Tolerance in the mouse to major histocompatibility complex-mismatched heart allografts, and to rat heart xenografts, using monoclonal antibodies to CD4 and CD8

Identification and characterization of the antigen-specific subpopulation of alloreactive CD4+ T cells in vitro and in vivo

We report the identification and characterization of the small subpopulation of alloantigen-specific T cells in vitro and in vivo. This subpopulation of T cells was distinguished by up-regulation of cell surface CD4 expression. These CD4high T cells were alloantigen specific in proliferation assays in vitro, and they expressed memory/activation markers, including CD44high and CD69high. Further studies demonstrated that these allospecific CD4high cells were also present (< or = 1% of CD4+ T cells) in vivo in BALB/c (H-2d) recipients of C57BL/6 (H-2b) skin allografts. CD4high T cells isolated from regional draining lymph nodes in these skin graft recipients reacted in a donor-specific fashion to C57BL/6 splenocyte stimulator cells in mixed lymphocyte culture. Adoptive transfer of CD4high, but not CD4normal T cells, just before skin engraftment in CD4 knockout mice, reconstituted rejection. The discovery that a small subpopulation of CD4high lymph node cells contained all of the alloantigen-specific T cells may allow study of tissue-specificity and subsequent alloantigen identification in transplantation.

Porcine embryonic brain cell cytotoxicity mediated by human natural killer cells

Intracerebral transplantation of porcine embryonic dopamine-producing neurons has been suggested as a method to treat patients with Parkinson's disease. Even though the brain is an immunologically privileged site, neuronal xenografts are usually rejected within a few weeks. T cells are important for this process, but the exact cellular events leading to rejection are poorly characterized. Brain cells from ventral mesencephalon of 26-27-day-old pig embryos were used as target cells in flow cytometry-assessed cytotoxicity assays using non- and IL-2-activated CD3- CD16+ CD56+ human natural killer (NK) cells as effector cells. The ability of human NK cells to kill pig embryonic brain cells by antibody-dependent cellular cytotoxicity (ADCC) in the presence of nondepleted and anti-Gal alpha1,3Gal antibody-depleted human blood group AB serum (AB serum) was evaluated using the same assay. Both nondepleted and anti-Gal alpha1,3Gal antibody-depleted AB serum could mediate ADCC of pig embryonic VM cells when human NK cells were used as effector cells. Nonactivated NK cells did not show any direct cytotoxic effect on freshly isolated VM cells, whereas IL-2-activated NK cells killed approximately 50% of the VM cells at an effector-to-target ratio of 50:1 in a 4-h cytotoxicity assay. Activation of VM cells by TNF-alpha did not change their sensitivity to human NK cell cytotoxicity. Human NK cells may thus contribute to a cellular rejection of pig neuronal xenografts by ADCC, or following IL-2 activation, by a direct cytotoxic effect.

Human natural antibodies cytotoxic to pig embryonic brain cells recognize novel non-Galalpha1,3Gal-based xenoantigens

Transplantation of porcine embryonic brain cells, including dopaminergic neurons, from ventral mesencephalon (VM) is considered a potential treatment for patients with Parkinson's disease. In the present study, we characterized the distribution among VM cells of the major porcine endothelial xenoantigen, the Galalpha1,3Gal epitope, and evaluated the cytotoxic effect of anti-Galalpha1,3Gal antibody-depleted and nondepleted human AB serum on VM cells. Overall levels of Galalpha1,3Gal-epitope expression was very low on the VM cell population using Bandeiraea simplicifolia IB(4) lectin staining of resuspended VM cells in flow cytometric analyses or staining of SDS-PAGE-separated, solubilized VM cell membrane proteins in Western blot analyses. Lectin-histochemical staining of sections of pig embryonal VM regions with BSA IB(4) lectin showed staining restricted to endothelial cells and microglia. In the presence of complement, both nondepleted and anti-Galalpha1,3Gal antibody-depleted AB sera were shown to be cytotoxic to VM cells as assessed in microcytotoxicity- and flow cytometry-based cytotoxicity assays. Purified IgM and IgG were both cytotoxic in the presence of complement. Three major VM cell membrane antigens of approximately 210, 105, and 50 kDa were reactive with natural IgM antibodies present in pooled human AB sera. Thus, antibody-dependent cytotoxicity may contribute to pig to human brain cell xenorejection, necessitating donor tissue modifications prior to a more widespread utilization of neural tissue xenografting.

Prediction of graft prolongation by mixed lymphocyte culture following anti-CD4 monoclonal antibody treatment among different donor-recipient combinations

This study was conducted to examine whether mixed lymphocyte culture (MLC) could be used as a predictor of the efficacy of anti-CD4 monoclonal antibody (MAb) immunosuppression in vivo in a mouse model. C57BL/10 or BALB/c hearts were transplanted into C3H.He recipients. Anti-CD4 MAb administration prolonged graft survival, but there was a clear difference between the two donor-recipient combinations studied, the median survival time (MST) being >100 days in the C57BL/10 --> C3H group, and 17 days in the BALB/c --> C3H group. Anti-CD8 MAb prolonged the survival of C57BL/10 hearts slightly to a MST of 22 days, but the BALB/c hearts were rejected at control rates. Combining anti-CD4 and anti-CD8 antibody therapy prolonged the survival of C57BL/10 hearts indefinitely, but had little effect on the survival of BALB/c grafts, achieving an MST of only 24 days. Next, MLCs were performed in the presence and absence of the MAbs and compared with the graft survival data. The inhibition rates in the MLC, being the C3H lymph node cell responder, correlated well with graft survival. When three kinds of C3H responder cells, namely lymph node (LN) cells, T cells, and CD4+ cells, were examined to determine which was the most suitable for predicting graft survival, the MLC results showed that the responses of LN cells correlated most closely with graft outcome. In conclusion, MLC using LN cells as the responder is a useful tool for predicting allograft survival induced by anti-CD4 MAb therapy.

Patterns of immune responses evoked by allogeneic hepatocytes: evidence for independent co-dominant roles for CD4+ and CD8+ T-cell responses in acute rejection

INTRODUCTION

This is the first in a series of reports that characterizes immune responses evoked by allogeneic hepatocytes using a functional model of hepatocyte transplantation in mice.

METHODS

"Donor" hepatocytes expressing the transgene human alpha-1-antitrypsin (hA1AT-FVB/N, H2q) were transplanted into C57BL/6 (H2b) or MHC II knockout (H2b) hosts treated with anti-CD4, anti-CD8, or a combination of anti-CD4 and anti-CD8 monoclonal antibodies (mAbs). Hepatocyte rejection was determined as a loss of circulating ELISA-detectable transgene product (hA1AT). In addition, some C57BL/6 mice underwent transplantation with FVB/N heterotopic cardiac allografts and were treated with anti-CD4 mAb. Cardiac allograft rejection was determined by palpation. Graft recipients were tested for donor-reactive alloantibodies and donor-reactive delayed-type hypersensitivity (DTH) responses.

RESULTS

The median survival time (MST) of allogeneic hepatocytes in normal C57BL/6 mice was 10 days (no treatment), 10 days (anti-CD4 mAb), 14 days (anti-CD8 mAb), and 35 days (anti-CD4 and anti-CD8 mAbs). The MST of hepatocytes in B6 MHC class II knockout mice was 10 days (no treatment) and 21 days (anti-CD8 mAb). The MST of cardiac allografts was 11 days (no treatment) and >100 days (anti-CD4 mAb). Donor-reactive DTH responses were readily detected in both untreated and mAb-treated recipients. Donor-reactive alloantibody was barely detectable in untreated hosts.

CONCLUSIONS

These studies demonstrate that allogeneic hepatocytes are highly immunogenic and stimulate strong cell-mediated immune responses by both CD4+ and CD8+ T cells, even when treated with agents that can cause acceptance of cardiac allografts. Indeed, CD4+ or CD8+ T cells seem to independently cause hepatocellular allograft rejection. Allogeneic hepatocytes evoked strong donor-reactive DTH responses but were poor stimuli for donor-reactive antibody production. This is an unusual pattern of immune reactivity in allograft recipients.

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.

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.

Thymus-dependent monoclonal antibody-induced protection from transferred diabetes

It is well established that long-term protection from insulin-dependent diabetes mellitus (IDDM) can be afforded to non-obese diabetic (NOD) mice by a short course of non-depleting (nd) anti-CD4 monoclonal antibodies (mAb). Since it is increasingly apparent that the CD8+ T cell plays a prominent role in the development of IDDM, we have investigated the effect of an anti-CD8 mAb (YTS 105) of the same isotype in both spontaneous and induced IDDM in NOD mice. Treatment with YTS 105 for 3 weeks was able to prevent the transfer of IDDM for a long period, and also substantially reduced spontaneous IDDM in female NOD mice. The role of the thymus in tolerance induction by these antibodies was studied. In the adult transfer model, thymectomized NOD mice, unlike their euthymic counterparts, were not protected long-term by treatment with YTS 105, and began to become overtly diabetic shortly after treatment. This was also true when the nd anti-CD4 mAb was used. Protection from spontaneous disease was not affected in the same way by thymectomy. The reasons for the observed effect of the thymus in the transfer model, and the differences between the two models that may explain the contrasting results are discussed.

Improvement in nerve regeneration by monoclonal antibodies to ICAM-1 and LFA-1 in allogeneic mice

We examined whether giving monoclonal antibodies (MoAb) to intercellular adhesion molecule 1 (ICAM-1) and leucocyte function associated antigen (LFA-1), which have important roles in the initial stage of rejection after allografts, can improve nerve regeneration in allogeneic mice. Fresh sciatic nerves were grafted using BALB/c mice as donors and C3H/He mice as recipients. Nerve regeneration at six weeks was significantly better in the mice given MoAbs at one and five days than in those given none (n = 5 in each group), although nerve regeneration even in the five-day group was significantly inferior to that in the syngeneic nerve graft group (n = 5). The survival time of the nerve donor skin graft at 12 weeks was not prolonged by treatment with MoAbs, indicating a failure to induce immunological tolerance. However, at 10 months after nerve grafting there were fewer Mac-1, Lyt-1, and Thy-1 positive cells in the five-day group and they showed less immunoreactivity than the untreated group. We conclude that giving MoAbs could effectively improve nerve regeneration in grafted allogeneic nerve segments, although it did not induce immunological tolerance.

Specific tolerance induction and organ transplantation

Induction of tolerance to histocompatibility antigens of an organ donor would eliminate the need for long-term administration of nonspecific immunosuppressive drugs associated with an increased risk of infection and malignancies. Recently, we established a murine model in which recipient mice were treated with a single dose of anti-CD3, anti-CD4, low dose of total body irradiation (TBI) and allogeneic bone marrow cells. Our results clearly demonstrate that stable multilineage mixed chimerism, immunocompetence and permanent donor-specific skin graft tolerance across full major histocompatibility (MHC) barriers can be successfully achieved in this way. The observations that the preparative regimen and skin transplantation can be performed on the same day, and that a significant reduction in irradiation dose is sufficient in haploidentical donor-recipient combinations (MHC-sharing effect), bring our protocol closer to clinical use.

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 Vβ8.2 and Vα10 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)×RAG-1−/− mice, while only CD4+ T cells were positively selected and found in the periphery of females. Female A1(M)×RAG-1−/− mice were able to rapidly reject (in &lt;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-γ 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 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.

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.

CD8+ Cells Are Not Necessary for Allograft Rejection or the Induction of Apoptosis in an Experimental Model of Small Intestinal Transplantation

Allospecific CTL can function as cellular effectors of solid organ graft rejection; however, the specific mechanisms of cell damage remain undetermined. In this study we examined the role of CD8+ T cells in apoptosis and rejection of small intestinal allografts. ACI rat intestinal grafts transplanted into Lewis rat recipients showed apoptosis of epithelial crypt cells on day 3 posttransplant as determined by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling staining. By day 7 numerous apoptotic crypt cells were detected in allografts, but were rarely observed in FK506-treated allograft recipients, isografts, or native intestine of allograft recipients. To further investigate the mechanism of rejection, recipient rats were depleted of CD8+ cells by treatment with OX-8 mAbs the day before and the day after transplantation of rat small intestinal allografts. Depletion of CD8+ cells from allograft recipients did not alter the tempo or the histologic features of rejection compared with those in the control (IgG-treated) group. Moreover, there was no difference in the number of apoptotic crypt epithelial cells in the grafts of control and CD8-depleted rats. Reverse transcriptase-PCR analyses determined there were similar levels of transcripts for Fas, Fas ligand, perforin, and granzyme B in control and CD8-depleted allograft recipients. By Western blot it was determined that the levels of Fas ligand protein were increased in the CD8-depleted group compared with those in control and FK506-treated allograft recipients. These data suggest that CD8 cells are not required for tissue injury or apoptotic cell death in small intestine allograft rejection.

Idarubicin-anti-CD3: a new immunoconjugate that induces alloantigen-specific tolerance in mice

BACKGROUND

In testing new anti-CD3 agents for transplantation tolerance induction, an anti-CD3 monoclonal antibody was used as a carrier for the cytotoxic drug idarubicin (IDA).

METHODS

Anti-CD3 (KT3) was covalently coupled with IDA, producing the IDA-KT3 immunoconjugate, which was tested for specificity by fluorometry and for inhibition of proliferation of CD3+ E3 cells ([3H]thymidine uptake). KT3 and IDA-KT3 were used to treat CBA recipients of BALB/c vascularized cardiac allografts. Mice with hearts surviving >100 days were challenged with donor and third-party (C57BL/6) skin.

RESULTS

Conjugation to IDA did not reduce binding of KT3 to E3 cells, although the toxicity of IDA was reduced by conjugation. In BALB/c to CBA cardiac allografts (rejected in 12-17 days), both KT3 and IDA-KT3 (0.25-0.5 mg/20 g mouse i.p. at the time of transplantation) induced tolerance. Hearts survived >100 days and skin graft challenge showed indefinite survival of donor grafts but not third-party grafts. KT3 was less toxic, as measured by tumor necrosis factor-a release and blood glucose levels, than equivalent dosages of 145-2C11. At lower dosages (0.1 mg/20 g mouse), KT3-treated animals rejected BALB/c allografts in 15 to 19 days, but IDA-KT3 induced long survival (>100 days) and donor-specific tolerance in 5 of 6 mice.

CONCLUSIONS

Coupling IDA to anti-CD3 reduced the in vivo toxicity of IDA and improved the immunosuppressive performance of KT3, reducing the side effects seen with other anti-CD3 agents. IDA-KT3 is a new, effective, nontoxic tolerogen in this donor-recipient combination.

Single dose anti-CD4 monoclonal antibody for induction of tolerance to cardiac allograft in high- and low-responder rat strain combinations

Repeated administration of monoclonal antibodies (mAb) directed against the CD4 lymphocyte receptor may induce specific, long-lasting unresponsiveness to fully MHC-mismatched cardiac allografts in rats without additional immunosuppression. We assessed the effect of a single dose of murine anti-rat depleting anti-CD4 mAb (OX-38) on allograft survival in high- and low-responder rat strain combinations. Isogenic strains of DA (RT1av1), PVG (RT1c), AUG (RT1c), and WF (RT1u) rats were used. Recipients in antibody treated groups were given one dose of 5 mg/kg OX-38 mAb on the day of transplant, a dose which was shown to effectively deplete (or block) circulating CD4+ T cells. Other groups were treated for 10 days with cyclosporin A (CsA) and/or Linomide, a novel immunomodulator, which is the first compound able to fully eliminate the effect of CsA in the rat cardiac allograft model. The DA strain was identified as a low-responder to the allogeneic haplotype RT1c (PVG or AUG), but not to RT1u (WF), and developed true tolerance following RT1c grafting and OX-38 or low-dose CsA (5 mg/kg) induction, as verified by the response to retransplantation of a graft from the same donor strain or a third-party challenge. PVG recipients of DA grafts were characterized by high response and only modest (OX-38; median 9.5 days) or moderate (CsA; 23.5 days) prolongation of graft survival. Contrasting graft survival results were obtained in the low-responder combination, either very early rejection (at 10 days) or permanent graft survival (> 100 days). Linomide challenge affected CsA treatment in the high-responder combination but not tolerance induction in the low-responder combination, or the effect of OX-38. It was concluded that in rat heart transplantation a single-dose anti-CD4 mAb therapy may induce permanent donor-specific unresponsiveness in a low-responder strain combination, and that anti-CD4 mAb seems to be unique among immunosuppressive agents while being resistent to challenge by Linomide.

Resistance to monoclonal antibody-induced CD8+ T-cell depletion in thymectomized MHC class II-deficient mice

BACKGROUND

CD8+ T cells are present at higher than normal levels in MHC class II-deficient (IIKO) mice.

METHODS

In this study, we have examined the sensitivity of CD8+ T cells to depletion induced by a single injection or multiple injections of an anti-CD8 monoclonal antibody (mAb) (2.43) in IIKO mice in vivo.

RESULTS

Thymectomized (ATX) IIKO mice showed the presence of a greater percentage of memory CD8+ T cells (CD44high, CD45RBlow, and MEL-14(-)) in peripheral blood lymphocytes (PBL) by 1 month after ATX compared with age-matched euthymic mice. Although CD8+ cells were not detectable in the periphery at 5 and 14 days after 2.43 injection, CD8+ T cell receptor alpha/beta+ cells expressing the memory phenotype had recovered markedly by 21 days after mAb injection in these ATX IIKO mice. The expression of CD8 beta-chain and Thy-1 as well as the absence of CD4 and of T cell receptor gamma/delta among most recovering CD8+ cells, and their varied Vbeta usage, suggested that these cells were derived from the thymus rather than from extra-thymic T-cell differentiation or from oligoclonal peripheral expansion. In addition, low numbers of CD8+ cells that were coated with mAb (2.43) were detected in the lymph nodes of ATX IIKO mice 7 days after mAb injection. Most of these nondepleted lymph node CD8+ cells expressed the memory phenotype and low levels of CD8beta. Furthermore, the levels of recovering CD8+ cells in PBL of ATX IIKO mice by 21 days after mAb treatment were markedly higher than those in PBL of simultaneously mAb-treated ATX wild-type (B10) mice.

CONCLUSION

Together, these studies indicate that memory CD8+ T cells are relatively resistant to mAb-induced depletion in vivo.

Effect of anti-CD4 monoclonal antibody combined with human CTLA4Ig on the survival of hamster liver and heart xenografts in Lewis rats

We investigated the effects of pretransplant anti-CD4 monoclonal antibody (mAb) combined with human (h) CTLA4Ig on the survival of hamster heart and liver xenografts. Pretransplant anti-CD4 mAb (5 mg/kg x 4 days) or hCTLA4Ig (0.5 mg/rat on days 1, 3, and 5 after transplantation) treatment alone prolonged the survival of hamster liver xenografts in Lewis rats (mean survival time [MST]=10.5 days, n=6, and MST=9.0 days, n=5, respectively, compared with untreated Lewis recipients of hamster liver grafts, MST=6.0 days, n=6). The same regimen could not prevent hamster heart xenorejection. Pretransplant anti-CD4 mAb (5 mg/kg x 4 days) combined with hCTLA4Ig (0.5 mg/rat x 4) treatments increased survival of hamster liver xenograft fourfold (MST=24.2 days, n=5). The current results also show that IgG in the sera from Lewis recipients of hamster liver grafts treated with anti-CD4 mAb and hCTLA4Ig was threefold reduced at 6 days after transplantation compared with untreated Lewis rats. These results suggest a synergistic effect of anti-CD4 mAb combined with hCTLA4Ig in a liver xenograft transplantation model.

Factors involved in rejection of concordant xenografts in complement-deficient rats

BACKGROUND

Factors that contribute to xenograft (Xg) rejection were investigated in complement C6-deficient (C-) PVG rats.

METHODS

First and second hamster hearts were transplanted in C6-deficient and C6-sufficient PVG rats. Xenoantibody (XAb) formation, hemolytic C (CH50) activity and immunohistochemistry were studied.

RESULTS

PVG C6-deficient rats rejected Xgs 3 days later than PVG C6-sufficient rats. Surprisingly, C activation participated in the rejection in PVG C- rats, as shown by partially recovered serum CH50 levels and deposition of C factors in the Xgs. As we found that cultured endothelial cells produced C6 in vitro, we hypothesized that Xg endothelial cells corrected the C6 defect in PVG C- rats. This was probably induced by IgM XAbs as: (1) it did not occur in immunosuppressed PVG C- rats in which XAb formation was prevented, and (2) transfer of IgM XAbs to naive, xenotransplanted PVG C- rats accelerated the recovery of CH50 and concomitantly Xg rejection. Thirty days after rejection of a first Xg, when no IgM XAbs or CH50 activity but high levels of IgG XAbs were detected in PVG C- rats, second Xgs underwent a hyperacute rejection. This time, complement was not involved, as no serum CH50 nor C deposition was found in the Xg. Instead, IgG antibody-dependent cellular cytotoxicity was involved as: (1) IgG XAbs were deposited in the Xg and (2) hyperacute rejection was induced in naive PVG C- rats by transfer of IgG XAbs, and (3) this rejection was delayed to 5+/-3 days if the adoptive hosts were first irradiated.

CONCLUSIONS

In the face of a defect of host C factors, IgM XAb may induce cells of the Xg to secrete C factors which may correct the C defect of the host. Even if activation of lytic C can be prevented, IgG XAb may still provoke an acute Xg rejection by antibody-dependent cellular cytotoxicity.

Rat pancreatic islet and skin xenograft survival in CD4 and CD8 knockout mice

The relative contributions of the CD4+ and CD8+ T cell subpopulations in xenotransplant rejection were studied using CD4 and CD8 knockout (KO) mice. Wistar Furth (WF, RT1a) rat pancreatic islet or skin xenografts were transplanted into either CD4 or CD8 KO recipients and compared to wild-type controls. Long-term survival of WF islet xenografts was observed in the CD4 KO mice (MST, >66+/-8 days) whereas CD8 KO mice rejected their islet xenografts within 8 days, similar to controls (MST, 7+/-0.2 days). In contrast, WF skin xenografts were rejected in both CD4 and CD8 KO recipients within 8 days. CD4 KO recipients which maintained xenoislets >90 days posttransplant rejected WF skin grafts within 9 days, without rejecting their original islet xenografts. These results suggest that CD4+ cells are essential for mediating islet xenograft rejection. These data also suggest that the absence of either CD4+ or CD8+ T cells is not sufficient to prevent rejection of skin xenografts.

Lack of cognate help by CD4+ T cells and anergy of CD8+ T cells are the principal mechanisms for anti-leukocyte function-associated antigen-1/intercellular adhesion molecule-1-induced cardiac allograft tolerance

Combined treatment with anti-leukocyte function-associated antigen-1 (LFA-1) and anti-intercellular adhesion molecule-1 (ICAM-1) monoclonal antibodies leads to allograft tolerance in murine cardiac transplantation. In the present study, we analyzed the mechanisms for this tolerance induction. In the tolerant mice, proliferative response of splenic T cells against donor-type cardiac myocytes and of CD8+ T cells against donor-type alloantigens was impaired as compared with responses in naive or rejected mice, but was completely restored with exogenous interleukin 2. This suggests that class I-restricted CD8+ T cells of tolerant mice were rendered anergic against donor-type alloantigens in the periphery. In contrast, proliferative response of CD4+ T cells against donor-type alloantigens in vitro was comparable between tolerant and naive mice. When heart and skin grafts from the same donor (BALB/c [H2d]) were simultaneously transplanted to C3H mice (H2k), both were rejected within 29 days, even though the mice were similarly treated with anti-LFA-1 and anti-ICAM-1 monoclonal antibodies. In contrast, when heart graft from BALB/c and skin graft from third-party donor (C57BL/6 [H2b]) were simultaneously transplanted to C3H mice under the same condition, the heart graft was accepted indefinitely and the skin graft was rejected. These findings suggest that the peripheral tolerance against cardiac allografts could be induced by selective inactivation of alloreactive CD8+ T cells resulting from the lack of cognate help by CD4+ T cells.

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.

CD4+ but not CD8+ cells are essential for allorejection

The generation of knockout mice with targeted gene disruption has provided a valuable tool for studying the immune response. Here we describe the use of CD4 and CD8 knockout mice to examine the role of CD4+ and CD8+ cells in initiating allotransplantation rejection. Pretreatment with a brief course of depletive anti-CD4 monoclonal antibody therapy allowed permanent survival of heart, but not skin, allografts transplanted across a major histocompatibility barrier. However, skin as well as heart grafts were permanently accepted in the CD4 knockout mice. Transfer of CD4+ cells into CD4 knockout recipient mice 1 d before skin engraftment reconstituted rejection, demonstrating that CD4+ cells are necessary for initiating rejection of allogeneic transplants. Major histocompatibility complex disparate heart and skin allografts transplanted into CD8 knockout recipients were rejected within 10 d. This study demonstrates that CD4+ but not CD8+ T cells are absolutely required to initiate allograft rejection.

Liver allograft rejection in rats depleted of CD8+ cells

The mechanism(s) of rejection or tolerance induction is a competitive, complex process that presumably involves interactions between multiple subpopulations of T lymphocytes. We investigated the roles of CD8+ cytolytic and CD4+ helper T cells in rat strains that tolerate liver allografts and that differ at both the major histocompatibility complex (MHC) (RT1) and minor histocompatibility genes. Orthotopic liver transplantation (OLT) with arterial reconstruction was performed with Brown Norway (BN) (RT1n) donors and Lewis (RT1(1)) recipients, some of which were untreated, others treated with anti-CD8 antibody, and still others treated with anti-CD4 antibody. Liver graft rejection was monitored for 28 days on the basis of two criteria: (1) serum levels of AST enzyme at 3-day intervals and (2) liver biopsies at weekly intervals and at the time of sacrifice at the end of the study period. In the untreated control group, an elevation of AST was found to peak at day 6 after grafting, and it remained elevated until day 28 (AST 542 +/- 72 U/l). Histologically, signs of severe rejection were first observed on day 9; these changed to moderate rejection about day 21 and to mild rejection by day 28, when the animals were sacrificed. Recipients pre-treated with anti-CD8 demonstrated a significant elevation of AST within 6 days that, unlike in the control recipients, continued to rise sharply through the observation period (AST 1127 +/- 181 U/l, P = 0.009 vs control group). Liver biopsies showed mild rejection at day 9 and moderate rejection at days 21 through 28. Recipients pretreated with anti-CD4 showed a time course of enzyme elevation and severity of rejection that was not significantly different from that observed in the control group. However, anti-CD4 treatment resulted in only 75% depletion of CD4+ cells in peripheral blood as compared to complete elimination of CD8+ cells following anti-CD8 treatment. Functional studies of spleen and liver-infiltrating lymphocytes obtained after 28 days showed low proliferative response in mixed lymphocyte culture with both BN and PVG stimulator spleen and lymph node cells. These results suggest that in this donor/recipient combination, removal of CD8+ cells increases the severity of rejection as demonstrated by a progressive rise in AST and histology. Moreover, OLT in this combination results in a profound, nonspecific inhibition of proliferative T-cell responses to MHC alloantigens.

Induction of persistent allograft tolerance in the rat by combined treatment with anti-leukocyte function-associated antigen-1 and anti-intercellular adhesion molecule-1 monoclonal antibodies, donor-specific transfusion, and FK506

We previously reported that a short course of treatment with anti-LFA-1 and anti-ICAM-1 monoclonal antibodies (mAbs) led to a persistent acceptance of mouse cardiac allografts, which resulted from the induction of allospecific tolerance. In the present study, we tested the effect of anti-LFA-1 and anti-ICAM-1 mAbs on rat allograft rejection and analyzed the mechanisms underlying allograft tolerance. In sharp contrast to the mouse case, a short course of treatment with anti-LFA-1 and anti-ICAM-1 mAbs led to a persistent acceptance in only half of the treated rats when MHC was compatible but mismatched for minor antigens, and was virtually ineffective when MHC was fully incompatible. However, treatment with these mAbs combined with donor-specific transfusion and FK506 consistently led to a persistent acceptance, even when the MHC was fully incompatible. Donor-specific tolerance was induced by this treatment, as estimated by skin challenging. In the tolerant rats, proliferative response and CTL generation against donor-type alloantigen were severely impaired but partially restored by exogenous interleukin-2. Limiting dilution analysis demonstrated that the precursor frequency of CTL was decreased in the tolerant rats, as compared with the naive rats. These results suggest that donor-reactive T cells were partially deleted and rendered anergic in the periphery.

CD4+ T cell mediated destruction of xenografts within cell-impermeable membranes in the absence of CD8+ T cells and B cells

Xenogeneic cells encapsulated in cell-impermeable diffusion chambers die within 3 weeks when implanted into immunocompetent animals but not when implanted into immunodeficient animals. To determine which cells are necessary for this observation, we depleted normal mice in vivo of either CD4+ or CD8+ T cells using monoclonal antibodies. We also reconstituted the immune system of athymic CBA mice (T-lymphocyte deficient) and C.B17 SCID mice (T- and B-lymphocyte deficient) with different cell subsets from normal CBA and BALB/C mice, respectively. Depleted or reconstituted mice were implanted with a diffusion chamber containing COS (monkey kidney) cells. Membrane enclosed xenografts survived in CD4+ T cell depleted mice but not in CD8+ T cell depleted or nondepleted control mice. Encapsulated xenografts survived when implanted into either athymic or SCID mice but were destroyed in reconstituted athymic and SCID mice. Furthermore, encapsulated xenogeneic cells were destroyed in athymic or SCID mice reconstituted with CD4+ cell preparations depleted of CD8+ cells and/or B cells. In contrast, encapsulated xenogeneic cells were not destroyed in athymic or SCID mice reconstituted with CD8+ cell preparations depleted of CD4+ cells. These studies highlight the critical role of CD4+ T cells, in the absence of CD8+ cells and B cells, in the processes leading to the ultimate destruction of encapsulated xenografts. Because of the use of cell-impermeable membranes in these studies, the most likely involvement of CD4+ T cells is in the indirect antigen recognition by these cells and subsequent stimulation of inflammatory cells.

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.

Indefinite survival of neural xenografts induced with anti-CD4 monoclonal antibodies

Xenografts of neural tissue are usually rapidly rejected when transplanted into the central nervous system of adult recipient animals. This study has examined the cell mediated immune response to both concordant (between closely related species) and discordant (between distantly related species) neural xenografts in the mouse, and has investigated the role of the CD4+ and CD8+ T lymphocyte subsets in this process using monoclonal antibodies specific for the CD4 and CD8 cell surface glycoproteins. We have established that: (1) in this model system concordant neural xenograft rejection occurs within 15-30 days; however, xenograft survival can be dramatically prolonged with CD4+, but not CD8+, T lymphocyte depletion; (2) the administration of two successive courses of a high dose of anti-CD4 monoclonal antibody treatment results in indefinite concordant neural xenograft survival; (3) the mechanism by which the high dose anti-CD4 monoclonal antibody therapy appears to function involves the depletion of intrathymic CD4+ cells; (4) anti-CD4 monoclonal antibody treatment enhances discordant neural xenograft survival, to beyond 60 days in many cases. These results demonstrate that CD4+ T lymphocytes are of central importance in the immune response to both concordant and discordant neural xenoantigens. Thus the use of anti-CD4 monoclonal antibody therapy is an effective strategy to prolong significantly the survival of xenogeneic neural transplants. Furthermore this treatment caused no obvious deleterious side-effects. These findings have implications for future cross-species studies in experimental neurobiology and, possibly, in clinical neural transplantation.

Immunopathogenesis and immunotherapy in rheumatoid arthritis: an area in transition

In years to come, new therapeutic modalities for the treatment of chronic arthritis will be launched for general clinical use. These therapies, until today only used in clinical studies, are based on knowledge obtained from animal models of chronic arthritis. This knowledge not only ushers therapeutic use in humans: in many settings, the animal studies have proven to be irreplacable tools to get insights into the pathogenesis of chronic arthritis. Rheumatoid arthritis (RA) shows a strong linkage of susceptibility to a certain epitope common to some HLA-DR beta chains; this immunogenetic linkage is the strongest evidence for specific, T-cell dependent immunity in the pathogenesis of the disease. Despite intense efforts, no unequivocal proofs of T-cell specificity or oligoclonality have been found in RA. Therapeutic efforts directed against T-cells or T-cell functions have also at the best showed partial effects. As compared to the local production of T-cell cytokines in the joint, monokine production is abundant. Therapies aimed at neutralizing the effects of the cartilage-devastating monokine TNF-a have showed remarkable results in small clinical trials. The possibility of increasing the presence of the regulatory cytokines IL-4, IL-10 and TGF-beta has also been explored, but only in animal studies. Immunology has also shed light on the mode of action of the commonly used 'disease modifying' drugs, and combinations of such drugs have shown increased potentials in recent clinical studies. The possibility of combining traditional anti-arthritic drugs with recent immunological tools seem promising for the future. This review discusses recent advances in the understanding of pathogenesis and delineate new therapeutic approaches for chronic arthritis from the point of view of the immunologically oriented clinician.

Tissue-specific effects of anti-CD4 therapy in induction of allograft unresponsiveness in high and low responder rats

In these experiments, we studied the role of anti-CD4 (Ox38) monoclonal antibody in the prevention of heart and/or kidney allograft rejection in low (ACI) and high (Lewis) responder rats. In low responder ACI rats, donor-specific tolerance for heart and kidney allografts (individually or in combination) was achieved by pretransplant anti-CD4 therapy. In high responder Lewis rats, anti-CD4 therapy alone (or combined with anti-CD8 (Ox8), thymectomy or total lymphoid irradiation) did not prevent first-set rejection of heart allografts. This difference was correlated with a more profound and longer lasting CD4+ cell depletion in the low responder strain. Anti-CD4 treatment, however, produced tolerance of kidney transplants in high responder rats. Additionally, anti-CD4 treatment induced tolerance to heart (as well as kidney) allografts in Lewis recipients of combined kidney and heart allografts from ACI. The effects of anti-CD4 treatment thus depend upon the recipient responder status as well as the organs transplanted and the order of transplantation.

The pharmacology of immunosuppressant drugs in skin transplant rejection in mice and other rodents

1. Skin transplantation in rodents is a convenient, widely used method, particularly in mice. It is used as much as an indicator of immune responsiveness as for pharmacological studies. 2. Many differences exist in experimental protocols, both for transplantation and drug administration and in this review, the increase in graft survival time with respect to control times is used to indicate drug effects, in an attempt to account for these differences. 3. The mechanisms underlying skin graft rejection in rodents are described, emphasising the crucial role of both helper and effector T cells. 4. The pharmacology of clinically-used immunosuppressants, including CsA, FK506, rapamycin and purine or pyrimidine synthesis inhibitors, in rodent models of skin transplantation is reviewed. 5. The effects of other potential immunosuppressants and compounds modulating immune responses are described, including the effects of UV light and involvement of platelet-derived factors, prostaglandins and thromboxanes.

Thymic epithelium induces full tolerance to skin and heart but not to B lymphocyte grafts

Athymic nude mice reconstituted at birth with allogeneic thymic epithelia (TE) from day 10 embryos (E10), show life-long specific tolerance to skin and heart grafts, but eliminate B lymphocytes of the TE donor haplotype, nearly as well as those from a third strain. Previous immunizations with B cells do not alter the state of tolerance to skin grafts, but specifically accelerate elimination of lymphocytes. In contrast, transplantation of E15 allogeneic thymuses already seeded by hematopoietic cells resulted in chimeras tolerant to both skin and B lymphocytes. In vitro reactivities towards stimulator spleen cells of the haplotype of the thymus were observed in both E10 TE and E15 thymus chimeras. We conclude that induction of full in vivo tolerance to B cells requires hematopoietic cells, while this is not the case for induction of tolerance to skin and heart tissues; furthermore, in vitro reactivity to stimulator spleen cells of the tolerized haplotype is independent of in vivo tolerance.

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.

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.

Monoclonal antibodies for the induction of transplantation tolerance

Non-lytic antibodies to CD4 and certain other T cell adhesion receptors can guide the immune system to become tolerant to foreign antigens, and to regain tolerance in autoimmunity. Tolerance is maintained lifelong through the action of regulatory T cells that in turn can influence naive T cells to acquire the same regulatory properties. A fuller understanding of the molecular basis of infectious tolerance could lead to the design of better immunosuppressive protocols.

The use of monoclonal antibodies to achieve immunological tolerance

Monoclonal antibodies are potentially useful immunosuppressive agents. Short courses of CD4/CD8 monoclonal antibody can be used to guide the immune system of experimental animals to accept organ grafts and to arrest autoimmunity. This reprogramming is accompanied by potent T-cell dependent, 'infectious' regulatory mechanisms. A goal for therapeutic immunosuppression should be to understand and harness these innate immunoregulatory mechanisms.

The use of monoclonal antibodies to achieve immunological tolerance

Monoclonal antibodies are potentially useful immunosuppressive agents. Short course of CD4/CD8 monoclonal antibody can be used to guide the immune system of experimental animals to accept organ grafts and to arrest autoimmunity. This reprogramming, reviewed by Herman Waldmann and Stephen Cobbold, is accompanied by potent T-cell dependent, 'infectious' regulatory mechanisms. A goal for therapeutic immunosuppression should be to understand and harness these innate immunoregulatory mechanisms.