3 - Recognition and Response Flashcards
Receptor-ligand binding
A receptor molecule attaches to its ligand by the same types of noncovalent chemical linkages that enzymes use to bind their substrates (hydrogen and ionic bonds, hydrophobic and van del Waals interaction). The key to the receptor-ligand interaction is that the sum of the bonding interactions holds the two interacting surfaces together with sufficient binding energy for sufficient time to allow a molecular signal to be received by the cell.
Receptor-Ligand interactions strenght
Dissociation constant (K_d) provides a quantitative measure of the strength of the ligand binding. Lower K_d = higher affinity. if [ligand]=K_d, 50% of the ligand is bound to the receptor
K_d= ([S]*[L])/[SL] S= free receptor sites L= free ligand SL = receptor-ligand pair
Interactions between receptors and ligands can be multivalent
BCR have more than one ligand-binding site per molecule, aka multivalent.
benefit: non-covalent binding interactions are reversible; ligand spends some time “on” and some time “off”. If there are multiple binding sites, it is less likely that the ligand will be “off” all the receptor sites simultaneously (i nwhich case the ligand can be released)
Avidity = overall strength of the collective binding interactions that occur duting multivalent binding.
monovalent receptors can cluster around multivalent ligands to avoid the ligand leaving.
Combinational expression of protein chains can increase ligand-binding diversity
By using different combinations of protein chains, the immune system can increase the variety of different receptor binding sites.
The combination of one receptor chain with multiple different partners
Adaptive immune receptor genes undergo rearrangement in individual lymphocytes
recombination of C and V genes, in addition to one C can combine with multiple different Vs in different cells (after priliferation after successful C?) = a gigantic variability
The DNA seq that the BCR and TCR are made up of are short (they make up the heavy and light chains). random combinations of these on different cells, in addition to random combinations of the protein chains (heavy and light chains).
Varying expression of receptors, include one interleukin example from lymphocytes
receptor expression patterns may change when a cell is activated, making it more or less responsive to particular signals
Most resting (non-Ag-activated) lymphocytes express a heterodimeric (two-chain) form of the IL-2 receptor, with intermediate affinity. This means that for physiological cytokine concentrations, IL-2 will not bind. Upon activation of the lymphocyte, the expression of a thirs chain of the IL-2 receptor increases, increasing the affinity for IL-2 and thus allows binding.
Local concentrations of ligands may be extremely high during cell-cell interactions
Cell-cell interactions allow the directional release of ligands, creating locally high concentrations and increasing signal strength.
Many immune receptors include immunoglobulin domains
The immunoglobulin superfamily of proteins includes BCRs, TCRs, adhesion molecules, and other receptors that function in the immune system.
CDRs = complementary-determining regions = the part of the BCR that makes contact with the Ag.
Immune Ag receptors locations
Immune receptors may be located on the plasma membrane, on intracellular membranes, in the cytosol, or even floating around in tissue fluids.
the locations are more variable in the innate than adaptive system
BCRs can be secreted. Its then called an antibody. Consists of two identical heacy chains and two identical light chains
BCR vs Ab
same specificity
B cell can make both soluble and membrane-bound Ig-receptors, encoded by the same gene and sharing a binding site.
Difference in membrane-bound and soluble form:
soluble have a hydrophobic AA seq of various lengths at the C-terminus. In membrane-bound, these are replaced by three regions:
1) extracellular, hydrophilig “spacer” seq (ca 26 AA)
2) hydrophobic transmembrane segments (25 AA)
3) very short cytoplasmic tail
In resting form, only membrane-bound are made. Upon activation, both are made.
3D structure of Ab
2 Light + 2 HEavy (identical)
disulfide bonds
2 Ag binding sites
L = two immunoglobulin domains, H = 4 or 5
Ag-binding site is made up by V_L and V_H.
CDR = complementary-determining regions.
two major classes of L-chain = (kappa + lambda).
the heavy chain = 5 seq types: gamma, alhpa, mu, epsilon and delta (GAMED).
Immature B cell: membrane bound AgR (BCR)
Mature, unstimulated B cells: IgM and IgD.
The expression of the others (IgG, IgA and IgE) requires an addtional and irreversible DNA recombination step (later chapter)
BRC coreceptors
CD21 = coreceptor, participates in the Ag-binding activity of the BCR complex.
binds to C3d on the Ag
cooperative binding occurs when Ag are first identified as foreign by the innate immune system (innate immune system covalently binds a C3d protein fragment to the pathogen). The CD21 specifically binds to C3d. The Ag is then bound directly through the BCR and indirectly via CD21.
BCR signal transduction mediators
ITAM = immunoreceptor tyrosine-based activation motifs. Short seq of AA.
The BCR complex includes IGalpha and IGbeta, ITAM-bearing proteins that transduce signals from Ag-binding to the interior of the cell
TCR structure
Ag-recgonizing (variable region)
“height-gaining” region (constant region)
transmembrane region
to chains (alpha+ beta, or gamma+delta)
C+V are held together by disulfide bonds between Cys-residues.
three CDRs (like BCRs) in their V domains.
gamma-delta TCR have more limited Ag-binding site diversity than BCR and alpha-beta-TCR.
TCR coreceptors
CD4 and CD8 are the only two accessory molecules that have direct involvement in Ag-rec. CD28 is also a coreceptor that decides activation, but is not directly involved with Ag-binding
CD4 = monomeric membrane glycoprotein. 4 extracellular Ig-like domains (D_1 -> D_4), a hydrophobic transmembrane region, and a long sytoplasmic tail.
CD8 is a disulfude-linked alpha-beta heterodimeric or alpha-alpha homodimeric glycoprotein. Each chain consists of a single, extracellular, Ig-like domain, a stalk region, a transmembrane region, and a cytoplasmic tail.
CD4 binds MCH II, CD8 binds MHCI
Activation of T cells requires CD28 coreceptor to engage its ligand, CD80 or CD 86, on the APC.
CD28 is requires so the TCR (which can’t distinguish between self and non-self) doesn’t bind self-peptides on MCH.
CD3 role in TCR signal transduction
signal transduction complex in T cells is composed of CD3 complex, which bears ITAMs, which become phosphorylated on tyrosine when activated.
Differences innate immune receptors and adaptive immune receptors
Receptors of innate immunity are expressed in a non-clonal fashion, while thos of adaptive immunity are clonally expressed. Wheras innate immune receptors are expressed on a diversity of cells, both immune and nonimmune in their primary funciton, BCRs and TCrs are borne by only B and T cells
Receipt of signals from innate immune receptors instructs the innate immune cell to destroy the invader as well as to secrete cytokines that inform the subsequenct adaptive immune response
PRRs recognize PAMPs (common for while classes of microbes), both pathogenic and non-pathogenic. Some can also recognize DAMPs
B and T cells express innate immune receptors in a nonclonal fashion simultaneously with the clonally expressed BCR or TCR.
innate receptors can be intracellular. Adaptive recognize soluble or cell-membrane bound stuff.
Cytokines
communicate among cells of the immune system
can induce a wide variety of responses
can cause changes in the expressioon of adhesion molecules and chemokine receptors on the target membrane, thus allowing the cell to move to a new location.
can signal to increase or decrease activity of an immune cell. can instruct cells to die.
high receptor affinity
chemokines = cytokines that attract the appropriate chemokine receptors to regions where the chemokine concentration is highest.
cytokines exhibit the properties of redundancy, pleiotropy, synergy, antagonism and cascade induction.
IL-1 family
promote proinflammatory signals
secreted by DCs, monocytes, magrophages early in the IR
signal liver to produce other cytokines, like type I IFNs, IL-6 and the chemokine CXCL8
interact with dimeric receptors to induce responses that are primarily proinflammatory
the physiological responses to some IL-1 family members are modulated by the presence of soluble forms og the receptos and soluble cytokine-binding proteins.
Class 1 Cytokines
share common structural motif. Four helix bundle structure
varied funcitons:
- signal the onset of T- and B-cell proliferation (IL-2)
- regulate TH functions (IL-4)
- call for B cell differentiation to plasma cells and Ab secretion (IL-6)
- initiate differentiation of particular leukocyte lineages (GM-CSF, G-CSF)
receptors for these cytokines are made up of at least two chains. In all cases, cytokine specificity is mediated by binding to the alpha-chain and signaling is mediated by one of three alternative chains (gamma_c, beta_c or gp130)
Class 2 Cytokines (3 families)
three families of IFNs
Type I IFNs = IFN-alpha and -beta. secreted by activated macrophages, DCs and virus-infected cells. bind to receptors in plasma membrane, inducing production of ribonucleases that destroy viral RNA, and inhibits cellular protein synthesis
Type II = IFN-gamma (IFN-y). produced by activated T cells and NK cells. indices the activation of macrophages (destruction of intracellular pathogens) + stimulates the differentiation of Tc cells + made by Th1 cells.
includes IL-10 (secreted by monocytes and T, B, DC cells.
Type III = IFN-lambda. like type I, they upregulate expression of genes controlling viral replication and host cell proliferation.
TNF family
soluble or membrane bound
various members of the TNF family of cytokines induce differentiation, survival, proliferation and apoptosis.
TNF-alpha
LT-alpha (lymphotoxins) and -beta. alpha = produced by activated lymphocytes, can give many signals. beta = imprortant for lymphocyte differentiation.
IL-17 family
primarily proinflammatory in action
secreted as dimers. Receptors may be dimeric or trimeric (homo- or hetero- di- or tri-meric units)
Chemokines
induce the directed movement of leukocytes
chemotaxis
chemoattractants
act on G-protein coupled receptors to promote chemoattractation, the movement of immune cells into, within or out of lymphoid organs
Ligand binding can induce dimerization of multimerization of receptors
binding of a ligand to a receptor binding site will often induce a conformational change that alters the receptors ability to bind to other receptor molecules.
tyrosine kinase activation
tyrosine kinase activation is a frequent early step in signal transduction. Phosphorylation may occur on the cytoplasmic domains of receptor proteins or on receptor-associated signaling proteins
Clustering of receptors at the cell membrane is an integral step in many signaling pathways. BCRs and TCRs, their coreceptors and their accessory proteins cluster in lipid rafts, areas of the membrane that have a high concentration of cytosolic enzymes.
Src-family Kinases
play important early roles in the activation of many immune cells
inadvertent activation of these enzymes can cause tumors.
cytosolic domains of receptors can be phosphorylated by Src-family kinases. dual mechanisms for regulation involving both a dephosphorylation and a subsequent, different autophosphorylation reactions.
Intracellular adapter proteins gather members of signaling pathways
adapter proteins bind multiple proteins, bringing receptors into physical proximity with downstream effectors
Common seq of downstream effector relays pass the signals into the nucleus
The components of a signalling pathway transduce a molecular signal from receptor-ligand binding at the cell surface to changes in enzyme and TF activity.
The specificity of the signal is generated by the specific combination of receptors, kinases, and downstream effector molecules activated by ligand binding
Not all ligand-receptor signals result in trc alterations
signal transduction pathways can culminate in cellular functions such as the release of effector molecules from preformed vesicles, the destruction or modification of particular proteins, the alteration of mRNA stability, or the initioation of apoptosis
The outcomes of immune system recognition (7)
1) changes in protein ezpression facilitate migration of leukocytes into infected tissues, aka to the site of immune activity
2) Activated macrophages and neutrophils may clear pathogens without invoking the adaptive immune response. They facilitate destruction of invading otganisms by upregulating phagolysosome activity and cytokine secretion. Oxidative burst
3) Antigen activation optimizes antigen presentation by DCs. Changes in protease specificity of their protasomes optimize the capacity of DCs to mediate Ag-presentation.
4) Cytokine secretion by DCs and T cells can direct the subsequent immune response. the nature of the innate immune receptor engaged in the DC determines what cytokines the DC secretes, and thus which type of T cell repsonse it activates
5) Ag stimulation by B and T cells promotes their longer-term survival by inhibiting apoptosis
6) Ag binding by T cells induces their division and differentiation. Different subtypes of T cells secrete different cytokines that direct varying aspects of immune effector responses
7) Ag binding by B cells induces their division and differentiation. the differentiation results in Ab-secreting plasma cells and memory B cells and the production of Ab of different classes and enhanced binding affinity
