A monoclonal antibody (A6) recognizing a unique epitope restricted to CD45RO and RB isoforms of the leukocyte common antigen family identifies functional T cell subsets

CD4+CD25+ T regulatory cells in renal transplantation

The immune response to an allograft activates lymphocytes with the capacity to cause rejection. Activation of CD4+CD25+Foxp3+T regulatory cells (Treg) can down-regulate allograft rejection and can induce immune tolerance to the allograft. Treg represent <10% of peripheral CD4+T cells and do not markedly increase in tolerant hosts. CD4+CD25+Foxp3+T cells include both resting and activated Treg that can be distinguished by several markers, many of which are also expressed by effector T cells. More detailed characterization of Treg to identify increased activated antigen-specific Treg may allow reduction of non-specific immunosuppression. Natural thymus derived resting Treg (tTreg) are CD4+CD25+Foxp3+T cells and only partially inhibit alloantigen presenting cell activation of effector cells. Cytokines produced by activated effector cells activate these tTreg to more potent alloantigen-activated Treg that may promote a state of operational tolerance. Activated Treg can be distinguished by several molecules they are induced to express, or whose expression they have suppressed. These include CD45RA/RO, cytokine receptors, chemokine receptors that alter pathways of migration and transcription factors, cytokines and suppression mediating molecules. As the total Treg population does not increase in operational tolerance, it is the activated Treg which may be the most informative to monitor. Here we review the methods used to monitor peripheral Treg, the effect of immunosuppressive regimens on Treg, and correlations with clinical outcomes such as graft survival and rejection. Experimental therapies involving ex vivo Treg expansion and administration in renal transplantation are not reviewed.

Multiple sclerosis patients have reduced resting and increased activated CD4+CD25+FOXP3+T regulatory cells

Resting and activated subpopulations of CD4⁺CD25⁺CD127^loT regulatory cells (Treg) and CD4⁺CD25⁺CD127⁺ effector T cells in MS patients and in healthy individuals were compared. Peripheral blood mononuclear cells isolated using Ficoll Hypaque were stained with monoclonal antibodies and analysed by flow cytometer. CD45RA and Foxp3 expression within CD4⁺ cells and in CD4⁺CD25⁺CD127^loT cells identified Population I; CD45RA⁺Foxp3⁺, Population II; CD45RA⁻Foxp3^hi and Population III; CD45RA⁻Foxp3⁺ cells. Effector CD4⁺CD127⁺ T cells were subdivided into Population IV; memory /effector CD45RA⁻ CD25⁻Foxp3⁻ and Population V; effector naïve CD45RA⁺CD25⁻Foxp3⁻CCR7⁺ and terminally differentiated RA⁺ (TEMRA) effector memory cells. Chemokine receptor staining identified CXCR3⁺Th1-like Treg, CCR6⁺Th17-like Treg and CCR7⁺ resting Treg. Resting Treg (Population I) were reduced in MS patients, both in untreated and treated MS compared to healthy donors. Activated/memory Treg (Population II) were significantly increased in MS patients compared to healthy donors. Activated effector CD4⁺ (Population IV) were increased and the naïve/ TEMRA CD4⁺ (Population V) were decreased in MS compared to HD. Expression of CCR7 was mainly in Population I, whereas expression of CCR6 and CXCR3 was greatest in Populations II and intermediate in Population III. In MS, CCR6⁺Treg were lower in Population III. This study found MS is associated with significant shifts in CD4⁺T cells subpopulations. MS patients had lower resting CD4⁺CD25⁺CD45RA⁺CCR7⁺ Treg than healthy donors while activated CD4⁺CD25^hiCD45RA⁻Foxp3^hiTreg were increased in MS patients even before treatment. Some MS patients had reduced CCR6⁺Th17-like Treg, which may contribute to the activity of MS.

Hitting the Holy Grail of Hematopoietic Cell Transplantation with Naive T-Cell Depleted Allografts-Graft Engineered Hematopoietic Stem Cell Transplant

Hematopoietic cell transplant is a potentially curative procedure for many benign and malignant conditions. The efficacy of allogeneic transplant relies in part on the cytotoxicity of the conditioning regimen and the graft versus tumor effect mediated by alloreactive donor T cells; the same cells are also implicated in the development of graft versus host disease (GVHD). Selective identification and depletion of the T cells implicated in GVHD, while preserving the T cells responsible for graft versus tumor effect has been the focus of many research groups in the recent years. Here we briefly review the physiology of T cells in transplantation, and comment on a recent clinical trial published by Bleakly et al. using a novel way of graft engineered allograft via naïve T cell depletion.

T Cells: Soldiers and Spies--The Surveillance and Control of Effector T Cells by Regulatory T Cells

Traditionally, T cells were CD4+ helper or CD8+ cytotoxic T cells, and with antibodies, they were the soldiers of immunity. Now, many functionally distinct subsets of activated CD4+ and CD8+ T cells have been described, each with distinct cytokine and transcription factor expression. For CD4+ T cells, these include Th1 cells expressing the transcription factor T-bet and cytokines IL-2, IFN-γ, and TNF-β; Th2 cells expressing GATA-3 and the cytokines IL-4, IL-5, and IL-13; and Th17 cells expressing RORγt and cytokines IL-17A, IL-17F, IL-21, and IL-22. The cytokines produced determine the immune inflammation that they mediate. T cells of the effector lineage can be naïve T cells, recently activated T cells, or memory T cells that can be distinguished by cell surface markers. T regulatory cells or spies were characterized as CD8+ T cells expressing I-J in the 1970s. In the 1980s, suppressor cells fell into disrepute when the gene for I-J was not present in the mouse MHC I region. At that time, a CD4+ T cell expressing CD25, the IL-2 receptor-α, was identified to transfer transplant tolerance. This was the same phenotype of activated CD4+ CD25+ T cells that mediated rejection. Thus, the cells that could induce tolerance and undermine rejection had similar badges and uniforms as the cells effecting rejection. Later, FOXP3, a transcription factor that confers suppressor function, was described and distinguishes T regulatory cells from effector T cells. Many subtypes of T regulatory cells can be characterized by different expressions of cytokines and receptors for cytokines or chemokines. In intense immune inflammation, T regulatory cells express cytokines characteristic of effector cells; for example, Th1-like T regulatory cells express T-bet, and IFN-γ-like Th1 cells and effector T cells can change sides by converting to T regulatory cells. Effector T cells and T regulatory cells use similar molecules to be activated and mediate their function, and thus, it can be very difficult to distinguish soldiers from spies.

IL-5 promotes induction of antigen-specific CD4+CD25+ T regulatory cells that suppress autoimmunity

Immune responses to foreign and self-Ags can be controlled by regulatory T cells (Tregs) expressing CD4 and IL-2Rα chain (CD25). Defects in Tregs lead to autoimmunity, whereas induction of Ag-specific CD4+CD25+ Tregs restores tolerance. Ag-specific CD4+CD25+ FOXP3+Tregs activated by the T helper type 2 (Th2) cytokine, IL-4, and specific alloantigen promote allograft tolerance. These Tregs expressed the specific IL-5Rα and in the presence of IL-5 proliferate to specific but not third-party Ag. These findings suggest that recombinant IL-5 (rIL-5) therapy may promote Ag-specific Tregs to mediate tolerance. This study showed normal CD4+CD25+ Tregs cultured with IL-4 and an autoantigen expressed Il-5rα. Treatment of experimental autoimmune neuritis with rIL-5 markedly reduced clinical paralysis, weight loss, demyelination, and infiltration of CD4+ (Th1 and Th17) CD8+ T cells and macrophages in nerves. Clinical improvement was associated with expansion of CD4+CD25+FOXP3+ Tregs that expressed Il-5rα and proliferated only to specific autoantigen that was enhanced by rIL-5. Depletion of CD25+ Tregs or blocking of IL-4 abolished the benefits of rIL-5. Thus, rIL-5 promoted Ag-specific Tregs, activated by autoantigen and IL-4, to control autoimmunity. These findings may explain how Th2 responses, especially to parasitic infestation, induce immune tolerance. rIL-5 therapy may be able to induce Ag-specific tolerance in autoimmunity.

Induction of passive Heymann nephritis in complement component 6-deficient PVG rats

Passive Heymann nephritis (PHN), a model of human membranous nephritis, is induced in susceptible rat strains by injection of heterologous antisera to rat renal tubular Ag extract. PHN is currently considered the archetypal complement-dependent form of nephritis, with the proteinuria resulting from sublytic glomerular epithelial cell injury induced by the complement membrane attack complex (MAC) of C5b-9. This study examined whether C6 and MAC are essential to the development of proteinuria in PHN by comparing the effect of injection of anti-Fx1A antisera into PVG rats deficient in C6 (PVG/C6(-)) and normal PVG rats (PVG/c). PVG/c and PVG/C6(-) rats developed similar levels of proteinuria at 3, 7, 14, and 28 days following injection of antisera. Isolated whole glomeruli showed similar deposition of rat Ig and C3 staining in PVG/c and PVG/C6(-) rats. C9 deposition was abundant in PVG/c but was not detected in PVG/C6(-) glomeruli, indicating C5b-9/MAC had not formed in PVG/C6(-) rats. There was also no difference in the glomerular cellular infiltrate of T cells and macrophages nor the size of glomerular basement membrane deposits measured on electron micrographs. To examine whether T cells effect injury, rats were depleted of CD8+ T cells which did not affect proteinuria in the early heterologous phase but prevented the increase in proteinuria associated with the later autologous phase. These studies showed proteinuria in PHN occurs without MAC and that other mechanisms, such as immune complex size, early complement components, CD4+ and CD8+ T cells, disrupt glomerular integrity and lead to proteinuria.

Transfer of Allograft Specific Tolerance Requires CD4+CD25+T Cells but Not Interleukin-4 or Transforming Growth Factor–β and Cannot Induce Tolerance to Linked Antigens

BACKGROUND

The mechanisms by which CD4+T cells, especially CD4+ CD25+T cells, transfer allograft specific tolerance are poorly defined. The role of cytokines and the effect on antigen-presenting cells is not resolved.

METHODS

Anti-CD3 monoclonal antibody (mAb) therapy induced tolerance to PVG heterotopic cardiac transplantation in DA rats. Peripheral CD4+T cells or CD4+ CD25+ and CD4+ CD25-T cell subsets were adoptively transferred to irradiated DA hosts grafted with PVG heart grafts. For specificity studies, tolerant CD4+T cells were transferred to hosts with Lewis or (PVGxLewis)F1 heart grafts. Cytokine mRNA induction and the requirement for interleukin (IL)-4 and transforming growth factor (TGF)-beta in the transfer of tolerance was assessed.

RESULTS

CD4+T cells transferred specific tolerance and suppressed naïve CD4+T cells capacity to effect rejection of PVG but not Lewis grafts. (PVGxLewis)F1 grafts had a major rejection episode but recovered. Later these hosts accepted PVG but not Lewis skin grafts. Adoptive hosts restored with tolerant or naïve cells had similar levels of mRNA expression for all Th1 and Th2 cytokines and effector molecules assayed. Transfer of tolerance by CD4+T cells was not blocked by mAb to IL-4 or TGF-beta. CD4+ CD25-T cells from either naïve or tolerant hosts effected rejection. In contrast neither tolerant nor naïve CD4+ CD25+T cells restored rejection.

CONCLUSIONS

Specific tolerance transfer required CD4+ containing CD4+ CD25+T cells. An inflammatory response with induction of mRNA for Th1 and Th2 cytokines plus cytotoxic effector molecules occurred, but IL-4 and TGF-beta were not essential. Inhibition of antigen presenting cells was not the sole mechanism as there was no linked tolerance.

Transplant Tolerance Associated With a Th1 Response and Not Broken by IL-4, IL-5, and TGF-β Blockade or Th1 Cytokine Administration

BACKGROUND

Specific transplant tolerance is mediated by CD4 T cells that die unless supported by T-cell derived cytokines and donor antigen. This study examined the role of Th1 and Th2 cytokines in the maintenance of tolerance.

METHODS

Tolerance to fully allogeneic PVG cardiac allografts in DA rats was induced by short-term anti-CD3 monoclonal antibody therapy. Responses of tolerant cells to donor and third party antigen were assessed in vivo by examination of the infiltrate in the heart and application of skin grafts, and in vitro in mixed lymphocyte culture. Cell subsets were stained, induction of cytokine mRNA assayed by reverse-transcriptase polymerase chain reaction and the role of cytokines determined by treating with blocking monoclonal antibody to cytokines or cytokine administration.

RESULTS

Tolerated grafts had a T cell and macrophage infiltrate with increased mRNA for Th1 cytokines, interleukin (IL)-2, and interferon (IFN)-gamma but not Th2 cytokines. Peripheral lymphocytes proliferated in mixed lymphocyte culture and expressed Th1 cytokine mRNA. Tolerant hosts accepted PVG and rejected Lewis skin allografts and the lymph nodes draining both these grafts had similar induction of Th1 and Th2 cytokine mRNA. Treatment of tolerant rats with Th1 cytokines IL-2, IFN-gamma, and IL-12p70 or monoclonal antibody that blocked IL-4, IL-5, and transforming growth factor-beta did not prevent acceptance of PVG skin grafts.

CONCLUSIONS

These studies in a model of tolerance regulated by CD4CD25 T cells demonstrated there was no defect in Th1 responses. Tolerance was due to regulation that was not solely dependent on IL-4, IL-5, or transforming growth factor-beta and was not inactivated or overwhelmed by administration of Th1 cytokines, IL-2, IFN-gamma or IL-12p70.

An anti-CD45RO/RB monoclonal antibody modulates T cell responses via induction of apoptosis and generation of regulatory T cells

The effects of a chimeric monoclonal antibody (chA6 mAb) that recognizes both the RO and RB isoforms of the transmembrane protein tyrosine phosphatase CD45 on human T cells were investigated. Chimeric A6 (chA6) mAb potently inhibited antigen-specific and polyclonal T cell responses. ChA6 mAb induced activation-independent apoptosis in CD4⁺CD45RO/RB^high T cells but not in CD8⁺ T cells. In addition, CD4⁺ T cell lines specific for tetanus toxoid (TT) generated in the presence of chA6 mAb were anergic and suppressed the proliferation and interferon (IFN)-γ production by TT-specific effector T cells by an interleukin-10–dependent mechanism, indicating that these cells were equivalent to type 1 regulatory T cells. Similarly, CD8⁺ T cell lines specific for the influenza A matrix protein-derived peptide (MP.58-66) generated in the presence of chA6 mAb were anergic and suppressed IFN-γ production by MP.58-66–specific effector CD8⁺ T cells. Furthermore, chA6 mAb significantly prolonged human pancreatic islet allograft survival in nonobese diabetic/severe combined immunodeficiency mice injected with human peripheral blood lymphocytes (hu-PBL-NOD/SCID). Together, these results demonstrate that the chA6 mAb is a new immunomodulatory agent with multiple modes of action, including deletion of preexisting memory and recently activated T cells and induction of anergic CD4⁺ and CD8⁺ regulatory T cells.

Il-4 therapy prevents the development of proteinuria in active Heymann nephritis by inhibition of Tc1 cells

The role of IL-4, a key Th2 cytokine, in promoting or inhibiting active Heymann nephritis (HN) was examined. HN is induced by immunization with Fx1A in CFA, and proteinuria in HN is associated with subepithelial IgG and C3 deposition and infiltration of CD8(+) T-cytotoxic 1 (Tc1) cells and macrophages into glomeruli, as well as induction of Abs to Crry. Treatment with rIL-4 from the time of Fx1A/CFA immunization stimulated an earlier IgG1 response to Fx1A, induced anti-Crry Abs, and up-regulated IL-4 mRNA in lymphoid tissue, but did not alter proteinuria. Treatment with MRCOx-81, an IL-4-blocking mAb, resulted in greater proteinuria, which suggests endogenous IL-4 regulated the autoimmune response. Delay of rIL-4 treatment until 4 wk post-Fx1A/CFA immunization and just before the onset of proteinuria prevented the development of proteinuria and reduced Tc1 cell infiltrate in glomeruli. Delayed treatment with IL-4 had no effect on titer or isotype of Abs to Fx1A or on Ig, C3, and C9 accumulation in glomeruli. Treatment with rIL-13, a cytokine that alters macrophage function such as rIL-4, but has no direct effect on T or B cell function, reduced glomerular macrophage infiltrate, but did not prevent proteinuria or CD8+ T cell infiltrate. Anti-Crry Abs were paradoxically only induced with rIL-4 therapy, not in HN controls with proteinuria. It was concluded that the rIL-4 effect was probably by inhibition of Tc1 cells, which normally mediate the glomerular injury that results in proteinuria.

Reversal of experimental allergic encephalomyelitis with non-mitogenic, non-depleting anti-CD3 mAb therapy with a preferential effect on T(h)1 cells that is augmented by IL-4

This study examined whether therapy with a non-mitogenic, non-activating anti-CD3 mAb (G4.18) alone, or in combination with the T(h)2 cytokines, could inhibit induction or facilitate recovery from experimental allergic encephalomyelitis (EAE) in Lewis rats. G4.18, but not rIL-4, rIL-5 or anti-IL-4 mAb, reduced the severity and accelerated recovery from active EAE. A combination of rIL-4 with G4.18 was more effective than G4.18 alone. The infiltrate of CD4(+) and CD8(+) T cells, B cells, dendritic cells, and macrophages in the brain stem was less with combined G4.18 and IL-4 than G4.18 therapy or no treatment. Residual cells had preferential sparing of T(r)1 cytokines IL-5 and transforming growth factor-beta with loss of T(h)1 markers IL-2, IFN-gamma and IL-12Rbeta2, and the T(h)2 cytokine IL-4 as well as macrophage cytokines IL-10 and tumor necrosis factor-alpha. Lymph nodes draining the site of immunization had less mRNA for T(h)1 cytokines, but T(h)2 and T(r)1 cytokine expression was spared. Treatment with G4.18, rIL-4 or rIL-5 from the time of immunization had no effect on the course of active EAE. MRC OX-81, a mAb that blocks IL-4, delayed onset by 2 days, but had no effect on severity of active EAE. G4.18 also inhibited the ability of activated T cells from rats with active EAE to transfer passive EAE. This study demonstrated that T cell-mediated inflammation was rapidly reversed by a non-activating anti-CD3 mAb that blocked effector T(h)1 cells, and spared cells expressing T(h)2 and T(r)1 cytokines.

Presence of B220 within thymocytes and its expression on the cell surface during apoptosis

B220 is the full-length splicing isoform of a tyrosine phosphatase CD45 and is predominantly expressed as a transmembrane protein on B cells. Other splicing isoforms of CD45 are yielded by alternative splicing of exons 4, 5 and 6. Recently, the expression of B220 on peripheral T cells during activation-induced cell death has been reported. To investigate whether B220 is implicated in apoptosis of immature T cells, we analysed (by flow cytometry using the anti-B220 monoclonal antibody, RA3-6B2) the expression of B220 on mouse thymocytes undergoing X-irradiation- and dexamethasone (DEX)-induced apoptosis. The expression of B220 on thymocytes positive for Thy-1 was induced by X-irradiation or DEX treatment and increased with length of incubation. The expression of B220 was pronounced on the apoptotic hypodiploid cells in the fraction showing lower forward scattering values. Reverse transcription-polymerase chain reaction detected mRNA containing exons 4, 5 and 6 of CD45 in normal thymocytes as well as those exposed to X-rays or DEX. Surprisingly, cytoplasmic B220 antigens were detected in a considerable fraction of normal thymocytes. Moreover, the expression level of the 220 000-MW protein in normal thymocytes was similar to that in the thymocytes undergoing apoptosis. During apoptosis, the expression level of B220 antigen was reduced in the cytoplasm but, conversely, up-regulated on the surface of thymocytes. These results suggest that B220 is constitutively expressed as a cytoplasmic form within thymocytes and possibly translocated to the cell membrane during apoptosis.

Thymus volume correlates with the progression of vertical HIV infection

BACKGROUND

The thymus is the organ responsible for the maturation and selection of T lymphocytes and is thus pivotal in allowing the development of a functional immune system. Because in HIV infection cell-mediated immune responses are severely impaired, we studied the role of thymus in the control of the progression of HIV infection to AIDS.

METHODS

Thymic volume was analysed by magnetic resonance imaging in 31 vertically HIV-infected children. Plasma HIV viral load and phenotypic and functional cellular immunity-defining parameters were examined in the same patients.

RESULTS

Thymic volume was not correlated with age or nutritional status; thymic volume was nevertheless correlated with CD4 T-lymphocyte counts and with the percentage and absolute number of CD45RA+CD62L+ (naive) T lymphocytes. In addition, the ability of peripheral blood mononuclear cells to proliferate upon tetanus stimulation was directly proportional to thymic volume. Finally, a negative correlation was detected between thymic volume and HIV viral load.

CONCLUSION

Because low HIV plasma viraemia and preserved immune function are favourable prognostic indices in HIV disease, these data indicate that an immunological, thymic-dependent control of the progression of HIV infection might be possible, at least in vertically transmitted HIV infection.

Expression of the sialosyl-Tn epitope on CD45 derived from activated peripheral blood T cells

The cell surface protein tyrosine phosphatase CD45 is a major target of IgM anti-T cell autoantibodies in systemic lupus erythematosus (SLE). The autoreactive determinants on CD45 are O-linked glycans expressed on activated T cells and certain T cell lines, rather than linear or conformational polypeptide epitopes or N-linked glycans. To identify oligosaccharide structures that may play a role in the functional interactions of CD45 or are candidate target epitopes of SLE anti-CD45 autoantibodies, autoreactive CD45 purified from Jurkat T cells and non-autoreactive CD45 purified from CLL B cells were tested by ELISA for expression of mucin-type O-glycan structures. Monoclonal antibodies (mAbs) directed against blood group A, type 1 H chains, type 2 H chains, T, Le(a), sialylated-Le(a), Le(b), sialylated-Le(c), Le(x), sialylated-Le(x), multi-fucosylated Le(x), Le(y), and sialylated-extended Le(v) failed to react with CD45 from either B cells or T cells. However, mAbs directed against Tn (galNAcalpha1-->O-ser/thr) or sialosyl-Tn (neuNAcalpha2-6gaINAcalpha1-->O-ser/thr) structures reacted with CD45 derived from Jurkat T cells, but not from CLL B cells. Anti-Tn mAbs also reacted in western blotting procedures with CD45 isolated from Jurkat T cells, but did not react with CD45 isolated from CEM, MOLT-3, or PEER T cells; Daudi, Raji, or CLL B cells; or resting or Con A-activated PBL. However, anti-sialosyl-Tn mAbs stained blots of CD45 isolated from Jurkat and CEM T cells and Con A-activated PBL, a pattern of reactivity similar to that of the anti-CD45 autoantibodies. Flow cytometric analyses demonstrated that the sialosyl-Tn epitopes are expressed on a subpopulation of CD4 +/CD8- T cells.

Permanent CD8(+) T cell depletion prevents proteinuria in active Heymann nephritis

Active Heymann nephritis (HN) is a rat model of human idiopathic membranous nephropathy in which injury is thought to be mediated by membrane attack complex of complement (MAC) activated by antibody (Ab) to glomerular epithelial cells. Recent work has shown that HN develops in C6-deficient rats which cannot assemble MAC, and that infiltration of activated cytotoxic CD8(+) T cells and macrophages into glomeruli coincides with proteinuria. This study examined the role of CD8(+) T cells in mediating glomerular injury in HN by permanent CD8(+) cytotoxic T cell depletion via adult thymectomy (ATx) and anti-CD8 mAb. Groups of rats were depleted of CD8(+) T cells either before immunization for HN or 6 wk after immunization when Ab responses and glomerular IgG deposition were well established. These were compared with groups of HN, ATx/HN, and complete Freund's adjuvant (CFA) controls. Neither group of CD8(+) T cell-depleted rats developed proteinuria, although there was normal development and deposition of Ab. CD8(+) T cell-depleted rats developed neither T cell or macrophage infiltrates nor their effector cytokines, which are present in glomeruli of rats with HN. Examination of lymph node (LN) draining sites of immunization showed these findings were not explained by altered immune events within these LNs. It was concluded that CD8(+) cytotoxic T cells are essential to the mediation of glomerular injury in HN and may be relevant to the pathogenesis and treatment of membranous nephropathy.

Anti-CD4 Monoclonal Antibody-Induced Tolerance to MHC-Incompatible Cardiac Allografts Maintained by CD4+ Suppressor T Cells That Are Not Dependent upon IL-4

Anti-CD4 mAb-induced tolerance to transplanted tissues has been proposed as due to down-regulation of Th1 cells by preferential induction of Th2 cytokines, especially IL-4. This study examined the role of CD4+ cells and cytokines in tolerance to fully allogeneic PVG strain heterotopic cardiac allografts induced in naive DA rats by treatment with MRC Ox38, a nondepleting anti-CD4 mAb. All grafts survived &gt;100 days but had a minor mononuclear cell infiltrate that increased mRNA for the Th1 cytokines IL-2, IFN-γ, and TNF-β, but not for Th2 cytokines IL-4 and IL-6 or the cytolytic molecules perforin and granzyme A. These hosts accepted PVG skin grafts but rejected third-party grafts, which were not blocked by anti-IL-4 mAb. Cells from these tolerant hosts proliferated in MLC and produced IL-2, IFN-γ, and IL-4 at levels equivalent to naive cells. Unfractionated and CD4+ T cells, but not CD8+ T cells, transferred specific tolerance to irradiated heart grafted hosts and inhibited reconstitution of rejection by cotransferred naive cells. This transfer of tolerance was associated with normal induction of IL-2 and delayed induction of IFN-γ, but not with increased IL-4 or IL-10 mRNA. Transfer of tolerance was also not inhibited by anti-IL-4 mAb. This study demonstrated that tolerance induced by a nondepleting anti-CD4 mAb is maintained by a CD4+ suppressor T cell that is not associated with preferential induction of Th2 cytokines or the need for IL-4; nor is it associated with an inability to induce Th1 cytokines or anergy.