The cell surface molecule recognized by the erythrocyte receptor of T lymphocytes. Identification and partial characterization using a monoclonal antibody.

A monoclonal antibody (mAb) to sheep red blood cells (SRBC), termed L180/1, is described that completely blocks rosette formation between SRBC and human or sheep T lymphocytes. L180/1 precipitated a minor glycoprotein of about approximately 42,000 mol wt from surface-labeled SRBC. This glycoprotein was partially affinity purified and found to block E rosette formation and to compete with anti-T11 mAb for the E receptor. The molecule detected by mAb L180/1 thus appears to be recognized by the E receptor and was given the preliminary name, T11 target structure (T11TS). Since the mAb to sheep T11TS blocks the binding of SRBC to both human and sheep T cells, and mAb to T11 blocks the binding of red cells from human and sheep to the human E receptor, we concluded that analogous receptor-ligand (T11-T11TS) systems exist in man and sheep that are crossreactive over the species barrier. The possibility is discussed that the E receptor, which is known to be involved in T cell activation, and T11TS function as complementary cell interaction molecules in T cell responses.

Monoclonal anti-Lyt-2.2 antibody blocks lectin-dependent cellular cytotoxicity of H-2-negative target cells.

The hypothesis that blocking of cytotoxic T lymphocyte (CTL)-mediated cytolysis with anti-Lyt-2 antibodies acts at the level of inhibiting the interaction of the Lyt-2-bearing structure with H-2 class I molecules was tested. In agreement with the findings of others, purified anti-Lyt-2.2 inhibited both antigen-specific lysis and lectin-dependent cellular cytotoxicity (LDCC). LDCC of H-2-positive and H-2-negative target cells was similarly inhibited by this antibody. As expected, this effect was specific for CTL expressing the Lyt-2.2 allele, in contrast to blocking with a rat monoclonal antibody to the murine LFA-1 antigen. The implications of this finding for the function of the Lyt-2 antigen in CTL-target cell interaction are discussed.

T cell receptor-mediated selection of functional rat CD8 T cells from defined immature thymocyte precursors in short-term suspension culture.

Recent results have indicated that positive and negative repertoire selection act on the major population of CD4,8 double-positive (DP) thymocytes that express 5-10-fold less T cell receptor (TCR) than mature T cells (i.e., they are TCRlow). Since DP cells obtained ex vivo are heterogeneous with regard to their stage within thymic selection, a homogeneous population of virgin DP cells suitable for selection studies was generated in vitro from their immediate precursors, the CD8 single-positive (SP) immature blast cells. To mimic TCR-mediated selection signals, these virgin DP cells were then cultured for another 2 d in the presence of immobilized anti-TCR monoclonal antibodies with or without interleukin 2 (IL-2). Daily monitoring of recovery and phenotype showed that without TCR stimulation, the cells remained DP and became small, TCRlow cells that were lost with a half-life of 1 d, regardless of the presence of IL-2. TCR stimulation resulted in rapid downregulation of CD4 and CD8, maintenance of a larger cell size, and induction of the CD53 antigen that marks mature and CD4,8 double-negative rat thymocytes. In the absence of IL-2, viability decreased as rapidly as without TCR stimulation. Addition of IL-2 rescued TCR-stimulated virgin DP cells and prevented CD8 downregulation, so that 50-80% of input DP cells were recovered after 2 d as CD4-8+53+ cells. After release from modulation, these in vitro generated CD8 SP cells quantitatively upregulated the TCR to the TCRhigh phenotype and were readily induced to proliferate and exhibit cytotoxic T lymphocyte (CTL) activity in a polyclonal readout. Evidence is presented implicating an IL-2 receptor (IL-2R) not containing the p55 chain (i.e., most likely the p70 intermediate affinity IL-2R) in the TCR plus IL-2-driven in vitro differentiation of virgin DP cells towards the mature CD8 SP phenotype.

Specificity of cytotoxic T cells from athymic mice.

In normal mice, self-H-2 antigens in the thymus have a profound influence on T cell specificity. We have therefore investigated the properties of cytotoxic T lymphocyte (CTL) precursors from athymic nude mice (5) with the notion that they may provide a model system for the study of T cells whose receptro specificity is closer to the germ-line-encoded repertoire. It was found that the precursors of nude CTL are, themselves, THy-1+ cells. The possibility that these nude t cells were derived from the phenotypically normal mother by placental transfer was ruled out. In the presence of T cell growth factor, nude CTL can be induced by polyclonal activation with concanavalin A or by stimulation with allogeneic or trinitrophenyl (TNP)-modified syngeneic stimulator cells, but not by stimulation with minor H antigens in the context of self-H-2. Alloreactive, nude CTL--like those from normal mice--recognize H-2K- and H-2D-region-encoded antigens in killer-target cell interactions, but, unlike normal CTL, did not cross-react on third-party target cells. Whereas the anti-TNP response of nude mice is H-2 restricted, it does not seem to be influenced by self-H-2 antigens in the same manner as in normal mice. This is suggested by the finding that the immunodominance of H-2k over H-2d in the anti-TNP-self response of normal (H-2d X H-2b)F1 mice is absent in (H-2d X H-2k)F1 nude mice. These observations are discussed in relation to the role of the thymus in the generation of the normal mature T cell receptor repertoire.

Self H-2 antigens influence the specificity of alloreactive cells.

We have tested Jerne's hypothesis (9) that the phenomenon alloreactivity is explained by the existence of T cells that express germline-encoded receptors specific for major histocompatibility complex antigens and that these cells undergo no change in specificity during thymic differentiation. T cells from [F1 leads to Parent] bone marrow radiation chimeras reactive to conventional antigens are known to have a self preference, i.e., [A X B leads to A] chimeras respond better to H-2A-plus-antigen than to H-2B-plus-antigen. We show here that alloreactive cells from such chimeras also have a self preference. Thus, H-2k-specific alloreactive T cells from [H-2b X H-2d leads to H-2b] and [H-2b X H-2d leads to H-2d] chimeras cross-react more on TNP-modified H-2b or H-2d targets, respectively. In contrast to Jerne's prediction, the results suggest that the receptor repertoire of alloreactive F1 cells is influenced by H-2 antigens on radiation-resistant cells present during T cell ontogeny. By this criterion of having a self preference in H-2 restriction, alloreactive T cells appear to be similar to T cells that respond to conventional antigens.

Immunopathology of interleukin (IL) 2-deficient mice: thymus dependence and suppression by thymus-dependent cells with an intact IL-2 gene.

Interleukin (IL) 2-deficient mice develop a fatal immunopathology characterized by lymphoadenopathy, splenomegaly, T cell infiltration of the bone marrow, loss of B cells, anemia, and inflammation of the gut. The thymus dependence of these disease symptoms was tested by introducing the IL-2 mutation into athymic mice. With the exception of an increase in CD8+ intrahepatic alpha/beta T cells, IL-2 deficiency had no detectable effect on leukocyte composition or health of athymic mice, indicating a key role for thymus-derived T cells in the initiation of disease and demonstrating that B cell development and survival are independent of IL-2. In adoptive transfer studies, lymph node and spleen cells from euthymic IL-2-deficient mice induced disease in athymic mice with an intact IL-2 gene, suggesting that thymus-independent IL-2-expressing cells are unable to control the development of immune pathology. Both IL-2+ and IL-2-/- bone marrow cells repopulated the thymus and the peripheral T cell compartment of the recombination activator gene 2-deficient recipients, and chimeras that had received IL-2-deficient bone marrow developed immune pathology. Disease development was, however, fully or at least partially prevented when 30% of the bone marrow inoculum was derived from mice able to express IL-2. These results demonstrate that the IL-2 deficiency syndrome depends on the intrathymic differentiation of T cells carrying the IL-2 mutation, and that the abnormal activation of IL-2-deficient lymphocytes can be controlled by thymus-derived but not thymus-independent lymphocytes.

Induction of interleukin 2 receptor beta chain expression by self-recognition in the thymus.

1-2% of adult mouse thymocytes express the T cell receptor alpha/beta (TCR-alpha/beta) together with the interleukin (IL) 2R beta (p70), but not the alpha (p 55) chain. We show that the previously described alpha/beta-TCR +CD4-8- and the partially overlapping Ly6C+ thymocytes are contained within this subset. Most IL-2R beta+ alpha/beta-TCR+ cells have a mature and activated (heat stable antigen [HSA]-, thymic shared antigen 1 [TSA-1]-, CD44high, CD69+) phenotype. Overrepresentation of V beta 8.2 in both CD4-8- and CD4 and/or CD8+ IL-2R beta+ thymocytes suggests that IL-2R beta expression is induced by a TCR-mediated activation event. In mice transgenic for an H-2Kb-specific TCR, IL-2R beta+ cells were abundant under conditions of mainstream negative selection, i.e., in the presence of Kb, but absent under conditions of mainstream positive selection or in a nonselecting environment. Together, these results show that in addition to clonal deletion, self-recognition by immature thymocytes leads to phenotypic maturation of a small subset of thymocytes expressing IL-2R beta. IL-2-deficient mice contain normal numbers of IL-2R beta+ alpha/beta-TCR+ thymocytes, indicating that like mainstream T cell development, this minor pathway of positive selection does not depend on IL-2. However, in the absence of IL-2, the CD4/CD8 subset composition of IL-2R beta+ thymocytes is skewed towards CD4-8+, mostly at the expense of CD4-8-. A possible relevance of this finding for the development of the immune pathology of IL-2-deficient mice is discussed.

Autoradiographic studies on the proliferation of antibody-producing cells in vitro.

A rapid and reliable autoradiographic technique for plaque-forming cells (PFCs) using (14)C rather than tritiated thymidine is described. Its application to PFCs developing in vitro shows that (a) practically all PFCs derive from precursors dividing steadily during the culture period, (b) PFC precursors divide in the absence of T-cell helper function, and (c) at least some PFCs may continue to divide.

Antigen recognition by cloned cytotoxic T lymphocytes follows rules predicted by the altered-self hypothesis.

Radiation chimeras prepared by injecting H-2 heterozygous F1 stem cells into lethally irradiated parental hosts show a marked, but not absolute, preference for host-type H-2 antigens in the H-2-restricted cytotoxic T lymphocyte (CTL) response to minor histocompatibility (minor H) antigens. We have selected for the anti-minor HCTL that are restricted to the parental H-2 type absent from the chimeric host and found that in two out of eight cases, such CTL lysed target cells of either parental H-2 type. From one of these CTL populations that lysed H-2d and H-2k target cells expressing BALB minor H antigens, clones were derived and further analyzed. The results showed that: (a) lysis of both H-2d and H-2k target cells was H-2 restricted; (b) H-2d restriction mapped to Dd, and H-2k restriction mapped to Kk; (c) testing against various H-2d and H-2k strains of different and partially overlapping minor H backgrounds as well as against the appropriate F1 crosses revealed that in Dd- and Kk-restricted killing, different minor H antigens were recognized. In a second system, a CTL population was selected from normal (H-2d x H-2k)F1 mice that was specific for H-2d plus minor H antigens and for H-2k plus trinitrophenylated bovine serum albumin. We interpret these findings in terms of the altered-self hypothesis: The association of one H-2 antigen with one conventional antigen X may be recognized by the same T cell receptor specific for the complex formed by a different H-2 antigen in association with a second conventional antigen Y. The implications of these observations for the influence of self H-2 on the generation of the T cell receptor repertoire are discussed.

Mechanism of T-cell help in the immune response to soluble protein antigens. I. Evidence for in situ generation and action of T-cell-replacing factor during the anamnestic response to dinitrophenyl keyhole limpet hemocyanin in vitro.

The involvement of a nonantigen-specific T-helper factor in the anamnestic immune response to dinitrophenyl keyhole limpet hemocyanin is demonstrated employing cultures of unseparated spleen cell populations. In such cultures of primed and boosted spleen cells, a good IgG anti-DNP response could be obtained if the hapten was presented on a heterologous carrier, provided that the homologous carrier was added simultaneously. T-cell depletion and reconstitution experiments show that such a factor, presumably identical with T-cell-replacing factor is produced by primed helper cells upon rechallenge and helps primed B cells, stimulated by soluble heterologous carrier hapten conjugates, to become IgG-secreting cells.

Mechanism of T-cell help in the immune response to soluble protein antigens II. Reconstitution of primary and secondary in vitro immune responses to dinitrophenyl-carrier conjugates by T-cell-replacing factor.

Spleen cells of dinitrophenyl keyhole limpet hemocyanin (DNPKLH) primed and boosted mice produced a nonantigen-specific helper factor upon in vitro challenge with DNPKLH. This helper factor displays all of the biological characteristics so far described for TRF produced by allogeneic or Concanavalin A stimulation of mouse spleen cells. It restores the primary anti-SRBC response in nude spleen cultures following the same kinetics of action as T-cell-replacing factor (TRF). Conversely, TRF restores the primary in vitro immune response of nude spleen cultures to DNPKLH. TRF also restores the secondary anti-hapten IgG response of T-cell-deprived spleen cell cultures derived from DNPKLH primed and boosted mice. Here the need for carrier specificity is fully overcome. The data therefore suggest that TRF, as a nonantigen-specific maturation signal, is involved in the primary and secondary immune responses to both particulate and soluble antigens.

T cell receptor ligation induces interleukin (IL) 2R beta chain expression in rat CD4,8 double positive thymocytes, initiating an IL-2-dependent differentiation pathway of CD8 alpha+/beta- T cells.

The role of interleukin (IL)2 in intrathymic T cell development is highly controversial, and nothing is known about IL-2R expression on thymocytes of the T cell receptor (TCR) alpha/beta lineage undergoing TCR-driven differentiation events. We analyze here IL-2R alpha and beta mRNA expression in an in vitro system where newly generated rat CD4,8 double positive (DP) thymocytes respond to TCR ligation plus IL-2 (but not to either stimulus alone) with rapid differentiation to functional CD8 single positive T cells (Hünig, T., and R. Mitnacht. 1991. J. Exp. Med. 173:561). TCR ligation induced expression of IL-2R beta (but not alpha) chain mRNA in DP thymocytes. Addition of IL-2 then lead to functional maturation and expression of the IL-2R alpha chain. To investigate if the CD8 T cells generated via this IL-2R beta-driven pathway in vitro correspond to the bulk of CD8 T cells seeding peripheral lymphoid organs in vivo, we compared their phenotype to that of lymph node CD8 T cells. Surprisingly, analysis of CD8 cell surface expression using a novel anti-CD8 monoclonal antibody specific for the alpha/beta heterodimeric isoform, and of CD8 alpha and beta chain mRNA revealed that T cells generated by TCR ligation plus IL-2 resemble thymus-independent rather than thymus-derived CD8 cells in that they express CD8 alpha without beta chains. These findings demonstrate that TCR crosslinking induces functional IL-2R on immature DP rat thymocytes. In addition, they show that at least in vitro, CD8 alpha/alpha T cells are generated from TCR-stimulated DP thymocytes (which express the CD8 alpha/beta in the heterodimeric isoform) along an IL-2-driven pathway of T cell differentiation.

A monoclonal antibody to a constant determinant of the rat T cell antigen receptor that induces T cell activation. Differential reactivity with subsets of immature and mature T lymphocytes.

mAb R73 detects a T cell-specific surface molecule consisting of two disulfide-linked subunits of 40 and 46 kD, respectively, on 97% of peripheral rat T cells, as defined by the OX-52 marker. Of the few OX-52+ R73- cells, none are CD4+ but many express the CD8 antigen known to be present on rat NK cells. mAb R73 is mitogenic for unseparated spleen cells and for purified T cells. In the absence of non-T "accessory cells", stimulation by R73 requires artificial crosslinking of the mAb and is largely dependent on exogenous IL-2. Overnight incubation of purified T cells with crosslinked R73 mAb induces blastoid transformation, IL-2-R expression, and modulation of the R73 antigen. In the rat thymus, mature medullary cells express the R73 determinant at the same level as peripheral T cells, whereas 94% of CD4-CD8- thymocytes are R73-. The major CD4+8+ thymocyte population contains 25% R73- and 70% R73low cells. Thymocytes of the CD4-CD8+OX-44- subpopulation that are the direct precursors of CD4+CD8+ cells display a continuum of R73 antigen density from undetectable to very low levels. We conclude that R73 is most likely directed at a constant determinant of the rat alpha/beta heterodimeric TCR and suggest that CD8+ immature thymocytes are the first cells in the T cell differentiation pathway to express this molecule at their surface.

The human immunodeficiency virus type 1 (HIV-1) Vpu protein interferes with an early step in the biosynthesis of major histocompatibility complex (MHC) class I molecules.

The human immunodeficiency virus type 1 (HIV-1) vpu gene encodes a small integral membrane phosphoprotein with two established functions: degradation of the viral coreceptor CD4 in the endoplasmic reticulum (ER) and augmentation of virus particle release from the plasma membrane of HIV-1-infected cells. We show here that Vpu is also largely responsible for the previously observed decrease in the expression of major histocompatibility complex (MHC) class I molecules on the surface of HIV-1-infected cells. Cells infected with HIV-1 isolates that fail to express Vpu, or that express genetically modified forms of Vpu that no longer induce CD4 degradation, exhibit little downregulation of MHC class I molecules. The effect of Vpu on class I biogenesis was analyzed in more detail using a Vpu-expressing recombinant vaccinia virus (VV). VV-expressed Vpu induces the rapid loss of newly synthesized endogenous or VV-expressed class I heavy chains in the ER, detectable either biochemically or by reduced cell surface expression. This effect is of similar rapidity and magnitude as the VV-expressed Vpu-induced degradation of CD4. Vpu had no discernible effects on cell surface expression of VV-expressed mouse CD54, demonstrating the selectivity of its effects on CD4 and class I heavy chains. VV-expressed Vpu does not detectably affect class I molecules that have been exported from the ER. The detrimental effects of Vpu on class I molecules could be distinguished from those caused by VV-expressed herpes virus protein ICP47, which acts by decreasing the supply of cytosolic peptides to class I molecules, indicating that Vpu functions in a distinct manner from ICP47. Based on these findings, we propose that Vpu-induced downregulation of class I molecules may be an important factor in the evolutionary selection of the HIV-1-specific vpu gene by contributing to the inability of CD8+ T cells to eradicate HIV-1 from infected individuals.

Superagonistic CD28 antibody induces donor-specific tolerance in rat renal allografts.

The ultimate goal of organ transplantation is to establish graft tolerance where CD4+CD25+FOXP3+ regulatory T (Treg) cells play an important role. We examined whether a superagonistic monoclonal antibody specific for CD28 (CD28 SA), which expands Treg cells in vivo, would prevent acute rejection and induce tolerance using our established rat acute renal allograft model (Wistar to Lewis). In the untreated or mouse IgG-treated recipients, graft function significantly deteriorated with marked destruction of renal tissue, and all rats died by 13 days with severe azotemia. In contrast, 90% of recipients treated with CD28 SA survived over 100 days, and 70% survived with well-preserved graft function until graft recovery at 180 days. Analysis by flow cytometry and immunohistochemistry demonstrated that CD28 SA induced marked infiltration of FOXP3+ Treg cells into the allografts. Furthermore, these long-surviving recipients showed donor-specific tolerance, accepting secondary (donor-matched) Wistar cardiac allografts, but acutely rejecting third-party BN allografts. We further demonstrated that adoptive transfer of CD4+CD25+ Treg cells, purified from CD28 SA-treated Lewis rats, significantly prolonged allograft survival and succeeded in inducing donor-specific tolerance. In conclusion, CD28 SA treatment successfully induces donor-specific tolerance with the involvement of Treg cells, and thus the therapeutic value of this approach warrants further investigation and preclinical studies.

Co-evolution of rat TAP transporters and MHC class I RT1-A molecules.

The genes for rat major histocompatibility complex (MHC) class I molecules are associated either with those for the A allele of the transporter associated with antigen processing (TAP-A), which can transport peptides with basic carboxy-terminal residues, or with those for TAP-B, which cannot [1-5]. To explore whether these associations have a functional basis, we compared the sequences of 13 rat MHC class la RT1-A cDNAs from nine MHC haplotypes. Of seven TAP-A- linked RT1-A molecules, six possess strongly acidic F pockets, and these bind a high proportion of peptides with basic carboxy-terminal residues. The F pockets of TAP-B-linked molecules, by contrast, were more basic. Furthermore, we identified six positions at the 'righthand end' of the peptide-binding groove, at which a majority of TAP-B-linked molecules diverge from the consensus sequence for class la molecules whereas, at these positions, all the TAP-A-linked molecules reflect the consensus sequence. Our results suggest that the linked rat class la and TAP genes have co-evolved to maximize the supply of appropriate peptides to the presenting molecules.

Systemic autoimmune disease as a consequence of defective lymphocyte death.

A major group of systemic autoimmune diseases is associated with abnormal lymphoproliferation, as a result of defects in the termination of lymphocyte activation and growth. Recent progress has been made in understanding the causes and consequences of these abnormalities. At the molecular level, the defects in CD95 and its ligand are only the most obvious reasons for the breakdown of 'clonal contraction' which in fact requires the participation of multiple gene products, including the IL-2-IL-2-receptor system, to set up a functional apoptotic machinery.

Combination of donor-specific blood transfusion with anti-CD28 antibody synergizes to prolong graft survival in rat liver transplantation.

Donor-specific blood transfusion (DST) has been shown to effectively induce tolerance to certain allografts. In addition, it is well known that blockade of costimulatory signals reduces the ability of T cells to respond to alloantigens, prolonging allograft survival in some transplant models. We assessed the effects of single or multiple DSTs in the absence or presence of anti-CD28 monoclonal antibodies (mAbs) on graft function and host survival in rat liver transplantation (LTx). Fully MHC-mismatched adult male Dark Agouti (DA) and Lewis (LEW) rats were used as donors and recipients, respectively. Heparinized DA blood was administered to naïve LEW rats 7 days before LTx [DST(-7d)], 14 and 7 days before LTx [DST(1 x 2)], twice a week for 2 weeks prior to LTx [DST(2 x 2)] and once a week for 4 weeks prior to LTx [DST(1 x 4)]. For some experiments, two different monoclonal antibodies (mAb) to rat CD28 (JJ316 and JJ319) were administered in combination with some DST treatments. We found that DST administration induced a time- and dose-dependent increase in host survival. Treatment of LEW rats with JJ316 or JJ319 mAb alone failed to prolong graft survival over untreated rats; however, the combination of DST(1 x 2) with JJ316 or JJ319 mAb induced indefinite survival at 100 days following surgery. We found that this protective effect was associated with increased numbers of splenic CD4+ CD45RC- but not CD4+ CD25+ foxp3+ T-cells in long-term survivors. Our data suggest that the combination of suboptimal DST with CD28 mAb induces donor-specific tolerance that correlates with enhanced numbers of regulatory T-cells.

T-cell receptor diversity in dendritic epidermal T cells in the rat.

The rat epidermis contains a population of dendritic CD3+ cells. For a better characterization of these cells and to investigate their relationship to epidermal lymphocytes of other species, we stained rat epidermal sheets using a variety of monoclonal antibodies against rat leukocyte differentiation antigens in an indirect immunofluorescence procedure. Additionally, we attempted to define their T-cell receptor (TCR) isotype at both the nucleic acid and protein level. Results obtained showed that the majority of the CD3+ dendritic epidermal cells are CD45+, CD2+, TCR alpha beta-, major histocompatibility complex class II-, Thy-1-, asialo GM1-, CD4-, CD5-, and CD8- lymphocytes. We further observed that, in contrast to the mouse system, the rat epidermis additionally harbors a small but distinctive portion of dendritic CD3+ cells that exhibit reactivity with an anti-pan TCR alpha beta monoclonal antibody. Our further finding that rat epidermal cells enriched for CD3+ lymphocytes express full-length C delta mRNA suggests that the vast majority of rat epidermal T cells carry surface-bound TCR gamma delta moieties. On the basis of these findings, one may speculate that the indigenous T-cell population of the epidermis is not necessarily programmed to uniformly express monomorphic TCR gamma delta molecules but, to effectively fulfill its role in host defense, is capable of adaptation to the specific challenges encountered by a given species.

Prolonged allograft survival but no tolerance induction by modulating CD28 antibody JJ319 after high-responder rat heart transplantation.

BACKGROUND: Allograft rejection depends on T cell immune responses requiring antigen recognition and costimulatory signals through accessory T cell receptors, including CD28. Inhibition of CD28 signaling with a CTLA-4-immunoglobulin (Ig) fusion protein has resulted in immunosuppression and occasional T cell anergy in mouse transplant models, but not in rats. Because this approach also inhibits a potentially tolerizing signal through CTLA-4, selective blockade of CD28 ligation might induce more profound immunosuppression and transplant tolerance. METHODS: The effects of escalating doses of the rat CD28 monoclonal antibody JJ319 on allograft survival were studied after vascularized heterotopic heart transplantation in a high responder strain combination (DA to Lewis). CD28 antigen modulation and circulating antibody levels were monitored by flow cytometry. RESULTS: CD28 antibody JJ319 markedly prolonged cardiac graft survival compared with untreated controls (7 days, range: 6-8). A strictly dose-dependent increase in median graft survival time was demonstrated with a maximum of 36 days (range: 30-40; p <0.001) after the administration of 8 x 1 mg JJ319 i.p. (days -1 to +6 before/after transplantation). However, indefinite graft survival and tolerance could not be induced by JJ319 treatment. At the maximal dose, flow cytometry showed complete down modulation of the CD28 receptor for 10-14 days without T cell depletion in close temporal relation to antibody presence in serum. In vitro, CD28-modulated T cells showed significantly reduced responses to activation. CONCLUSIONS: CD28 antibody JJ319 induces profound immunosuppression after rat heart transplantation, however without development of transplant tolerance. The underlying mechanism seems to be receptor modulation during primary alloantigen recognition. While still potentially applicable clinically, there are no qualitative or quantitative differences to the treatment with CTLA-4/lg or the blockade of CD2 or LFA-1, as reported elsewhere. Thus, a CD28-modulating approach seems not to allow therapeutic exploitation of a tolerizing signal delivered by CTLA-4 but may still be clinically applicable, especially in combined immune interventions.