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Restriction-modification systems in bacteria
Sites on bacterial chromosome
protected by proteins/methylation
G A A T T C
C T T A A G
EcoRI
Sites on phage DNA/RNA unprotected and cut
Penicillin and other antibiotics
•
Discovered by Fleming in 1929 and turned into a useful treatment by Florey
and Chain (1940s)
•
Prevents the formation of the bacterial cell wall (by interfering with
peptidoglycan cross-linking)
– New cells subject to osmotic lysis
– Only acts against Gram-positive bacteria
•
Other antibiotics
– Streptomycin from bacterium Streptomyces acts on protein synthesis
– Tetracycline, also from Streptomyces, also acts on protein synthesis on a
wide range of bacteria
The immune system of Drosophila
•
Body fluid of insects contains haemolin
– a member of the immunoglobulin superfamily, which binds to microbes
•
Also several short peptides that play an unknown role in antibacterial action
– Defensins – like scorpion venom taxins
– Cecropins and attacins (disrupt cell membrane)
– Drosomycin (antifungal) – similar to plant substances
– Rapid evolution of some these proteins indicates an ongoing evolutionary
arms race
•
Cellular immunity
– Haemocytes secrete antimicrobial peptides and are involved in coagulation,
phagocytosis and encapsulation
Immune systems in vertebrates - overview
•
Mechanical barriers
– Collagen/skin , mucous membranes, secretions
•
Non-specific defence (innate immunity)
– Phagocytosis by neutrophils (70% WBC)
– Macrophages (5% WBC)
– The inflammatory response (histamine, chemokines, fever)
– Antimicrobial proteins (complement, interferons)
•
Specific immune defence (adaptive immunity)
– Humoral (antibodies, B cells)
– Cell-mediated (Cytotoxic T cells, Helper T cells)
Antigen (1st exposure)
Engulfed by
Free antigens
directly activate
Antigens displayed by
infected cells activate
Macrophage
Stimulates
B cell
Gives rise to
Stimulates
Helper T cell
Stimulates
Memory helper T cell
Stimulates
Cytotoxic T cell
Gives rise to
Stimulates
Antigen
(2nd exposure)
Plasma cells
Antibodies
Memory
B cells
Stimulates
Memory
T cells
Active cytotoxic
T cells
B cells (made in bone marrow)
•
B cells secrete antibodies which recognise intact foreign particles outside cells
and block their action/target them for destruction
Infected cell
B cell
Antibody
Antigenic determinant
Each antigen can present many
different antigenic determinants
and be recognised by many different
Antibodies
Antigen
The structure of antibodies
The diversity of antibodies
•
Collectively referred to as immunoglobulins
– IgM (pentamer) first to appear in response to infection
– IgG (monomer) most abundant circulating form, confers maternal immunity
– IgA (dimer) produced by mucous membranes
– IgD (monomer) Differentiation of B cells
– IgE (monomer) stimulate mast cells and histamine
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Each has own heavy chain (a, d, e, g, m)
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There are 2 types of light chain (k and l)
•
Both light and heavy chains are encoded by libraries of subunits which are
shuffled to generate diversity
Light-chain diversity
Variable (40 units)
Joining (5 units)
Constant
Somatic recombination with imprecise joining
TGGCTGGA
x
TGGCCAG
TGGCTGGCAG
TGGAG
GCAGCC
‘Fine-tuning’ by somatic hypermutation
(error-prone DNA repair process)
Possible diversity – (prior to mutation) about 1012
Cell memory and clonal selection
•
Acquired immunity ensures that self-antigens are not attacked through
– Clonal selection
– Clonal deletion
– Repeated editing
– Absence of Helper T cells ensures that immune response is not mounted
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Initially, B cells produce membrane bound antibodies
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Cells stimulated by an antigenic response proliferate and differentiate into
effector cells that secrete antibody
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A fraction of the simulated cells differentiate instead into memory cells that
are rapidly induced in subsequent antigenic challenges
T cells (made in thymus)
•
Only respond when foreign antigen displayed on surface of self-cells in
periperal lymphoid organs
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Recognise fragments of foreign particles displayed by MHC proteins
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Cytotoxic T cells recognise and destroy infected cells
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Helper T cells stimulate other cells
– Macrophages
– B cells
– Cytotoxic T cells
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T cell receptors are very like antibodies, but are membrane bound ONLY and
are less variable
Cytotoxic killing
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Cytotoxic cells release perforin (related to C9) which makes the infected cell
permeable to secreted serine proteases
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Activate apoptosis (cell death) through capsase family
MHC proteins
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Major histocompatibility complex (chromosome 6 in humans)
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Class I MHC present to Cytotoxic T cells
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Class II MHC present to Helper T cells
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The most polymorphic loci in humans (some have >200 alleles)
6p21.3
DP
DQ DR
4 Mbp c. 127 genes
C4 C2 TNFa,b HLA-B HLA-C HLA-A
HLA-D
18 genes
Class II
Class III
Class I
Rapid evolution of MHC genes
In both humans and house mice, the antigen-binding site (ABS) of class I and II MHC molecules
(light blue) have a high rate of nonsynonymous versus synonymous nucleotide substitutions, which
is the opposite pattern for genes under purifying selection, such as nonantigen-binding sites of MHC
molecules (dark blue). Adapted from Potts WK and Wakeland EK (1990) Evolution of diversity at
the major histocompatibility complex. Trends in Ecology and Evolution 5: 181–187.
Helper T cell activation
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Two signals are required
– MHC bound peptide
– Costimulatory signal
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Different costimulatory receptors occur in different cell types
– CD3 complex on all T cells
– CD4 on helper T cells
– CD8 on cytotoxic T cells
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Cytokines secreted by T cells (and B cells) provide communication between
elements of the immune system inducing and suppressing reactions
– Interleukins
– Interferons
– TNF-a
The immunoglobulin superfamily
How to evade the immune system
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Once in, replicate really fast
– Bacteria, viruses
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Replicate within cells that are trying to destroy you
– Mycobacterium tuberculosis replicates in macrophages
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Trick normal cells into taking you in
– Yersinia pseudotuberculosis expresses a protein that binds to E-cadherin
and stimulates cells into forming a cell junction, through which the bacterium
can enter
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Present very diverse surface proteins which immune systems are unlikely to
have experienced before
– Trypanosomes somatic switching