Defense against pathogens tumour immunology transplantology Martin Lika

Defense against pathogens, tumour immunology, transplantology Martin Liška

Extracellular microorganisms • Typically bacteria or parasites • For defense against extracellular microbes and their toxins, specific humoral immune response is important

Humoral immune response • Recognition of antigen by specific Ig, bound in cell membrane of naive B lymphocyte • The binding of antigen cross-links Ig receptors of specific B cells and then biochemical signal is delivered to the inside B cell; a breakdown product of the complement protein C 3 provides necessary „second signal“ • Clonal expansion of B cell and secretion of low levels of Ig. M

Humoral immune response • Protein antigens activate CD 4+ T helper cells after presentation of specific antigen • T helper cells exprime CD 40 L on their surface and secrete cytokines → proliferation and differentiation of antigen-specific B cells, isotype switching, affinity maturation

Phases of humoral immune response

Effector functions of antibodies • Neutralization of microbes (incl. viruses) and their toxins • Opsonization of microbes (binding to Fc receptors on phagocytes; at the same time, stimulation of microbicidal activities of phagocytes) • ADCC (Antibody-dependent cell-mediated cytotoxicity) – Ig. G opsonized microb is destroyed by NK cells after its binding to IC • Activation of the complement system (classical pathway)

Defense against extracellular pathogens (bacteria and unicellular parasites) a/ non-specific (innate) immune system - monocytes/macrophages, neutrophils, complement system, acute phase proteins (e. g. CRP) b/ specific (adaptive) immune system - antibodies (opsonization, neutralization)

Defense against multicellular parasites • Production of Ig. E → coating and opsonization of parasites • Activation of eosinophils - they recognize Fc regions of the bound Ig. E, then they are activated and release their granule contents (MBP, ECP, EPO), which kill the parasites • Th 2 -lymphocytes support this type of immune response

Intracellular microorganisms • Initially: non-specific immune response (ingestion by phagocytes) • Some microorganisms are able to survive inside phagocytes (e. g. some bacteria, fungi, unicellular parasites, viruses) – they survive inside phagosomes or enter the cytoplasm and multiply in this compartment • The elimination of these microorganisms is the main function of T cells (specific cell-mediated response)

Processing and presentation of antigen • Professional antigen-presenting cells: macrophages, dendritic cells, B lymphocytes (they express constitutionally class II MHC) a/ exogenous antigens – e. g. bacterial, parasitic - hydrolysed in endosomes to linear peptides → presentation on the cell surface together with class II MHC to CD 4+ T lymphocytes

Processing and presentation of antigen b/ endogenous antigens – e. g. autoantigens, foreign antigens from i. c. parasites or tumorous antigens - hydrolysed to peptides → associated with class I MHC → presentation on the cell surface to CD 8+ T lymphocytes

T cell-mediated immune response • Presentation of peptides to naive T lymphocytes in peripheral lymphoid organs → recognition of antigen by naive T lymphocytes • At the same time, T lymphocytes receive additional signals from microbe or from innate immune reactions → production of cytokines → clonal expansion → differentiation → effector & memory cells → effector cells die after elimination of infection

T cell-mediated immune response • TCR (T cell receptor) – T cell antigen-specific receptor - TCR recognizes (together with co-receptors - CD 4 or CD 8) the complex of antigen and MHC - a signal is delivered into the cell through molecules associated with TCR and co-receptors (CD 4 or CD 8) after antigen recognition

T cell-mediated immune response • APC exposed to microbes or to cytokines produced as part of innate immune reactions to microbes express costimulators that are recognized by receptors on T cells and delivered necessary „second signals“ for T cell activation • Activated macrophages kill ingested bacteria by reactive oxygen intermediates, NO and lysosomal enzymes

Function of Th 1 and Th 2 lymphocytes

Activation of T lymphocytes

Mechanisms of resistance of intracellular microbes to cell-mediated immune response • Inhibiting phagolysosome fusion • Escaping from the vesicles of phagocytes • Inhibiting the assembly of class I MHCpeptide complexes • Production of inhibitory cytokines • Production of decoy cytokine receptors

Defense against intracellular pathogens (bacteria and unicellular parasites) • Intracellular bacteria (Mycobacteria, Listeria monocytogenes), fungi (Cryptococcus neoformans), parasites (Plasmodium falciparum, Leishmania) • Specific immune response is necessary

Anti-viral defense • Viruses may bind to receptors on a wide variety of cells and are able to infect and replicate in the cytoplasm of these cells, which do not possess intrinsic mechanisms for destroying the viruses • Some viruses can integrate viral DNA into host genome and viral proteins are produced in the infected cells (e. g. Retroviruses)

Tumor immunology Tumor antigens • Mutant proteins (the products of carcinogen- or radiationinduced animal tumors) • Products of oncogenes or mutated tumor suppressor genes (Bcr/Abl fusion protein) • Overexpressed or aberrantly expressed self protein (AFP in hepatomas) • Oncogenic virus products (HPV products in cervical CA)

Tumor immunology Mechanisms of defense • The principle is formation of cytotoxic T lymphocytes clone (CTL) specific for tumor antigens • The cooperation of naive CD 8+ T cells and APC (costimulation) and T-helper cells of the same antigenic specifity (cytokines) is required • APC enables formation of antigen-specific CD 8+ T cells and CD 8+ T cells = cross presentation

Tumor immunology • Ig, activated macrophages and NK-cells also participate in anti-tumor-defense • Immunotherapy of tumors – aims to enhance anti-tumor immunity passively (by providing immune effectors) or actively (vaccination with tumor antigens or with tumor cells engineered to express costimulators and cytokines)

Tumor immunology How tumors evade immune responses: a/ lack of T cell recognition of tumor - generation of antigen-loss variant of tumor cells - mutations in MHC genes or genes needed for antigen-processing b/ inhibition of T cell activation - production of immuno-suppressive proteins

Transplantation immunity – alloimmune reaction • Recipient’s T cells recognize donor’s allogeneic HLA molecules that resemble foreign peptideloaded self HLA molecules • Graft antigens are recognized: a/ Directly – donor’s HLA molecules on graft APC bind peptide fragments of allogeneic cellular proteins (different from that ones that recipient’s APC bind)→ they are recognized by recipient’s T cells as foreign b/ Indirectly – graft antigens are presented by recipient’s APC to recipient’s T cells

Transplantation immunity alloimmune reaction Antibodies against alloantigens • They can react with HLA molecules or with another surface polymorphic antigens • Especially the complement binding antibodies have harmful effect (cytotoxic) • Possible presence of preformed antibodies (e. g. after blood transfusion, repeated pregnancy)

Immunologically privileged tissue • In allogeneic transplantation, some tissues are rejected less frequently (e. g. CNS, cornea, gonades). • The mechanisms of protection from the immune system: separation from the immune system (haematoencephalic barrier); the preference of Th 2 - and suppression of Th 1 -reactions; active protection from effector T cells • The privileged status is not absolute (see MS)

Types of transplantation • Autologous – within the same individual (e. g. a skin graft from an individual’s thigh to his chest); that is, they are not foreign • Syngeneic – in genetically identical individuals (e. g. identical twins); that is, they are not foreign • Allogeneic (alloantigens) – in genetically dissimilar members of the same species (e. g. a kidney transplant from mother to daughter); it is foreign • Xenogeneic (heterogeneic) – in different species (e. g. a transplant of monkey kidneys to human); it is foreign

Immunological examination before transplantation • HLA is the most important • MHC genes are highly polymorphic = there is a great number of gene variants (alleles) in the population • MHC haplotype = a unique combination of alleles (at multiple loci) encoding HLA molecules, that are transmitted together on the same chromosome

HLA typing 1/ Sera typing – identification of specific class I and class II HLA molecules using sera typing - less time-consuming method, however, also less accurate 2/ DNA typing – human DNA testing by PCR - low resolution (groups of alleles), high resolution (single alleles) - more time-consuming method, however, also highly accurate

Transplantation immunity - tests • Mixed lymphocyte reaction = T cells from one individual are cultured with leukocytes of another individual → the magnitude of this response is proportional to the extent of the MHC differences between these individuals • Cross match = preformed antibodies detection test (donor’s serum is mixed with recipient’s lymphocytes in the presence of complement proteins → if preformed cytolytic antibodies are present in serum then the lysis of donor’s leukocytes occurs)

Rejection = rejection of the graft by recipient’s immune system, which considers it as non-self • Hyperacute rejection (minutes) – mediated by preformed antibodies (natural or generated after previous immunization) → complement fixation → endothelial injury; activation of haemocoagulation → thrombosis of graft vessels → accumulation neutrophils → amplification of inflammatory reaction • Acute rejection (days or weeks) – mediated by T cells (→ graft cells and endothelial injury) and by antibodies (thes bind to endothelium) • Chronic rejection (months or years) – mediated by alloantigen-specific T cells → cytokines, stimulating growth of vascular endothelial and smooth muscle cells and tissue fibroblasts

Graft versus Host reaction (Gv. H) = donor’s T cells, present in graft recognize recipient’s tissue antigens as non-self and, therefore, they react to them 1/ Acute Gv. H – days or weeks after transplantation → liver, skin and intestinal injury 2/ Chronic Gv. H – moths or years after transplantation → chronic vascular, skin, organs or glands inflammation → replacement of functional by fibrous tissue, disorder of graft’s blood circulation → loss of tissue function

Therapeutic approaches of Gv. H prevention and treatment • The selection of an appropriate donor • Immunosupression – the development of coexistence is possible later • Donor’s T cells replacement from the graft

Maternal-foetal tolerance • Th 2 -type responses in mother • Protective effect of hormones – h. CG, estrogens • Specific placental (Bohn’s) proteins – immunosuppressive effect • Blocking antibodies • Sialomucinous membrane – between mother and fetus

Rh incompatibility Mother Rh-/Foetus Rh+: delivery → senzitization (anti-D antibodies); next pregnancy (Rh incompatibility) → anti-D Ig. G pass through the placenta into foetal circulation → destruction of foetal erythrocytes → hemolytic anemia Prevention: administration of anti-D to Rhmother after delivery of Rh+ child