EFFECTOR FUNCTIONS

• There are various immune system effector functions:

– helper T cells (TH1 and TH2)

– regulatory T cells (Treg)

– cytokines

– cytotoxic T cells (CTL)

– NK cells and other innate immune effector functions

– antibody-mediated effector functions

Helper T cell activity – TH1 and TH2 responses

• After mature CD4 T cells are activated by contact with antigen and APC, they are induced to proliferate and differentiate into effector cells – cells that can exert helper activity.

• Most of the effector function of CD4 T cells is mediated by the secretion of cytokines, and by the expression of various cell surface stimulatory molecules.

 • Activated CD4 T cells can differentiate down two pathways,

becoming either type 2 (TH2) helper cells, helper cells for humoral (antibody) responses, or type 1 (TH1) helper cells, which are helper cells for cell-mediated responses:

TH1 cells induce cellular and inflammatory responses, activating macrophages, NK cells, cytotoxic CD8 T cells, and other effector cells

Antibody production is enhanced in TH2 responses:

Initially, mature naïve CD4 T cells are not committed to being either TH1 or TH2 cells

After activation, these cells proliferate and differentiate into TH1 or TH2 cells

• Factors that are thought to be important in TH1/TH2 differentiation include:– the type of infectious agent and genetics– together, these result in the pattern of cytokines

produced by APC and by other cells:

• IL-12, IFN: TH1 cells; IL-4 and IL-6: TH2 cells

Therefore, the nature of the initial innate immune response determines whether TH1 or TH2 cells are stimulated

• TH1 and TH2 cells are defined by the pattern of cytokines that they produce - there is no unambiguous phenotypic profile for TH1 or TH2 cells, based on the expression of cell surface markers:

– TH1 cells produce various cytokines that are macrophage-activating (IFN, TNF) and/or enhance cellular responses (IL-2).

 – TH2 cells produce various cytokines that are B

cell-activating factors (IL-4, IL-5, IL-6, IL-10).

TH1 cytokines (IFN, TNF) are potent macrophage-activating factors.

TH2 cytokines (IL4, IL5, IL10) are potent B cell-stimulatory cytokines.

Therefore, TH1 and TH2 cells not only have stimulate different types of immune responses, they also enhance the activation of different sorts of APC:

Cytokines produced by TH1 or TH2 cells not only exert the effector functions associated with these helper T cells, they also can act on the opposite type of helper T cell:cross-regulation

• Both TH1 and TH2 responses play important roles in defense against pathogens.

• Also, all immune responses contain TH1 and TH2 components

 • However, there can be pathological consequences

that result from a TH1 or a TH2-dominated response.

• In leprosy, a TH2-dominant response is associated with disease progression, and a TH1 response is associated with a better disease outcome.

Regulatory T cells - Treg

• Regulatory T cells:

– CD4-positive

– produce cytokines that are inhibitory for other T cells, such as IL-10 or TGF

Cytokines

• CD4 effector functions are mediated by the production and secretion of cytokines.

• Different types of CD4 cells produce different cytokines, and these cytokines have different effects on their target cells.

• However, many types of cells, both immune system cells and non-immune system cells, can produce cytokines.

• Therefore, cytokine production is not restricted to helper T cells, although these cells seem to be specialized to secrete cytokines that are important for inducing and enhancing immune responses.

• Cytokines are a heterogenous group of protein mediators that play important roles in the generation and maintenance of immune responses.

• While heterogenous, cytokines do share some features:– low-intermediate molecular weight (10-80 kDa)– interact with high-affinity receptors, specific for each

cytokine– glycosylated, secreted proteins– produced during innate and specific adaptive immune

responses– mediate and regulate immune and inflammatory responses– produced briefly and locally, in a self-limiting manner

• Cytokines can act as autocrine, paracrine, or less frequently, as endocrine factors:

• Cytokines can act on multiple cell types - pleiotropic (many target cell types, as well as many biological activities):

• Their biological actions often are redundant - several different cytokines may exert the same or similar function:

• Although heterogenous, cytokines fall into groups of structurally-similar, evolutionarily-related molecules - cytokine super-families:

– hematopoietins (IL-2, IL-4, IL-6, IL-10, IL-12, IFN)

– tumor necrosis factor-like cytokines (TNF, fas-ligand, CD40L)

– chemokines (IL-8, SDF-1, RANTES, etc)

– IL-1-like cytokines (IL-1, IL-1, IL-1RA)

– transforming growth factor-like cytokines(TGF, etc)

• Cytokine receptors also fall into families of related

molecules.

 

• Cytokines interact with high-affinity, specific cell surface receptors; cytokine binding results in signal transduction, and ultimately in altered cellular behavior, by inducing a new pattern of gene expression:

• The hematopoietin cytokine superfamily is the largest cytokine family, and contains many of the cytokines that play critical roles in immune responses:– IL-2, IL-4, IL-6, IL-10, IL-12, G-CSF– interferons (IFN, IFN, IFN)

• hematopoietins are monomers with a parallel -helical “bundle” structure

• In addition to this, several hematopoietin cytokines (such as erythropoietin or leptin) operate primarily outside of the immune system.

• While the hematopoietins share a similar molecular structure, they clearly have diverse, and some times opposing, biological effects.

 

• Another cytokine family that contains several cytokines known to play important roles in immune responses is the tumor necrosis factor (TNF) cytokine family:– CD40-ligand– fas-ligand– TNF

• cytokines in this large family typically are ~20kDa

• have extensive -pleated sheet structure

• homotrimers

 

• Other cytokine families that contain important cytokines are the IL-1-like cytokines, TGF-like cytokines, and chemokines (chemoattractant cytokines: IL-8 and similar cytokines).

• IL-2 plays a critical role in T cell growth, following the activation of T cells by exposure to Ag+MHC and co-stimulatory signals.

• Following T cell activation, T cells begin to produce IL-2, as well as to express a higher-affinity, more responsive, form of the IL-2 receptor complex, resulting in proliferation and differentiation to become effector T cells.

• IL-2 interacts with the IL-2 receptor complex.

• The IL-2 receptor can exist in a high-affinity, as well as in lower-affinity, versions.

• The high-affinity receptor is composed of three polypeptides, the , , and IL-2 receptor chains.

• On resting T cells, only the and chains are expressed (lower affinity).

• Activation of T cells results in the expression of the chain, which when associated with the and chains (high affinity form).

• Following activation, CD4 T cells can go on and produce a wide variety of cytokines, in addition to IL-2.

• Overproduction of cytokines in response to infection can result in effects that are detrimental to the host.

• In particular, the overproduction of TNF in response to infection with gram-negative bacteria can result in a systemic response that can be life-threatening, septic shock.

• In recent years, several cytokines have become available commercially, for clinical use.

• EPOGEN®, recombinant human erythropoeitin used to treat anemia – a member of the hematopoietin family

• INFERGEN®, bioengineered, non-naturally occurring type-I interferon used in the treatment of chronic hepatitis C virus.

• NEUPOGEN®, a recombinant human granulocyte colony-stimulating factor (G-CSF) used for treatment of severe chronic neutropenia.

• Unfortunately, the availability of some of these cytokines has resulted in significant problems with abuse.

Cytotoxic T cell activity - CD8 cells

• Stimulation of naïve CD8 T cells by antigen + MHC class I on effective APC results in the induction of effective killer T cells, or cytotoxic T cells (CTL).

 • CD8 cytotoxic T cells can be stimulated in several ways:

– dendritic cells acting as APC– CD4 helper cell-activated APC– high concentrations of IL-2 produced by helper T cells

• The most effective APC for CD8 cells are dendritic cells.

• Dendritic cells express a wide range of co-stimulatory molecules, such as B7, and can act as effective APC, resulting in the efficient induction of cytotoxic T cells:

Activated CTL detect potential targets by binding to cells that express a foreign antigen together with their MHC class I molecule:

• When a CTL detects a target cell expressing antigen, it kills this cell by releasing:

– cytokines that can stimulate apoptosis (TNF, TNF, LT, IFN, fas ligand),

– cytotoxins, such as perforin and granzymes, which are directly cytotoxic and apoptosis-inducing molecules

• CTL store cytotoxins in lytic granules:

 

• The interaction of the CTL with the target cell results in selective killing of virus-infected cells, which is induced by programming the infected cell to die by apoptosis.

• CTL can kill sequentially kill several target cells at the site of viral infection:

Innate immune effector mechanisms

Innate immune responses are critical in combating infection:

There are several forms of innate immunity:

natural killer (NK) cells

complement system

inflammatory cytokine response by macrophages

acute-phase response

Innate immune effector mechanisms – Natural killer (NK) cells

• NK cells are another type of cytotoxic cells.

• NK cells are morphologically large granular lymphocytes (LGL).

• They do not express the T cell receptor for antigen: are not antigen-specific in their activity.

• NK cells are sensitive to cells that have lost MHC class I expression, and can kill these cells via cytotoxic mechanisms similar to those exerted by CTL.

• Some viruses attempt to subvert the immune system by down-regulating MHC class I expression, making these virus-infected cells “invisible” to conventional CD8 CTL

• NK cells are believed to fill this gap in immunity, by automatically killing such virus infected cells

NK cells also can be armed by antibody, and exert an effector function called antibody-dependent cellular cytotoxicity (ADCC), in which antibody bound to an antigen (tumor cell, virus infected cell) can bind via their Fc (non-antigen-binding) portion to NK cells, which express Fc receptors that specifically allow binding to antibody.

Complement

• Complement:– complex group of plasma proteins that are pre-formed (not made

in response to infection)– found in serum and body fluids– produced mainly by liver cells– can be thought of as a form of innate humoral immunity

 • Activation of complement results in a cascade of molecular events,

which results in:– enhanced phagocytosis of microbes– recruitment of inflammatory cells– direct lysis of bacteria

 

• There are three different activation pathways for complement:– alternative pathway (induced by direct interactions with

molecules in the surface of pathogens)– classical pathway (antibody-mediated)– lectin pathway (induced by the mannose-binding protein, an

acute-phase reactant)

• The poorly-named alternative pathway is perhaps the most active and important means of activating complement, in that it can act early in infection, before the stimulation of other responses.

 

• Some complement components are activation/proteolytic enzymes.

• Other complement components are membrane-binding proteins that act as opsonins.

• Other complement components are mediators of inflammation.

• Still others form part of what is called the membrane-attack complex, which can lyse microbes.

• All three complement pathways intersect at the point of having some enzyme (a C3-convertase) that can cleave a molecule called C3 into C3b and C3a, as well as an enzyme that can cleave C5 (C5 convertase).

 • C3b is a potent opsonin.

• C3a and C5a are peptide mediators of inflammation.

• C5b forms part of the membrane attack complex

Macrophage-driven inflammatory responses, Acute Phase Response

Antibody Effector Functions

• Antibodies can exert a wide variety of effector functions, including helping induce complement activation via the classical pathway, enhancing phagocytosis, as well as arming NK cells in ADCC

 • In addition to this, antibody molecules can carry out a

wide range of biological functions. Antibodies can directly neutralize viruses, bacteria, or bacterial toxins

The different immunoglobulin isotypes have different biological properties, and are selectively distributed throughout the body:

IgM is found primarily in blood, and is very efficient at activating complement.

IgE can activate mast cell degranulation, inducing allergic responses.

IgA is readily transported across epithelial barriers and serves as the primary immunoglobulin type in gut and mucosal surfaces. IgA also is found in colustrum and breast milk.

IgG is the only isotype that is transported across the placenta.

Together, these passively-transferred antibodies provide a significant level of immune protection in the newborn, as newborn humans cannot produce their own antibodies for some time after birth: