United Network for Organ Sharing

See also Kidney Diseases  See also MHC

In 1901, Landseiner discovered ABO blood group antigens. In 1954, Murray performed the first successful kidney transplant between identical twin brothers. In 1958, Dausset discovered the major histocompatibility complex in human, the human leukocyte antigens (HLA). In 1963, the first liver allograft transplant by starzl and the first lung transplant was performed by Hardy. In 1967 the first heart allograft transplant was performed by Barnard. In 1972, Borel discovered immunosupressive properties of Cyclosporine (isolated from fungus in Norway). In 1983, Baby Fay received the first baboon heart and survived for 21 days. In 1999, Rapamycin was approvied for clinical kindey transplantaion. These are all milestones in transplantation therapy, improving graft transplantation.

Today, a 80% 1 year survival rate for kidney, liver, heart and pancreas transplantation is obtained. However, chronic rejection of transplants remains a large problem. Another major problem is organ shortage. The other problems in translantation are infections due to overimmunosupression. The solution to all of these problem would be to make organs accepted to reduce the need to make transplantations.

Definitions

Autologous graft (“autograft”): is a graft transplanted between two genetically identical individuals.

Allograft (allogeneic graft): is a graft transplanted between two genetically different inidividuas of the same species. In conventional transplantation for allegeneic recipients, multiple HLA class I and class II proteins must be matched for histocompatbility. 

Zenograft: is a graft transplanted between inidividuals of different species. 

Tolerance means the absence of a destructive response to the allograft in an immunocompetant host. , although easy to acheive in small animal models, has been extremely difficult in large animal models and humans. There is also no assay to measure tolerance (no simple assay). There is also a problem of compatbility of tolerance induction strategies with conventional immunosuppression. So any new treatments have to be compatible with immunosupressive drugs people are already taking.

Acute versus Chronic rejection: 

Chronic GVHD usually appears 100 days post transplantation and sevral factors are thought to be involved including upregulation of TGF-beta which casues fibrosis and upregulation of OX40 ligand (OX40L), a TNF family member. In the acute form of the disease, mature T cells present in the bone marrow recognize the donor tissue as foreign. which via APCs casue the activation and proliferation of the donor T cells. Onset for the acute form is usually within 100 days of transplantation (Campbell, US 2017/0327587)

Chronic rejection is poorly defined that involves immune and non-immune components. It is a slow process that occurs months to years after transplantation and characterized by arteriole thickening and interstitial fibrosis. Current immunosuppressive are ineffective at treating chronic rejection.

Mechanisms/Pathways of graft Rejection

Activation of T cells: Allografts are rejected in part by the activation of T cells. The transplant recipient mount a rejections response following CD4+ T cell recognition of foreing antigens in the allograft. These antigens are encoded by the major histocompatibility complex (MHC). There are both class I and Class II MHC molecuels. In human the class I MHC molecules are HLA-A, B and C. The class II MHC molecules are called HLA-DR, DQ and DP. (Rother, WO2005/110481). 

There are important differences in HLA expression between T and B cells, which influence the interpretation of a crossmatch. T cells do not sonstitutively express HLA class II; so the result of a T cell crossmatch generally reflects antibodies to HLA class I only. B cells express both HLA class I and II. Becasue of this, a positive B cell crossmatch is more difficult to interpret thatn a positive T cell crossmatch. It may be due to antibodies directed agaisnt HLA class I, II or both. (Frey, US Patent Applicaiton No: 16/340,453, published as US 2019/0276524).

–Role of Costimulatory molecules and T cell activation: For example, CD40 on APC and CD154 is an important interaction. Antibodies against CD154 to block this interaction has been used in animal studies. Blockage of CD28/B7 and CD40/Cd40L pathways shows synergy in prolonging graft survival.

OX40L is not constituitively expressed but can be induced on professional APCs such as DCs and macrophages. Other types of cells such as Langerhans and NK cells can be induced to expressed OX40L. The OX40L receptor, OX40, is expressed on activted T cells (CD4 and CD8, TH1, TH2 and TH17), which may provide essential signals for the generation of memory T cells, the enhcnacement of the TH2 response and the prolongation of the inflammatory response. OX40 signals into responder T cells renders them resistant to Treg meidated suppression. (Campbell, US 2017/0327587)

Antibody-mediated rejection (AMR): 

AMR, also called “accelerated humoral rejection” is characterized by markedly elevated circulating donor reactive antibodies, microvascular thrombosis, and C4d deposition in the graft.  (Wang “Inhibition of terminal complement components in presensitized transplant recipients prevents antibody-mediated rejection leading to Long-Term graft survival and accomodation” J Immunol 2007, 179: 4451-4463).

In AMR, complement is suggested to be activated by the classical pathway and to play a key role in the pathogenesis. Rother (WO2005/110481) dicloses using a mose model for AMR and showing that anti-C5 mAb in combination with CsA and CyP achieved indefinite heart graft survival over 100 days. 

–In Kidney Transplantation:

Frey, (US Patent Applicaiton No: 16/340,453, published as US 2019/0276524) dicloses a method of treating/reducing antibody mediated rejeciton (AMR) in a human kidney transplant recipient that includes administering an anti-C5 antibody such as ecullizumab at a doese of 1200 mg for 3 hours prior to kidney allograft reperfusion, 900 mg 18-30 hours after reperfusion of the kidney allograt and 7, 14, 21 and 28 post transplantatin and 1200 mg dose adminsitered at week 5, 7 and 9 post transplantation. wherein the recipient is sensitive to a human living donor, received desensitization therapy prior to transplantation for 11 days or more but not post-transplation for at least 9 week. 

Rother (US Patent Application No: 15/243,290, issued as US 9,771,418) discloses a method of treating AMD in a patient having a kidney transpant which includes intravenously adminsitering to the pateint 1200 mg of eculizumab less than 24 hours beofre or during the transplant operation and one post-operative dose within 24 hours of the transplant, 900 mg once a week for four weks and then 1200 mg of eculizumab on week five and bi-weekly therafter. The eculizumab is adminsitered in an amount and with a frequency to maintin at least 50 ug of eculizumab per milliliter in the patient’s blood. 

Stegall (American J of Transplantation, 2011, 11: 2405-2413) discloses an eculizumab dosing region from PNH for AMR which includes 1200 mg immediately prior to transplantation, 600 mg on posoperative day 1 and 600 mg weekly therafter for 4 weeks. At week 4, assessment of DSA levels are performed. Eculizumab was dicontinued in patients whose DSA had signifcantly decreased and eclizumab treatment contined (1200 mg week 5 and then every 2 weks) in patients with persistently high DSA. 

–In Heart Transplantation:

All individuals who reeive a heart tranpslant are at risk for developing antibody-mediated rejeciton (AMR). The growing proportion of sensitized cardiac recipients presents an additional challenge to the transplant practitioner attempting to minimize the occurrence of AMR. Pateints pre-exposed or “sensitived” to antigen exposing events (i.e: lgood transfusions, multiple pregnancies, prior organ transplantations, ventricular support devices) are more likely to both possess preformed and develop antibodies. NCT02013037 by Alexion Pharmaceuticals discloses a non-randomized, open-label efficacy trial investigating use of eculizumab alongside conventional therapy to prevent antibody mediated rejection. Eculizumab as admisntiered 1200 mg at the time of transplantation, 900 mg day 1 post-transplant, on day 5 post-transplant, 1 g/Kg for 2 consecutive days, on days 7, 14 and 21 900 mg and then days 28, 42 and 56 1200 mg. 

Acute/Classical pathway: This is cell mediated (predominantly CD4 and CD8 T cells) directed at donor MHC antigens. CD4 T cells recognize these classes (allosensitization) which help develop effector functions (B cells, CD8T cells, Macrophages) which destroy the graft. Current immunosupression is targeted at acute rejection.

In allorecognition the TCR is working as the antigen. This means the precursor frequency is different. 1-% of T cells will become activated rather then with nominal antigen where 1 in about 10000 T cells are active to respond. The frequency of antigens presented on a cell is much greater then in the traditional sense. In direct recognition, the T cell recognizes an allo APC presenting a foreign pathway. The direct pathway is predominant (greater than 90%). It activates both CD4 and CD8 T cells and is sufficient to cause rejection. There is also an indirect recognition (lower frequency) where peptides are presented by self (host) APC. This is probably important in chronic rejection and may have a role in tolerance induction. This pathway involves epitope spreading.

Hyperacute rejection (HAR): occurs minutes to hours after transplantation due to antibody mediated and complement dependent pathways. HAR is rapid and severe and represents one of the largest obstacles to the success of xenotransplantation techniques. HAR is for the most part mediated by antibodies and complement, there being natural human antibodies, predominantly IgG and IgM subclasses which react with nuermous molecuels on xenotransplant cells, particularly endothelial cells, in vascularized transplants. These are preformed antibodies specific for MHC antigens (eg.., graft before, etc). It is now generally accepted that all or most of the HAR reaction is due to the presence of human antibodies directed against the carbohydrate epitope Gal-alpha (1,3) Gal.

The target of the response is vacular endothelium. This can be avoided by prescreening (taking serum from patient and reacting it against donor cells). Attempts to eradicate HAR have included removal or neutralization of complement and antibody.  

Dendritic Cells in Alloreactivity: DCs are specialized, exptremely potent APC that stimulate both CD4 and CD8 T cells in mixed luekocyte response. 30 years ago it was noted if depleted DCs before transplantation, found greater survival rate. Both donor and host CD contribute to alloactivation (direct and indirect pathways above).

Trying to take advantage of idea that certain DCs can promote tolerogenicity, BM DCs were derived in low dose GM-CSF and shown that they are poor stimulators of T cells. Low dose gave rise to what looked like immature DCs. High does gave rise to mature and immature DC. The low dose, immature DCs transplanted at day 3 prolonged graft survival.

Role of autoantigens in alloreactivity: In one report, gave rats allograft and then challenge rate with specific antigen to see whether ear swelling occurred which indicates T cell response.

Cross-Matching Prior to Transplantation:

Cross-matching was developed in an attempt to identify recipients hwo are likley to develop acute vacular rejection as a graft form a given donor. This phenomenon, HAR, is a result of preformed antiobdies against the donor; referred to as donor-specific antibodies (DSA). Such antibodies are usually formed as the result of previous expsure to HLA, generally through pregnancy, blood transfusion or previous transplantation. Preformed antibodies cause rejection by binding to HLA antigens expressed on the endothelium of vessels in the transplanted kidney, resulting in activaton of the complement cascade with resultant thrombosis and infarction of the graft. (Frey, US Patent Applicaiton No: 16/340,453, published as US 2019/0276524).

Complement Dependent Cytotoxicity Crossmatch: A CDC crossmatch involves placing recipient serum (potentially containing donor-specific anti-HLA antibodies) onto donor lymphocytes (containing HLA antigens). A cytotoxic reaction suggests the presence of preformed donor specific antibodies (DSA).  (Frey, US Patent Applicaiton No: 16/340,453, published as US 2019/0276524).

Treatment Strategies:

The principal stragegies to prevent GVHD center around the depletion of donor T cells. However, this may lead to the loss of GVHD (graft verus tumor effect) and to an increased risk of infections and graft failure. Recently focus has been on host DCs as key stimulators of donor T cells, inducing GVHD. 

Desensitization: refers to donor specific human antigen (HLA or DSAs) reduction techniques used to facilitate kidney transplantation for recipients who are sensitized to their donor organs by lowering the amount of circulating DSA. DSA techniques include for example direct antibody removal by plasmapheresis (PP), immune modulation using intravenous immunoglobulins (IVIg) and attempts to deplete B cells using a variety of immunosuppressive agents. (Frey, US Patent Applicaiton No: 16/340,453, published as US 2019/0276524).

 Antibodies against OX40L: Campbell, (US 2017/0327587) disclsoes antibodies against OX40L which are useful for treatment graft versus host disease.