Immunology Lecture 16 Ashraf Khasawneh Faculty of Medicine ...

Immunology Lecture 16

Ashraf KhasawnehFaculty of Medicine

The Hashemite University

Transplant immunology

Introduction

• Alexis Carrel (France) (1910-1930): Work on vascular suture

and the transplantation of blood vessels and organs. (1912)

• Peter Brian Medawar and Frank Macfarlane Burnet (1949):

Discovery of acquired immunological tolerance. (1960)

• Joseph E. Murray (1954-1965): Discoveries concerning organ

transplantation in the treatment of human disease. (1990)

• Today, the following transplantations are performed among

non-identical individuals with ever increasing frequency and

success:

– Chest: heart and lung

– Abdomen: kidney, pancreas, liver, intestine, stomach, testis, and penis.

– Tissue, cells and fluids: Hand, bone marrow, and cornea, skin, bone,

heart valves, blood and blood vessels, and islets of Langerhans.

Definition

• Graft or Transplant: Transfer of living cells,

tissues and organs from one part of the body to

another or from one individual to another.

Types of Transplantation:

Autologous grafts (Autografts): Grafts transplanted from one part of the body to another in the same individual

Syngeneic grafts (Isografts): Grafts transplanted between two genetically identical individuals of the same species

Allogeneic grafts (Allografts): Grafts transplanted between two genetically different individuals of the same species

Xenogeneic grafts (Xenografts): Grafts transplanted between individuals of different species

Transplantation Antigens

1. Major histocompatibility antigens (MHC

molecules)

2. Minor histocompatibility antigens

3. Other alloantigens including blood grouping

antigens

1. Major Histocompatibility

Antigens

• Main antigens of grafts rejection

• Cause fast and strong rejection

• Gene complex whose alleles encode polymorphic cell surface glycoproteins involved in antigen recognition and presentation

• MHC-matching between transplant donor and recipient greatly reduces likelihood of rejection

• Class I antigens: constitutively expressed on surface of most cells

• Class II antigens: expressed on cells of lymphoid system

• HLA (human Leukocyte antigen) – class I: A, B, C;

– class II: DR, DQ, DP, DM, DO

2. Minor

Histocompatibility

Antigens

• small peptides that are found on the cell surface in association with class I or class II major histocompatibility complex (MHC) molecules.

• Single amino acid differences are detectable by T cells that can become immunoreactive and cause GVHD in the context of HLA-matched allogeneic stem cell transplantation.

• Although >100 minor histocompatibility antigens have been identified and sequenced, few data are available on the global role of minor histocompatibility antigen disparities on the development of GVHD.

• Can cause grafts rejection, but slow and weak

3. Blood Groups Antigens

• Human ABO blood group antigens are complex carbohydrate (polysaccharide) structures on surface of many cell types including graft cells & RBC

• Some tissue specific antigens: Skin＞kidney＞heart＞pancreas＞liver

– Antigen:

• H antigen: base chain; defines blood type O

• A trisaccharide on H chain: blood type A or A1

• B trisaccharide on H chain: blood type B

• A and B trisaccharides

– Transplantation of ABO-incompatible organs leads to hyperacute rejection

Mechanism of Allograft Rejection

1. Cell-mediated Immunity

2. Humoral Immunity

3. Role of NK cells

1. Cell-mediated Immunity

• Recipient's T cell-mediated cellular immune

response against alloantigens on grafts

• T cells of the recipient recognize the

allogenetic MHC molecules

• Many T cells can recognize allogenetic MHC

molecules

• Direct Recognition

• Indirect Recognition

Recognition of Alloantigens

• Direct Presentation

– Recognition of an intact MHC molecule displayed

by donor APC in the graft

– Basically, self MHC molecule recognizes the

structure of an intact allogeneic MHC molecule

• Indirect Presentation

– Donor MHC is processed and presented by

recipient APC

– Basically, donor MHC molecule is handled like

any other foreign antigen

Direct and Indirect Recognition

Figure 16-3

Difference between Direct Recognition and

Indirect Recognition

Direct

Recognition

Indirect

Recognition

Allogeneic MHC

molecule

Intact allogeneic

MHC molecule

Peptide of allogeneic

MHC molecule

APCs Recipient APCs are

not necessary

Recipient APCs

Activated T cells CD4＋T cells and/or

CD8＋T cells

CD4＋T cells and/or

CD8＋T cells

Roles in rejection Acute rejection Chronic rejection

Degree of rejection Vigorous Weak

Role of CD4＋T cells and CD8＋T cells

• CD4+ differentiate into cytokine producing

effector cells

– Damage graft by reactions similar to DTH

• CD8+ cells activated by direct pathway kill

nucleated cells in the graft

• CD8+ cells activated by the indirect pathway

are self MHC-restricted

• MΦ activation and recruitment

2. Humoral Immunity

• Important role in hyperacute rejection

(Preformed antibodies)

– Complements activation

– ADCC

– Opsonization

• Enhancing antibodies/Blocking antibodies

3. Role of NK cells

• Killer cells immunoglobulin

like receptors (KIR) can’t

recognize allogeneic MHC

on graft

• CKs secreted by activated

Th cells can promote NK

activation

The multiple role of NK cells in kidney transplantation. (A) Graft rejection. NK cells can promote ADCC against the graft by interacting with Donor specific antibodies (DSA) bound to the graft endothelial cells, thus driving antibody-mediated rejection (ADCC). OR through the secretion of pro-inflammatory molecules such as IFN-g. (B) Transplant tolerance. Activated NK cells can directly kill donor-derived dendritic cells, thus promoting transplant tolerance. (C) Immunosuppression. Immunosuppressive drugs might modulate the phenotype of NK cells that can retain their ability to respond to stimulation. Moreover, immunosuppression can reduce the number of NK cells after kidney transplantation. Monitoring NK cell numbers and functions in transplanted patients under specific immunosuppressive regiments is important to control and predict the onset of infections and neoplasia.

Classification of Allograft

Rejection

• Host versus graft reaction (HVGR):

Conventional organ transplantation

• Graft versus host reaction (GVHR)

– Bone marrow transplantation

– Immune cells transplantation

I. Host Versus Graft Reaction (HVGR)

– Hyperacute rejection

– Acute rejection

– Chronic rejection

1. Hyperacute Rejection

Occurrence time: Occurs within minutes to hours after host blood vessels are anastomosed to graft vessels

Pathology: Thrombotic occlusion of the graft vasculature, ischemia, denaturation, necrosis

Mechanisms

1. Preformed antibodies Antibody against ABO blood type antigen

Antibody against vascular endothelial cells (VEC) antigen

Antibody against HLA antigen

2. Complement activation: Endothelial cell damage

3. Platelet activation: thrombosis, vascular occlusion, ischemic damage

2. Acute Rejection

• Occurrence time: Occurs within days to 2 weeks after transplantation, 80-90% of cases occur within 1 month

• Pathology– Acute humoral rejection: Acute vasculitis

manifested mainly by endothelial cell damage

– Acute cellular rejection: Parenchymal cell necrosis along with infiltration of lymphocytes and MΦ

• Mechanisms

• Vasculitis – IgG antibodies against alloantigens on endothelial

cell

– CDC complement dependent cytotoxicity

• Parenchymal cell damage – Delayed hypersensitivity mediated by CD4+Th1

– Killing of graft cells by CD8+Tc

3. Chronic Rejection

• Occurrence time: Develops months or years after acute rejection reactions have subsided

• Pathology: Fibrosis and vascular abnormalities with loss of graft function

• Mechanisms

– Not clear

– Extension and results of cell necrosis in acute rejection

– Chronic inflammation mediated by CD4+T cell/MΦ

– Organ degeneration induced by non-immune factors

II.Graft Versus Host Reaction (GVHR)

Graft versus host reaction (GVHR) Allogenetic bone marrow transplantation

Rejection to host alloantigens

Mediated by immune competent cells in bone marrow

Graft versus host disease (GVHD)A disease caused by GVHR, which can damage the host

Conditions: Enough immune competent cells in grafts

Immunocompromised host

Histocompatability differences between host and graft

1. Acute GVHD

• 10-100 days post transplant

• Occur in 30-50% of allogenic transplant recipients

• Endothelial cell death in the skin, liver, and gastrointestinal

tract

• Rash (palms and soles),nausea, vomiting, diarrhea, loss of

appetite, jaundice, abdominal pain, wt loss, and

gastrointestinal hemorrhage

• Graded I-IV (skin, liver, gut). Poor prognosis IV

• Mediated by mature T cells in the grafts

2. Chronic GVHD• 90-600 days post transplant

• In addition to the symptoms seen in acute GVHD the pts might

have increased liver enzymes, dry eyes, bacterial infections, and

small airway blockages

• Fibrosis and atrophy of one or more of the organs

• Eventually complete dysfunction of the affected organ

• Graft loss HLA-DR (6 months), HLA-B (2 yrs), HLA-A (long

term).

Laboratory Studies

• ABO and Rh blood typing

• Crossmatching (Preformed antibodies)

• HLA typing – HLA-A and HLA-B

– HLA-DR

• Lymphocytotoxicity testdetermine whether a patient already has any specific HLA antibodies, a lab specialist will test a patient's blood (serum) against lymphocytes obtained from a panel of about 100 blood donors.

• Mixed leukocyte reaction

• Screening for Presence of Preformed Antibodies to allogeneic HLA

Identifying MHC Polymorphisms

(Tissue typing)• Tissue typing ensures that an organ from a donor

will be compatible with its recipient. The process

starts with identifying the unique human leukocyte

antigens (HLAs) for the organ donor and recipient,

either from blood or tissue.

• Formerly determined by antibodies

against MHC molecules

• Now by DNA testing: allele-specific

PCR, sequencing

Mixed Lymphocyte Reaction:

• Strong Proliferation--->High

incompatibility

• Weak proliferation--->Low

incompatibility

• No proliferation---> 100%

compatibility

• Helps to identify any antigenic

differences between donor and

recipient

DonorRecipient

(Irradiate) Cell Proliferation+

Immunosuppressive Therapy

• Cyclosporine(CsA), FK506: Inhibit nuclear

factor of activated T-cells (NFAT) transcription

factor

• Azathioprine, Cyclophosphamide: Block the

proliferation of lymphocytes

• Ab against T cell surface molecules: Anti-CD3

mAb----Deplete T cells

• Anti-inflammatory agents: Corticosteroids----

Block the synthesis and secretion of cytokines

Induction of Immune

Tolerance

• Inhibition of T cell activation

– Soluble MHC molecules

– CTLA4-Ig

– Anti-IL2R mAb

• Th1 cytokines

– Anti-TNF-α，Anti-IL-2，Anti-IFN-γ mAb

• Microchimerism

– The presence of a small number of cells of donor, genetically distinct from those of the host individual

Removal of T cells from Marrow Graft

• Methods to overcome GVHR:

– Treat bone marrow to deplete T cells.

– Use autologous bone marrow.

– Use umbilical cord blood.

Why is Fetus not Rejected?

• Placenta acts as a barrier or filter. It filters anti-MHC Abs.

• Trophoblast---outermost layer of fetal tissue---is in direct contact with maternal blood. Trophoblast expresses weak or no MHC.

• Progesterone---hormone---immunosuppressive.

• Uterine epithelium and trophoblast* secrete cytokines that suppresses TH1

• Spontaneous abortions are sometimes triggered by maternal immune response against fetus.