Constitutive and acquired resistance to calcineurin inhibitors in renal transplantation: role of P-glycoprotein-170

Cellular pharmacodynamics of immunosuppressive drugs for individualized medicine

The therapeutic effects of immunosuppressive drugs are known to deviate largely between patients, but efficient strategies for the differentiation of patients who show clinical resistance to immunosuppressive therapies have not been established. Accordingly, a considerable number of patients receive treatment with immunosuppressive drugs despite the onset of serious side effects and poor responses. Cellular pharmacodynamics of immunosuppressive drugs in vitro using peripheral lymphocytes derived from each patient, an attractive way to distinguish resistant patients, is respected and has been applied to the carrying out of individualized immunosuppressive therapy. In this article, I summarize experimental procedures for assaying immune cell responses to immunosuppressive drugs in vitro, and highlight the relationship between cellular sensitivity to immunosuppressive drugs and the therapeutic efficacy of drugs in organ transplantation and several immunological disorders. I will also overview the molecular mechanisms and genetic bases for cellular and clinical resistance to immunosuppressive drugs. Lastly, the future clinical prospects for the application of in vitro drug sensitivity tests for "patient-tailored" immunosuppressive therapies are discussed.

Optimizing the use of cyclosporin in allogeneic stem cell transplantation

Cyclosporin remains the most widely used immunosuppressive agent in patients undergoing allogeneic stem cell transplantation (SCT). The increased awareness of the impact of the intensity of post-transplant immunosuppression on determining outcome after allogeneic SCT has resulted in a re-examination of whether cyclosporin is currently being optimally used in this population of patients. Recent studies in solid organ transplantation have questioned whether the use of trough levels provides the most accurate reflection of the immunosuppressive actions of cyclosporin and alternative strategies to monitor cyclosporin dosage after liver and kidney transplantation are increasingly being used. As a result there is now interest in examining whether there is scope for translating these advances into the arena of haematopoietic transplantation. In this paper, we will review the rationale underlying the current schedules for dosing and monitoring cyclosporin after allogeneic SCT and identify specific areas in which the use of cyclosporin requires re-evaluation. These include evaluation of whether patient outcome would be improved by using peak cyclosporin levels to determine dosing schedules, analysis of optimal cyclosporin dosing schedules in patients undergoing reduced intensity allografts and investigation of surrogate markers of cyclosporin's immunosuppressive activity.

Does pharmacogenetics have the potential to allow the individualisation of immunosuppressive drug dosing in organ transplantation?

The immunosuppressive drugs used in organ transplantation have a narrow therapeutic index, with rejection occurring as a consequence of underdosing and infection, malignancy and a number of drug-specific side effects with excessive dosing. Significant heterogeneity in the dose of drug required to achieve therapeutic blood concentrations adds to the complexity of the problem, which has been partly resolved by therapeutic drug monitoring. Single nucleotide polymorphisms have been identified in genes encoding metabolic enzymes, drug efflux pumps and drug targets for most of the drugs in widespread use. A pharmacogenetic approach to immunosuppressive drug prescribing remains to be tested. Based on current evidence, the most promising strategy would be use of the cytochrome P450 3A5 expressor genotype to guide initial dosing with tacrolimus.

An assessment of P-glycoprotein expression and activity in peripheral blood lymphocytes of transplant candidates

BACKGROUND

P-glycoprotein (P-gp) is involved in the transport of the xenobiotic immunosuppressive agents and many cytokines, such as IL-2 and IFN-gamma. Hence, P-gp activity on peripheral blood lymphocytes (PBLs) could affect the pharmacologic response to xenobiotic immunosuppressants and immune responsiveness. The objectives of this study were to (1) determine the level of P-gp expression and activity on PBLs of kidney transplant candidates; and (2) determine whether P-gp expression correlates with P-gp activity.

METHODS

We measured P-gp expression and activity on CD3(+)/CD8(+), CD3(+)/CD4(+), B lymphocytes, and NK cells of 36 kidney transplant candidates using a flow cytometric assay. P-gp activity was determined for each subpopulation of cells by the ratio of the mean Rhodamine 123 fluorescence (MFI Rh123) in the presence of verapamil divided by the MFI Rh123 in the absence of verapamil. P-gp expression was noted as the percentage of P-gp(+) cells.

RESULTS

NK cells exhibited the greatest amount of P-gp activity (MFI Rh123 = 20.2 +/- 16.4) compared with other cell populations (P < .05). P-gp efflux activity was also significantly elevated in CD3(+)/CD8(+) cells (13.9 +/- 10.5) compared with B lymphocytes (4.9 +/- 2.7; P < .05) and CD4/CD3(+) cells (2.4 +/- 1.0; P < .05). P-gp expression was significantly higher in B lymphocytes (11.7 +/- 9.5) and NK cells (10.2 +/- 7.3) when compared with CD3(+)/CD8(+) cells (7.3 +/- 6.9) and CD3(+)/CD4(+) cells (6.4 +/- 3.8). P-gp expression was highly variable and did not correlate with P-gp activity (P > .05).

CONCLUSIONS

CD3(+)/CD8(+) cells and NK cells, exhibited significantly increased P-gp activity compared with the other cell populations. P-gp expression is not a good correlate of P-gp activity. These findings may have important implications for the use of immunosuppressive drugs posttransplant and immune responsiveness after transplantation.

P-GLYCOPROTEIN ACTIVITY IS DECREASED IN CD4+ BUT NOT CD8+ LUNG ALLOGRAFT-INFILTRATING T CELLS DURING ACUTE CELLULAR REJECTION

BACKGROUND

Modulation of P-glycoprotein (P-gp) activity in graft-infiltrating T cells may alter their susceptibility to immunosuppression.

METHODS

P-gp activity was measured by rhodamine efflux in T-cell subsets from bronchoalveolar lavage (BAL) of five healthy volunteers and 27 lung allograft recipients. The effect of T-cell activation on P-gp activity was modeled by stimulation of peripheral blood mononuclear cells with staphylococcal enterotoxin B.

RESULTS

Most BAL T cells expressed memory-effector markers. Patients had a lower proportion of CD4 T cells (P = 0.005), whereas control subjects had CD4-to-CD8 ratios similar to peripheral blood. In controls, basal P-gp activity was greatly increased in both CD4 (35% P-gp active) and CD8 (63%) lung T cells compared with peripheral T cells. Basal P-gp activity was elevated in patient BAL T cells but was lower than control BAL activity (CD4, P = 0.07; CD8, P = 0.03). Lung T cells from transplant patients had modest (CD4) or marked (CD8) increases in substrate-induced P-gp activity compared with normal lung, indicating that P-gp was not irreversibly inhibited. Patients with acute cellular rejection (ACR) had reduced P-gp activity in CD4, but not CD8, BAL T cells compared with patients without ACR (P = 0.004). To determine the relationship between T-cell activation on P-gp modulation, P-gp activity was measured in staphylococcal enterotoxin B-stimulated peripheral blood mononuclear cells. P-gp activity was abrogated in CD71 cycling cells but remained high in a persistent but minor population of resting naive T cells.

CONCLUSIONS

Lung T cells have increased in vivo P-gp activity and therefore may eliminate substrate drugs, resulting in local resistance to immunosuppressive therapy. However, P-gp function is reduced during T-cell activation, providing a window of susceptibility to treatment during ACR.

Whole-blood cultures from renal-transplant patients stimulated ex vivo show that the effects of cyclosporine on lymphocyte proliferation are related to P-glycoprotein expression

BACKGROUND

Cyclosporine (CsA) is a substrate for the MDR-1 gene product P-glycoprotein (P-gp). CsA efficacy may be modulated by lymphocyte P-gp expression levels. In this study, CsA inhibition of lymphocyte proliferation in whole-blood cultures ex vivo has been related to (1) lymphocyte P-gp expression and (2) the C3435T polymorphism in the MDR-1 gene, which has been reported to alter P-gp function.

METHODS

In 30 renal-transplant recipients taking CsA monotherapy, P-gp expression was measured by flow cytometry. Whole-blood samples were stimulated with purified protein derivative (PPD) and phytohemagglutinin (PHA). CsA resistance ex vivo was defined as less than 10% reduction in proliferation with either PPD or PHA at 2 hours compared with 0 hours.

RESULTS

CsA resistance was associated with greater P-gp expression using either PPD (median expression, resistant 1.89 vs. sensitive 0.96, P =0.02) or PHA (1.66 vs. 0.96, respectively, P =0.02). Whole-blood CsA levels in resistant and sensitive patients were similar. The C3435T polymorphism did not affect inhibition of proliferation by CsA (P >0.05 for all between genotype group comparisons).

CONCLUSIONS

Our results indicate that lymphocyte P-gp expression determines the degree of inhibition of proliferation by CsA ex vivo; whether this also affects CsA effectiveness in vivo and therefore graft survival requires further study.

CD154 is expressed during treatment with calcineurin inhibitors after organ transplantation

BACKGROUND

CD154 (CD40 ligand) monoclonal antibody prevents allograft rejection in rodents and monkeys. Inasmuch as calcineurin inhibitors (CI) inhibit CD154 expression by pharmacologic agents in vitro, we investigated whether CD154 is also inhibited by CI in vivo and in vitro during allogeneic stimulation.

METHODS

CD154 expression was determined by immunohistochemistry and flow cytometry in human lymph nodes and spleen sections from rhesus monkeys with or without CI treatment. The effect of CI on induction of CD154 expression was studied by stimulating lymphocytes with phorbol 12-myristate 13-acetate (PMA) and ionomycin or with allogeneic monocyte-derived mature dendritic cells.

RESULTS

Lymph nodes from patients with or without CI cyclosporine (CsA) or FK506 (FK) treatment showed comparable CD154 expression, which was present on the cell surface of T cells. CD154-expressing cells were also present in spleens from monkeys treated with CsA in comparable numbers to those in the nontreated group. Moreover, in several liver transplant rejection biopsies taken during CI therapy CD154-expressing cells were observed. In vitro, CsA and FK strongly inhibited induction of CD154 expression on peripheral blood mononuclear cells by pharmacologic stimuli. Maximum inhibition was found at 50 ng/ml CsA and 20 ng/ml FK. CD154 expression induced by dendritic cells in peripheral blood mononuclear cells or spleen cells was also almost completely inhibited by CsA.

CONCLUSION

Although CI strongly suppressed pharmacologic and allogeneic induction of CD154 expression on T cells in vitro at concentrations at approximately clinical trough levels, CD154 is prominently expressed during CI therapy in lymphoid tissue and (sporadically) in liver allografts. This suggests that the CD154-CD40 pathway remains functional during CI therapy, which may contribute to allograft rejections in the clinical setting.

P-glycoprotein (P-gp) is upregulated in peripheral T-cell subsets from solid organ transplant recipients

Immunosuppressive agents such as cyclosporine, tacrolimus, sirolimus, and corticosteroids are substrates for the transmembrane multidrug resistance pump P-glycoprotein (P-gp). Experience in oncologyhas suggested that chronic exposure to P-gp substrates induces upregulation of P-gp activity, which could result in resistance to immunosuppressive drugs. The authors investigated P-gp function in CD4+ and CD8+ T cells from the peripheral blood of solid organ transplant recipients (SOTX). Subjects included 14 stable SOTX (10 liver, 4 lung) and 16 healthy controls. Four-color flow cytometry was used to simultaneously measure intracellular concentration of the fluorescent P-gp substrate Rhodamine 123 (Rh123) and surface expression of CD45RO (nominal memory/effector), CD45RA (naive), and either CD4 or CD8. P-glycoprotein function was measured by a dye efflux assay in which activity was inferred from a decrease in Rh123 fluorescence. CD4+ and CD8+ T cells from patients and control subjects eliminated Rh123, and this activity was inhibited by verapamil, a known P-gp substrate. CD8+ T cells had greater P-gp activity than CD4+ cells, and naive and transitional T cells displayed greater activity than memory T cells. Activity was bimodal in CD8+ CD45RO+ T cells, with a subset of these cells expressing the greatest P-gp activity. Patient CD8+ naive and transitional T cells had upregulated P-gp activity compared to control subjects. We conclude that (1) P-gp activityis significantly upregulated in specific T-cell subsets (CD8+/CD45RA+) in the peripheral blood of SOTX, and (2) the bimodal nature of P-gp response in CD8+ T cells complicates analysis of the effect of chronic administration of P-gp substrates to SOTX.