Exam Flashcards
Phagocytosis
“To eat”. The process by which a cell engulfs a large particle e.g. bacteria, dead tissue, small particles (> 0.5uM) into a phagosome by wrapping plasma membrane around the material and internalizing it. Early innate immune defense performed by phagocytes.
Neutrophils as phagocytes
patrol blood stream and migrate rapidly to tissue during infection. Phagocytosis and intracellular killing. Granules with proteolytic enzymes and ROS. Antimicrobial peptides.
Monocytes as phagocytes
In the blood. Migrate into tissue and differentiate to macrophages.
Macrophages- tissue resident that sense infection and kill microbes by phagocytosis. Aided by complement. Produce inflammatory mediators/cytokines to recruit/activate other cells. Antigen presentation to effector T cells.
Dendritic cells as phagocytes
Process antigen and present it to T cells. Provide co-stimulation and activate T cells. Produce ROS, IFN, Cytokines.
What role does C3 play in an immune response?
C3 is a central protein in the complement system that is cleaved in all three complement pathways by a C3 convertase. When complement pathways are activated it is cleaved into C3a and C3bàInflammation, opsonization, lysis by MAC.
C3a is an anaphylotoxin (inflammation). C3b acts as an opsonizing agent (phagocytosis) and is needed to cleave C5 which allows assembly of the membrane attack complex (bacterial lysis).
What consequences would C3 deficiency likely have for a person?
People with defective C3 are susceptible to bacterial infection. Severe, pyogenic infections early in life. Recurrent infection by Neisseria gonorrhoeae or Neisseria meningitides due to lack of MAC formation.
Dendritic cells are often referred to as the cells that bridge innate and adaptive immunity. Briefly explain why these cells have earned this description
Dendritic cells are innate immune cells that phagocytose microbes, but they are also important for initiating the adaptive immunity since they are crucial presenting antigen and in providing the costimulation needed to activate CD4 T cells, and thereby mount an adaptive immune response that provides memory. Thus, they bridge innate and adaptive immunity.
Describe two main roles of MHCI (major-histocompatibility complex I).
Ag presentation: Display foreign peptide on MHCI to show that the cell is infected and to engage Tc cells
* Display self on MHCI to demonstrate that the cell is healthy (NK cells)
* To display a self-peptide to test developing T cells for autoreactivity (primary lymphoid
organs; thymus and BM)
* Display self-peptide to maintain tolerance to self (secondary lymphoid organs; lymph
nodes, spleen +++)
Which cells express MHCII (major-histocompatibility complex II)
Macrophage, B cells and Dendritic cells
How are antigens loaded onto MHCII? 1
Peptides are generated from internalized antigens in endocytic vesicles. Particles are taken in within endosomes and endoscopes are fused with lysosome. The contents are degraded Simultaneously, MHC class II molecules are produced and exported from the ER
in vesicles
How are antigens loaded onto MHCII? 2
Invariant chain guides transport of class II MHC molecules to endocytic vesicles
– Invariant chain (Ii, CD74) prevents peptides from binding to the groove too early in the ER
– Ii also uses sorting signals in its cytoplasmic tail to direct MHCII–containing vesicles to peptide-containing endocytic compartments
How are antigens loaded onto MHCII? 3
Peptides assemble with class II MHC molecules
– Ii is initially degraded by proteolytic activity within endocytic compartments to
class II–associated invariant chain (CLIP)
– HLA-DM exchanges CLIP out of the groove for a peptide fragment
Which cells recognize antigen presented on MHCII?
CD4+ T helper cells by recognizing Ag with TCR (signal 1)
Describe two additional signals that trigger survival, proliferation and differentiation of these cells. 2 + 1.5 +1.5
Co-stimulation through B7(CD80/CD86) ++
à Survival and proliferation à IL-2 transcription
à Stabilization of IL-2 mRNA
3. Cytokine release (IL12, IFNg; IL4; IL23; IL6 etc)
à Direct differentiation of Th cells à Th1, Th17, Th2, TFH, Treg
Describe the process of positive selection of thymocytes
CD4+CD8+ DP thymocytes undergo positive selection - Selects for thymocytes with receptors that can bind self-MHC with low affinity: «self-MHC restriction» Thymocytes are constantly in contact with cTECs (cortical thymic epithelial cells) that express MHC I and II DP thymocytes can have one of three fates:
1. TCR does not bind to any of the MHC/self-peptides during selection => apoptosis («death by neglect»)
2. TCR binds with very high affinity to MHC/self-peptides => apoptosis («clonal deletion») 3. TCR binds with ”just the right” affinity to MHC/self-peptide complexes: Cells receive survival signal («positive selection») and mature to single positive CD4+ or CD8+ T cells (lineage commitment)
Explain the term “Self- MHC restriction”.
T cells are restricted to recognizing peptides presented in the context of self MHC alleles
– only processed antigen peptide fragments presented by APC can be recognized
– T cells recognize antigen specifically.
– Antigen must be presented by self MHC alleles
Why are loss-of-function mutations in the autoimmune regulator AIRE associated with autoimmune disorders?
AIRE is important in Negative selection (“self-tolerance”) in the thymus which involves eliminating autoreactive T cells which have high-affinity receptors for self-MHC/selfpeptide complexes
AIRE: Autoimmune regulator, transcription factor with the unique ability to turn on expression of tissue-specific proteins that would normally not be found in the thymus -> thymic cells can present tissue-specific auto-antigens
Negative selection is responsible for central tolerance; errors in negative selection can cause autoimmune disorders (e.g. Type I diabetes) since T cells won’t be negatively selected for
recognition of self proteins found outside the thymus.
HIV-infection eventually leads to progressive loss of CD4+ T cells. Why is this this loss fatal?
CD4+ T cells are essential for mounting an adaptive immune response which provides memory. Without these cells, no effector CD4+ T cells; Th1, Th2, Th17, Tfh, Treg
Affects B2 cells and humoral response and memory since Tfh are needed.
Affects activation of macrophages and recruitment of neutrophils.
Affects activation of CD8+ T cells since this requires CD4+ Th cells as well. Loss of effector cytokines produced by the CD4+ Th subsets.
Describe the main characteristics Th1 cells and their role in an immune response.
TH1 regulate immunity to intracellular bacteria and viruses
* Differentiation induced by IL12, IFNg
* Help macrophages to destroy intracellular pathogens (e.g. mycobacteria)
Macrophage presents antigen to Th1 cell and gets activating signals back: – T cell cytokines (IFNg)
– CD40-CD40L interaction
àphago-lysosome fusion, ROS production, increased expression of MHC class II, B7, CD40…
* Major effector cytokine: IFN-γ
* Provide help to activate CD8+ T cytotoxic cells (co-stimulation and IL2)
What is a hematopoietic stem cell?
a stem cell that can differentiate to any blood cell (myeloid or lymphoid).
What are two important abilities of hematopoietic stem cells?
Self-renewing – they can divide into clones
- Multipotential – they can divide to form daughter cells that are more differentiated -> can de-
velop to all cells of the blood.
Briefly describe five mechanisms that generate antibody diversity in naive B2 cells during development.
Combinatorial diversity of multiple gene segments. V, D and J for heavy chain, V and J for light chains loci.
2. Heavy chain - Light chain combinatorial diversity
- Independent combination of different heavy and light chains
3. P-nucleotide addition. P=palindromic. Result of assymetric cleavage of hairpin-sealed DNA by Artemis
4. Loss of nucleotides at V-D-J and V-J junctions due to exonucleases
5. N-nucleotide addition by TdT. N=nontemplate.
Why is class-switch rearrangement important for mounting an effective humoral response?
Class-switch rearrangement provides the generation of all the isotypes of antibodies, IgGs, IgA, IgE, in addition to IgM. All have different functions depending on which FC receptor they bind and which effector cell expresses the given effector response. This provides the great variety in antibody-mediated effector responses that can be mounted (neutralization, opsonization, complement activation and ADCC).
IgM
pentavalent “5-armed” with 10 Ag binding sites in total.
a. First Ab produced in a primary response
b. Tend to be lower affinity
c. Very good at complement fixation leading to MAC formation and target lysis
d. Also efficient at forming dense Ab-pathogen complexes that are efficiently engulfed
by macrophage (agglutination)
e. Large. Cannot cross placenta.
IgA
IgA1 monomer (serum), IgA2 dimer (secretions)
Major isotype found in secretions
Effective at neutralizing toxins and pathogens
Protects epithelial surfaces from infectious agents
Does not fix complement, so does not drive inflammation
Long half-life in secretions due to protease-resistant amino acid sequence in Fc region
Mediates ADCC
Triggers degranulation of granulocytes
List three cellular locations of PRRs. Give one example of a PRR in each location and name
a pathogenic structure that it recognizes
Compartments: Cell surface (examples TLR1,2,4,5,6… + ligand), endosome (examples TLR7,8,9 + ligand), cytosol (NLRs, RIG-I, MDA5, cGAS/STING… + ligand), 0.5P for each location, 0.5P for a PRR example in each location and 0.5P for a correct ligand
Inflammasomes
Inflammasomes are cytosolic multiprotein complexes of the innate immune system that induce inflammatory responses
The general activation process and the outcome of Inflammasomes
Nod-like receptors (NLRs) family of cytosolic sensors, recognize intracellular PAMPs. Inflammasome consists of NLR, adaptor Ask and caspase-1, activation of Il-1beta and IL-18 (at least IL-1beta should be mentioned), two signals are required for activation – one drives expression of IL-1beta and the second signal induces inflammasome assembly and cleavage/activation of IL-1beta. Major outcome is release of IL-1b (and IL-18).
Terms that should be mentioned to get all points: NLRs (1P), (cytosolic) assembly of multiprotein complex (1P), 2-step process/caspase cleavage (1P), IL-1b release (1P)
Name the three different pathways by which the complement system can be activated.
What is the initiation event in each of the three pathways?
- Lectin pathway: initiated by mannose binding lectin (MBL or MBP) which binds to man-nose on microbes
- Classical pathway: initiated by C1 complex when C1q binds directly to surface compo-nents of some bacteria (e.g. lipoteichoic acid of G+ bacteria) or to CRP that binds bacte-rial surface
- Alternative pathway: initiated by C3b binding to microbial surfaces. C3b can be generat-ed in two ways: by the classical pathway or the lectin pathway, and by spontaneous hy-drolysis of serum C3
