Defense against pathogens, tumour immunology, transplantology

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Defense against pathogens, tumour immunology,
transplantology

• Martin Liška

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Extracellular microorganisms

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

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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 C3 provides
  necessary second signal
• Clonal expansion of B cell and secretion of low
  levels of IgM

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Humoral immune response

• Protein antigens activate CD4 T helper cells
  after presentation of specific antigen
• T helper cells exprime CD40L on their surface and
  secrete cytokines ? proliferation and
  differentiation of antigen-specific B cells,
  isotype switching, affinity maturation

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Phases of humoral immune response
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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) IgG opsonized microb is destroyed
  by NK cells after its binding to IC
• Activation of the complement system (classical
  pathway)

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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)

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Defense against multicellular parasites

• Production of IgE ? coating and opsonization of
  parasites
• Activation of eosinophils - they recognize Fc
  regions of the bound IgE, then they are activated
  and release their granule contents (MBP,ECP,EPO),
  which kill the parasites
• Th2-lymphocytes support this type of immune
  response

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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)

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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 CD4 T lymphocytes

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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 CD8 T
  lymphocytes

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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

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T cell-mediated immune response

• TCR (T cell receptor) T cell antigen-specific
  receptor
• - TCR recognizes (together with co-receptors -
  CD4 or CD8) the complex of antigen and MHC
• a signal is delivered into the cell through
  molecules associated with TCR and co-receptors
  (CD4 or CD8) after antigen recognition

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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

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Function of Th1and Th2 lymphocytes
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Activation of T lymphocytes
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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 MHC-peptide
  complexes
• Production of inhibitory cytokines
• Production of decoy cytokine receptors

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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

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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)

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Anti-viral defense

• a/ non-specific (innate) immune system
• - monocytes/macrophages, NK cells, interferons
  (IFN-g ? activation of macrophages, IFN-a ?
  antiviral activity, activation of NK cells)
• b/ specific (adaptive) immune system
• - T cells, antibodies (e.g. neutralization of
  viruses)

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Tumor immunology

• Tumor antigens
• Mutant proteins (the products of carcinogen- or
  radiation-induced 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)

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Tumor immunology

• Mechanisms of defense
• The principle is formation of cytotoxic T
  lymphocytes clone (CTL) specific for tumor
  antigens
• The cooperation of naive CD8 T cells and APC
  (co-stimulation) and T-helper cells of the same
  antigenic specifity (cytokines) is required
• APC enables formation of antigen-specific CD8 T
  cells and CD8 T cells cross presentation

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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 co-stimulators and cytokines)

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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

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Transplantation immunity alloimmune reaction

• Recipients T cells recognize donors allogeneic
  HLA molecules that resemble foreign
  peptide-loaded self HLA molecules
• Graft antigens are recognized
• a/ Directly donors HLA molecules on graft APC
  bind peptide fragments of allogeneic cellular
  proteins (different from that ones that
  recipients APC bind)? they are recognized by
  recipients T cells as foreign
• b/ Indirectly graft antigens are presented by
  recipients APC to recipients T cells

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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)

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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
  Th2- and suppression of Th1-reactions active
  protection from effector T cells
• The privileged status is not absolute (see MS)

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Types of transplantation

• Autologous within the same individual (e.g. a
  skin graft from an individuals 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

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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

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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

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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
  (donors serum is mixed with recipients
  lymphocytes in the presence of complement
  proteins ? if preformed cytolytic antibodies are
  present in serum then the lysis of donors
  leukocytes occurs)

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Rejection

• rejection of the graft by recipients 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

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Graft versus Host reaction (GvH)

• donors T cells, present in graft recognize
  recipients tissue antigens as non-self and,
  therefore, they react to them
• 1/ Acute GvH days or weeks after
  transplantation ? liver, skin and intestinal
  injury
• 2/ Chronic GvH moths or years after
  transplantation ? chronic vascular, skin, organs
  or glands inflammation ? replacement of
  functional by fibrous tissue, disorder of grafts
  blood circulation ? loss of tissue function

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Therapeutic approaches of GvH prevention and
treatment

• The selection of an appropriate donor
• Immunosupression the development of coexistence
  is possible later
• Donors T cells replacement from the graft

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Maternal-foetal tolerance

• Th2-type responses in mother
• Protective effect of hormones hCG, estrogens
• Specific placental (Bohns) proteins
  immunosuppressive effect
• Blocking antibodies
• Sialomucinous membrane between mother and fetus

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Rh incompatibility

• Mother Rh-/Foetus Rh delivery ?
• senzitization (anti-D antibodies) next
• pregnancy (Rh incompatibility) ? anti-D IgG
• pass through the placenta into foetal
• circulation ? destruction of foetal
• erythrocytes ? hemolytic anemia
• Prevention administration of anti-D to Rh-
• mother after delivery of Rh child