Immunosuppressive strategies in transplantation

  Immunosuppressive strategies

in transplantation 

Denton, Mark D; Magee, Colm C; Sayegh, Mohamed H

Lancet 1999; 353: 1083-91

Islet cell transplantation

• Of the 267 allografts transplanted since 1990.(1990-1999)

• 12.4 percent have resulted in insulin independence for periods of more than one week.

• 8.2 percent--more than one year.

• The majority of regimen of immunosuppression consisted of

1.antibody induction with an antilymphocyte globulin

2.cyclosporine

3.azathioprine

4.glucocorticoid

Edmonton protocol

• A glucocorticoid-free immunosuppressive protocol

1.Sirolimus

2.Tacrolimus

3.Daclizumab-- a monoclonal antibody against the interleukin-2 receptor(IL-2).

Mechanisms of allograft rejection: central role for T cells

• The CD4 T cell is crucial in both the initiation and the coordination of the rejection response.

• Recipient mounts a rejection response following CD4 T cell recognition of foreign antigens derived from the allograft.

• These antigens are encoded predominantly by highly polymorphic loci within the major histocompatibility complex on the short arm of chromosome 6.

Mechanisms of allograft rejection: central role for T cells

• Direct allorecognition

1. activate a much larger proportion of the T cell pool

2. cause the vigorous immune response in acute rejection.

• Indirect allorecognition

1. generates smaller numbers of alloreactive T cell clones.

2. may lead to the insidious immune response that occurs in chronic rejection.

Second (costimulatory) signal • When both signals are provided, the T

cell secretes optimum concentrations of interleukin 2. (IL-2)– A potent autocrine growth factor– Induces 1.T cell proliferation,2.clonal expansion, 3.cytokine production.

In the absence of Second signal,

CD4 T cell becomes1. unresponsive to further exposure

to antigen.2. fails to secrete cytokines. 3. may undergo programmed cell

death (apoptosis).

Second (costimulatory) signal

• The best-characterised costimulatory signal – CD28 on the surface of

the CD4 T cell. – a member of the B7 fa

mily of molecules (B7-1 or B7-2) on the APC.

• Another important costimulatory signal – CD40-CD154 interaction

s• A mutational deficiency of

CD154 (CD40 ligand)T cell defence against intra

cellular microorganisms is impaired

Mechanisms of allograft rejection: central role for T cells

• Alloactivated CD4 T cells interact with effector cells of the rejection response via direct cell-cell contact and cytokine secretion.

• By an increase in the activation and function of B cells, cytotoxic CD8 T cells and monocyte/macrophages, alloactivated CD4 T cells promote alloantibody production, antigen-specific cell lysis, and delayed type hypersensitivity responses, respectively.

Current immunosuppressive drugs

1. T cell activation

2. cytokine production

3. clonal expansion.

Standard transplantation immunosuppressive protocols

1. initial therapies

2. maintenance therapies

Initial immunosuppression

• initiated at high levels in the immediate post-transplant period – risk of graft rejection is greatest.

• initial immunosuppression consists of– higher doses of the agents used in maintenance

therapy.– potent anti-T-cell antibody preparations (Table

1).

Higher risk of acute rejection

• Children

• Sensitised patients

– second or subsequent transplants

– previous pregnancies

– have received multiple blood transfusions.

• post-transplant acute tubular necrosis,

– delayed introduction of full-dose cyclosporin or tacrolimus.

– nephrotoxic effects

Initial immunosuppression• 7-14 day course of 1. polyclonal antilymphocyte antibodies –

a. antithymocyte globulinb. antilymphocyte globulin

2. monoclonal anti-CD3 antibodiesa. reducing the incidence and severity of early acut

e rejection.• Routine use is not indicated

– serious infection – malignant disease.

Table-1 Induction immunosuppressive drugs

Rodent antibodies to IL-2 receptor

• Advantage 1. Effective as induct

ion therapy2. better tolerated th

an antithymocyte globulin

3. safe use of low initial doses of cyclosporin.

• Disadvantage 1.short plasma half-life

-limited the efficacy to several days.

2.generation of human anti-rodent AB.-overcome by chimeric

and humanised Ab to IL-2 receptor.

IL-2 receptor Ab

• Two preparations, – basiliximab – Daclizumab--- phase III clinical studies.

• Compared with placebo in combination with standard cyclosporin and corticosteroids.

IL-2 receptor Ab

• The results of these trials were strikingly similar:

1. well tolerated

2. reduced the incidence of acute rejection by roughly 35%

3. neither increase of infection or other adverse events within the 1-year follow-up period.

4. effective interleukin-2-receptor blockade is achieved for several weeks after the last dose

IL-2 receptor Ab

• Several questions remain unanswered.

1. Is it beneficial when combined with potent mycophenolate mofetil and tacrolimus?

2. IL-2 Ab circumvent the need for calcineurin inhibitors (cyclosporin or tacrolimus) ?

• Preliminary data show a fairly high rate of acute rejection in patients receiving IL-2 Ab with mycophenolate mofetil, and steroids in the absence of calcineurin inhibition.

•Figure 2. Stages of T cell activation: multiple targets for immunosuppressive agents: •Signal 1: Stimulation of T cell receptor (TCR) results in calcineurin activation, a process inhibited by cyclosporin (CyA) and tacrolimus. Calcineurin dephosphorylates NFAT (nuclear factor of activated T cells) enabling it to enter nucleus and bind to interleukin-2 promotor. Corticosteroids inhibit cytokine gene transcription in lymphocytes and antigen-presenting cells by several mechanisms. •Signal 2: Costimulatory signals are necessary to optimise T cell interleukin-2 gene transcription, prevent T cell anergy, and inhibit T cell apoptosis. Experimental agents but not current immunosuppressive agents interrupt these intracellular signals. Signal 3: Interleukin-2 receptor stimulation induces cell to enter cell cycle and proliferate. •Signal 3 may be blocked by interleukin-2 receptor antibodies or by sirolimus, which inhibits second messenger signals induced by interleukin-2 receptor ligation. Following progression into cell cycle, azathioprine and mycophenolate mofetil (MMF) interrupt DNA replication by inhibiting purine synthesis.

Maintenance immunosuppression

• combinations of immunosuppressive agents.

• minimise side-effects of any single drug

Maintenance immunosuppression

1. calcineurin inhibitor

• cyclosporin or tacrolimus.

2. antiproliferative agent

• azathioprine or mycophenolate mofetil.

3. Corticosteroids.

Maintenance immunosuppression

1. type of organ transplanted

2. degree of HLA incompatibility

3. recipient race

4. history of previous acute rejection

• 30-40% of transplant recipients are predicted to develop neoplasia over 30 years.

Calcineurin inhibitors

• Keystone of most immunosuppressive regimens in organ transplantation.

1. cyclosporin 2. tacrolimus• bind to cytoplasmic receptors (cyclophylin and F

K-binding protein 12 [FKBP-12]) – resulting complexes inactivate calcineurin– Calcinerin inhibition prevents interleukin-2 gene trans

cription—inhibit T cell IL-2 production.

cyclosporin

• Increased 1-year graft survival for cadaveric kidney allografts by 10%.

• ubiquitous tissue distribution of cyclophylins.

• Such side-effects include– acute and chronic nephrotoxic effects– Hypertension– dyslipidaemia.

cyclosporin

• Acute nephrotoxicity– intrarenal vasoconstrict

ion

• exacerbate acute tubular necrosis in cadaveric renal transplantation.

• Acute nephrotoxic effect are reversible

• Chronic nephrotoxicity– persistent renal vasoconstri

ction, ischaemia, – induction of fibrogenic grow

th factor, transforming growth factor-beta.

• Histology– obliterative vasculopathy an

d interstitial fibrosis. • chronic effects are irreversible.

cyclosporin• Monitoring of trough concentrations is essential.• Poor and variable oral absorption• High doses are advocated in the initial 6-12 months.

– appropriate long-term dosage is contested.

• Registry data--lower maintenance doses are associated with poorer graft outcome.

• A recent prospective trial--lower cyclosporin concentrations – did not compromise long-term (5 year) graft survival – associated with a reduction in the incidence of neoplasia.

Microemulsion cyclosporin --Neoral

• Better and more consistent bioavailability– more reproducible daily exposure to cyclosporin. – lower acute rejection rates.

• Lead to an increase in long-term toxic effects?– not been borne out in large multicentre studies.

• Whether patients stabilised on standard cyclosporin should be converted to the microemulsion formulation depends mainly on patient's and physician's preference.

Tacrolimus (FK506)• An alternative agent to cyclosporin, • More potent than cyclosporin:

– presumably owing to a greater affinity of its complex with FKBP-12 to calcineurin

– greater water solubility• Lesser dependence on bile salt • Its exact role as primary maintenance therap

y outside the area of liver transplantation is not yet clearly defined.

Tacrolimus (FK506)

• Large US and European multicentre trials in both liver and kidney transplantation– tacrolimus-based therapy with a lower incidenc

e of acute rejection than cyclosporin-based therapy.

• US renal transplant study(3 year follow up)– improved graft survival in tacrolimus-treated pa

tients.

Tacrolimus (FK506)

• In renal transplantation, – rescue therapy for refractory acute rejection on cycl

osporin-based therapy. – 75% rescue rate in a mean 5-year follow-up period.

• In combined kidney and pancreas transplantation

– impressive preliminary results

• In the limited heart and lung transplantation– favourably than cyclosporin.

Tacrolimus (FK506)

• Tacrolimus and cyclosporin – different side-effect,– roughly equivalent nephrotoxicity.

• Tacrolimus-treated patients – a higher incidence of diabetes mellitus and neurotox

ic reaction.– a lower incidence of hypertension, hyperlipidaemia,

hirsutism, and gum enlargement.

Azathioprine

• A purine analogue– inhibits DNA synthesis,

• Used as an immunosuppressive agent since 1960s.

• Principal side-effect is bone-marrow suppression.

Mycophenolate mofetil(MMF)

• Replacing azathioprine in standard immunosuppression protocols for new kidney and pancreas-kidney transplants.

• A selective inhibitor of the de-novo pathway of purine biosynthesis.

• More specific and potent inhibition of T cell and B cell proliferation.

Mycophenolate mofetil

• Three large multicentre trials in renal transplant 1. Compared MMF with azathioprine or placebo pa

tients receiving cyclosporin and corticosteroids. – a lower incidence of acute rejection (50% reduction) i

n the first year.– a lower requirement for high-dose steroids and antily

mphocyte antibody therapy to treat rejection.– over 3 years follow-up, have not shown improved lon

g-term graft survival with mycophenolate mofetil, perhaps owing to lack of adequate statistical power.

Mycophenolate mofetil

• In cardiac transplantation,

– a prospective randomised trial – a benefit of mycophenolate mofetil over a

zathioprine when results were examined on a treated but not on an intention-to-treat basis.

Mycophenolate mofetil

• Provisional renal-transplantation data suggest that MMF may be useful in the treatment of – acute cellular rejection

– refractory acute rejection in patients on azathioprine.

• In animal models, MMF prevents the development of chronic rejection– it may have a similar effect in human transplantation.

Mycophenolate mofetil

• Side-effects

1. gastrointestinal upset

2. diarrhoea (may be dose-limiting)

3. tissue invasive cytomegalovirus infection.

• have not shown a reduced incidence of bone-marrow suppression.

Sirolimus (rapamycin)

• Sirolimus (rapamycin)– a highly potent immunosuppressive agent curre

ntly undergoing phase III clinical trials.

• cyclosporin and tacrolimus inhibit T cell production of cytokines, sirolimus inhibits T cell responses to these cytokines.

Sirolimus

• Inhibits intracellular signalling distal to the interleukin-2 receptor

• inhibits progession of the T cell into the S phase of the cell cycle.

• designed for use with cyclosporin rather than with tacrolimus – sirolimus and tacrolimus bind to the same intrac

ellular protein (FKBP-12), although an antagonist effect in vivo is disputed.

Sirolimus

• Phase I trials showed that sirolimus is well tolerated.

• Side effect – hyperlipidaemia – thrombocytopenia

Sirolimus

• A phase II trial has shown that addition of sirolimus to a cyclosporin and corticosteroid – less than 10% incidence of acute renal allograft re

jection.

• 1-year follow-up data– steroid withdrawal can be achieved successfully i

n the majority. – Such low rates of acute rejection can be maintaine

d even when low doses of cyclosporin are used.

Sirolimus

• a phase III European multicentre trial in renal transplantation – compare sirolimus with cyclosporin used with a

zathioprine and corticosteroids.

• Preliminary data show– similar incidences of acute rejection between gr

oups, albeit higher than when both agents are combined. ??

Sirolimus

• Experimental data suggest that– sirolimus may prevent the development of graf

t atherosclerosis, the sine qua non of chronic rejection.

• This encouraging finding may be related to a non-specific antiproliferative effect of the drug.

Corticosteroids

• non-specific anti-inflammatory agents. • inhibit cytokine production by T cells and macrop

hages, disrupting T cell activation and macrophage-mediated tissue injury.

• This effect is mediated through inhibition of nuclear factor kappa B activation and by binding to glucocorticoid response elements in the promoter regions of cytokine genes.

Corticosteroids

• Corticosteroids have numerous well-known side-effects ( Table 2).

• Hypertension, dyslipidaemia, and glucose intolerance are side-effects shared by both corticosteroids and calcineurin inhibitors and undoubtedly contribute to the high prevalence of cardiovascular disease in transplant recipients.

Corticosteroids

• The fact that cardiovascular disease is the major cause of death in this population of patients must be stressed.

• These complications have prompted investigators to assess the effects of gradual corticosteroid withdrawal in stable renal allograft recipients maintained on cyclosporin and azathioprine.

Corticosteroids

• Most studies show that corticosteroid withdrawal 1 year after transplantation is associated with a small but significant risk of acute rejection.

• Furthermore, the potential deleterious effects of steroid withdrawal on long-term graft function have raised concerns over this practice. [60]

Corticosteroids

• Hopefully, the development of immunological monitoring techniques may allow the identification of patients in whom safe withdrawal of steroids is achievable.

• In addition, introduction of newer agents such as tacrolimus, mycophenolate mofetil and sirolimus may allow safer withdrawal of steroids, an issue currently being studied.

Table 2 maintenance immunosuppressive drugs

New immunosuppressive strategies in transplantation• inhibit CD4 T cell activation.

• ultimate aim--inhibit only those T cells that respond to donor antigen, achieving immunological unresponsiveness to the transplant (donor-specific tolerance).

New immunosuppressive strategies in transplantation• Several ligand/receptor interactions occur b

etween the T cell and the antigen-presenting cell during antigen presentation ( fig-3).

• While some simply mediate cell-cell adhesion, others transduce activational signals to either the T cell or the antigen-presenting cell.

•Figure 3. Receptor: ligand interactions between T cell and antigen-presenting cell: Cell surface molecules on CD4 T lymphocytes interact with corresponding ligands on antigen-presenting cell. Some of these ligands such as LFA3 and ICAM-1 are also located on endothelial cells. Interactions between these molecules are important in cell-cell adhesion and/or transmission of signals (including costimulatory signals) into immune cells. Monoclonal antibodies against many of these surface receptors are under investigation as immunodulatory agents.

•From:   Denton: Lancet, Volume 353(9158).March 27, 1999.1083-1091

1. Blockade of T cell costimulation

2. Blockade of T cell accessory molecules

3. Peptide-mediated immunosuppression

4. Gene therapy in transplantation

5. Tolerance induction

New immunosuppressive strategies

Conclusion

• Large multicentre trials--incorporation of antibodies to interleukin-2 receptor, tacrolimus, or MMF into immunosuppressive regimens significantly lower acute rejecton.

• The capacity of IL-2 receptor blockade added to MMF and steroids to obviate use of calcineurin inhibitors is being assessed.

Conclusion

• Several important issues have yet to be addressed.

1.Are these agents providing more specific or simply more potent immunosuppression?

2.How can these newer agents best be combined to achieve maximum efficacy and to keep side-effects to a minimum?

Conclusion

.Will newer agents prevent chronic rejection and improve long-term graft survival?

• Finally, the issue of cost-effectiveness cannot be ignored. Is the routine use of newer, more expensive drugs justified if evidence of improved long-term outcome is lacking?

Table 3 Novel immunosuppressive drugs

Table 4 Maintenance immunosuppressive regimens