# Lecture 28: Transplantation Immunology
## Immunology
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Define the types of grafts (autograft, isograft, allograft, xenograft) and the immunological basis of graft rejection
2. Explain the direct and indirect pathways of allorecognition
3. Describe the mechanisms and classification of graft rejection (hyperacute, acute, chronic)
4. Explain the principles of HLA matching and crossmatching in transplantation
5. Describe immunosuppressive strategies to prevent rejection and the concept of graft-versus-host disease (GVHD)
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## Lecture Content
### I. Overview of Transplantation
**Transplantation** is the transfer of cells, tissues, or organs from a donor to a recipient. Grafts are classified according to the genetic relationship between donor and recipient. An **autograft** involves the same individual as both donor and recipient, as in a skin graft or saphenous vein used for coronary artery bypass grafting, and does not provoke rejection. An **isograft (syngeneic graft)** is exchanged between genetically identical individuals such as identical twins, and similarly avoids rejection. An **allograft** is transferred between genetically different individuals of the same species and is subject to rejection. A **xenograft** crosses species boundaries, as when a pig valve is placed in a human, and provokes vigorous rejection.
Most clinical transplantation involves allografts of kidney, liver, heart, lung, pancreas, cornea, or bone marrow/hematopoietic stem cells. The major barrier to successful transplantation is the **immune response against alloantigens**, principally foreign MHC molecules and minor histocompatibility antigens.
### II. Alloantigens
The **major histocompatibility complex (MHC) antigens** are the primary targets of allorejection. **MHC class I** molecules (HLA-A, HLA-B, HLA-C) are expressed on all nucleated cells and serve as targets for CD8+ CTLs. **MHC class II** molecules (HLA-DR, HLA-DQ, HLA-DP) are expressed on APCs, endothelium, and activated cells, and serve as targets for CD4+ T cells. MHC molecules are extremely polymorphic, making it highly probable that any donor-recipient pair will differ at multiple loci. A remarkable feature of transplant immunology is that **1 to 10 percent of the T cell repertoire** recognizes allogeneic MHC, an extraordinarily high frequency compared to the roughly 1 in 100,000 T cells specific for any given microbial antigen. This high frequency exists because T cells selected on self-MHC can cross-react with foreign MHC molecules, which structurally resemble self-MHC presenting a foreign peptide.
**Minor histocompatibility antigens (mHAs)** are polymorphic self-peptides presented on MHC molecules that differ between individuals. Even HLA-identical siblings differ at these antigens, which can cause slower but still clinically significant rejection. H-Y antigens, encoded on the Y chromosome, place male-to-female transplants at increased risk.
### III. Allorecognition Pathways
The immune system recognizes alloantigens through three distinct pathways. In **direct allorecognition**, recipient T cells directly recognize intact donor MHC molecules on donor APCs, known as passenger leukocytes, within the graft. CD4+ T cells recognize donor MHC class II and CD8+ T cells recognize donor MHC class I. This pathway generates a very strong response due to the high precursor frequency of alloreactive T cells, and it dominates **acute rejection** early after transplant. However, it decreases over time as donor APCs are depleted from the graft.
In **indirect allorecognition**, recipient APCs phagocytose shed donor MHC molecules and other alloantigens, process them, and present donor-derived peptides on recipient MHC class II to CD4+ T cells. This follows the same pathway as conventional antigen presentation. Because donor antigens are always present in the graft, indirect recognition persists indefinitely and dominates **chronic rejection**. It is also important for generating alloantibodies through T-dependent B cell responses.
The **semi-direct pathway** involves recipient DCs acquiring intact donor MHC molecules through cell-cell contact or exosomes, a process called cross-dressing. These recipient DCs then present intact donor MHC to recipient T cells, similar to direct recognition but occurring on recipient APCs. This pathway may contribute to both acute and chronic rejection.
A diagram comparing the direct and indirect pathways of allorecognition. Left panel (Direct pathway): A donor dendritic cell (expressing donor MHC class I and class II loaded with donor peptides) migrates from the graft to the recipient's draining lymph node. A recipient CD4+ T cell directly recognizes intact donor MHC class II on the donor DC via its TCR. A recipient CD8+ T cell directly recognizes intact donor MHC class I. Both are activated, proliferate, and migrate to the graft to cause damage. Right panel (Indirect pathway): Donor cells or shed donor MHC molecules from the graft are taken up by a recipient dendritic cell. The recipient DC processes donor MHC proteins into peptide fragments and presents donor-derived peptides on recipient MHC class II to recipient CD4+ T cells. These CD4+ T cells provide help for B cells to produce anti-donor antibodies (alloantibodies) and help for CD8+ T cells. Center annotation: Direct pathway dominates early (acute rejection); indirect pathway persists long-term (chronic rejection). A small inset shows the semi-direct pathway: a recipient DC with acquired intact donor MHC molecules on its surface.
### IV. Types of Graft Rejection
**Hyperacute rejection** occurs within minutes to hours after transplantation. The mechanism involves pre-formed antibodies in the recipient that bind donor endothelial antigens, activating complement and causing thrombosis, graft ischemia, and necrosis. These pre-formed antibodies arise from prior blood transfusions, pregnancies, or previous transplants (anti-HLA antibodies), or from natural antibodies (anti-ABO blood group). Prevention relies on ABO blood type matching and pre-transplant **crossmatch testing**, in which recipient serum is mixed with donor lymphocytes. If the recipient has anti-donor antibodies, the crossmatch is positive and the transplant is contraindicated. Thanks to crossmatching, hyperacute rejection has been essentially eliminated in modern transplantation. In **xenotransplantation**, natural anti-Gal antibodies directed against alpha-galactose on pig cells cause hyperacute rejection, leading to the development of genetically modified pigs lacking alpha-1,3-galactosyltransferase.
**Acute rejection** occurs days to months after transplantation, most commonly in the first 6 months, and has two components. **Acute cellular rejection** is T cell-mediated, with CD8+ CTLs infiltrating and killing graft cells while CD4+ Th1 cells activate macrophages, producing graft inflammation and parenchymal damage. **Acute antibody-mediated rejection (AMR)** involves de novo donor-specific antibodies (DSA) that activate complement on graft endothelium, causing vasculitis. C4d deposition on biopsy is the diagnostic hallmark. Treatment for acute rejection includes high-dose corticosteroids as first-line therapy, anti-thymocyte globulin (ATG) for steroid-resistant cases, and plasmapheresis/IVIG for antibody-mediated rejection.
**Chronic rejection** develops over months to years and is mediated predominantly by indirect allorecognition driving CD4+ T cell help for alloantibody production, along with chronic endothelial injury and T cell-mediated vascular damage. The pathology shows intimal fibrosis and smooth muscle proliferation in graft blood vessels (transplant vasculopathy or arteriopathy), along with interstitial fibrosis and tubular atrophy in kidney grafts. Progressive decline in graft function leads to eventual graft loss. Chronic rejection is the **leading cause of late graft failure**, and unfortunately there is no effective treatment once established, making prevention the key strategy.
### V. HLA Matching and Transplant Immunology Workup
**HLA typing** is performed using molecular methods (PCR-based) to determine HLA-A, HLA-B, and HLA-DR alleles as a minimum for kidney transplant, with more loci typed for HSCT. The degree of matching significantly affects graft survival, with a 6/6 match (2 alleles each for HLA-A, -B, -DR) providing the best outcomes. The **panel reactive antibody (PRA)** measures the percentage of donor HLA antigens against which the recipient has antibodies; a high PRA indicates a highly sensitized patient for whom finding a compatible donor is difficult. The **crossmatch** is the final test before transplant, mixing recipient serum with donor lymphocytes. A positive crossmatch indicating antibody-mediated cell killing represents an absolute contraindication to proceeding.
The importance of HLA matching varies by organ. It is most critical for **kidney** transplantation, where it strongly impacts survival, and for **HSCT**, where a 10/10 allele match is preferred. The liver is relatively resistant to antibody-mediated rejection, so matching has less impact. For heart and lung transplantation, time constraints limit the degree of matching possible.
### VI. Immunosuppressive Therapy
Immunosuppressive therapy is organized into induction and maintenance phases. **Induction therapy** provides intense immunosuppression at the time of transplant and includes anti-IL-2Ralpha (basiliximab) to block T cell proliferation, anti-thymocyte globulin (ATG) to deplete T cells, and alemtuzumab (anti-CD52) for broad lymphocyte depletion.
**Maintenance therapy** typically involves lifelong triple therapy. **Calcineurin inhibitors**, including tacrolimus (FK506) and cyclosporine, block calcineurin and prevent NFAT activation, thereby inhibiting IL-2 transcription and T cell activation. They form the backbone of immunosuppression. **Antiproliferatives** such as mycophenolate mofetil (which inhibits IMPDH to block purine synthesis and lymphocyte proliferation) and azathioprine prevent lymphocyte expansion. **Corticosteroids** provide broad anti-inflammatory effects by inhibiting NF-kappaB and reducing cytokine production. **mTOR inhibitors** (sirolimus/rapamycin and everolimus) block IL-2 signaling downstream to inhibit T cell proliferation and additionally promote Tregs.
The **side effects of immunosuppression** are significant. Patients face increased infection risk from opportunistic organisms including CMV, BK virus, Pneumocystis, and fungi. There is increased cancer risk, particularly skin cancer and lymphoma/post-transplant lymphoproliferative disorder (PTLD). Drug-specific toxicities include nephrotoxicity from calcineurin inhibitors, diabetes, hyperlipidemia, and bone marrow suppression.
### VII. Graft-Versus-Host Disease (GVHD)
Graft-versus-host disease occurs after **allogeneic hematopoietic stem cell transplantation** (and rarely after blood transfusion in immunocompromised patients). The mechanism involves donor T cells in the graft recognizing recipient alloantigens and attacking recipient tissues. The **three requirements** described by Billingham's criteria are: the graft must contain immunocompetent cells (T cells), the recipient must express antigens not present in the donor, and the recipient must be unable to reject the donor cells (immunocompromised).
**Acute GVHD** occurs within 100 days of transplant and is T cell-mediated, targeting the skin (producing rash), liver (causing jaundice), and GI tract (causing diarrhea). **Chronic GVHD** develops after 100 days and resembles autoimmune disease, with scleroderma-like skin changes, sicca syndrome, bronchiolitis obliterans, and hepatitis.
An important therapeutic consideration is the **graft-versus-leukemia (GVL) effect**, in which donor T cells also attack residual leukemia cells, providing a therapeutic benefit. Balancing GVHD prevention with preservation of the GVL effect remains a key challenge in allogeneic HSCT.
A timeline diagram illustrating the three types of graft rejection after solid organ transplantation. A transplanted kidney is shown at time 0. Hyperacute rejection (minutes to hours): the graft immediately turns dark/necrotic. Histology inset shows thrombosis in graft vessels, neutrophil margination, and complement deposition. Mechanism labeled: pre-formed anti-donor antibodies + complement. Acute rejection (days to months): the graft becomes swollen and tender. Histology inset shows dense T cell infiltrate in the renal tubules and interstitium (cellular rejection) and C4d staining in peritubular capillaries (antibody-mediated rejection). Mechanism labeled: T cell-mediated and/or de novo donor-specific antibodies. Chronic rejection (months to years): the graft progressively fibroses and atrophies. Histology inset shows intimal fibrosis and smooth muscle proliferation in a graft artery (transplant vasculopathy), interstitial fibrosis, and tubular atrophy. Mechanism labeled: indirect allorecognition, alloantibodies, chronic inflammation. A graph below shows graft function over time, with acute rejection episodes (treatable, function recovers) and gradual chronic decline.
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