time compared with the control group. Light microscopic observation of the graft sections stained with the DNA nick-end labelling method showed that the apoptosis in the control allografts was mainly induced in hepatocytes, while that in the FTY720-treated allografts was in infiltrated lymphocytes. The rejection therapy with FTY720 did not alter the expression of IL-2, IFN-gamma, and perforin mRNAs, but slightly decreased granzyme B expression. Our results suggest that FTY720 does not alter the intrinsic lymphocyte function to produce the rejection-related cytokines, but strongly induces apoptotic cell death in the activated lymphocytes. Thus, FTY720 affords new insight into the mechanisms underlying improvements in immunosuppressive treatments.

Transgenic anti-CD4 monoclonal antibody secretion by mouse segmental pancreas allografts promotes long term survival

To compare the effectiveness of transgenic and systemic monoclonal antibody therapy for pancreas transplantation, vascularised segmental pancreas allografts from wild-type or transgenic pancreatic tissue that secreted monoclonal anti-CD4 were placed in CBA recipients in which diabetes had been induced chemically by streptozotocin (STZ, non-autoimmune diabetes). In untreated CBA recipients, wild-type BALB/c or C57BL/6 bml pancreas transplants were rejected in a mean survival time (MST) of 27 and 30 days, respectively. BALB/c and C57BL/6 graft survival improved when recipients were given a short course of T cell depleting monoclonal anti-CD4 antibody, (GK 1.5, 2 mg total on days -1, 0, 1, 2 with grafting on day 0) with MST +/- S.D. of 71 +/- 29 and 44 +/- 36 days, respectively. Thus, transient depletion of CD4 was effective in delaying pancreas allograft rejection in these strain combinations. The use of C57BL/6 bml mice transgenic for a rat anti-CD4 antibody (GK5 mice) as pancreas donors provided allografts that secreted sufficient anti-CD4 antibody to cause CD4 T cell depletion in the recipients (CD4 cells decreased from 30 to < 5% of small lymphocytes). This degree of depletion was not sustained and the CD4 recovery inversely correlated with graft survival. Mice with > 20% CD4 cells in the splenic lymphocyte population 4 weeks post-transplant rejected their grafts (3 of 10 mice). However, in 7 of 10 mice CD4 cells remained low (< 15%) and allografts survived for > 80 days. The GK5 allografts survived significantly longer than those from non-transgenic bml controls (MST 83 +/- 32 days, compared with 30 days, P < 0.0005). This survival time was similar to that of BALB/c allografts in CBA recipients treated with a high dose of anti-CD4 antibody. Thus, transgenic secretion of anti-CD4 antibody by the pancreas allograft was very effective in prolonging its survival.

Non-depleting anti-CD4, but not anti-CD8, antibody induces long-term survival of xenogeneic and allogeneic hearts in alpha1,3-galactosyltransferase knockout (GT-Ko) mice

The anti-galactose-alpha1,3-galactose (Gal) antibody (Ab) response following pig-to-human transplantation is vigorous and largely resistant to currently available immunosuppression. The recent generation of GT-Ko mice provides a unique opportunity to study the immunological basis of xenograft-elicited anti-Gal Ab response in vivo, and to test the efficacy of various strategies at controlling this Ab response [1]. In this study, we compared the ability of non-depleting anti-CD4 and anti-CD8 to control rejection and antibody production in GT-Ko mice following xenograft and allograft transplantation. Hearts from baby Lewis rat or C3H mice were transplanted heterotopically into GT-Ko. Non-depleting anti-CD4 (YTS177) and anti-CD8 (YTS105) Abs were used at 1 mg/mouse, and given as four doses daily from day -2 to 1 then q.o.d. till day 21. Xenograft rejection occurred at 3 to 5 days post-transplantation in untreated GT-Ko recipients, and was histologically characterized as vascular rejection. Anti-CD4, but not anti-CD8, Ab treatment prolonged xenograft survival to 68 to 74 days and inhibited anti-Gal Ab as well as xeno-Ab production. In four of the five hearts from anti-CD4 mAbs-treated GT-Ko mice, we observed classic signs of chronic rejection, namely, thickened intima in the lumen of vessels, significant IgM deposition, fibrosis and modest mononuclear cell infiltrate of Mac-1+ macrophages and scattered T cells (CD8>CD4). Xenograft rejection in untreated, as well as anti-CD4- and anti-CD8-treated, recipients was associated with increased intragraft IL-6, IFN-gamma and IL-10 mRNA. C3H allografts were rejected in 7 to 9 days by untreated GT-Ko mice and were histologically characterized as cellular rejection. Treatment with anti-CD4 and anti-CD8 mAb resulted in graft survivals of >94.8 and 11.8 days, respectively. Anti-CD4 mAb treatment resulted in a transient inhibition of alloreactive and anti-Gal Ab production. The presence of circulating alloreactive and anti-Gal Abs at >50 days post-transplant was associated with significant IgM and IgG deposition in the graft. Yet, in the anti-CD4 mAb-treated group, the allografts showed no signs of rejection at the time of sacrifice (>100 days post-transplantation). All rejected allografts had elevated levels of intragraft IL-6, IFN-gamma and IL-10 mRNA, while the long-surviving anti-CD4-treated allografts had reduced mRNA levels of these cytokines. Collectively, our studies suggest that the elicited xeno-antibody production and anti-Gal Ab production in GT-Ko mice are CD4+ T-cell dependent. The majority of xenografts succumbed to chronic rejection, while allografts survived with minimal histological change, despite elevated levels of circulating alloAbs. Thus, immunosuppression with anti-CD4 mAb therapy induces long-term survival of allografts more effectively than to xenografts.

Suppression of induction of experimental immune mediated blepharoconjunctivitis by tolerogenic conjugates of the antigen and monomethoxypolyethylene glycol

AIM

Covalent conjugates consisting of diverse antigens coupled to optimal numbers of monomethoxypolyethylene glycol (mPEG) molecules have been shown to suppress antigen specific antibody formation. In this study, the possibility was examined that the same conjugates might prevent experimental immune mediated blepharoconjunctivitis (EC, formerly EAC) which had been shown to be caused by CD4(+) T cells-that is, to cell mediated immunity.

METHODS

6-8 week old male Lewis rats were used. The test groups of rats received two intravenous injections, each of 300 microg, of a conjugate of ovalbumin mPEG (OVA(mPEG)(11)) in phosphate buffered saline (PBS), 14 and 28 days before the single immunisation with OVA in complete Freund's adjuvant. The rats were challenged 3 weeks later by eye drops containing OVA; 24 hours later they were sacrificed, and their eyes, blood, and lymph nodes were harvested for histological examination and determination of anti-OVA antibody titres and levels of cellular immunity. Two control groups received PBS or OVA in PBS before immunisation. Furthermore, the possibility that OVA(mPEG)(11) may have induced OVA specific suppressor cells was tested by establishing the effects of the co-transfer of splenocytes from OVA(mPEG)(11) treated rats with OVA primed lymph node cells on the manifestations of EC.

RESULTS

Either PBS or OVA pretreated rats, which had not received OVA(mPEG)(11), developed high levels of antibodies and cell mediated immune responses to OVA, and application of eye drops led to blepharoconjunctivitis with massive cellular infiltration. In contrast, pretreatment with OVA(mPEG)(11) prevented cellular infiltration into the lids and conjunctivas, as well as the formation of detectable humoral and cellular immunity against OVA. Co-transfer of splenocytes from OVA(mPEG)(11) treated rats with OVA primed lymph node cells suppressed the cellular infiltration on application of OVA on the conjunctiva.

CONCLUSIONS

These data indicate that intravenous injection of OVA(mPEG)(11) conjugates suppressed both humoral and cellular immunity by the effects of antigen specific suppressor cells, thus leading to the inhibition of development of EC.

Cure of progressive murine leishmaniasis: interleukin 4 dominance is abolished by transient CD4(+) T cell depletion and T helper cell type 1-selective cytokine therapy

Progressive infection with Leishmania major in susceptible BALB/c mice is mediated by interleukin (IL)-4-producing T helper cell type 2 (Th2) CD4(+) T cells that, once established, become resistant to Th1-deviating therapies with recombinant (r)IL-12 and/or neutralizing anti-IL-4 antibodies. We sought to restore protective immunity in advanced leishmaniasis by depletion of Th2-biased CD4(+) populations and by cytokine-directed reconstitution of Th1 cellular responses during lymphocyte recovery. Treatment with cytolytic GK1.5 anti-CD4 mAb alone did not reverse disease in 3 wk-infected BALB/c mice, but GK1.5 combined with anti-IL-4 antibody and intralesional rIL-12 cured cutaneous lesions in 80% of mice and established a Th1-polarized cytokine response to L. major antigen protective against reinfection. The curative effects of GK1.5 were not replaced by cytotoxic anti-CD8 monoclonal antibody 2.43 or nondepleting anti-CD4 mAb YTS177, confirming that depletion of CD4(+) cells was specific and essential for therapeutic effect. Finally, combined CD4(+) depletion and IL-4 neutralization were curative, indicating that neither increased parasite burden nor altered accessory cell function independently biased towards Th2 reconstitution in advanced leishmaniasis. Advanced leishmaniasis can be cured by T cell depletion and cytokine-directed recovery of Th1 cellular responses, suggesting novel interventions for other immune-mediated diseases and identifying distinct roles for CD4(+) T cell and non-T cell in the maintenance of Th2 and Th1 phenotypes.

Antagonism of cytotoxic T-lymphocyte activation by soluble CD8

The CD8 co-receptor is important in the differentiation and selection of class I MHC-restricted T cells during thymic development, and in the activation of mature T lymphocytes in response to antigen. Here we show that soluble CD8alphaalpha receptor, despite an extremely low affinity for MHC, inhibits activation of cytotoxic lymphocytes by obstructing CD3 zeta-chain phosphorylation. We propose a model for this effect that involves interference of productive receptor multimerization at the T-cell surface. These results provide new insights into the mechanism of T-cell activation and evidence that CD8 function is exquisitely sensitive to disruption, an effect that might be exploited by molecular therapeutics.

Assessment of peripheral tolerance in anti-CD4 treated C57BL/6 mouse heart transplants recipients

The study was designed to compare second heart and skin grafts and in vitro assays as a means of assessing peripheral tolerance in C57BL/6 mice. Vascularized heterotopic BALB/c hearts were placed in C57BL/6 recipients treated with anti-CD4, GK1.5 (1 mg total per 20 g mouse i.p. on days 0, 1, 2, 3). Those mice in which hearts survived for >60 days were challenged with donor and third-party (CBA) skin grafts or with second heart grafts, of donor or third-party origin, attached to the carotid artery and jugular vein. In vitro alloreactivity was assessed by mixed lymphocyte reactions (MLR) and cell mediated lympholysis (CML) using recipient spleen cells. Parenchymal damage, cellular infiltration and vascular disease were assessed from the histology of long-term allografts and isografts. Allografts in untreated recipients were rapidly rejected while isografts survived > 100 days. Primary allografts in anti-CD4 treated recipients also survived > 100 days, as did donor strain secondary heart transplants given at >60 days after the first graft. Third-party hearts were rapidly rejected, as were donor and third-party skin grafts placed on recipients with long-term allografts. These recipients showed low MLR response to both donor and third-party stimulators and donor-specific suppression of CML at 60 days post graft. Long-surviving heart allografts all showed evidence of parenchymal damage and vascular intimal thickening. Thus in the BALB/c to C57BL/6 donor-recipient strain combination, hearts, but not skin grafts, could be used to demonstrate peripheral tolerance, which seemed to be both organ and major histocompatibility complex (MHC) specific. Despite long survival, BALB/c hearts all showed evidence of parenchymal damage and vascular intimal thickening, a sign of chronic rejection.

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.

Redefining peripheral tolerance in the BALB/c to CBA mouse cardiac allograft model: vascular and cytokine analysis after transient CD4 T cell depletion

BACKGROUND

To evaluate cardiac allografts from recipients that had achieved peripheral tolerance after transient CD4+ T cell depletion, we analyzed cellular infiltrate, cytokine expression, and vascular thickening. Long-surviving cardiac allografts from tolerant recipients were compared with acutely rejecting allografts and isografts.

METHODS AND RESULTS

In CBA mice treated with anti-CD4 (GK1.5, 0.5 mg intraperitoneally on days 1-28), BALB/c cardiac allografts survived >100 days. These recipients were tested for tolerance at >70 days, by challenge with donor and third-party (C57BL/6) skin grafts. BALB/c skin grafts survived >30 days, although C57BL/6 skin was rejected in <12 days, reflecting alloantigen-specific peripheral tolerance. When vascular thickening in graft arteries was assessed and computerized measurements performed, heart allografts from tolerant recipients showed significantly increased percentage of luminal occlusion compared with isografts (47% compared with 1.2%). Semiquantitative reverse transcriptase-polymerase chain reaction was used to assess normalized intragraft mRNA transcripts for cytokines and T cell markers, with immunoperoxidase staining of frozen sections to confirmed the presence of protein. Compared with rejecting grafts, well-preserved hearts from tolerant mice had lower levels of macrophage and T cell infiltration and decreased transcription of interferon-gamma, interleukin (IL)-2, IL-10, and inducible nitric oxide synthase. IL-4 expression was similar in both groups.

CONCLUSIONS

The degree of tolerance achieved allowed specific acceptance of donor skin grafts, preserved primary graft function, and reduced inflammatory activation. Tolerance did not, however, completely prevent macrophage and T cell infiltration of the graft or the development of vascular lesions typical of chronic rejection.

Effect of IL-4 deletion on cardiac allograft survival in the BALB/c to 129Sv x C57BL/6 strain combination

To investigate the effect of IL-4 deletion on cardiac allograft survival, vascularized BALB/c cardiac allografts were placed in C57BL/6, 129Sv x C57BL/6 (IL-4 +/+) or 129Sv x C57BL/6 IL-4 knockout mice (IL-4-/-). Untreated recipients rejected allografts in < 15 days while isografts survived indefinitely (> 100 days). Treatment with anti-CD4 (GK1.5) for 4 days at the time of allografting increased mean survival to > 100 days in C57BL/6, 90+/-16 days in 129Sv x C57BL/6 (IL-4 +/+) and 68 +/- 36 days in 129Sv x C57BL/6 (IL-4-/-) recipients. Although there was a trend towards shorter survival times in the IL-4-/- mAb-treated mice, survival in the three recipient groups was not significantly different (P = 0.07). A 30-day course of anti-CD4 did not further prolong BALB/c heart survival. All long-surviving hearts had histological evidence of parenchymal damage and transplant vascular disease. None of these recipients developed antigen-specific tolerance, since both donor and third party skin graft challenges were rejected when challenged at > 60 days post-graft and all primary grafts failed by 120 days. Thus the effects of IL-4 deletion were subtle and were seen only with low doses of immunosuppression in this high responder strain combination.

Therapy with Monoclonal Antibodies. II. The Contribution of Fcγ Receptor Binding and the Influence of CH1 and CH3 Domains on In Vivo Effector Function

An in vivo model is used to define Fc motifs engaged by mAbs to deplete target cells. Human IgG1 and human IgG4 were very potent, and mutations within a motif critical for FcγR binding (glutamate 233 to proline, leucine/phenylalanine 234 to valine, and leucine 235 to alanine) completely prevented depletion. Mouse IgG2b was also potent, and mutations to prevent complement activation did not impair depletion with this isotype, as previously shown for human IgG1. In contrast, a mutation that impaired binding to mouse FcγRII (glutamate 318 to alanine) eliminated effector function of mouse IgG2b and also reduced the potency of human IgG4. To reveal potential contributions of domains other than CH2, domain switch mutants were created between human IgG1 and rat IgG2a. Two hybrid mAbs were generated with potencies exceeding anything previously seen in this model. While their mechanism of depletion was not defined, their activity appeared dependent upon interdomain interactions in the Fc region.

Molecular requirements for lineage commitment in the thymus--antibody-mediated receptor engagements reveal a central role for lck in lineage decisions

Recent experiments in our laboratory have focused on the receptor engagements required for the differentiation of fully mature, single positive thymocytes from their double positive precursors. We have used a novel approach which involves the ligation of surface receptors on immature thymocytes with genetically engineered F(ab1)2 reagents, which, unlike conventional antibodies, do not aggregate the CD3 complex to such an extent as to induce extensive deletion of these cells. The experimental data presented in this review indicate that differentiation of the two mature CD4 and CD8 lineages occurs in response to distinct intracellular signals induced by particular receptor engagements. The data suggest that the tyrosine kinase p56lck (lck) plays a crucial role in determining lineage choice, in that maturation of thymocytes into the CD4 lineage occurs upon recruitment of active lck to the T-cell receptor (TCR)/CD3 complex, whereas CD8 maturation can be induced by CD3 ligation in the absence of co-receptor-mediated lck recruitment. A central role for lck activity in determining the threshold for differentiation of the CD4 lineage is revealed in experiments with thymi deficient for a regulator of lck activity, CD45. A model of thymocyte differentiation is presented in which we propose that the relative balance of signals delivered by TCR engagement and lck activation determines lineage choice.

Requirement of CD4 cells for induction and maintenance of unresponsiveness in islet xenografted mice

Long-term survival of islet xenografts in the hamster to mouse model can be induced by a short-course treatment with a nondepleting anti-CD4 mAb but not with a depleting anti-CD4 mAb [Lu et al. Xenotransplantation 1998; 5:154-163]. Although CD4 cells are known to play a key role in the rejection of islet xenografts, it remains unclear whether CD4 cells are also required for the induction and/or maintenance of specific unresponsiveness to xenografts. To investigate this problem, islets were isolated from golden hamsters and transplanted into streptozotocin-induced diabetic CBA/J mice. Nondepleting mAb YTS 177.9 was used to block CD4 cells for the induction of islet xenograft unresponsiveness and subsequently depleting mAb GK1.5 to deplete CD4 cells in the unresponsive recipients. First, we now confirm that second donor-strain xenografts were permanently accepted in recipients that had been unresponsive to the first grafts, whereas Lewis rat islet xenografts, used as third-party grafts, were rejected like a primary graft within 7-8 days. Second, we depleted CD4 cells in recipient mice, which had been treated perioperatively with the nondepleting mAb YTS 177.9 and became unresponsive to their primary hamster islet graft, by using a depleting anti-CD4 mAb at different time points post-transplant. Depletion of CD4 cells in the unresponsive recipients by the depleting anti-CD4 mAb GK1.5 did abrogate this unresponsive state, since the grafts were always rejected within an average of 25.5 days after the mAb GK1.5 injections. Therefore, our results strongly suggest that CD4 positive cells play an active suppressive role and that their presence in the recipients appears essential for both induction and maintenance of long-term islet xenograft survival or specific unresponsiveness.

A novel monoclonal antibody against rat LFA-1: blockade of LFA-1 and CD4 augments class II MHC expression on T cells

Previously, we have shown that myelin basic protein (MBP)-specific Lewis rat GP2.E5/R1 (R1) T cells cultured with antigen and irradiated syngeneic splenocytes (IrrSPL) in the presence of anti-CD4 and LRTC1 monoclonal antibodies (MAbs) become highly effective antigen presenting cells (APC). The purpose of these studies was to identify the ligand for the LRTC1 MAb and to determine whether this MAb affected MBP-stimulated IL-2 production and expression of MHC class II molecules by T cells. In the current studies, we show that the LRTC1 MAb specifically immunoprecipitated molecular species of approximately 95, 150, and 180 kD. Commercially available anti-CD18 (beta2 integrin, beta-chain of LFA-1, MAC-1, and p150, 95) and LRTC1 MAb immunoprecipitated proteins with identical mobilities on 1-D and 2-D SDS-PAGE gels. Moreover, anti-CD18 and LRTC1 immunoprecipitates also showed identical mobilities on 1-D gels after enzymatic cleavage of N-linked oligosaccharides and thereby had the same patterns of differential glycosylation. Anti-CD4 MAb W3/25 and LRTC1 MAb synergistically inhibited T-cell IL-2 mRNA and IL-2 bioactivity, but augmented antigen-stimulated surface I-A on R1 T cells. In conclusion, these studies describe the characteristics of a novel anti-LFA-1 MAb, LRTC1, which should prove useful in studying costimulatory and adhesion pathways among rat leukocytes.

Cardiac Allografts from IL-4 Knockout Recipients: Assessment of Transplant Arteriosclerosis and Peripheral Tolerance

To study the role of IL-4 in tolerance induction and transplant arteriosclerosis, BALB/c hearts were transplanted into C57BL/6J wild-type or IL-4 knockout (IL-4−/−) recipients. A 30-day course of anti-CD4/8 mAb was used to induce long term graft survival. Primary graft survival was 50% (5 of 10) in IL-4−/− recipients comparable to 63% (5 of 8) in wild-type recipients. Mice with allografts surviving &gt;80 days were tested for tolerance by challenge with a second donor or third party (CBA) heart. Secondary donor-strain heart grafts survived &gt;30 days, but showed histologic evidence of ongoing alloimmune response. Third party hearts rejected rapidly. Although immunostaining and 32P RT-PCR assays showed no differences in the mononuclear cell infiltration and T cell activation between IL-4−/− and wild-type tolerant recipients, some monokines (IL-12, TNF-α, and allograft inflammatory factor-1) were up-regulated in grafts from IL-4−/− recipients. Computer-assisted analysis of elastin-stained vessels revealed that the severity of vascular thickening (percentage of luminal occlusion, mean ± SD, n = 329) was similar in grafts from IL-4−/− (63.7 ± 16.9%) and wild-type (69.5 ± 17.6%) recipients. Thus, IL-4 deficiency did not alter primary or secondary graft survival, infiltration, or vascular thickening. The selective alterations in monokine expression suggests that alternative pathways are activated and may compensate in IL-4−/− mice.

How do monoclonal antibodies induce tolerance? A role for infectious tolerance?

One of the major goals in therapeutic immunosuppression has been to achieve long-term benefit from short-term therapy. The discovery in the mild-1980s that CD4 antibodies can induce immunological tolerance without depleting CD4+ T cells has reawakened interest in the use of nondepleting monoclonal antibodies for reprogramming the immune system in autoimmunity and in transplantation. Since that time, antibodies to CD11a, CD4OL, CD25, CD3, and CTLA4-Ig have all been shown capable of facilitating tolerance. In order to apply to principle of reprogramming in the clinic, we have sought to understand the mechanisms that are involved in its induction and its maintenance. In a number of allogeneic transplant models (heart, skin, bone marrow) anti-CD4 (+/- CD8) antibodies can be shown to block the rejection process while selectively promoting the development of CD4+ regulatory T cells responsible for a dominant tolerance that is reflected in findings of linked suppression and infectious tolerance. In these models, T cells that have never been exposed to CD4 antibodies become tolerant to grafted antigens by experiencing antigen in the microenvironment of regulatory T cells. Dominant tolerance is not the only mechanism that can be facilitated by CD4 Mab therapy. If allogeneic marrow is given at high cell doses under the umbrella of CD4 and CD8 antibodies, then tolerance can be achieved through clonal deletion. The mechanism by which regulatory CD4+ T cell suppress is not yet defined but could be active or passive. We have proposed the "civil service model" to explain how tolerant T cells might interfere with the responses of competent T cells in such a way as to render them tolerant.

Long-term survival of hamster islet xenografts in mice under short-course treatment with nondepleting versus depleting anti-CD4 monoclonal antibodies

Xenogeneic grafts provide a potential alternative to the current shortage of human organs for transplantation. However, the prevention of rejection and tolerance induction of xenografts still remain to be further explored. Islet xenografts appear more promising than vascularized whole organ xenografts and additionally also more resistant to the recurrence of autoimmune disease than allografts. Recently, the nondepleting monoclonal antibody (mAb), which blocks the CD4 molecule on lymphocytes, was reported to be able to induce tolerance in allotransplantation and CD4 positive cells were further confirmed to be a major factor responsible for cellular xenograft rejection. Therefore, we hypothesize that anti-CD4 nondepleting mAb could also be effective in protecting cellular xenografts and inducing unresponsiveness of recipients. We studied the effect of the nondepleting anti-CD4 mAb YTS177.9 on islet xenograft survival by using the hamster-to-mouse islet transplantation model. Results were compared with that of the depleting anti-CD4 mAb GK1.5 that was shown to have similar binding sites on the CD4 molecule to mAb YTS177.9. Our data show that mAb YTS177.9 did effectively prolong the survival of islet xenografts and, in addition, also successfully did induce long-term acceptance of 40% grafts after only three perioperative injections of 0.5 mg mAb per mouse. The average survival of the graft was markedly prolonged to >66.8+/-37.1 days compared with controls (8.3+/-1.4 days) or with the depleting anti-CD4 mAb GK1.5 (25.7+/-5.5 days). However, the latter displayed a more profound inhibition in in vitro and ex vivo mixed lymphocyte xenoreaction than mAb YTS177.9. Moreover, the activity of this nondepleting mAb was found to be dose-dependent and 80% of grafts survived permanently when the dose was increased to six injections of 0.5 mg mAb. Like mAb GK1.5, mAb YTS177.9 also prevented rejection when given after a delay of two days posttransplant. In addition, we found that neither depleting nor nondepleting anti-CD8 mAb was effective in this model. Our results strongly suggest that an anti-CD4 nondepleting or blocking mAb alone is able to induce long-term acceptance of islet xenografts and that blocking the CD4 molecule is significantly superior to depleting CD4 positive cells for the protection of islet xenografts. This may indicate that CD4 cells play a major role in xenograft tolerance induction.

CD4 and CD8: modulators of T-cell receptor recognition of antigen and of immune responses?

The response of T cells to antigen involves the participation of a number of distinct receptor-ligand engagements. The major players in the recognition of complexes of major histocompatibility complex molecules and peptide antigens are the T-cell receptors and the co-receptors CD4 and CD8. Progress in understanding the physical structures of these molecules, and how complexes between them are formed, is helping our understanding of how they participate in regulating the signals transduced to T cells.

Humanized anti-CD4 monoclonal antibody therapy of autoimmune and inflammatory disease

We have investigated the biological and therapeutic properties of a humanized anti-CD4 MoAb, hIgG1-CD4, in patients with refractory psoriasis and rheumatoid arthritis (RA). hIgG1-CD4 is a modulating, non-depleting MoAb, which induced a first-dose reaction in most patients treated. It provided brief symptomatic relief in both conditions, and psoriasis appeared easier to control with conventional agents after MoAb therapy. At the doses used, hIgG1-CD4 did not synergize therapeutically with the panlymphocyte MoAb CAMPATH-1H (C1H) in patients with RA treated sequentially with both agents. There were no serious adverse effects definitely attributable to therapy. Our results are compared with those of other CD4 MoAb studies, and factors influencing the outcome of therapy are discussed.