Assignment 7 Flashcards
what is a germinal center
sites within secondary lymphoid organs where mature B lymphocytes proliferate, differentiate, mutate their antibodies (through somatic hypermutation), and switch the class of their antibodies (for example from IgM to IgG) during a normal immune response to an infection.
where does B cell activation take place in a primary immune response?
secondary lymphoid tissue
- Antigen entry affects site of immune response
- Blood: spleen
- Mucosal tissue: MALT (resp. GI, etc.)
- Lymph: lymph node
where does B cells predominate in secondary lymphoid tissue?
follicles
where do T cells predominate in secondary lymphoid tissue
regions surrounding follicles
where does naive B cell activation initiation begin in the secondary lymphoid tissue
outer areas of follicles - so T cells can help out
what occurs during antigen recognition and processing (should already be familiar with)
- Cross linking of BCR by antigen
- Endocytosis into endosome & fusion of lysosomes w/ endosome
- Formation of chimeric vescicle (endosome w/ antigenic peptides & MHC class II)
occurring in - Formation of MHC class II-antigen peptide complex & translocation to B cell surface
interfollicular - Presented to TCR of activated CD4+ TH cell (Th2)
what adhesion molecules stabilize the T cell/B cell conjugate? (role of the CD4+ T cell)
ICAM-1/LFA1
LFA-3/CD2
what costimulatory molecules of B cells also act on the Tcell/B cell conjugate? (role of the CD4+ T cell)
B7/CD28
CD40/CD40 Ligand
when is clonal expansion of B cells detectable
within 24 hours of immunization
-following burst of proliferation, activated B cells and T cells migrate to primary follicles where enhanced proliferation of B cells leads to the formation of germinal centers (hall mark of secondary follicles)
define clonal expansion
an immunological response in which lymphocytes stimulated by antigen proliferate and amplify the population of relevant cells.
what areas are associated with clonal expansion, isotype switching, affinity maturation, differentiation to plasma and differentiation to memory B cells ?
germinal centers
What is the role of CD40/CD40 ligand interaction?
delivers an essential signal for T cell-dependent immunoglobulin class switching, memory B cell development, and germinal center formation.
what happens if CD40/CD40 ligand is disrupted?
steps in the differentiation pathway are prevented
results in hyper IgM => because of no isotype switching
what do secreted antibodies lack (from plasma cells) compared to expressed antibody surface receptors (B cells)?
- carboxy terminal cytoplasmic sequences
- transmembrane domain required for anchorage to plasma membrane
when does differentiation of plasma cell stage occur and what it the event marked by
4 days after immunization
marked by secretion of IgM antibodies. Later, B cells undergo isotype switching and then B cell differentiation to the plasma cell stages is characterized by the secreted antibody isotype to which class switching occurred.
what antibody is NOT secreted?
IgD
define isotype switching
process by which cells expressing IgM and IgD are modified at the genomic level such that they produce antibodies of different isotypes (IgA, IgE, IgG)
what gene region/part of the antibody is modified during isotype switching?
the heavy chain constant region
what is switch recombination?
same as isotype switching?
occurs at unique switch regions, located between each gene encoding a constant segment of heavy chains
where does isotype switching/switch recombination occur and when?
germinal centers
1 week after initial B cell activation by T dependent antigen
what makes isotype switching irreversible? explain?
the segment of DNA that is downstream (3’) of the variable region, but upstream (5’) of the “new” switch region is deleted form the genome
what decides the selection of one heavy chain constant region over another during switch recombination?
- signal generated during B/Tcell conjugation
- T cell derived cytokines present in microenvironment
disruption of the IL-4 gene in mice does what
inhibits switch recombination to IgE
define affinity maturation/somatic mutation?
process that leads to the gradual accumulation, with time after immunization, of higher affinity antibodies for the immunizing antigen
production of an antibody with increased affinity for the immunizing antigen occurs randomly as a result of _______ which gives rise to point mutations
somatic mutation
*point mutations - occur in “VDJ” regions of both light and heavy chain
where does somatic mutation/affinity maturation occur and when
germinal center
7-10 days following B cell activation
where does differentiation of activated B cells to memory B cells occur and when
germinal centers
1 weeks after antigenic challenge with a T dependent antigen
why do memory B cells usually express high affinity immunoglobulin and isotypes other than IgM?
because memory B cell differentiation occurs within the same time frame as isotype switching and affinity maturation
where do memory B cells hang out (2 places generally)
secondary lymphoid tissue
most join the pool of recirculating B cells that circulate from blood, lymph, and tissues that were the primary sites of previous antigen encounter.
how do memory B cells enter lymph nodes
afferent lymphatics (no HEV) - why? because they have reduced expression of L-selectin which is the molecules that facilitates extravasation of naive B cells fro the blood to the lymph nodes at the HEVs
what is the time memory B cells can survive without stimulation and are they in the cell cycle or not
survive for weeks or months without stimulation
not in cell cycle
what is the major role of BLys (B cell stimulator/ BAFF B cell activating factor)?
role in B cell selection and survival
what cells types produce BLys?
monocytes/macrophages
dendritic cells
some lymphocytes
how can BLyS be expressed?
- first as a cell surface protein
- released as soluble molecule
what triggers the release of BLyS from monocytes/macrophages and dendritic cells?
cytokines or by crosslinking of FCgammaR by IgG (opsonin) => link between innate and adaptive immunity and B cell survival
what receptors does BLyS bind to on peripheral B cells? which receptor has the highest affinity?
BR3, BCMA, TACI
BR3 - highest affinity
*each receptor = different cascade = each has different outcomes
but all 3 receptors play a key role in B cell differentiation in the bone marrow
what does BR3 signaling do?
leads to B cell survival by preventing apoptosis
when is BR3 upregulated?
following crosslinking of the B cell antigen receptor
what happens when abnormal production of BLyS occurs?
alters immune tolerance by allowing the survival of autoreactive B cells = triggering autoimmune disorders
Rheumatoid arthritis (RA)
systemic lupus erythematosus (SLE)
MS
what does down regulation of B cell activation require
colligation of Fcgamma R II B-IgG/antigen with membrane bound immunoglobulin/antigen = decreases BLyS receptor levels = negatively impacts effects of BLyS and B cell survival
define negative signaling
inhibition of B cells following antigen induced activation
what is the mechanism of negative signaling?
when secreted IgG crosslinks the membrane bound antibodies (same specificity) with the integral membrane receptor, FcgammaR (low affinity FcgammaR) => secreted immunoglobulin serves as a negative feedback regulator to inhibit further activation of the naive cells and subsequent secretion of antibodies
what is the only ligand for BR3?
BLyS
what ligands bind to TACI and BCMA
APRIL and BLyS - APRIL is higher affinity than BLyS
where doe secondary immune responses occur?
outside of the secondary lymphoid tissue
is secondary immune response faster or slower than primary immune response
much faster - the progeny is nearly one log greater than that generated in primary immune response
what are characteristics of secondary immune responses?
- shorter lag time
- lead to a higher antibody titer
- secretion of antibodies of the new isotype
- can occur anywhere in the body!
what are T independent antigens?
antigens that can induce naive B cell activation int he absence of cognate interaction with T cells
leads mainly to IgM production
what are examples of T independent antigens?
- lipopolysaccharide (LPS) - bacterial cell wall component = T-indep at low concentration (at high concentrations = mitogenic)
- pneumococcal polysaccharide form Streptococcus pneumoniae
The immune response to an antigen can be enhanced in the presence of ____
adjuvants => important for deliberate immunization like when administering vaccines - maybe causes slow release of antigen so antigen presentation to T cell is prolonged
-examples = alum precipitate, Freund’s complete adjuvant
what are mitogens?
naturally occurring molecules that have the capacity to bind to and trigger proliferation of many clones of lymphocytes
B cell mitogens -pokeweed - LPS - at high concentration T cell mitogens -concanavalin A (Con A) -phytohemagglutinin (PHA) -pokeweed
what are oligoclonal acitvators
superantigens (bacterial products) that act in a non-antigen specific nut non-polyclonal (oligoclonal) fashion on T cells
bind to regions of the T cell receptors called Vbeta
triggers the activation of all the clones that express any given Vbeta = will lead to the clonal expansion of many clones (oligoclonal) but not ALL clones
