Cell-mediated immunity

CMI

• is an immune response that does not involve antibodies or complement but rather involves the activation of macrophages, natural killer cells (NK), antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in response to an antigen

Mechanisms of CMI

• activating antigen-specific cytotoxic T-lymphocytes that are able to induce apoptosis in body cells displaying epitopes of foreign antigen on their surface, such as virus-infected cells, cells with intracellular bacteria, and cancer cells displaying tumor antigens;

• activating macrophages and natural killer cells, enabling them to destroy intracellular pathogens; and

• stimulating cells to secrete a variety of cytokines that influence the function of other cells involved in adaptive immune responses and innate immune responses

T cell activation

• Naïve mature T cells constantly recirculate between secondary lymphoid organs and blood and lymphatic circulations.

• To activate T cells, APC must deliver two distinct signals, one to TCR via peptide on MHC, and a second co-stimulatory signal.

• Adaptive T cell-mediated immunity is driven by activation of T cells – cytotoxic T cells activated by endogenous antigen to

kill infected cells – helper T cells activated by exogenous antigen to

stimulate macrophage killing of endosomal pathogens

• The professional APC that can provide co-stimulation are dendritic cells (DC), macrophages, and B cells. Of these cell types, DC deliver the best co-stimulation and are probably responsible for activating most naïve T cells.

• APC use the membrane molecule B7 (B7.1 = CD80 and B7.2= CD86 are slightly different forms of this molecule) to deliver a co-stimulatory signal through T cell membrane CD28

• pathogens targeted by cellular immunity are protected from antibody and complement binding by their intracellular locations.

• Adaptive humoral immunity also usually involves T cell activation to produce cytokines that stimulate B cell antibody synthesis.

Naïve resting T cells are stimulated by – antigen peptide presented on Class I MHC to

cytotoxic CD8 T cells or Class II MHC to helper CD4 T cells,

– co-stimulatory signals from APC, to proliferate and differentiate into clones of fully activated effector T cells.

Only professional APC (dendritic cells, macrophages, and B cells) have both Class I and Class II MHC and can deliver co-stimulatory signals.

Activated T cells perform their effector functions when they encounter MHC-presented peptide on their target cells.

• the same cell must deliver the antigen signal and the co-stimulatory signal simultaneously to the T cell for activation to occur

• T cells which bind antigen without co-stimulation become anergic, and cannot in the future respond to antigen even if proper co-stimulation is received.

• Once a T cell becomes activated, it expresses other receptors which enhance co-stimulation. A key T cell molecule expressed early in activation is CD40 ligand (CD40L). CD40L binds APC CD40 and transmits activation signals to the T cell; T cells in mice that cannot express CD40L cannot go beyond the early stages of T cell activation. CD40L binding to APC CD40 signals the APC to express more B7 molecules to provide even more co-stimulation to the T cells; signaling to B cells and macrophages through CD40 also promotes their activation.

• Resting macrophages express little Class II MHC and B7, but increase expression of these molecules once they have phagocytosed bacteria due to signaling through receptors such as TLR-4.

• Pathogens which are phagocytosed most efficiently bind to receptors on macrophages recognizing patterns of common bacterial antigens such as LPS and carbohydrates: CD14, mannose receptor, scavenger receptor, and complement receptors

• Macrophages migrate to secondary lymphoid organs from infection sites to present antigen to T cells

• B cells are unique among APC because they use their membrane Ig to specifically bind and internalize soluble antigens. Antigen cross-linking of BCR stimulates endocytosis and antigen presentation by B cells. B cells constitutively have high levels of Class II MHC. Co-stimulatory molecule expression increases following contact with some bacterial carbohydrates and LPS, making it unlikely that B cells will activate T cells to respond to self peptides in the absence of infection. B cells enter secondary lymphoid organs by passing through the T cell areas, where they are likely to contact T cells. B cells activate T cells at lower antigen concentrations than do macrophages. They present predominantly bacterial and insect toxins and allergens.

Once T cells are activated by peptide + MHC and receive co-stimulation, they begin to synthesize the a chain of IL-2 receptor

and to secrete IL-2. IL-2 can signal the activated T cell to begin clonal

proliferation and differentiation into an armed effector cell.

The expression of more and higher affinity cytokine receptors by activated cells compared to resting cells keeps the immune response antigen-specific, even though the same cytokines are made in response to many different antigens.

The response of T cells to a cytokine they secrete is called an autocrine response.

T cell inactivation• When a T cell becomes activated it expresses CTLA-4( a molecule

very similar to CD28 and also able to bind B7, alongside its CD28 molecules.

• CTLA-4 binds B7 about 20 times more avidly than does CD28, and CTLA-4 binding to B7 sends an inactivation signal to the T cell.

• The immune response, therefore, depends on the "push" of antigen stimulation to activate T cells into effectors and the "pull" of negative signals that curtail T cell responses.

• These combinations of activation and inactivation signaling mechanisms are common in the immune system and are there to control a very powerful response that can damage the body if not restricted.

• Mice born with a defective CTLA-4 gene suffer from over-proliferation of lymphocytes.

Armed Effector T Cells • T cells differentiate into armed effector cells

that no longer need co-stimulation to perform their effector functions after 4-5 days of rapid clonal expansion

• Three classes of armed effector cells – CD8 CTL kill infected cells displaying cytosolic

pathogen peptides on Class I MHC – CD4 Th1 cells activate macrophages with persistent

vesicular pathogens whose peptides are displayed on Class II MHC

– CD4 Th2 cells activate B cells that have used their membrane Ig to internalize specific antigen and display peptides on Class II MHC.

Functions of distinct subsets of effector Th cells

• Th1 CD4 T cells or Inflammatory T cells produce IL-2, IFNg, and TNFb, which activate Tc and macrophages to stimulate cellular immunity and inflammation. – also secrete IL-3 and GM-CSF to stimulate bone

marrow to produce more leukocytes and signal B cells to produce opsonizing antibodies (IgG1 and IgG3 in humans and IgG2a and IgG2b in the mouse).

• Th2 CD4 T cells or Helper T cells activate naïve B cells to divide and secrete IgM; – also secrete IL-4, IL-5, and IL-6, which stimulate

neutralizing antibody production by B cells

• T cells are initially activated as T0 cells, which produce IL-2, IL-4 and IFNg.

• Factors which influence a Th0 cell to become a Th1 or Th2 are not well understood, but include the cytokines elicited by the pathogen, co-stimulatory signals, and the nature of the MHC:peptide presented to the Th cell.

• Th1 and Th2 cytokines have antagonistic effects on Th cells. The Th1 cytokine IFNg inhibits proliferation of Th2 cells, while the Th2 cytokine IL-10 inhibits Th1 secretion of IFNg and IL-2.

• The balance between Th1 and Th2 activity steers the immune response in the direction of cell-mediated or humoral immunity.

• M. leprae can survive and replicate in macrophage phagolysosomes. People infected with M. leprae who produce a Th1 response make minimal antibody but their cellular response keeps pathogen numbers low and their disease progresses slowly (tuberculoid leprosy). People infected with M. leprae who make primarily a Th2 response have high levels of M. leprae-specific antibodies, but the bacteria proliferate unchecked in their macrophages and disease progression is rapid (lepromatous leprosy).

Cytotoxic T Cells

• are important for killing intra-cytoplasmic parasites that are not accessible to secreted antibody or to phagocytes. Examples include all viruses, rickettsias (causes of Rocky Mountain spotted fever and typhus), some obligate intracellular bacteria (Chlamydia), and some protozoan parasites which export their proteins from macrophage vesicles to the cytoplasm (Toxoplasma gondii). The only way to eliminate these pathogens is to kill their host cells

• CTL induce apoptosis in their targets. Cells undergoing apoptosis undergo chromatin condensation and membrane vesicle shedding. DNA is cut into pieces in multiples of 200bp,

• During their maturation into effector CTL, CD8 cells synthesize cytotoxic molecules and store them in cytoplasmic granules for quick release upon target cell binding. – Perforin has sequence homology with complement C9, and

polymerizes to form pores in the target cell membrane through which water and salts can enter. At high perforin concentrations, this may be enough to destroy the target by osmotic lysis, but physiological levels of perforin are believed to be too low to induce osmotic lysis. Instead, perforin pores allow granzymes to enter the target cell.

– Granzymes are serine proteases which trigger the apoptotic cascade leading to DNA fragmentation and membrane-bound vesicle shedding.

• Binding of CTL membrane Fas ligand (FasL) to target cell Fas induces target cell apoptosis. Since activated Th1 and Th2 cells also express FasL, they may also have cytotoxic activity. Because mice with defects in Fas or FasL have a higher incidence of lymphoproliferative disease and autoimmunity, this cytotoxic mechanism may be important for regulating immune responses

• CTL also regulate immune responses by releasing IFNg, TNFa, and TNFb. IFNg inhibits viral replication, activates IL-1 and TAP expression by infected cells to promote antigen presentation, and recruits and activates macrophages as APC and effector cells. TNFa and TNFb act with IFNg to activate macrophages and to directly kill some target cells.

• Describe the initial encounter of mature naïve resting T cells with antigen..

• List the three professional APC and compare and contrast their antigen-presenting capabilities.

• Describe co-stimulation of T cells and explain why this

requirement is important in regulating T cell activity. • Compare and contrast naïve and effector T cell

stimulation. • Describe the functions of effector Tc, Th1, and Th2 cells.

Be able to sketch the interactions with target cells and describe the functions of the effector molecules.

• Explain how effector T cell functions are localized to specific target cells.

• Give specific examples of pathogens which elicit effector Tc, Th1, or Th2 cells.