Effects of cyclosporine and rapamycin on immunoglobulin production by preactivated human B cells

Risks and ways of preventing kidney dysfunction in drug-induced immunosuppression in solid organ recipients

Immunosuppressive therapy (IMT) is the cornerstone of treatment after transplantation. The goal of immunosuppression is to prevent acute and chronic rejection while maximizing patient survival and long-term graft function. However, the expected effects of IMT must be balanced against the major adverse effects of these drugs and their toxicity. The purpose of this review is to summarize world experience on current immunosuppressive strategies and to assess their effects on renal function.

Cyclosporine a directly affects human and mouse b cell migration in vitro by disrupting a hIF-1 αdependent, o2 sensing, molecular switch

BACKGROUND

Hypoxia is a potent molecular signal for cellular metabolism, mitochondrial function, and migration. Conditions of low oxygen tension trigger regulatory cascades mediated via the highly conserved HIF-1 α post-translational modification system. In the adaptive immune response, B cells (Bc) are activated and differentiate under hypoxic conditions within lymph node germinal centers, and subsequently migrate to other compartments. During migration, they traverse through changing oxygen levels, ranging from 1-5% in the lymph node to 5-13% in the peripheral blood. Interestingly, the calcineurin inhibitor cyclosporine A is known to stimulate prolyl hydroxylase activity, resulting in HIF-1 α destabilization and may alter Bc responses directly. Over 60% of patients taking calcineurin immunosuppressant medications have hypo-gammaglobulinemia and poor vaccine responses, putting them at high risk of infection with significantly increased morbidity and mortality.

RESULTS

We demonstrate that O 2 tension is a previously unrecognized Bc regulatory switch, altering CXCR4 and CXCR5 chemokine receptor signaling in activated Bc through HIF-1 α expression, and controlling critical aspects of Bc migration. Our data demonstrate that calcineurin inhibition hinders this O 2 regulatory switch in primary human Bc.

CONCLUSION

This previously unrecognized effect of calcineurin inhibition directly on human Bc has significant and direct clinical implications.

Antibody-mediated rejection in the cardiac allograft: diagnosis, treatment and future considerations

PURPOSE OF REVIEW

This review summarizes the latest publications dealing with antibody-mediated rejection (AMR) and defines areas of controversy and future steps that may improve the outcome for patients with this virulent form of rejection.

RECENT FINDINGS

Recent progress includes publication of standardized pathologic criteria for acute AMR by the International Society for Heart and Lung Transplantation (ISHLT) and guidelines for treatment of acute AMR by the American Heart Association, endorsed by ISHLT as well. Recently published review articles emphasize the important role of innate immune mechanisms, clarify the role of viral infection and provide insights into vascular biology and the role of innate effector populations, macrophages and dendritic cells.

SUMMARY

Strategies for future studies are discussed in the context of these new findings and similar efforts undertaken by renal and liver allograft investigators.

Use of sirolimus in pediatric heart transplant patients: A multi-institutional study from the Pediatric Heart Transplant Study Group

BACKGROUND

Proliferation signal inhibitors, such as sirolimus, are increasingly used in solid-organ transplantation. However, limited data exist on sirolimus-treated pediatric patients. We aimed to describe sirolimus use in pediatric heart transplant patients and test the hypothesis that sirolimus use is associated with improved outcomes.

METHODS

A retrospective review and propensity-matched analysis of the Pediatric Heart Transplant Study database was performed on patients undergoing primary heart transplantation from 2004 to 2013 with at least 1 year of follow-up comparing patients treated vs not treated with sirolimus at 1 year after transplant. The primary outcome of interest was patient survival, with secondary outcomes including cardiac allograft vasculopathy, rejection, malignancy, and renal insufficiency.

RESULTS

Between 2004 and 2013, 2,531 patients underwent transplantation. At least 1 year of follow-up was available for 2,080 patients, of whom 144 (7%) were on sirolimus at 1 year post-transplant. Sirolimus-treated and non-treated patients had similar survival in the overall cohorts and in the propensity-matched analysis. The secondary outcomes measures were also similar, including a composite end point of all outcome measures. There was a trend toward increased time to cardiac allograft vasculopathy (p = 0.09) and decreased time to infection (p = 0.05) among sirolimus-treated patients in the overall cohort (p = 0.19) but not in the propensity-matched cohort (p = 0.17).

CONCLUSIONS

Sirolimus was used in less than 10% of patients at 1 year post-transplant. Overall outcomes of sirolimus treated and non-treated patients were similar with respect to survival and major transplant adverse events. Further study of sirolimus in pediatric heart transplant patients is needed.

Differential Effects of Tacrolimus versus Sirolimus on the Proliferation, Activation and Differentiation of Human B Cells

The direct effect of immunosuppressive drugs calcineurin inhibitor (Tacrolimus, TAC) and mTOR inhibitor (Sirolimus, SRL) on B cell activation, differentiation and proliferation is not well documented. Purified human B cells from healthy volunteers were stimulated through the B Cell Receptor with Anti-IgM + anti-CD40 + IL21 in the absence / presence of TAC or SRL. A variety of parameters of B cell activity including activation, differentiation, cytokine productions and proliferation were monitored by flow cytometry. SRL at clinically relevant concentrations (6 ng/ml) profoundly inhibited CD19⁺ B cell proliferation compared to controls whereas TAC at similar concentrations had a minimal effect. CD27⁺ memory B cells were affected more by SRL than naïve CD27^- B cells. SRL effectively blocked B cell differentiation into plasma cells (CD19⁺CD138⁺ and Blimp1⁺/Pax5^low cells) even at low dose (2 ng/ml), and totally eliminated them at 6 ng/ml. SRL decreased absolute B cell counts, but the residual responding cells acquired an activated phenotype (CD25⁺/CD69⁺) and increased the expression of HLA-DR. SRL-treated stimulated B cells on a per cell basis were able to enhance the proliferation of allogeneic CD4⁺CD25⁻ T cells and induce a shift toward the Th1 phenotype. Thus, SRL and TAC have different effects on B lymphocytes. These data may provide insights into the clinical use of these two agents in recipients of solid organ transplants.

Cyclosporine A normalizes mitochondrial coupling, reactive oxygen species production, and inflammation and partially restores skeletal muscle maximal oxidative capacity in experimental aortic cross-clamping

OBJECTIVE

By binding to cyclophilin D, cyclosporine A (CsA) inhibits mitochondrial permeability transition pore (mPTP) opening and prevents mitochondrial dysfunction and ultimately cell death after ischemia-reperfusion (IR) injury in cardiac muscle. This study tested whether CsA would decrease skeletal muscle oxidative stress and mitochondrial dysfunctions after aortic cross-clamping related IR.

METHODS

Forty-five Wistar rats were investigated. The sham group (n = 8) had aortic exposure but no ischemia, the IR group (n = 10) had aortic cross-clamping for 3 hours followed by 2 hours of reperfusion, and the IR+CsA group (n = 9) had two intraperitoneal injections of 10 mg of CsA at 90 and 150 minutes of ischemia before reperfusion. Mitochondrial coupling (acceptor control ratio) and mitochondrial respiratory chain complexes' activities were measured. Reactive oxygen species (ROS) production, cyclophilin D expression, and muscle inflammation were determined using dihydroethidium staining, Western blot, and immunohistochemistry, respectively. An additional 18 sham rats were investigated to determine CsA blood levels and the effects of CsA on mitochondrial respiration and calcium retention capacity, a marker of mPTP opening, both in myocardium and gastrocnemius with and without CsA.

RESULTS

Compared with sham, IR decreased mitochondrial coupling (1.38 ± 0.06 vs 1.98 ± 0.20; P = .0092), increased ROS production (3992 ± 706 arbitrary units [AU] vs 1812 ± 322 AU; P = .033), was associated with macrophage infiltration, and decreased maximal oxidative capacity (V(max): 4.08 ± 0.38 μmol O(2)/min/g vs 5.98 ± 0.56 μmol O(2)/min/g; P = .015). Despite IR, CsA treatment totally restored mitochondrial coupling (1.93 ± 0.12; P = .023 vs IR), normalized ROS (1569 ± 348 AU; P = .0098 vs IR), and decreased inflammation. The V(max) was slightly enhanced (5.02 ± 0.39 μmol O(2)/min/g; P = .33 vs IR; P = .35 vs sham). Compared with myocardium, gastrocnemius muscle was characterized by a decreased cyclophilin D content (-50%) associated with an earlier opening of mPTP (calcium retention capacity increased from 10.85 ± 1.35 μM/mg dry weight [DW] to 12.11 ± 2.77 μM/mg DW; P = .65; and from 11.07 ± 1.67 to 37.65 ± 11.41 μM/mg DW; P = .0098 in gastrocnemius and heart, respectively).

CONCLUSIONS

Cyclosporine A normalized ROS production, decreased inflammation, and restored mitochondrial coupling during aortic cross-clamping. Incomplete Vmax protection might be due to low cyclophilin D expression in gastrocnemius, preventing CsA from blocking mPTP opening.

The role of B cells in solid organ transplantation

The role of antibodies in chronic injury to organ transplants has been suggested for many years, but recently emphasized by new data. We have observed that when immunosuppressive potency decreases either by intentional weaning of maintenance agents or due to homeostatic repopulation after immune cell depletion, the threshold of B cell activation may be lowered. In human transplant recipients the result may be donor-specific antibody, C4d+ injury, and chronic rejection. This scenario has precise parallels in a rhesus monkey renal allograft model in which T cells are depleted with CD3 immunotoxin, or in a CD52-T cell transgenic mouse model using alemtuzumab to deplete T cells. Such animal models may be useful for the testing of therapeutic strategies to prevent DSA. We agree with others who suggest that weaning of immunosuppression may place transplant recipients at risk of chronic antibody-mediated rejection, and that strategies to prevent this scenario are needed if we are to improve long-term graft and patient outcomes in transplantation. We believe that animal models will play a crucial role in defining the pathophysiology of antibody-mediated rejection and in developing effective therapies to prevent graft injury. Two such animal models are described herein.

Use of rapamycin in the induction of tolerogenic dendritic cells

Rapamycin (RAPA), a macrocyclic triene antibiotic pro-drug, is a clinically-utilized 'tolerance-sparing' immunosuppressant that inhibits the activity of T, B, and NK cells. Furthermore, maturation-resistance and tolerogenic properties of dendritic cells (DC) can be supported and preserved by conditioning with RAPA. Propagation of murine bone marrow (BM)-derived myeloid DC (mDC) in clinically relevant concentrations of RAPA (RAPA-DC) generates phenotypically immature DC with low levels of MHC and significantly reduced co-stimulatory molecules (especially CD86), even when exposed to inflammatory stimuli. RAPA-DC are weak stimulators of T cells and induce hyporesponsiveness and apoptosis in allo-reactive T cells. An interesting observation has been that RAPA-DC retain the ability to stimulate and enrich the regulatory T cells (Treg). Presumably as a result of these properties, alloantigen (alloAg)-pulsed recipient-derived DC are effective in subverting anti-allograft immune responses in rodent transplant models, making them an attractive subject for further investigation of their tolerance-promoting potential.

Prevention and inhibition but not reversion of chronic allograft vasculopathy by FK778

BACKGROUND

This study aimed at investigating the efficacy of the novel immunosuppressant FK778 to prevent the development and progression of chronic allograft vasculopathy (CAV).

METHODS

Orthotopic aortic transplantations were performed in the PVG-to-ACI rat model and followed over the course of 120 days. Immunosuppression with FK778 (20 mg/kg) or sirolimus (2 mg/kg) was either started early or delayed when CAV was already present. Trough levels were monitored. Aortic luminal obliteration was quantified using computer morphometry and intragraft cytokine profiles were analyzed with Western Blotting. Donor-reactive antibodies were quantified by flow cytometry.

RESULTS

Untreated animals developed CAV with luminal obliteration of 25.2+/-13.6% and 41.4+/-23.3% after 80 and 120 days, respectively. Continuous immunosuppression with FK778 or sirolimus effectively prevented the development of vasculopathy. When the start of the immunosuppressive regimen was delayed until postoperative day 80, FK778 and sirolimus inhibited a progression of established CAV but did not reverse the luminal obliteration. Intragraft tumor growth factor-beta activity increased over the course of time in untreated recipients but was significantly suppressed after continuous immunosuppression with either agent. Expression of platelet-derived growth factor, intercellular adhesion molecule-1, and vascular adhesion molecule-1 also was moderately suppressed. A stable elevation of donor-reactive IgG-antibody levels was found over 120 days in the absence of treatment. With FK778 or sirolimus, antibody levels were effectively decreased. FK778 was very well tolerated and only sirolimus showed side effects with elevation of BUN, cholesterol, triglycerides, and ALT after 120 days.

CONCLUSIONS

FK778 prevents the development of CAV and inhibits a progression of established disease. It shows a similar efficacy but a safer drug profile when compared to sirolimus.

Prevention of murine lupus disease in (NZBxNZW)F1 mice by sirolimus treatment

Sirolimus is a new immunosuppressive drug used to avoid allograft rejection. The immunosuppressive effect of sirolimus is due to inhibition of the mammalian target of rapamycin, necessary for the proliferation and clonal expansion of activated T-cells. Because T-cells play a central role in the pathogenesis of autoimmune disease developed in (NZBxNZW)F1 mice, we evaluated the therapeutic use of sirolimus in such mice. (NZBxNZW)F1 female mice received 1mg/kg/day of sirolimus from 12 to 37 weeks of age. The development of autoimmune disease was evaluated by measuring the serum levels of auto-antibodies (autoAbs) and their immunoglobulin isotypes, prevalence of glomerulonephritis and mortality rates. Sirolimus directly inhibited production of autoAbs, glomerular deposits of immunoglobulins and development of proteinuria; also the survival of these mice was prolonged. Our results demonstrate the beneficial effects of sirolimus in preventing the development of lupus disease in (NZBxNZW)F1 female mice.

The relative effects of different systemic immunosuppressives on skin cancer development in organ transplant patients

The immunosuppressive medications used in organ transplant recipients (OTRs) to prevent allograft rejection are associated with an increased incidence of skin cancer in this population. Several classes of immunosuppressive agents exist, and the selection of which medication to use varies by organ type and with time. The tumorigenic potential and relative effects of different immunosuppressive medications on skin cancer risk have been studied. This article reviewed the history of the immunosuppressive agents and their mechanisms of action. A summary of the current literature on the relationship between skin cancer and the different immunosuppressives were included. Guidelines for management of OTRs in dermatologic practice were discussed.

Role of siroliumus, a novel immunosuppressive drug in heart and lung transplantation

Lung and heart transplantation has become an accepted therapeutic option for patients with end-stage disease. However, the calcineurin-inhibitor-based immunosuppression often causes renal impairment. Therefore, sirolimus, a novel immunosuppressive agent, may serve as an alternative or complementary agent to calcineurin inhibitors. The aim of this review was to summarize the role of sirolimus in lung and heart transplantation. Although only a few, small studies have been conducted so far, the drug's mechanisms of action and low-toxicity profile make it a highly promising option.

Incidence and specificity of HLA antibodies in multitransfused patients with acquired aplastic anemia

BACKGROUND

This study aimed to establish the prevalence and characteristics of anti-HLA in antibody acquired aplastic anemia patients following cessation of antithymocyte globulin therapy and to characterize antibody in terms of epitope specificity.

STUDY DESIGN AND METHODS

One hundred and fifty multitransfused, untransplanted patients from eight European centers were investigated by serologic methods.

RESULTS

Sixty-two percent were antibody positive. Eighteen HLA-Class-I-specific antibodies (15 IgG, 3 IgM) were identified in 13 patients; 13 antibodies were specific for HLA-A epitopes and 5 for HLA-B. Epitope analysis identified significant correlation between serum reactivity and amino acid substitutions associated with HLA-Class-I epitopes. An excess of antibodies to HLA-A1-associated cross-reactive groups was identified. There was no significant difference in antibody frequency in patients taking cyclosporine compared to those who were not.

CONCLUSION

Data suggested a contribution from B cell memory of alloantigens introduced during pregnancy. In some cases, antibody production continued many years after the last transfusion, and although the target varied between individual patients, the antibody to HLA was focused on a few specific Class I epitopes, the majority of which mapped to the HLA-A molecule.

Sirolimus: its discovery, biological properties, and mechanism of action

Sirolimus is the USAN-assigned generic name for the natural product rapamycin. Sirolimus is produced by a strain of Streptomyces hygroscopicus, isolated from a soil sample collected from Rapa Nui commonly known as Easter Island. Although sirolimus was isolated as an antifungal agent with potent anticandida activity, subsequent studies revealed impressive antitumor and immunosuppressive activities. Sirolimus demonstrates activity against several murine tumors, such as B16 43 melanocarcinoma, Colon 26 tumor, EM ependymoblastoma, and mammary and colon 38 solid tumors. Sirolimus is a potent inhibitor of antigen-induced proliferation of T cells, B cells, and antibody production. Demonstration of the potent immunosuppressive activity of sirolimus in animal models of organ transplantation led to clinical trials and subsequent approval by regulatory authorities for prophylaxis of renal graft rejection. Interest in sirolimus as an immunosuppressive therapy in organ transplantation derives from its unique mechanism of action, its unique side-effect profile, and its ability to synergize with other immunosuppressive agents. The molecular mechanism underlying the antifungal, antiproliferative, and immunosuppressive activities of sirolimus is the same. Sirolimus forms an immunosuppressive complex with intracellular protein, FKBP12. This complex blocks the activation of the cell-cycle-specific kinase, TOR. The downstream events that follow the inactivation of TOR result in the blockage of cell-cycle progression at the juncture of G1 and S phase.

In vivo and in vitro effects of SAR 943, a rapamycin analogue, on airway inflammation and remodeling

No current therapy is considered to be satisfactory for severe asthma, and alternative approaches are still required for what is a major unmet medical need. In this study, we compared the effect of a rapamycin derivative, SAR 943, with budesonide, using a murine model of lung inflammation and remodeling. Allergen challenge of ovalbumin-sensitized BALB/c mice induced an increase in the levels of interleukin-5 and interleukin-4; numbers of eosinophil, neutrophil, and lymphocyte; cellular fibronectin; lung epithelial cell proliferation and mucus hypersecretory phenotype; as well as hyperreactivity to methacholine. Both SAR 943 and budesonide, when given intranasally 1 hour before and 24 hours after the aerosol challenge, inhibited all of these parameters with a similar potency (effective dose 50% of 1 mg/kg). In primary cultured smooth muscle cells from human airways, SAR 943 dose dependently inhibited epidermal growth factor-induced proliferation but did not affect the basal cell proliferation. Neither the basal nor stimulated proliferation of a human bronchial epithelial cell line (16HBE14o-) was affected by SAR 943. In conclusion, SAR 943 is as effective as budesonide in inhibiting both lung inflammation and remodeling in a murine model of asthma. Hence, this class of compound could offer beneficial effects in patients with severe asthma.

Rapamycin in transplantation: a review of the evidence

Rapamycin in transplantation: A review of the evidence. The calcineurin inhibitors have been the mainstays of immunosuppression for solid organ transplantation over the last two decades, but nephrotoxicity limits their therapeutic benefit. Rapamycin is a new drug with both immunosuppressant and antiproliferative properties that has a unique mechanism of action distinct from that of the calcineurin inhibitors. It has a role as a maintenance immunosuppressant either alone or in combination with a calcineurin inhibitor and can also be used to treat refractory acute rejection. Theoretical evidence suggests that it may limit the development and progression of chronic rejection in transplant recipients, but this has yet to be confirmed. This review examines the current in vitro animal and human work underlying the use of rapamycin and, in addition, comments on the pharmacokinetics and side-effect profile of this promising new agent.

Rapamycin inhibits hepatic stellate cell proliferation in vitro and limits fibrogenesis in an in vivo model of liver fibrosis

BACKGROUND & AIMS

The accelerated course of hepatic fibrosis that occurs in some patients after liver transplantation is a major clinical problem. This response may be caused by the antirejection therapeutics, and in an earlier report we showed that FK-506 enhanced the fibrogenic process in in vivo and in vitro models of liver fibrosis. In the present study, the aim was to determine whether a new immunosuppressive agent, rapamycin, enhances or inhibits liver fibrosis.

METHODS

Effects of rapamycin were investigated in a carbon tetrachloride model of hepatic fibrosis in rats and on hepatic stellate proliferation in vitro.

RESULTS

Rapamycin inhibited extracellular matrix deposition in the rat model of fibrogenesis as determined by histological analysis, collagen content, messenger RNA levels of procollagen and transforming growth factor beta1, and tissue transglutaminase activity. Moreover, rapamycin decreased platelet growth factor-induced proliferation of hepatic stellate cells.

CONCLUSIONS

These findings indicate that the new antirejection agent rapamycin inhibits hepatic fibrosis and thus may become a valuable addition to the immunosuppression armamentarium.

The 1997 Annual Renal Transplantation in Children Report of the North American Pediatric Renal Transplant Cooperative Study (NAPRTCS)

This report of the North American Pediatric Renal Transplant Cooperative Study (NAPRTCS), covering the years 1987-96 (January 15, 1997), analyzed data on 4898 patients who received 5362 transplants. Over the 10 yr of the study, 21.3% of the patients were under the age of 6 yr. Of the disorders that lead to end-stage renal disease (ESRD), structural and developmental anomalies of aplasia, dysplasia, and obstructive uropathy accounted for over 1500 patients. Despite the potential therapies for focal segmental glomerulosclerosis (FSGS), there has been little change in its incidence and this lesion continues to be the most common cause of renal failure and transplantation among acquired diseases. Eighty-nine per cent of patients with a functioning graft continue to receive cyclosporin A (CsA) 5 yr post-transplantation, and 84% of patients continue to receive azathioprine (AZA), whereas 26% of patients receive alternate-day steroid therapy at 4 yr post-transplant. Thirty-seven per cent of the living donor (LD) recipients and 47% of cadaver donor (CD) recipients were treated with the polyclonal T-cell antibody ATG/ALG for induction, and the monoclonal T-cell antibody, OKT3, was utilized for induction in 12% of LD patients and in 19% of CD patients. Twenty-five per cent of the 5362 transplants (1333) have failed over the 10-yr period. Graft survival is 90% at 1 yr and 74% at 6 yr for LD kidneys, and is 80% at 1 yr and 58% at 6 yr for CD patients. The most common cause for graft failure (30%) is chronic rejection. For recipients of LD grafts, relative risk (RR) factors for graft survival are African-American race (RR = 2.1, p < 0.001) and greater than five prior transfusions (RR = 1.6, p < 0.001). Prophylactic anti-T-cell antibody reduces the risk of graft failure (RR = 0.76, p = 0.009). For recipients of cadaver kidneys, risk factors are: recipient age < 2 yr (RR = 2, p < 0.001), donor age < 6 yr (RR = 1.3, p = 0.005), and absence of induction T-cell antibody (RR = 1.31, p < 0.001).

Rapamune (RAPA, rapamycin, sirolimus): mechanism of action immunosuppressive effect results from blockade of signal transduction and inhibition of cell cycle progression

OBJECTIVE

Rapamune is a novel immunosuppressive agent in Phase III clinical trial in renal transplantation. Its unique mechanism of action has created great interest in its use as a biochemical probe of signal transduction pathways that has provided insight into its molecular mechanism of action. This article reviews the current state of our understanding of the mechanism of action of rapamune.

Comparison of binding characteristics of four rapamycin metabolites to the 14 and 52 kDa immunophilins with their pharmacologic activity measured by the mixed-lymphocyte culture assay

OBJECTIVES

To compare the binding characteristics of four rapamycin (RAPA) metabolites to the 14 and 52 kDa minor immunophilins with their pharmacologic activity, as measured by the mixed-lymphocyte culture (MLC) assay.

METHODS

Four RAPA metabolites were isolated by HPLC from the urine of renal transplant patients. Each metabolite was evaluated at 40 micrograms/L for its pharmacologic activity using the MLC assay. The results of the MLC assay were compared to those obtained using the radioreceptor assay (RRA), which measured the binding characteristics of equal concentrations of the metabolites to the 14 and 52 kDa minor immunophilins.

RESULTS

Each of the four RAPA metabolites showed low immunosuppressive activity by MLC. RM2 showed the highest activity, with 9% of parent RAPA activity. RM1, 3, and 4 showed 2%, 8%, and 4% activity, respectively. Only RM1 was found to bind significantly to either minor immunophilin, with 21% of parent binding to the 14 kDa protein and 25% of parent binding to the 52 kDa protein. RM2, 3, and 4 bound to both proteins with < or = 2% of parent binding.

CONCLUSION

We have demonstrated that the RRA for these four RAPA metabolites shows little cross-reactivity. There is no commercially available immunoassay for RAPA at present. The RRA, therefore, provides an excellent way to rapidly assess efficacy/toxicity of RAPA in patients receiving the drug.

Mechanism of action of the immunosuppressant rapamycin

Rapamycin has potent immunosuppressive properties reflecting its ability to disrupt cytokine signaling that promotes lymphocyte growth and differentiation. In IL-2-stimulated T cells, rapamycin impedes progression through the G1/S transition of the proliferation cycle, resulting in a mid-to-late G1 arrest. Two major biochemical alterations underlie this mode of action. The first one affects the phosphorylation/activation of the p70 S6 kinase (p70s6k), an early event of cytokine-induced mitogenic response. By inhibiting this enzyme, whose major substrate is the 40S ribosomal subunit S6 protein, rapamycin reduces the translation of certain mRNA encoding for ribosomal proteins and elongation factors, thereby decreasing protein synthesis. A second, later effect of rapamycin in IL-2-stimulated T cells is an inhibition of the enzymatic activity of the cyclin-dependent kinase cdk2-cyclin E complex, which functions as a crucial regulator of G1/S transition. This inhibition results from a prevention of the decline of the p27 cdk inhibitor, that normally follows IL-2 stimulation. To mediate these biochemical alterations, rapamycin needs to bind to intracellular proteins, termed FKBP, thereby forming a unique effector molecular complex. However, neither(p70s6k) inhibition, nor p27-induced cdk2-cyclin E inhibition are directly caused by the FKBP-rapamycin complex. Instead, this complex physically interacts with a novel protein, designated "mammalian target of rapamycin" (mTOR), which has sequence homology with the catalytic domain of phosphatidylinositol kinases and may therefore be itself a kinase. mTOR may act upstream of (p70s6K) and cdk2-cyclin E in a linear or bifurcated pathway of growth regulation. Molecular dissection of this pathway should further unravel cytokine-mediated signaling processes and help devise new immunosuppressants.

Potentiation of in vitro synthesis of human IgE by cyclosporin A (CsA)

In this study, we investigated the modulatory effects of CsA on in vitro synthesis of IgE, IgG1 and IgG4 by human peripheral blood mononuclear cells (PBMC). In contrast to its known immunosuppressive effect, we have demonstrated that a low dose of CsA (10(-7) M, 120 ng/ml) potentiated IgE production by up to 40-fold (i.e. from 33 +/- 4.5 to 1346 +/- 290 ng/ml). This potentiation was specific for IgE since no such effect was demonstrable with IgG1 and IgG4. Potentiation of IgE synthesis by CsA in the PBMC cultures was partly due to CsA acting on T cells, as demonstrated by the addition of CsA-treated T cells to T cell-depleted cultures. However, potentiation was also demonstrable in a T cell-depleted, anti-CD40-stimulated culture (four-fold increase from 400 +/- 48 to 1606 +/- 127 ng/ml). Our data therefore suggest that there are at least two mechanisms for CsA-induced potentiation of IgE synthesis, one T cell-dependent and the other T cell-independent. The clinical implications of these findings are discussed with regard to the use of CsA in the treatment of Th2-mediated diseases.