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 C3 provides necessary „second signal“

• Clonal expansion of B cell and secretion of low levels of IgM

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

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) – IgG 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 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

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 CD4+ 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 CD8+ 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 - 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

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 Th1and Th2 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 MHC-peptide 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 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)

Tumor immunologyMechanisms 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

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)

Tumor immunologyHow 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 peptide-loaded 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 Th2- and suppression of Th1-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 (GvH)

= donor’s T cells, present in graft recognize recipient’s 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 graft’s blood circulation → loss of tissue function

Therapeutic approaches of GvH 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

• Th2-type responses in mother• Protective effect of hormones – hCG,

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 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