OVERVIEW OF THE IMMUNE SYSTEM
OVERVIEW OF IMMUNE SYSTEM
 Normal system is protective against non-self
o defends against microbes and foreign antigens
 Deficient system: develop infections, possibly tumors
 Hyperactive system: possibly fatal, e.g. bee sting
 Blind system: does not distinguish self from non-self
o result: autoimmune diseases
IMMUNE SYSTEM: MAJOR SUBSETS
 Innate (natural): springs into action immediately
o upon breach of epithelial barrier by bacteria
o attacks invaders even before infection established
 Adaptive (acquired)*: second line of defense
o even more powerful than the innate system
o activated days after breach of the perimeter.
o recognizes both bacterial and viral antigens
o * Note: Also known as the immune response
ROLE OF COMPLEMENT
 Innate immunity:
o binds bacteria using alterative pathway
 Adaptive immunity:
o binds antibodies using classical pathway
CELLS OF ADAPTIVE IMMUNITY: T CELLS
 T lymphocytes (thymus-derived): live in
o blood (60-70% of lymphocytes)
o lymph nodes (paracortical region)
o periarteriolar sheath of spleen
o other sites
 How did they get to their various destinations?
o have receptors for locally-produced cytokines
EDUCATION OF THYMUS-DERIVED T CELLS
• By exposure to self- antigens in thymus, maturing T-cells are trained to respect them.
• They are therefore “educated”
o Q:How are the T-cells exposed to these self-antigens?
o A:Self-antigens ( with self-MHC mols) are presented to them by thymic epithelial cells
AND by intrathymic dendritic cells
• Those that develop receptors for self-antigens (or become hyper-avid for them) during
maturation, undergo apoptosis
o Q: how many cells die daily ?
o A: 95% of those produced!
T LYMPHOCYTES
• Each T-cell has a receptor for a cell-bound antigen
• 95% have ab TCRs
• Receptors are antigen-specific
 substructures:
o
disulfide-linked heterodimers, a/b polypeptides*
 *T-cells with ab -TCRs cannot process sol. agns unless agns have been
processed and are attached to cells.
o
variable & constant regions (variable binds cells)
• ab-TCRs recognize peptide antigens displayed by major MHCs.
• These MHCs must be located on / attached to the surfaces of antigen-presenting cells.
• In 5% of T lymphocytes, receptors are gd-TCRs
o react with peptides, microbial phospholipids
• DISPLAY BY MHC PROTEINS IS NOT REQUIRED
• These cells are found under epithelial surfaces of
o respiratory, gastrointestinal tracts;
o may be trying to block would-be invaders
• Function-related molecules are also expressed:
o CD2
o CD4
o CD8
o CD28
o integrins
• CD4 & CD8 appear on 2 mutually exclusive subsets
o CD4: 60% of mature CD3+ T cells (helper cells)
o CD8: 30% of mature CD3+ T cells (cytotoxic cells)
• CD4
o binds to epitopes on class II MHCs
• CD8
o binds to epitopes on class I MHCs
• Each of the CDs can therefore recognize epitopes only in the context of their appropriate MHC
mols.
WHAT IS A CLUSTER DESIGNATION (CD) ?
• It is a protein molecule or antigen, located on the surface of a cell. It has a designated number.
• Within each antigen are one or more tiny stretches of amino acids called epitopes (alias?)
• Such stretches are the “hooks” by which the CD antigens can attach to / interact with other
cells.
• A single antigen molecule may have many epitopes- and usually does.
MORE ON EPITOPES…..
• Epitopes (or antigenic determinants) are small discrete sites on surfaces of much larger agns
• They are tiny, immunologically active regions of much larger, more complex molecules
• B-cells can recognize epitopes on free-floating antigens
• T-cells can mostly recognize epitopes when presented by MHC mols on the surfaces of cells
QUERIES
• Q: Where are MHC Class II molecules located?
• A: on MΦs, dendritic cells, B-cells
•
•
•
Q: Where do MΦs get their load of antigens?
A: by eating and processing microbes,
absorbed bits of protein
•
•
Q: What are the products of such processing?
A: polypeptides (bits of which are epitopes)
•
•
Q How would a typical presentation go down?
A: a cell with class l MHC mols and attached peptides would present to a CD8 + T cell
•
•
Q What are the professional APCs ?
A: dendritic cells, macrophages and B cells
• Q What about APCs with class ll MHCs and attached peptides?
• A: they would present to naïve CD4 + T cells
SIGNALS
• Signal I:
o CD4 / MHC II or
o CD8 / MHC I
• Signal II: CD28*
o interaction with B7-1 or B7-2 (CD 80 or CD 86) on antigen-presenting cells
o *In its absence, T-cells become nonreactive or apoptotic.
MECHANISM
• After successful encounters with B cells exhibiting antigens on their surfaces:
 T-cells secrete cytokines ( IL-2 among others)
 these cause proliferation and differention of B cells
 the B cells ultimately differentiate into:
o
effector cells (function?)
o
memory cells (function?)
CD 4 SUBSETS
• What do CD4 subsets do?:
o TH1 cells secrete IL-2 and IFN-g
o TH2 cells produce IL-4, IL-5, IL-13
• What is the ultimate goal of these subsets?
o TH1 cells are involved in / facilitate:
• delayed hypersensitivity
• synthesis of opsonins, C-fixing abys
o TH2(better helpers than TH1 cells) facilitate:
• synthesis of IgE
• activation of eosinophils
CD 8+ SUBSETS
• What do CD8+ subsets do?
o mainly cytotoxic, i.e. kill target cells such as
 virus-infected
 malignant
 transplanted (as in unmatched grafts)
HOW DOES A CD8+ CELL KILL?
• First, it recognizes agn. associated with MHC1
• Then, LFA-1 on CD-8 attaches to ICAM on target cell
o with formation of a conjugate
• Death apparatus on CD-8 cell then does its thing!
• By the perforin-granzyme system
• By the Fas / Fas-ligand system
QUERIES
• Q: What is an LFA?
• A: This represents a leukocyte function antigen. It is better known as an integrin and acts as a
receptor for a variety of ligands.
•
•
Q: What is an ICAM?
A: This stands for an intercellular adhesion molecule, most commonly described on an
endothelial cell but in the present context, any cell targeted for death. It acts as a ligand for
LFA.
NUMERIC FACTOIDS
• Each T-cell has 105 unique agn-binding TCRs
o all TCRs are identical
o every antigen bears one or more epitopes
o epitopes must be presented by cells bearing MHCs
• altered self-cells (virus-infested, cancerous)
• MHC I - bearing
• agn-presenting cells (B-cells, MØs, dendritic cells)
• MHC II - bearing
OTHER FUN STUFF
• Flu virus: takes 20-30 different epitopes to combine with an MHC molecule
• Thus: 20-30 different targets for T-cell recognition
• One in a million T-cells will recognize an epitope
• With help (?), division will start, 2 / day
• By day 7, 14 divisions will have occurred ( = 214)
• Result: 10,000 cells from 1 cell
B-LYMPHOCYTES
Where do they live?
o blood (10-20% of blood lymphocytes)
o LN, spleen, tonsils, Peyer’s patches
NAÏVE OR RESTING B-CELL
o In Go
o Activation must occur before B-cell can function
o How is this accomplished?
o By an encounter with an antigen
o What happens next?
o Cell enters cycle !
SUBSTITUENTS OF RECEPTORS
o IgM and IgD (on all naïve B cells)
o agn-binding component of B-receptor complex
o CD21
o = complement receptor-2
o = also receptor for EBV (consequence?)
o Fc
o CD 40 (see later)
ASSORTED FACTOIDS l
o Each B-lymphocyte has 1.5 x 105 unique agn-binding receptors (Igs) for soluble antigens.
o all are identical in any one lymphocyte
o diversity of response achieved by juggling V-D-J genes
o epitopes bind at NH2 (variable) ends of H & L chains
o Each B-lymphocyte also has Class II MHC surface molecules
o for purpose of presenting processed agns to helper T-cell
o elapsed time from internalizing agn. to presenting with MHC:
 30-60 minutes
o V–D–J rearrangements  > 1010 antigenic specificities
o number later reduced in marrow (how?)
 Reduced on encountering agns attached to cells in BM
 result: apoptosis
 Note: B-cells only do soluble, unattached agns
o binding  cloning of cells specific for 1 epitope
o Progeny:
o memory lymphocytes?:
 are formed during primary response
o Longevity:
 “long time” for memory lymphs (lifetime in Go)
 1-2 weeks for plasma cells (Goldsby et al.)
o function?
 memory lymphs, on re-exposure to foreign antigen:
o induce a more rapid / intense immune response,
ASSORTED FACTOIDS ll
o Q) Must presentation of an antigen by an MHC precede activation?
o A) Not necessarily: it’s a matter of whether the assistance of a helper T-cell is required (later)
IMMUNOGLOBULINS: GENERAL
o Membrane bound: on B-cells
o Free in plasma: secreted by plasma cells
o Because most agns are complex, with many epitopes:
o several B-cell clones may be needed to bind 1 agn
o each clone may then bind a single epitope
o antibodies so produced are polyclonal
WHAT IS A MONOCLONAL ANTIBODY?
o One specific for a single epitope of an antigen
PLASMA CELLS
• Make 1000-2000 molecules of Ig / cell / sec
• By another estimate, make up to 1010 / hr
• Note: May take B-cell 3-4 days to bump into its agn. Thereafter, differentiation to plasma cell
is rapid.
WHAT IS A HAPTEN?
• A hapten is a small organic molecule that is antigenic though not immunogenic. Coupling with
a carrier results in an immunogenic conjugate.
MACROPHAGES
 Eat microbes and protein antigens
• process and present fragments to T-cells
 Activated by IFN-g produced by TH1 cells
• their ability to kill is thereby enhanced (kill what?)
• Microbes
• Tumor cells
 May need opsonins to phagocytose certain microbes
• what opsonins?
• Fc portion of IgG
• C3b
DENDRITIC CELLS: INTERDIGITATING
 Immature forms live in epithelium (Langerhans cells)
 Mature forms live under epithelia (why?)
 Because that location is well-suited capturing would-be invaders, such as microbes and
other foreign antigens.
 Also found in interstitia of all tissues
 All have many receptors (for microbes, TLRs, etc.)
• This is a toll-like receptor that recognizes a component of a bacterial cell wall.
• TLR2 recognizes lipopolysaccharides (LPS), part of surface of Gram-negative bacterial
cell walls.
 Respond to same cytokine receptors as T-cells
• thereby recruited to T-cell zone
• where they present to helper T-cells
 Have high levels of MHC II ,MHC l , B7-1, B7-2 mols. Sig?
DENDRITIC CELLS: FOLLICULAR
 Located in germinal centers of spleen, LNs
 Do not have Class ll MHCs.So what ?
o cannot present agn to helper T cells.
INSTEAD
 Have Fc receptors for IgG and receptors for C3b
o can trap agns bound to antibodies or C and
o present to B-cells with highest affinity
OTHER CELL-CELL INTERACTIONS
 Viral-infected cell and Tc lymphocyte
 Macrophage – TH lymphocyte
NATURAL KILLER CELLS I
• Part of the innate defense system
• 10-15% of blood lymphocytes. Morphology?
• Capable of killing
• virus- infected cells
• variety of tumor cells
• CD3-, CD56+,
CD16+
• Significance of CD16 positivity?
• Is Fc receptor for IgG
• NK cells can lyse IgG-coated cells
• ADCC
Other Functions
• Produce:
o INF-g
 activates MØs to destroy ingested bugs
 promotes naïve CD4+  TH1 cells (effects?)
o TNF
o GM-CSF
AGAIN, FUNCTION OF TH1 CELLS?
• TH1 involved in/facilitate:
o delayed hypersensitivity
o synthesis of
 opsonins
 complement-fixing abys
WHAT TURNS ON NK CELLS?
• To proliferate: IL-2, IL-15
• To kill: IL-12
• To secrete IFN-γ: IL-12
CYTOKINES: GENERAL
• Lymphocyte-derived (lymphokines)
• Monocyte-derived (monokines)
• Cytokines (origins not otherwise defined)
• Others (polypeptides)
• Note: Some are called interleukins.Why?
MEDIATING IMMUNITY
• Innate: IL-1, IL-6, TNF (TNF-a), Type I interferons
• IL-1 and TNF recruit pmns
• IL-6 upregulates formation of CRP
• interferons protect against viral infections
• Innate and adaptive
• IL-12 and INF-g act against intracellular organisms
REGULATING LYMPHOCYTES
•
IL-2, IL-4, IL-12, IL-15, TGF-b
• IL-2: growth of T-cells
• IL-4: differentiation to TH2 pathway
• IL-12: differentiation to TH1 pathway
• IL-15: growth and activity of NK cells
• TGF-b: downregulates immune response
ACTIVATING INFLAMMATORY CELLS
• IFN-g: activates macrophages
• IL-5: activates eosinophils
• TNF-a
• TNF-b
• TNF –alpha and beta: act on neutrophils & endothelial cells
AFFECTING WBC MOVEMENT
• Called chemokines
• C–X–C family
o produced by activated MΦs & endothelium
• C–C family
o produced by T-cells
• Recruit different WBC types to different arenas
• Note: Also affect distribution of lymphocytes. Whither?
o B- and T-lymphocytes to appropriate regions of lymph node
STIMULATING HEMATOPOIESIS
• CSFs acting on committed progenitor cells
 GM-CSF
 G-CSF
• CSF acting on pluripotent (totipotent) stem cells
 c-kit ligand (SCF) also acts on pluripotent cells
WHAT CELLS PRESENT ANTIGENS?
• Cells with MHC class 1 molecules:
 all nucleated cells (except neurons)
 about 105 mols. per cell
• When a cell becomes infected with a virus:
 processed virus becomes attached to MHC-l
 complex moves to cell surface for presentation
 dumb move:

cell is then attacked / killed by CD8+ T-cells
 Cells with MHC class ll molecules
o B cells, MΦs, dendritic cells
o soluble proteins, microbe fragments processed
o products then attached to Class II mols and
o presented to CD4+ T-cells
WHERE ARE MHCs LOCATED?
 Class I mols:
o lymphocytes (5 x 105 mols/cell) >> muscle, liver
o present in most tissues in low concentration
o absent from neurons
 Class II mols:
o relatively low levels in MΦs and B-cells
  by interaction with antigens
MHC CLASS l vs MHC CLASS ll MOLS.
 Class l mol. on left can bind 8-10 amino acids
o blue on top represents β2 microglobulin
o white represents HLA-A2
 Class ll mol. on right can bind 13-18 amino acids
o mol. shown is HLA-DR1 with long groove
o white = DRα and blue = DRβ
 Red = peptides from HIVrt (L) and FLU hemaggl.(R)
HISTOCOMPATIBILITY MOLECULES
 T-cells can only recognize
o membrane-bound antigens ( B-cells cannot )
 Proteins to be processed in cells and presented
o include viral, peptide fragments, soluble proteins
 Presentation must be by histocompatibility molecules
 T-cells can then do their thing
HLA AND DISEASE ASSOCIATIONS
 HLA-BW47; congenital adrenal hyperplasia gene!
o progesterone

11-deoxycorticosterone
HLA-B27; ankylosing spondylitis x 90
WHY THESE HLA DISEASE ASSOCIATIONS?
 Disease genes are mapped inside HLA complex:
o 21 hydroxylase deficiency (HLA-BW47)
 Ankylosing spondylitis (HLA-B27); unknown
 Location to a degree would explain associations
o autoimmune diseases (DR locus)
GOOD NEWS / BAD NEWS Re HLA
 Good:
o inherited ability to bind particular bacterial peptides
o many provide resistance by evoking protective abys via T-helper cells
 Bad:
o inherited ability to bind peptide from ragweed pollen
o may cause genetically-determined allergy to ragweed
CTLs vs TUMOR ANTIGENS
 CTLs (aka?) recognize tumor-produced, “foreign” agns
o can kill tumor cells, at least in vitro
o class I MHC agns are required
TUMOR STEALTH
 Tumors produce “foreign” antigens.
o Q: How therefore can they evade a CTL attack?
o A: Many ways to fly under enemy radar
ANTIBODIES AS RX AGAINST TUMORS
 Antibodies can provide basis for immunoRx against
o “differentiation” agns of lymphoid cells
 anti CD20 for B-cell lymphomas (Retuximab)
ANTIBODIES vs TUMOR ANTIGENS
 Antibodies can provide basis for immunoRx against
o “differentiation”….
o HER2R: amplified in 25% of breast Cas
 Herceptin gives 3 mos delay in disease progression
o VEGF activity in metastatic colon Ca
 Avastin gives 4 months increase in survival
o Artificial autoimmunity in advanced melanoma = sentinel node positive
 induced by interferon alpha 2b (mechanism of action unknown)
 dramatic increase in survival
o a4 b1 integrins for MS (Natalizumab):
 prevents lymphocytes from entering CNS
BACK TO ERRANT T CELLS
 T-cells born in marrow mature in thymus
 Only those that can recognize self-MHC mols exported
 Two subpopulations die by apoptosis in thymus, those
o with high affinity for self-MHC molecules
o with high affinity for self-agn presented by self-MHCs
 What would happen if these subpopulations did not die and were exported…….?
CONSEQUENCES……
Q Yes,children,what would happen?
A These are dangerous renegades and could cause tissue damage.
Q How could they be “muzzled” or “neutralized” ?
A One way would be by the intervention of a cadre of patrolling, regulatory police cells
A CADRE OF POLICE CELLS?
• Such a cadre exists,probably trained by previous experience in the thymus.
• It consists of a subset of CD4 cells known as regulatory T cells (T-regs).
• These are CD25 positive.
• Full designation :CD4+, CD25+ T-regs.
 FUNCTION
 Suppress wide variety of immune system cells
o impede multiplication
o prevent excess cytokine secretion
 Key component: Foxp3,a transcription factor.
o mutation thereof leads to IPEX syndrome
IPEX SYNDROME
• Immune dysregulation
• Polyendocrinopathy (thyroid, pancreas)
• Enteropathy (CIBD)
• X-linkage
CD4+ CD25+ T CELLS (T-REGS)
 Also influence the immune response to
o infectious agents
o transplanted organs
o pregnancy
o cancer
T-REGS: IMMUNE SYSTEM RESPONSE TO
 Infections: system on hair-trigger to attack GI bugs
o if no T-regs, would do so; if blunted: H.Pylori (?)
 Transplants: Rx with T-regs could prevent rejection!
o permanent: no need for drugs!
 Pregnancy: T-regs more active.
o if too few,could be a cause of spontaneous abortion?
 Cancer: may impede immune surveillance (? increased)
o decreased numbers could also help in TB or AIDS
SLIPPAGE OF CENTRAL B-CELL TOLERANCE
• B-cells with receptors for many self-agns may escape from bone marrow (what self-agns?)
• The self-antigens in question are:
 Thyroglobulin
 Collagen
 DNA
• B-cells bearing receptors for the above are found in the blood of healthy individuals. They have
probably been ‘muzzled’. Others have been deleted.
IMMUNE REACTIONS AGAINST SELF
 1%-2% of US population. Variety of diseases.
 Can have autoantibodies without disease (innocuous)
o especially in elderly
o following tissue damage
WHAT IS PATHOLOGIC AUTOIMMUNITY?
 There are 3 requirements:
o autoimmune reaction has been documented
o must be primary (not 2° to infection)
o other well-defined causes have been excluded
PATHOPHYSIOLOGY IN BROAD TERMS
• Antibodies against self antigens (humoral)
• T-cell attack on self cells (cell-mediated)
IMMUNOLOGIC TOLERANCE: DEFINITION
• “Inability to mount an immune response to a specific antigen”
• May be due to a defect in any component of the immune system
• Net result: immunodeficiency state
T-CELL TOLERANCE: STRATEGIES
Peripheral Tolerance
• Anergy
• Subject: (self) agn-specific T-cells
• To do damage, they must dock with would-be victims
• their CD28s must bind to victim ligands B7-1, B7-2
• Normal tissues may have few or no such ligands
• The encounter has two consequences:
• would-be victims are unharmed
• dangerous T-cells become anergic, i.e.are muzzled
• Subject: peripheral B-cells with receptors for self-agns
• To become activated, they must have help from
• specific helper T-cells (CD4+)
• If they encounter self-antigens without helper action:
• net result is ANERGY
• Subsequently, may be excluded from fine restaurants !
• Suppression by regulatory T-cells
• “Regulatory T-cells” (circulating policemen)
• Prevent reactions against self-agns
• probably “trained” by previous exposure in thymus
• This cadre of cells is CD4+ CD25 +
• Do their thing by secreting cytokines which inhibit
• lymphocyte activation / effector functions
• Clonal deletion
• Bad guys: CD4+ cells recognizing self-agns
• When these cells are “activated”, they express FasL
• Many cells including lymphocytes express Fas (CD95)
• When activated CD4+ cells interact with Fas-bearing cells
• autoreactive bad guys undergo apoptosis
• Cool: self-reactive B-cells expressing Fas are also deleted after encountering FasLbearing T-cells.
• Antigen sequestration
• Certain agns in testis, eye are “sequestered”
• do not communicate with immune system
• Exposure to the elements may occur following
• trauma
• infection
• Net result: immune response leading to chronic
• post-traumatic orchitis
• uveitis
IF NOT TOLERANCE, THEN WHAT?
HORROR AUTOTOXICUS
AUTOIMMUNE DISEASE