Transplant Immunobiology Douglas Stahura DO Grandview Hospital November 6, 2002

Goals Review definitions Review basic mechanisms of immunologic system Understand T-cell immunity Review transplant medications and mechanisms

Definitions Autograft – transplantation of one’s own tissue to another site Syngraft – transplantation of graft between two genetically identical individuals Allograft – transplantation of graft between individuals of the same species Xenograft – transplantation of graft between individuals of different species

Definitions Alloantigens – antigens recognized as foreign on allografts Alloreactive – lymphocytes that recognize and respond to alloantigens

Definitions Major Histocompatibility Complex (MHC) In humans, known as HLA system Central role in antigen recognition Principal immunologic function of MHC gene product is to present antigens as fragments of foreign proteins, forming complexes that can be recognized by T lymphocytes

MHC MHC molecules are membrane associated Antigen-specific T lymphocytes recognize fragments of antigens only when bound to the surface of other cells that bear MHC molecules Mature T lymphocytes recognize and react to foreign antigens and not to self Clonal deletion Self tolerance

MHC Located on short arm of chromosome 6 Six separate genes; Class I = HLA-A,B,C Class II = DR,DQ,DP Each gene is represented by one of two codominant alleles (paternal +maternal) Haplotype – alleles of HLA system passed to offspring via meiosis

MHC HLA Class I Found on all nucleated cells Function as immunorecognition site for endogenously synthesized foreign protein antigens Recognized by CD8+ T cells

Figure 3 (facing page). Antigen Processing. Panel A shows the principal pathways of generating petides for loading onto HLA class I molecules. Worn-out or defective proteins in the cytosol are degraded into e tides in proteasomes. Selected petides are then transported into the endoplasmic reticulum, where they are loaded onto newly synthesized class I molecules. The HLA– petide complexes are exported by way of the Golgi aparatus to the surface of the cell. In tissues infected with a virus, viral particles are taken up by cells and uncoated. The viral DNA or RNA enters the nucleus and replicates within it. The viral messenger RNA (mRNA) then enters the cytosol and is transcribed into proteins. Some of the proteins are subsequently degraded in proteasomes, and the petides are delivered into the endoplasmic reticulum, where they are loaded onto class I molecules for export to the surface of the cell. Panel B shows the processing of extracellular proteins. Self or foreign proteins are taken up by endocytosis (or phagocytosis) and sequestered into endosomes. Class II molecules synthesized in the endoplasmic reticulum are delivered by way of the Golgi aparatus into primary lysosomes, which fuse with the early endosomes to form the major-histocompatibility-com lex (MHC) class II com artment. Enzymes brought into this compartment by the lysosomes degrade the engulfed roteins into e tides. HLA- M molecules synthesized in the endoplasmic reticulum and delivered into the MHC class II compartment by trans ort vesicles hel load the petides onto the class II molecules. The HLA– petide com lexes are then exported to the surface of the cell.

MHC HLA Class II Found on APC (antigen presenting cells) B monocytes, macrophages, dendritic cells, mesangial cells, Kupffer’s cells, alveolar type 2 cells central role in the initiation of the immune response to transplantation antigens

T cell activation

T cell activation Allorecognition – recognition of TRANSPLANT antigens by T cells TCR – T cell receptor “recognizes” MHC/antigen complex CD3 complex initiates intracellular signalling

T cell activation CD3 complex – five peptide chain closely arrayed in cell membrane alongside TCR TCR binding induces conformational change in CD3 which initiates intracellular signal pathways

T cell activation Accessory molecules Stabilize the interaction between cytotoxic T cells and their target cell Provide a non-antigen “second” signal for T cell activation Adhesion molecules enhance antigen recognition by increasing affinity between T cell and MHC cell TCR recognition without accessory molecule co-stimulation results in clonal anergy or apoptosis

T cell Costimulation

Immunosuppressive Medications Drugs target the cellular mechanisms that are known Signal ONE Calcineurin inhibitors Cylosporine (Sandimmune, Neoral) Tacrolimus (FK506, Prograf) Monoclonal Antibodies OKT3 – targets CD3 molecule Polyclonal Antibodies (ALG) Thymoglobulin

Immunosuppressive Medications Signal Two – no currently approved Signal Three Sirolimus (Rapamycin) Monoclonal Ab that target IL-2 receptors Anti-metabolite Azathioprine (Imuran) nonspecific inhibitor of purine synthesis Mycophenolate mofetil (Cellcept) lymphocyte selective inhibitor of de novo purine synthesis

Immunosuppressive Medications Corticosteroids Blocks T-cell and APC cytokine and cytokine receptor expression (inhibits transcription) Blocks migration of lymphocytes to tissues (anti-inflammatory)