Lab 8 - Immunogenetics Flashcards
Innate immune system
Includes phagocytes, NK cells, complement system. Is an early part og the immune response and recognizes general features of invading microorganisms.
Adaptive immune system in found only in
Vertebrates
Key components of the adaptive immune system
T-cells and B-cells from primary lymphoid organs.
B lymphocyte component of the immune system is also called — because?
Humoral immune system because it produces antibodies that circulate in the blood stream.
Helper T cells stimulate what? What is the T-cell component sometimes called?
B cells and T cells to respons to infections more effectively, and cytotoxic T lymphocytes that can directly kill infected cells.
Cellular immune system (because of direct interaction with infected cells)
How does the adaptive immune response begin?
Begins when antigen-presenting cells (APCs), which include macrophages and dendritic cells, engulf invading microbes and then present peptides from these microbes on their cell surface.
How does the APC alert the adaptive immune system?
1) The foreign peptide is transported to the surface of the APC by a class II major histocompatibility complex (MHC) molecules, which carries the foreign peptide in a specialized groove. This complex is recognized by helper T cell, which secrete signaling proteins- cytokines. These cytokines stimulate B-cells whose receptors (immunoglobulins) can bind to the invading peptides –> additional DNA sequence variations is generated via somatic hypermutation –> produce alterations in the receptors binding characteristics.
2) Creation of memory B cells. High-affinity binding B cells that persists in the body after the infections has subsided. Have already been selected, more rapid response.
What become plasma cells?
B-cells that produce variants with a higher level of binding affinity for the microorganism. Secrete their cell-surface receptors, or immunoglobulins, into the blood stream.
How many days does the differentiation and maturation into AB-producing plasma cells require?
5-7 days.
Why are vaccinations effective?
Induce the formation of memory cells that can response to a specific pathogen
Key member of the cellular immune response? Found where?
Class I MHC molecule. Found on the surface of nearly all cells of the body
How does MHC I molecule work?
Binds with small peptides derived from the interior of the cell, carrying the peptide with it and displaying it outside of the cell.
MHC-peptide complex- function
Binds to receptors on the appropriate T cell’s surface, which prompts the T cell to release a chemical that destroy the infected cell (cytotoxic/killer T lymphocytes or CD8+ T cells)
How are T cells alerted to the presence of an infection?
When circulating dendritic cells present the foreign peptide on their cell surface and migrate to secondary lymphoid tissue, where most of the T cells residue.
Helper T cells respons to the presence of pathogens
Secreting cytokines –> stimulate B cells and cytotoxic T cells
Th1
Primarily intracellular pathogens, secrete IL-2, IF-gamma, TNF-B
Th2
Multicellular parasites. Involved in allergic responses, secrete IL-4, IL-5, IL-6 and IL-13
TH17 and TH 22
Secrete IL-17
Secrete IL-22
Memory T cells- function
Quickly respons to a foreign pathogen in the future
Regulatory T cells
Help to regulate the immune system so that self-peptides are not inadvertently attacked
Cellular and humoral immune system - comparison
Humoral immune system is specialized to fight extracellular infections, such as circulating bacteria and viruses. The cellular immune system fights intracellular infections, such as parasites and viruses within cells.
Gene expression
Process by which information from a gene is used in the synthesis of a functional gene product. Products are often proteins, but in nonprotein coding genes (tRNA,mRNA, snRNA) the product is functional RNA.
Bi-allelic expression
Both alleles are transcribed and expressed at the same time.
Monoallelic expression
When only one of the two copied of a gene is active, while the other is silent. Frequently initiated in the development of an organism and stably maintained thereafter.
Ex: X chromosome inactivation, occurs in the development of most female mammals.
Dosage compensation
In females, most of the genes on one X chromosome are inactivated in every cell. As a result, the overall dosage (level of transcription) of most X-linked genes is equal in males and females.
Genomic imprinting
Epigenetic phenomenon that causes genes to be expressed in a parent-of-origin-specific manner. Inherit only working copy. Either the copy from mom or dad is silenced. Improper imprinting results in having two active or two inactive copies. Can lead to severe abnormalities, cancer etc. Ex: Prader-Willi and Angelman syndrome
Allelic exclusion
Process by which only one allele of a gene is expressed while the other allele is silenced.
Allelic exclusion ensures that functional B cells never contain more than one what
VHDHJH and VLJL. Essential for antigenic specificity
What is an antibody (Ig) molecule composed of?
Identical pair of longer heavy chains and an identical pair of shorter light chain, which are linked together by disulfide bonds
What does immature B-lymphocytes produce?
only IgM, but as they mature, a rearrangement of heavy chain genes –> class switching, occurs.
What does Ig differ in?
amino avis composition, charge, site and carbohydrate content.
Where is the constant and variable region in the heavy and light chains located?
C- and N-terminal ends
What is the variable region responsible for?
antigen recognition and binding
Genes that encode for the light chain
Constrant, Variable, and J, which joints the constant and variable regions
Genes that encode for the heavy chain
C, V, J and a Diversity region located between J and V.
Somatic recombination (VDJ recombination)
As immunoglobulin molecules are formed during B lymphocyte maturation, a specific combinations of a single V and J segments is selected for the light chains, and another combination of V, D, and J segments is selected for the heavy chain. This is accomplished by deleting the DNA sequences separating the single regions before they are transcribed into mRNA.
What carries out the deletion process in the VDJ recombination? Function
Recombinases, which initiate double-strand DNA breaks at specific DNA sequences that flank the V and D segments.
What happens after the deletion in VDJ recombination? What is the process known as?
All segments except one V, D, and J are deleted, the non-deleted segments are joined by ligases. Somatic recombination
Recombinases are encoded by what?
RAG1 and RAG 2 genes
How does the mature B lymphocytes vary in comparison to most other cells of the body?
Vary in terms of their rearranged Ig DNA sequences. Because there are many possible combinations of single VJD segments –> 100 000- 1 000 000 different types of antibody molecules
Class-switching (isotype switching)
After antigenic stimulation of B-cell, the heavy-chain DNA can undergo further rearrangement in which the VHDHJH unit can combine with any CH segment.
Where is the DNA flanking sequences (switch regions) located? What are these regions composed of?
Upstream for each C-region, except for Cdelta.
Composed of multiple short repeats (GAGCT and TGGGG)
What is switch factors?
Intracellular regulatory proteins known as cytokines. Plays major role in determining the particular Ig class that is expressed. Ex. IL-4 induces switch from Cmu to Cgamma1 or Cepsilon
Somatic hypermutation
A small subset of B cells has cell-surface receptors (Ig) that can bind to a specific foreign antigen, and their binding affinity is usually low. Once B-cells is stimulated by a foreign antigen, they undergo an affinity maturation process characterized by hypermutation of V segments of Ig genes. End results is a population of mature plasma cells that secrete antibodies that are highly spesific to the invading pathogen
Activation-indueced deaminase causes what?
Cytosine bases replaced with uracil
Clonal selection
Antigen selectivity causes activation, division and differentiation only in those cells that express receptors with which it combine
Membrane-bound and secreted form of immunoglobulins differ in?
Amino acid sequence of the heavy-chain carboxyl-terminal domains
What produces membrane bound-antibody and secreted antibodies?
Mature naive B cells produce membrane-bound
Differentiated plasma cells produce secreted
M1 and M2
located 1,8 kb downstream from the 3’ end of the Cmu4 exon.
M1 encodes transmembrane segment
M2 encodes cytoplasmic segment of the CH4 domain in membrane-bound IgM
Downstream M1 and M2
encode transmembrane and cytoplasmic segments
Primary transcript produced by transcription of a rearranged mu heavy chain gene contains what?
2 poly-A sites, in the Cmu segment.
Cleavage of primary transcript and addition of poly-A tail at site 1 leads to
M1 and M2 exons are lost
Excision of introns and splicing of the remaining exons produces mRNA encoding the secreted form of the heavy chain
Cleavage of primary transcript and addition of poly-A tail at site 2 leads to
Splicing removes the S sequence at the 3’ end of Cmu4 exon, which encodes the hydrophilic carboxyl-terminal end of the secreted form, and joins the remainder of the Cmu4 exon with M1 and M2 exons, producing mRNA for the membrane form of the heavy chain
What determines whether the secreted or membrane form of an immunoglobulin will be produces?
Differential processing of a common primary transcript
Simultaneous expression of membrane bound Igm and IgD by mature B cells depends on what?
Differential RNA processing
Transcription of rearranged heavy-chain genes in mature B cells produce primary transcript containing?
Both Cmu and Cdelta gene segments. Close together, and the lack of a switch site between them persists the entire VDJCmuCdelta region to be transcribed into a single primary RNA transcript, which contains 4 poly-A sites.
Site 1,2,3,4 (IgM and IgD expression)
1 and 2: Cmu3
3 and 4: Cdelta
Heavy-chain transcript cleaves and polyadenylated at site 2 after Cmu exons, mRNA encodes what?
membrane bound IgM
Polyadenylation at site 4, after Cdelta exons will produce mRNA encoding for what?
membrane bound IgD
What does mature B cells express on its membrane?
both IgM and IgD
Cleavage and polyadenylation at site 1 and 3 leads to?
1: secreted form of IgG
3: secreted form of IgD
What does T-cell acitivation require?
presentation of foreign peptide along with an MHC molecule
What are T-cell receptors composed of?
alpha and beta chain or gamma and delta chain
What does not occur in the genes that encode the T-cell receptors than occurs in the generating of immunoglobulin diversity? why?
Somatic hypermutation, because this helps to avoid the generation of T cells that would react against the body’s own cells.
Where does the MHC lie?
4- Mb region on the short arm of chromosome 6
What is class I MHC molecules composed of?
A single heavy glycoprotein chain and a single light chain called b2-microglobulin
Role of Class I MHC
Form a complex with foreign peptides that is recognized by receptors on the surface of cytotoxic T lymphocytes. Essential for cytotoxic T cell response.
Class I MHC are encoded by
Human leukocyte antigens A,B,C on chromosome 6
Role of MHC I in transplantation
Matching of the donor’s and recipient’s class I alleles increase the probability of graft or transplant tolerance.
Where are class II MHC found?
surfaces of the immune system’s APCs (phagocytes and B lymphocytes)
Role of class II MHC
When ass with foreign peptides, they stimulate helper T cell activity after binding to the T cells’ receptors.
What does the class II MHC consist of?
an alpha and a beta chain, each of them encoded by different genes located on chromosome 6
What kind of genes does the class II MHC include?
HLA-DP, -DQ, -DR and genes that encode peptide transporter proteins (TAP1 and TAP2) that help to transport peptides into the ER, where they initially form complexes with class I molecules before migrating to the cell surface.
Advantage of having great variety of MHC molecules
A person who expresses a greater variety of MHC molecules has a better chance of dealing effectively with a variety of infectious organism. Some variants bind peptide from a given pathogen more effectively than others. Ex: homozygous for each of major class I loci (A,B,C) expresses 3 different class I MHC molecules in each cell. heterozygous expresses 6.
MHC restriction. What complement is not MHC restricted?
T-cell receptors recognize peptides only in combination with MHC molecules on cell surfaces.
Complement system
Some virus-infected cells and tumor cells take advantage of MHC restriction, how?
They suppress the expression of MHC molecules on their surfaces in an attempt to evade detection by T cells. Fortunately, NK cells are activated by the absence, rather than the presence of MHC molecules on cell surfaces.
Activation of NK cells in the absence of MHC molecules
Mediated by killer cell immunoglobulin-like receptors (KIR). They inhibit NK cells when they bind to MHC class I molecules on the surfaces of normal cells, but activate them when MHC class I molecules are absent.
Most important genes involved in class III MHC
Genes encoding the complement system
Difference between distribution of T-cell receptors and immunoglobulins and MHC molecules
Whereas immunoglobulins and T-cell receptors vary between cells within an individual, MHC molecules vary between individuals.
