Apoptosis of human polymorphonuclear neutrophils accelerated by dialysis membranes via the activation of the complement system

BACKGROUND

Haemodialysis (HD) with bioincompatible cellulosic membranes like Cuprophan (CU) is considered to influence negatively the clinical outcome of acute and chronic renal failure. In this effect, apart from the disturbance of phagocytosis or oxygen species production by leukocytes, increased apoptosis also has been implicated recently. The objective of this study was to study the effect of HD membranes on apoptosis induction in polymorphonuclear neutrophils (PMN).

METHODS

PMN from healthy donors and uraemic patients were isolated and apoptosis was induced by co-incubation with CU, Hemophan or polyamide hollow fibres in the presence of serum from healthy or uraemic humans. Apoptosis was quantified by flow cytometry using Annexin V-FITC and propidium iodide staining and was confirmed by the detection of DNA fragmentation on gel electrophoresis. The deposition of immunoglobulins (Ig) and complement factors on hollow fibres was detected by direct immunofluorescence.

RESULTS

Heat inactivation or the depletion of complement components or Ig significantly reduced apoptosis, indicating its dependence on classical complement activation. The detection of IgG on hollow CU fibres and the restored acceleration of apoptosis by the appropriate replenishment of Ig-deficient sera additionally confirmed these findings. Inhibition experiments revealed that caspases were necessary mainly, but not exclusively, for apoptosis to occur after complement activation. Uraemia led to increased PMN apoptosis in the presence of bioincompatible, but not biocompatible, membranes.

CONCLUSIONS

Our results suggest that the acceleration of PMN apoptosis in the presence of CU is mediated via an antibody-dependent activation of the classical complement pathway mobilizing both caspase-dependent and -independent pathways.

Janus kinase-3 (JAK3) inhibition: a novel immunosuppressive option for allogeneic transplantation

Current immunosuppressive therapy in clinical organ transplantation is based on drugs that suppress various functions of immunocompetent cells but still affect cells and organ compartments other than the immune system. Hence, these drugs have considerable side effects which lead to increased morbidity and reduced life-quality of transplant recipients. A major step forward in the rationale design of clinical immunosuppression resides in the elucidation of molecular targets that play a critical role specifically within the immune system. Recently, Janus kinase 3 (JAK3) has been identified as such a molecule. Genetic absence or ablation of this tyrosine kinase is associated with defective T-cell immunity that results in severe combined immunodeficiency (SCID) without apparent changes in other organ systems. Furthermore, pharmacological inhibition has significantly prolonged allograft survival in several experimental models of organ transplantation. The present review provides an overview of the emerging role of JAK3 in the immune system and the development of JAK3-inhibiting drugs. The potential clinical application of JAK3 inhibitors in organ transplantations is discussed in the light of a recent series of successful kidney transplantations in non-human primates immunosuppressed solely with a novel JAK3 inhibitor.

Hyporesponsiveness in alloreactive T-cells by NF-kappaB inhibitor-treated dendritic cells: resistance to calcineurin inhibition

Dendritic cells (DC) are the most potent antigen-presenting cells (APC) initiating primary T-cell responses. Beyond this immunostimulatory function, certain DC subsets have been shown to induce T-cell tolerance in vitro and in vivo. In this study, immature monocyte-derived DC were activated in the presence of the NF-kappaB inhibitor pyrrolidine dithiocarbamate (PDTC) and characterized with regard to phenotype, cytokine production and allostimulatory potential. Furthermore, the functional consequences of calcineurin inhibition were studied in T cells exposed to PDTC-modulated DC. We demonstrate that PDTC treatment of DC leads to an arrest in maturation as reflected by down-regulated major histocompatibility complex (MHC) antigens and costimulatory molecules, suppressed immunostimulatory cytokines and an impaired capability to support allogeneic T-cell activation. Allogeneic T cells challenged with PDTC-treated DC are refractory upon restimulation with alloantigens but not to polyclonal stimuli. Interestingly, the successful establishment of alloantigenic hyporesponsiveness is not prevented by concomitant calcineurin inhibition in vitro as well as in T cells from patients under cyclosporine A (CsA)-based immunosuppression ex vivo. These data may have important implications for the design of clinical regimens for the establishment of antidonor hyporeactivity in organ transplantation using in vitro-modulated DC.

Prevention of CD40-triggered dendritic cell maturation and induction of T-cell hyporeactivity by targeting of Janus kinase 3

Pharmacological targeting of Janus kinase 3 (JAK3) has been employed successfully to control allograft rejection and graft-vs.-host disease (GVHD). Recent evidence suggests that in addition to its involvement in common-gamma chain (cgamma) signaling of cytokine receptors, JAK3 is also engaged in the CD40 signaling pathway of peripheral blood monocytes. In this study, we assessed the consequences of JAK3 inhibition during CD40-induced maturation of myeloid dendritic cells (DCs), and tested the impact thereof on the induction of T-cell alloreactivity. Dendritic cells triggering through CD40 induced JAK3 activity, the expression of costimulatory molecules, production of IL-12, and potent allogeneic stimulatory capacity. In contrast, JAK3 inhibition with the rationally designed JAK3 inhibitor WHI-P-154 prevented these effects arresting the DCs at an immature level. Interestingly, DCs exposed to the JAK3-inhibitor during CD40-ligation induced a state of hyporeactivity in alloreactive T cells that was reversible upon exogenous IL-2 supplementation to secondary cultures. These results suggest that immunosuppressive therapies targeting the tyrosine kinase JAK3 may also affect the function of myeloid cells. This property of JAK3 inhibitors therefore represents a further level of interference, which together with the well-established suppression of cgamma signaling could be responsible for their clinical efficacy.

Selective inhibition of T cell activation via CD147 through novel modulation of lipid rafts

The plasma membrane is compartmentalized into microdomains and the association/dissociation of receptors and signaling molecules with/from these membrane domains is a major principle for regulation of signal transduction. By following the reorganization of microdomains on living cells and performing biochemical studies, we show that Ab targeting of the T cell activation-associated Ag CD147 prevents TCR stimulation-dependent reorganization and clustering of microdomains. Triggering CD147 induces a displacement of the GPI-anchored coreceptors CD48 and CD59 from microdomains in human T lymphocytes. This perturbation of microdomains is accompanied by a selective inhibition of TCR-mediated T cell proliferation. The CD147-inhibited cells secret normal levels of IL-2 but acquire reduced amounts of the IL-2 receptor alpha-chain CD25. These results indicate that negative regulating signals can modulate microdomains and suggest a general mechanism for inhibition of receptor signaling.

Antineutrophil cytoplasmic autoantibody-negative antiproteinase 3 syndrome presenting as vasculitis, endocarditis, polyneuropathy and Dupuytren's contracture

Antiproteinase 3 antibodies (antiPR3) are assumed to be subtypes of antineutrophil cytoplasmic autoantibodies (ANCA), with a high specificity for active Wegener's granulomatosis and microscopic polyangiitis. Thus, antiPR3 positivity in ELISA, together with negativity in indirect immunofluorescence (IIF) is a rare finding. A 56-year-old man with Dupuytren's contracture and polyneuropathy was admitted for leukocytoclastic vasculitis. Echocardiography, performed because of fever and dyspnea, detected aortic valve endocarditis. Because of severe aortic insufficiency the valve was replaced. Blood cultures and bacteriologic investigations of the explanted valve were negative. AntiPR3 were elevated (123-163 U/ml; normal <6 U/ml), together with negativity in IIF. This case shows that antiPR3 elevation with negative ANCA may be associated with vasculitis, endocarditis, polyneuropathy and Dupuytren's contracture. A causal relationship between the clinical presentation and antiPR3 elevation is likely. In order not to miss such cases of vasculitis, combined screening by IIF and ELISA is recommended in selected cases.

Suppression of early T-cell-receptor-triggered cellular activation by the Janus kinase 3 inhibitor WHI-P-154

BACKGROUND

Therapeutic targeting of Janus kinase 3 (JAK3) has received particular attention, because it is associated with the common gamma signaling of cytokine receptors and thus vitally influences T-cell growth and survival. Recent evidence, however, indicates a critical role for JAK3 in signaling linked to the T-cell antigen receptor.

METHODS

In this study we investigated whether targeting JAK3 with a rationally designed inhibitor affects early T-cell activation events. T cells were stimulated by CD3 and CD28 cross-linking, and interleukin (IL)-2 production, activation marker expression, increase of free intracellular Ca2+ concentration, activation of the extracellular-related kinase, and nuclear translocation of transcription factors were evaluated.

RESULTS

We found that JAK3 inhibitor treatment dramatically impaired T-cell-receptor (TCR)-induced IL-2 production, surface activation marker expression (CD69, CD154), and homotypic T-cell aggregation. Accordingly, mRNA production of IL-2, interferon-gamma, and IL-10 was profoundly inhibited. Molecular analysis revealed that TCR-triggered phosphorylation of phospholipase C-gamma1, increase in cytoplasmic Ca2+ concentration, and activation of extracellular-related kinase were markedly reduced by the JAK3 inhibitor, resulting in substantially decreased DNA binding of nuclear factor of activated T cells and alkaline phosphatase-1 and subsequent IL-2 promoter activation. Remarkably, on TCR-independent stimulation, IL-2 production, CD69 expression, and blast formation were completely insensitive to JAK3 inhibitor treatment.

CONCLUSION

These data indicate that pharmacologic targeting of JAK3 uncouples early TCR-triggered signaling from essential downstream events, which may have important implications for the use of such compounds in T-cell-mediated disorders such as allograft rejection or graft-versus-host disease.

Suppression of T cell signaling by polyunsaturated fatty acids: selectivity in inhibition of mitogen-activated protein kinase and nuclear factor activation

Polyunsaturated fatty acids (PUFAs) are known to suppress inflammatory and autoimmune responses and, therefore, clinical applications of PUFAs as immunomodulatory substances are extensively studied. PUFAs are known to inhibit T cell responses, but with respect to TCR/CD3-mediated signal transduction only a block in CD3-induced phospholipase Cgamma1/calcium signaling has been shown so far. In this study, we investigated PUFA-mediated changes in downstream T cell signal transduction. We show that among the mitogen-activated protein kinase families activation of c-Jun NH(2)-terminal kinase, but not phosphorylation of extracellular signal-regulated kinase-1/-2 or p38 is inhibited. CD3/CD28-induced activity of NF-AT was markedly reduced by PUFA treatment, while activation of other nuclear receptors (AP-1 and NF-kappaB) remained unaltered. Furthermore, IL-2 promoter activity, IL-2 and IL-13 mRNA levels, IL-2 secretion, and IL-2R alpha-chain expression were significantly diminished by PUFA treatment, whereas the expression of IFN-gamma, IL-4, IL-10, and CD69 remained essentially unaffected by PUFAs. In conclusion, PUFA treatment of T cells inhibits selectively c-Jun NH(2)-terminal kinase and NF-AT activation, resulting in diminished production of IL-2 and IL-13.

Flow cytometry based detection of HLA alloantibody mediated classical complement activation

Complement-dependent cytotoxicity (CDC) panel reactive antibody (PRA) testing is used to assess recipient presensitization and post-transplant alloantibody formation in transplant recipients. However, CDC test results can be affected by false-positive reactions brought about by autoantibodies or antilymphocyte reagents. As an alternative to the CDC-PRA assay, detection of HLA alloantibodies using HLA antigen-coated microbeads (FlowPRA test) was recently established. FlowPRA testing, however, does not distinguish between (presumably more harmful) complement-fixing and noncomplement-fixing alloantibodies. In this study, we established a novel assay allowing flow cytometric detection of HLA alloantibody dependent classical complement activation using the FlowPRA test. For the detection of complement activation, FlowPRA beads were incubated with sera from highly sensitized dialysis patients (CDC-PRA reactivity >60%) and then stained for C4 (C4d, C4c) and C3 (C3d, C3c) fragments, as well as C1q deposition using indirect immunofluorescence. We demonstrate alloantibody induced induction of C4 fragment, and in parallel C1q deposition to HLA class I or class II beads. As shown by immunoblotting, C4 staining was not due to the presence of preformed C4 fragment-IgG/M complexes. Indeed, C4 fragment deposition in our in vitro system was demonstrated to result from de novo complement activation. First, inactivation of C4 by treatment of sera with methylamine, which inhibits cleavage of the internal thioester, completely abolished C4 fragment deposition. Second, C4 fragment deposition was not observed in the evaluation of C4-free immunoadsorption eluates obtained from highly sensitized dialysis patients. After supplementation with complement, however, eluates induced C4 deposition. Deposition of C4 split products and C1q was temperature-dependent with maximum binding after incubation at 4 degrees C for 60 min. In contrast, maximum C3 fragment deposition was found at 37 degrees C. At this temperature, C3 deposition occurred in an alloantibody and C4-independent fashion, presumably as a result of alternative complement activation. In summary, we describe a novel cell-independent and easy-to-perform PRA test that permits flow cytometry based detection of alloantibody induced classical complement activation. Future studies will have to evaluate its possible relevance as an alternative to CDC-PRA testing in clinical transplantation.

Engagement of Na,K-ATPase beta3 subunit by a specific mAb suppresses T and B lymphocyte activation

In order to identify new molecules involved in regulation of T cell proliferation, we generated various mAb by immunization of mice with the T cell line Molt4. We found one mAb (termed P-3E10) that down-regulated the in vitro T cell proliferation induced by CD3-specific OKT3 mAb. The P-3E10 mAb was also able to inhibit IFN-gamma, IL-2, IL-4 and IL-10 production of OKT3-activated T cells. The antigen recognized by P-3E10 mAb is broadly expressed on all hematopoietic as well as on all non-hematopoietic cell lines tested so far. Within peripheral blood leukocytes, the P-3E10 antigen was detected on lymphocytes, monocytes and granulocytes. Human umbilical vein endothelial cells (HUVEC) also scored positively. By evaluating the effect of P-3E10 mAb on these cell types we found that it also inhibited anti-IgM-induced B cell proliferation. However, it did not block growth factor-mediated proliferation of HUVEC, and spontaneous proliferation of SupT-1, Jurkat, Molt4 and U937 cell lines. Moreover, it did not influence phagocytosis of human blood monocytes and granulocytes. Biochemical analysis revealed that the P-3E10 antigen is a protein with a mol. wt of 45-50 kDa under non-reducing and 50-55 kDa under reducing conditions. By using a retroviral cloning system, the P-3E10 antigen was cloned. Sequence analysis revealed the P-3E10 antigen to be identical to the beta3 subunit of the Na,K-ATPase.

Novel mode of interference with nuclear factor of activated T-cells regulation in T-cells by the bacterial metabolite n-butyrate

The transcription factor nuclear factor of activated T-cells (NF-AT) plays an essential role in the activation of many early immune response genes. A dynamic equilibrium between calcineurin and cellular kinases controls its phosphorylation and thus regulates its activity by determining its subcellular localization. Here, we demonstrate that T-cell activation in the presence of the bacterial metabolite n-butyrate, which leads to inhibition of interleukin-2 transcription, is characterized by the maintenance of the activity of counter-regulatory kinases glycogen synthase kinase 3 and protein kinase A as well as persistence of intracellular cAMP levels, whereas calcium response and mitogen-activated protein kinase activation were indistinguishable from cells stimulated in the absence of n-butyrate. Nuclear binding of NF-AT was decreased but other transcription factors implicated in interleukin-2 expression such as AP1 and nuclear factor kappaB were unaffected. The effect on NF-AT binding appeared to be the result of increased nuclear export because the export inhibitor leptomycin B completely restored nuclear binding of NF-AT. We, therefore, provide first evidence for interference with NF-AT regulation alternative to the currently understood inhibition of nuclear import. This mechanism might represent a bacterial strategy to subvert host defense, which could be of particular clinical importance in the gastrointestinal tract where high amounts of n-butyrate are physiologically present.

Short-chain fatty acids: bacterial mediators of a balanced host-microbial relationship in the human gut

The luminal compartment of the gastrointestinal tract is colonized by a large and highly complex microflora providing not only nutritional advances, but also representing a potential immunological challenge for the host. Under physiological conditions, the immune cells of the colonic mucosa do not defeat the microflora. In contrast, in cases of inflammatory bowel disease (IBD), the intestinal microflora appears to be the target of immune reactivity as demonstrated in various genetic studies and animal models of mucosal inflammation. The mechanisms responsible for the maintenance of this immunological unresponsiveness in the mucosal compartment are still largely enigmatic though recent studies indicate that luminal components might control this peculiar state. The bacterial fermentation product n-butyrate has been identified as such as critical molecule. Apart from its essential nutritional function for colonocytes, an anti-inflammatory activity of this short-chain fatty acid (SCFA) has been recognized in vitro and in vivo. Regarding its molecular mode of action, an interference with transcription factors critical for the production of pro-inflammatory cytokines has been found. This overview discusses the physiological functions of this bacterial metabolite and its emerging role as a potent regulator of mucosal homeostasis. Special emphasis is laid on potential therapeutic implications of SCFA in the treatment of several forms of colitis.

Bacterial metabolite interference with maturation of human monocyte-derived dendritic cells

Dendritic cells (DC), the most potent APC, are central to antimicrobial immunity. Because of evolutionary pressure, it is reasonable that pathogens have evolved strategies to also subvert this host-defense mechanism. In the present study, we describe a novel way of bacterial interference with DC maturation. The bacterial metabolite n-butyrate, which occurs physiologically in high concentrations in the gastrointestinal tract and has well-known anti-inflammatory effects, is able to prevent LPS-induced maturation of DC resulting in a reduced capability to stimulate T cells. In particular, n-butyrate prevents homotypic DC clustering, inhibits IL-12 while sparing IL-10 production, and at the molecular level, blocks NF-kappa B translocation. These results demonstrate efficient targeting of DC function by a bacterial metabolite, which might explain the particular type of immune responsiveness in the presence of this bacterial agent as exemplified in the gastrointestinal tract.

Characterization of CDw92 as a member of the choline transporter-like protein family regulated specifically on dendritic cells

CDw92 is a 70-kDa surface protein broadly expressed on leukocytes and endothelial cells. In this manuscript, we present the molecular cloning of the CDw92 molecule by using a highly efficient retroviral expression cloning system. Sequence analysis of the CDw92 cDNA revealed a length of 2679 bp. The 1959-bp open reading frame encodes a protein of 652 amino acids. Computational analysis of the CDw92 protein sequence indicates 10 transmembrane domains, three potential N-linked glycosylation sites, and an amino acid stretch in the C-terminal region that is related to the immunoreceptor tyrosine-based inhibitory motif. Comparison of the sequence of the CDw92 clone presented in this study with various database entries show that it is a C-terminal variant of human choline transporter-like protein 1, a member of a recently identified family of multitransmembrane surface proteins. Furthermore, we found that CDw92 is stably expressed on monocytes, PBLs, and endothelial cells, as we did not yet find modulation of expression by various stimuli on these cells. In contrast to this factor-independent expression of CDw92, we detected a specific regulation of CDw92 on monocyte-derived dendritic cells (Mo-DCs). Maturation of Mo-DCs by ionomycin or calcium ionophore resulted in down-regulation of CDw92 and incubation of these cells with IL-10 in a specific re-expression. Moreover, targeting of CDw92 on LPS-treated Mo-DCs by CDw92 mAb VIM15b augmented the LPS-induced IL-10 production 2.8-fold. Together, these data suggest a crucial role of the CDw92 protein in the biology and regulation of the function of leukocytes in particular DCs.

Endothelial C4d deposition is associated with inferior kidney allograft outcome independently of cellular rejection

BACKGROUND

Capillary deposition of complement split product C4d has been suggested to be a valuable marker for humoral rejection. In this retrospective study we evaluated the clinical impact of C4d deposition in renal allografts with special emphasis on associations between C4d staining patterns and histological features of acute rejection.

METHODS

One hundred and two allograft biopsies obtained from 61 kidney transplants (1-532 days after transplantation; median 14 days) were examined by immunohistochemistry on routine paraffin sections using a novel anti-C4d polyclonal antibody (C4dpAb).

RESULTS

Fourty-two of 102 biopsies showed endothelial C4d deposits in peritubular capillaries (PTC). Histopathological analysis revealed a significantly lower frequency of positive C4d staining in biopsies with rather than in those without acute cellular rejection defined by the Banff grading schema (P<0.01). For clinical evaluation, patients were classified according to C4d staining in allografts (C4d(PTC) positive in at least one biopsy, n=31 vs C4d(PTC) negative in all biopsies, n=30). C4d(PTC) positive patients had significantly higher serum creatinine levels than C4d negative patients. Even in the absence of morphological evidence for rejection, differences in serum creatinine levels between C4d(PTC) positive and negative recipients were significant (6 months: 2.01+/-0.75 vs 1.41+/-0.27 mg/dl; 12 months: 1.95+/-0.60 vs 1.36+/- 0.34 mg/dl; 18 months: 1.98+/-0.50 vs 1.47+/-0.31 mg/dl; P<0.05). All patients with rejection resistant to conventional therapy (n=4) were in the C4d(PTC) positive subgroup. All recipients with panel reactive antibodies (PRA) >50% (n=8) were C4d(PTC) positive.

CONCLUSIONS

Our data indicate that endothelial C4d deposition is associated with inferior graft outcome. We provide evidence that this immunohistochemical finding and its clinical impact are not associated with morphological signs of cellular rejection.

Serum proteins modified by neutrophil-derived oxidants as mediators of neutrophil stimulation

Reactive oxygen intermediates (ROI) released during inflammation may act as important mediators of neutrophil effector functions. The objective of this investigation was to evaluate the influence of ROI generation on neutrophil adhesion molecule regulation and degranulation. Induction of the neutrophil oxidative burst via Fcgamma receptor cross-linking was accompanied by up-regulation of neutrophil surface CD11b, CD35, and CD66b only in the presence of selected serum proteins, such as purified human C4, C5, or human serum albumin (HSA). Scavenging of ROI attenuated protein-dependent receptor regulations. Moreover, exogenous hydrogen peroxide was effective to increase neutrophil CD11b expression in a protein-dependent way. HSA exposed to neutrophil-derived ROI displayed signs of oxidative modification in terms of carbonyl formation. Such modified HSA transferred to resting neutrophils bound readily to the cell surface and effected receptor modulation as well as cellular spreading. In contrast, neither native HSA nor HSA protected against oxidation by the tocopherol analog Trolox exhibited agonistic properties. In conclusion, we demonstrate that neutrophil-derived ROI modify selected serum proteins, which, in turn, act as proinflammatory mediators of neutrophil stimulation.

CD14(dim)/CD16(bright) monocytes in hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is an extremely rare and highly lethal chronic inflammatory disease, which is mediated by proinflammatory cytokines. In the peripheral blood of a boy suffering from HLH, a chronic expansion of CD14(dim)/CD16(bright) inflammatory monocytes was detected. Compared with CD14(bright) monocytes, their immunophenotype correlated with more mature monocytic cells differentiating to macrophages: they showed lower expression of CD11b, CD64 and CD35. Such CD14(dim)/CD16(bright) monocytes produce the inflammatory cytokines IL-1beta, IL-6 and TNF-alpha. They fit in well with the pathophysiological concept of HLH as an inflammatory state of lymphocytes and of the monocyte/macrophage system. In the presented patient the percentage of these circulating inflammatory monocytes decreased over time during clinical response to immunosuppressive therapy. This finding may indicate that CD14(dim)/CD16(bright) monocytes represented the degree of inflammation in this extremely rare and highly lethal disease.

Long-term evaluation of proliferative donor antigen-specific reactivity in cadaveric kidney transplant recipients

Development of donor-specific proliferative hyporeactivity has been evaluated in many studies for its usefulness in identifying transplant recipients at low risk of immunological complications. These studies often result in controversial conclusions, however. The authors claim that the discrepancy in the predictive value of mixed lymphocyte culture- (MLC) reactivity might partly be due to differences in presentation and interpretation of results. The purpose of this study is to investigate the usefulness of a normalized evaluation of antigen-specific donor-reactivity in a small number of kidney transplant recipients. This could then serve as a basis for an extended clinical study. Ten cadaveric kidney recipients were tested for proliferative reactivity to donor- and third-party antigens up to 20 months posttransplantation. Expressing donor-specific reactivity as a relation between the percentage of pretransplant responses towards donor splenocytes and the percentage of pretransplant responses towards third-party donor cells should minimize influences of e. g. uremia, current immunosuppression or infections on the evaluation of specific reactivity and thus should allow an evaluation of the donor-specificity of T-cell alloresponses independently of fluctuations in global responsiveness. Four of ten recipients acquired a state of donor-specific hyporeactivity ( < 75 % relative specific reactivity) at 20 months post-transplantation (61 +/- 12%, mean +/- SD). Six patients were classified non-hyporeactive (98 +/- 10% mean relative specific reactivity). Relative specific reactivity did not correlate with the levels of general reactivity. Three of the four hyporeactive and four of the six non-hyporeactive patients developed acute rejection. Stable graft function at 20 months posttransplantation (serum creatinine < or = 2 mg/dl) was not closely related to the reactivity status, as five of eight patients with well-functioning grafts did not develop relative specific hyporeactivity. One recipient with chronic rejection was classified hyporeactive. One non-hyporeactive patient lost his graft due to non-immunological causes. Our data suggest that post-transplant relative specific reactivity does not predict acute rejection. Downregulation of donor-specific reactivity might not be a prerequisite for stable graft function but could help identifying recipients who require less immunosuppression. This, however, remains to be established in a prospective immunosuppression-weaning study.