Functions of Th Cells, Th1 and Th2 Cells, Macrophages, Tc Cells, and NK Cells; Immunoregulation

Critical Role of Th Cells in Specific Immunity Select effector mechanisms Induce proliferation in appropriate effectors Enhance functional activities of effectors APC T h cell B cell Tc cell NK Ag NK cell Cytokines Granulocyte Macrophage Cytokines Antigen-presenting cell

Naïve Th Cells Can Differentiate Into Th1 or Th2 Cells T h P cell ThO cell Th1 cell Th2 cell ThM cell IL-12 IL-4 IL-2 IFNγ IL-2 IL-4 IL-5 IL-10 IL-4 IL-5 IL-6 IL-10 IL-2 IFNγ IL-2 Naive Th cellsShort-term stimulation Chronic stimulation Long term Memory cells

Functions of Th1 and Th2 Cells Th1 cell Th2 cell Macrophage B cell IFNγ Activates IL-4 IL-5 IL-10 Activates Inhibits production Inhibits proliferation Mast cell Eosinophil Antibodies (including IgE)

Cytokines Regulate Ig Class Switching Fc region of antibodies determines effector function in different anatomical locations Class (isotype) switching produces class or subclass of antibody most effective in host defense Cytokines acting alone or in combination regulate class switching

Macrophage Th cell Macrophage Cytokines Lymphokines Cytokines Anti-microbial functions Anti-tumor functions Activate Invading agentAntigen presentationActivated macrophage Central Role of Macrophages in Natural and Specific Immunity Involved in initial defense and antigen presentation and have effector functions

Detailed Functions of Macrophages Inflammation – Fever, Production of: IL-6, TNF-alpha, IL-1 – act as pyrogen Immunity Selection of lymphocytes to be activated: IL-12 results in Th1 activation IL-4 results in Th2 activation Activation of lymphocytes: Production of IL-1 Processing and presentation of antigen Reorganization of tissues, Secretion of a variety of factors: Degradative enzymes (elastase, hyaluronidase, collagenase) Fibroblast stimulation factors Stimulation of angiogenesis Damage to tissues Hydrolases, Hydrogen peroxide production Complement C3a TNF alpha production Antimicrobial action O2–dependent production of: hydrogen peroxide, superoxide, hydroxyl radical, hypochlorous acid O2-independent production of: acid hydrolases, cationic proteins, lysozyme Anti-tumor activity produced by: Toxic factors Hydrogen peroxide Complement C3a Proteases, Arginase Nitric oxide TNF alpha

Macrophage Activation Macrophage activation results from alterations in gene products that govern new functions. Two major mechanisms that activate macrophages: IFN-γ produced by Th or Th1 cells plus bacterial endotoxin (LPS) IFN-γ produced by Th or Th1 cells plus TNF-α

Mechanism of Macrophage Activation Macrophage Activated Macrophage Bacterial endotoxin (lipopolysaccharide) triggers cytokine production Th1 cell 1 IFN gamma 2 Macrophage Activated Macrophage TNF alpha Various products

Cytolytic T (Tc) Cells Tc exiting the thymus are pre-Tc cells, i.e. have TCR that can recognize antigen, but are not mature and cannot kill until “armed” To become armed requires two signals: 1.Recognition by TCR of specific antigen associated with class I MHC, and 2.Exposure to cytokines (IL-2 and IFN-γ)

Mechanism of Arming Tc Cells Pre-Tc cell Tc cell T helper cell Class I MHC Class II MHC APC 1. Cell expressing class I MHC presents antigen ( ) to a pre-Tc cell IFN IL-2 2. Antigen-presenting cell presents antigen in association with class II MHC to Th cell 3. Th cell makes cytokines 4. Pre-Tc cell differentiates to functional Tc cell 5. Tc recognizes antigen on class I MHC-expressing target cell 6. Target cell is killed

Features of Tc Killing Antigen-specific Requires cell-cell contact Each Tc capable of killing many target cells

Main Mechanism of Tc Killing Tc granules contain perforin and granzymes Upon contact with target cell, granule contents released, perforin polymerizes and forms channel in target cell membrane Granzymes (serine proteases) enter target cell through channel, activate caspases and nucleases, lead to apoptosis of target cell

Mechanism of Tc Killing Tc cell Ca++ Perforin monomers Perforin polymerizes Polyperforin channels Tc cell Target cell Granzymes Target cell

Steps in Tc Killing Tc cell 1. Tc recognizes antigen on target cell Target cell Tc cell 2. A lethal hit is delivered by the Tc using agents such as perforin or granzyme B Target cell Tc cell 3. The Tc detaches from the target cell Target cell 4. Target cell dies by apoptosis Target cell

Natural Killer (NK) Cells Derived from bone marrow Lack most markers for T and B cells (do not have TCR) Do not undergo thymic maturation Express CD56, a specific NK marker Express a receptor for Fc portion of IgG, called FcRIII (CD16) Cytokines (IL-2) promote differentiation into lymphokine-activated killer (LAK) cells

NK Cell Effector Mechanisms Mechanism of killing similar to those of Tc cells Not MHC-restricted Susceptibility of target cell to killing is inversely proportional to expression of class I MHC (killer inhibitory receptors (KIR) on NK cells recognize class I MHC and prevent killing)

NK Effector Mechanisms (continued) IgG-coated target cells recognized by FcRIII (CD16) are killed by antibody- dependent cell-mediated cytotoxicity (ADCC) Lymphokine-activated killer cells (LAK) kill broader range of cells than do NK cells

Regulation of Immune Responses Magnitude determined by balance between the extent of lymphocyte activation and tolerance induced by an antigen Nature determined by specificities and functional classes of lymphocytes activated Regulatory mechanisms may act at the recognition, activation, or effector phases of an immune response