Transplantation is the process of taking cells, tissues, or organs from one individual and transferring them to the same or different individual
Blood transfusions are a common form of transplantation practiced in veterinary medicine
Allograft Rejection
Host vs. graft disease
Autologous and syngeneic grafts are accepted by the Recipient’s (host) immune system because the graft (Donor) antigens are identical to recipient antigens
Graft rejection occurs when graft antigens are different than Recipient antigens and, thus, recognized as foreign
The genetics of graft rejection (an experiment)
Autologous and syngeneic grafts are readily accepted
Allografts between mouse strains are rejected in 7-10 days
Graft from an inbred parental strain to the F1 hybrid progeny is accepted
Graft from the F1 progeny to the inbred parental strain is readily rejected
If Recipient has never been exposed to Donor antigens before, allograft rejection typically requires 7 – 10 days
Once sensitized to Donor antigens by the first graft, the Recipient will reject a second (and all subsequent grafts from same Donor) in about 3 days – second set rejection
This reaction is due to rapid response by memory lymphocytes
If lymphocytes are isolated from a Recipient strain B mouse that has already rejected a graft from a strain A mouse, adoptive transfer of those cells to a naïve strain B mouse will cause that mouse to reject a graft from strain A in about 3 days
Which Donor antigens (alloantigens) do Recipient lymphocytes recognize and how are these alloreactive Recipient lymphocytes activated?
Classes of alloantigens
Major histocompatibility (MHC) antigens
Donor MHC molecules in the graft play a role completely unrelated to antigen presentation
Most powerful alloantigens involved in activation of Recipient lymphocytes
MHC molecules were initially identified this way
Minor histocompatibility antigens
Direct presentation
Donor APCs from the graft traffic to regional lymph nodes and present antigen to Recipient T cells
Donor MHC molecules are intact and interact directly with Recipient TCRs
Up to 2% of Recipient T cells express TCRs that interact directly with antigenic determinants formed by either Donor MHC molecules or Donor MHC – peptide complexes
Even when the antigen determinants are formed only by the Donor MHC molecules, MHC-bound peptide is required for stable expression of the Donor MHC molecules
TCR : MHC interaction provides the first signal of Recipient T cell activation
The second signal for Recipient T cell activation is provided by costimulatory molecules (B7-1) expressed on the surface of Donor APCs
Direct presentation evokes the most powerful T cell response
During normal antigen presentation (i.e. viral infection) only a small fraction (~1%) of MHC molecules on an APC are loaded with viral peptide while the rest are loaded with self antigens
Thus, only a small number of host lymphocytes are specific for the MHC – viral peptide complex
Takes 7-10 days for this small population of lymphocytes to clear the infection
In contrast, up to 2% of Recipient T lymphocytes can directly interact with the thousands of copies of Donor MHC molecules present on graft APCs
The combination of more Recipient lymphocytes capable of responding to Donor MHC molecules and the abundance of MHC molecules on the surface of Donor APCs yields an extremely powerful immune response
Indirect presentation
Recipient APCs travel to graft, collect Donor antigens, and travel back to regional lymph node for T cell presentation
Donor antigen (either Major or Minor histocompatibility molecules) are processed by Recipient dendritic cells and presented to Recipient T cells
Only a small fraction (~1%) of Recipient MHC molecules on a given APC are loaded with Donor antigens
Only a small number of Donor antigen-specific lymphocytes (~.0001?) can interact with this MHC – Donor peptide complex
Thus, it takes 7-10 days for this small population of lymphocytes to expand and attack the graft
Mixed Lymphocyte Response (MLR)
Excellent way to demonstrate the response of alloreactive Recipient T cells to Donor MHC molecules
Peripheral blood mononuclear cells (PBMCs) are collected from unrelated (i.e. have different MHC molecules) Donors X and Y
Donor Y PBMCs are rendered unable to proliferate via sub lethal radiation or incubation with mitomycin C
Donor X PBMCs are added to inactivated Donor Y PBMCs in tissue culture
Up to 2% of Donor X CD4+ and CD8+ cells are capable of recognizing MHC molecules on Donor Y APCs
The robustness of the lymphocyte proliferation response is proportional to the degree of MHC molecule difference between Donors X and Y
This crude test can also be used to determine if two individuals have exactly the same MHC molecules (i.e. PBMCs collected from monozygotic twins will not proliferate in an MLR)
Effector Mechanisms of Graft Rejection
Graft rejection is classified on the basis of how long it takes for the graft to be rejected
Graft blood vessels (which are surgically attached to the Recipient circulatory system) are the first structures to be exposed to the Recipient immune system and are, thus, the initial location of graft rejection
Hyperacute rejection
Characterized by thrombotic occlusion of graft vasculature within minutes to hours after Recipient blood vessels are linked to the graft blood vessels
Antibodies in the Recipient that recognize Donor antigens are alloantibodies
Before tissue cross-matching became routine, graft rejection was mediated by preexisting “natural antibodies” of the IgM class that bound to graft vascular antigens. These antibodies arose in response to carbohydrate antigens expressed by gut microflora and recognized foreign blood groups. However, hyperacute rejection die to blood group incompatibility has largely been eliminated by ensuring the Donor and Recipient have the same blood type
Today, hyperacute rejection is mediated by IgG antibodies specific to donor antigen from previous exposure (i.e. previous transplantation, blood transfusion, multiple pregnancies)
Mechanism of action
Recipient alloantibodies bind alloantigens expressed on endothelium of Donor vasculature
Activation of the clotting cascade, platelet accumulation (thrombotic occlusion)
Recruitment of neutrophils to Donor vasculature
How to prevent hyperacute rejection?
Check for pre-existing anti-donor antibodies via crossmatching
Major crossmatch – Donor erythrocytes incubated with recipient serum
Minor crossmatch – Donor serum, recipient erythrocytes
Acute rejection
Characyerized by vascular and parenchymal injury mediated by T cells and antibodies that usually begins a week after transplantation
Mechanism of action
Recipient alloreactive T cells and/or antibodies develop aftertransplantation
Alloreactive T cells recognize Donor alloantigens (particularly MHC molecules) expressed by the endothelium of the graft blood vessels and parenchymal cells
Alloreactive CD8+ cells can directly kill the graft cells
Alloreactive CD4+ and CD8+ cells can become activated and produce cytokines
Microscopically, this appears as non-suppurative endothelitis (inflammation of the blood vessel wall mediated by mononuclear cells) and is a hallmark of graft rejection
Alloantibodies bind to graft endothelium, activate complement, resulting in transmural vascular necrosis (i.e. entire vessel wall thickness) and neutrophil recruitment
Chronic rejection
Because the use of immunosuppressive drugs and tissue-typing methods to obtain optimum match of Donor and Recipient tissue increases survival of allografts, chronic rejection has become the most common forms of graft rejection
Characterized by arterial occlusion as a result of intimal smooth muscle proliferation and/or fibrosis that occurs over time, months to years after transplantation
Mechanism of action
Recipient alloreactive T lymphocytes develop after transplantation
T cells can induce monocytes or macrophages to secrete smooth muscle growth factors
Fibrosis
Alloreactive T cells can infiltrate the graft and promote fibroblast proliferation
May occur in response to transmural vascular necrosis following acute rejection
Prevention and Treatment of Allograft Rejection
Match Donor and Recipient MHC molecules as closely as possible
Because MHC molecules are the most powerful alloantigens associated with graft rejection, precise matching of Donor and Recipient MHC molecules reduces the incidence and severity of graft rejection
Inhibit T cell activation by blocking co-stimulation
CTLA4-Ig – binds B7-1 on APCs and prevents T cell co-stimulation via CD28
Immunosuppressive drugs
Drugs that Inhibit T lymphocyte proliferation
Cyclosporine
FK506
RapamycinAnti-CD25
Azathioprine
Mycophenolate mofetil
Drugs that kill T lymphocytes
Anti-CD3 (OKT3)
Immunosuppressive drugs that cause non-specific immunosuppression
Corticosteroids – inhibit secretion of cytokines by many cell types
Bone Marrow Transplantation
Reasons for bone marrow transplantation
Treat hematologic malignancies
Treat solid tumors (experimental)
Correct inherited deficiencies of enzymes or proteins
Recipient is “prepared” to receive graft via high dose radiation and chemotherapy
Kills Recipient bone marrow and leukocytes populating lymphoid organs
Prevents Recipient from rejecting bone marrow graft
Even a few residual Recipient leukocytes (particularly NK cells) can promote rejection
Provides “space” in the bone marrow and lymphoid organs for the grafted cells to grow
Harvest Donor transplant cells
Whole bone marrow was traditionally used, though this led to a high incidence of graft vs. host disease because marrow contained large populations of mature, alloreactive lymphocytes
Now, purified CD34+ bone marrow stem cells are used
Monoclonal antibodies are used in this process to deplete Donor marrow of mature alloreactive lymphocytes, decreasing the incidence of graft vs. host disease
New technique using umbilical cord CD34+ stem cells
Cells are less differentiated and, thus, less likely to contain alloreactive lymphocytes
New technique of treating Donor with bone marrow-stimulating cytokines (i.e. GMCSF) to stimulate CD34+ stem cell production and release into peripheral blood
Peripheral blood is collected, then sorted for CD34+ cells (avoid general anesthesia, less painful procedure)
CD34+ stem cells differentiate into mature RBCs, granulocytes, megakaryocytes, NK, T, and B cells
Treat Recipient with immunosuppressive drugs to prevent rejection
Recipient is treated for an extended period of time and gradually weaned off
Danger of long-term immunosuppression is susceptibility to infection
Support Recipient until grafted marrow starts repopulating blood with new RBCs, WBCs, and platelets
Occurs when alloreactive Donor T cells and NK cells of a graft attack Recipient tissues
In both forms, CD8+ T cells and NK cells may be responsible for the observed epithelial injury
Acuteform
Results in major organ necrosis and dysfunction (liver, kidneys, GI tract, skin, lungs)
Chronicform
Results in organ atrophy and/or fibrosis
Methods to minimize graft versus host disease
Carefully match Donor and Recipient at all MHC loci
Deplete Donor marrow of mature lymphocytes and NK cells
Treat Recipient with immunosuppressive drugs
Beneficial effects of graft versus host disease
In leukemia patients, a low level of graft versus host disease is shown to be beneficial because grafted T lymphocytes kill any residual leukemia cells in the Recipient
