Immunology 2 Flashcards
(43 cards)
Antibiotics
ANTIBIOTICS à an antimicrobial agent produced by a microorganism that kills or inhibits other microorganisms
o Bactericidal è kills bacteria
o Bacteriostatic è stops bacteria growing
Cell Wall - Beta lactams
- Penicillin and methicillin
- Inhibits bacterial cell wall peptidoglycan formation
- Binds transpeptidases (penicillin-binding proteins)
- Problems w/ penicillin resistance -> beta lactamase
DNA - Fluoroquinolones
- Broad spectrum, bactericidal
- Inhibits DNA replication
DNA gyrase in gram -ve - Topoisomerase IV in gram +ve
Ribosomes - Variety (CAMT)
o Chloramphenicol (50S) à prevents protein elongation by inhibiting peptidyl transferase activity
o Aminoglycosides (30S) à affects RNA proofreading and causes damage to cell membrane
o Macrolides (50S) à prevents amino-acyl transfer and truncation of polypeptides
o Tetracyclines (30S) à inhibit translation – stops binding of aminoacyl-tRNA to mRNA translation complex
Antivirals -> Acyclovir
o Can work against HSV, varicella zoster (chickenpox)
o Converted to acyclovir triphosphate only in HSV infected cells due to presence of thymidine kinase
o Acyclovir triphosphate à inhibits DNA polymerase à prevents viral DNA synthesis
· Antiviral drugs we use have to be highly specific for purpose
o Viruses evolve very quickly
o Viruses = obligate intracellular parasites
o Outside completely inert –> can’t do anything
o Genome can be made of DNA or RNA
o Inside cell virus replicates its genome
o Has to co-opt cellular machinery existing inside cell
o Have to find things which are unique to viruses to target like with antibiotics (selective toxicity)
Generic Viral Replication Diagram
o Virus is on outside, attached to cell and gets in
o Virus falls apart so important viral genome is exposed (nucleocapsids)
o Genome is replicated and used to produce mRNA using host cell nuclear synthetic machinery e.g. Enzymes, ribosomes, vesicles etc.
o Too small to carry synthetic machinery
o Replicated genomes and new capsids reassemble to create new viruses which leave the cell and infect other cells
Prophylaxis
preventing disease before cause is acquired e.g. Vaccination or drug before infection
Therapy
treating disease once host has been infected
How antiviral drugs work
Antiviral drugs target viral enzymes, often substrate analogues such as nucleoside analogues (stops building of DNA/RNA)
· Substrate analogue - looks like real substrate to enzyme but has chemical modifications on it
· Increased understanding of structure of viral components and enzymes can lead to rational drug design
Why acyclovir is the best antiviral agent
o Nucleoside analogue
o Mimics guanosine but lacks 3’ -OH groups
o Chain terminator
o Next base can’t be attached due to lack of 3’ OH group
o Given as a prodrug - given as an unphosphorylated form –> not incorporated into strand until it becomes tri-phosphorylated.
o Virus produces enzymes called thymidine kinase which attaches first phosphate to acyclovir –> subsequent phosphorylation to ACVTP by cellular enzymes
o These enzymes only in cells that are affected by virus (like herpes virus)
o Particular substrate has better affinity for viral DNA polymerase so more likely to be copied into and terminate strand of viral genome as opposed to host cell genome
o Resistance rare but maps to thymidine kinase
· Look for specific viral proteins which you could devise small molecules against which only affect what virus is doing
Drugs used against Influenza - Amantadine/Rimantadine
§ Cyclic amines w/bulky cage-like structures
§ Byproducts of petroleum refinement
§ Active against Influenza A only
§ Block replication of influenza
§ M2 protein - tetrameric ion channel involved in uncoating of virus - drug blocks this ion channel so protons can’t get through and unlock viral core
§ Virus locked in endosomes and won’t initiate infection
§ Single amino acid change in M2 can make virus resistant e.g. Serine to asparagine @ position no. 31, amino acid 31 (S31N mutation has little/no cost to fitness of virus)
§ H3N2 subtype causes vast majority of seasonal flu, and all have M2 proteins with S31N mutation
Drugs used against Influenza - Relenza + Tamiflu
§ Crystal structure of neuraminidase protein allowed rational drug design
§ Design a substrate analogue which looks like substrate - sialic acid
§ Neuraminidase cleaves sialic acid form virus otherwise sialic acid binds virus back down onto cell as it’s trying to leave at end of replication
§ Cleavage of sialic acid allows release of new virus particles and continued virus replication in next cell or next host
§ RELENZA - guanidine positive charge
· Modifying it so it sticks more avidly than natural substrate sialic acid (plug drug)
· Enzyme can no longer turn over
§ TAMIFLU - also blocks enzyme but w/diff chemistry
Drugs used against influenza - Baloxavir
§ Inhibits PA endonuclease (polymerase acidic endonuclease)
§ Resistance
§ Single point mutation PA I38T in PAG target
§ Changes isoleucine to threonine makes virus resistant
§ Common in H3N2 virus especially in children
HIV antivirals
o Inhibit virus entry
o Fusion inhibitors stop virus envelope fusing with plasma membrane
o Reverse transcriptase inhibitors (make RNA into DNA)
o Integrase another unique enzymes (DNA gets integrated into our own DNA)
o Protease inhibitors (assembly of virus requires protein to be chopped into pieces)
o AZT or zidovudine - nucleoside analogue
o Combination therapy: to avoid rapid selection for resistance to antiviral drugs
§ You have a drug, which targets a single place on HIV genome, 10,000 nucleotides long with a reverse transcriptase error rate 1 in every 10,000.
§ It’s only going to take one round of replication for the virus to generate the mutation which escapes your drug. But that same genome has another target here,
§ for a second drug.
§ The chances that that genome generate the mutation to your first drug and the mutation to your second drug in the given time, very slim.
§ Now, if you target a different gene of the virus with a third drug that requires a third mutation, the chances that one genome gets all three, very slim.
§ Of course, you’ll get one or the other or the other. But so long as you’re taking all the drugs at once, each singly changed one can’t escape
Antibiotic Resistance
High use of antibiotics = selection pressure
o High no. of bacteria, few are resistant to antibiotic à antibiotic kills pathogenic bacteria as well as good bacteria within body (probiotics) à antibiotic resistant bacteria proliferate w/o competition à bacteria can transfer antibiotic-resistance gene to other bacteria via plasmids (conjugation)
4 mechanisms of resistance
o Altered target site à methicillin-resistance involved alternative penicillin binding protein (PBP2a) w/low affinity for beta-lactams
o Inactivation of antibiotic à B-lactamase destroys beta lactam ring
o Altered metabolism à increased production of PABA = resistance to sulphonamides
o Decreased drug accumulation à antibiotic efflux pump
Inhibition of cell wall synthesis
Penicillins
Cephalosporins
Bacitracin
Vancomycin
Inhibition of protein synthesis
Chloramphenicol
Erythromycin
Tetracyclins
Streptomycin
Inhibition of nucleic acid replication and transcription
Quinolones
Rifampin
Inhibition of synthesis of essential metabolites
Sulfanilamide
Trimethoprim
Bacteria evading host defence mechanisms
o More likely to replicate + propagate their genes
o More likely to cause disease (their pathogenicity)
o Pathogenicity à ability to cause disease
§ Depends on virulence and infectivity
§ Virulence à features that enhance disease causation
§ Infectivity à general features favouring disease causation
3 methods of evading host defences - Evade antibody opsonisation
§ 1. Hide antigens
o Bacteria coated w/polysaccharide capsule so antigens are hidden and can’t be recognised
§ 2. Disrupt functions
o Express proteins that means antibody binds to pathogen incorrectly i.e. via Fc and not Fab
o Neutrophils and other immune system components can’t access Fc region so no result from antibody binding (only recognise Fc so if this isn’t shown nothing can happen)
§ 3. Prevent detection
o Secrete proteins that cover up Fc receptors
o E.g. for S. aureus this protein is SSL10 which binds to IgG to cover up Fc receptors
o Also used by TB
§ 4. Degrade antibodies
o Secrete enzymes (proteases) which chop up antibodies so they’re ineffective
§ 5. Modify antigenicity
o Antigenicity = capacity of antigen to produce an immune response inside body
o Determined by how it binds to antibodies (B cell response) or receptors (T cell response)
o Antigen variation à genetic mutations (i.e. recombination) to produce antigen that is diff. in structure and can’t be recognised
o Gram -ve bacteria particularly good at this
—Lipopolysaccharide membrane in gram -ve bacteria
—Variation in sugar content for O antigen part accounts for different serotype of the pathogen
3 methods of evading host defences - Evade complement opsonisation
§ 1. Inhibit convertases
o Secrete proteins which inhibit C3 and C5 convertases - as a result this prevents:
§ C3b deposition
§ C3a formation
§ C5a formation
o Therefore, less chemoattraction
o S. aureus protein SCIN binds to C3bBb and inhibits
formation of C3 convertase and C5 convertase
o C3 and C5 convertases convert substrates into C3 and C5 which in turn can lead to chemoattraction of neutrophils and also inflammation
§ 2. Inhibit complement components
o Secrete proteins which prevent:
§ Binding of factor B to C3 à essential for C3 convertases to recognise C3 and convert it into C3a
§ C3dg binding CR2 à CR2 is a complement receptor expressed on B cells, if C3dg can’t bind to CR2 – no B cells are activated
o S. aureus protein Efb binds C3d in C3 which induces conformational change which prevents binding of factor B to C3 and C3dg binding to CR2
§ 3. Degrade complement components
o Proteases cleave C3 or C5 into 2 non-functional forms
§ 4. Recruit host-derived regulators
o Recruit regulators of complement via surface proteins, inactivates components e.g. C3b
o Genetic mutations – express copies of human complement regulators à can turn off complemen
3 methods of evading defences - Evade neutrophil functions
§ 1. Inhibit chemotaxis
o Inhibiting chemotactic receptors
o Secrete proteins which inhibit chemotaxis
o S. aureus CHIPs inhibits chemotaxis and activation
§ 2. Inhibit detection of bacteria
o Blocking Fc receptors preventing detection of IgG opsonised bacteria
o Secrete proteins that bind to Fc receptors on neutrophils –
preventing phagocytosis
o Neutrophils need to bind to Fc region of antibody (which is bound to pathogen) to trigger phagocytosis of that pathogen
o S. aureus FLIPr and SSL5 block Fc receptors
§ 3. Kill neutrophils
o Kill neutrophils à by releasing toxins
o Express surface proteins that are stimulatory receptor antagonists
o Express surface proteins that activate inhibitory receptors
o Secrete molecules that neutralise toxins
o Manipulate intracellular signalling à escape endosome or phagosome
o Prevent fusion of phagosome w/lysosomes
o Survive well in phagolysosome – mainly applies to intracellular pathogens
o Change their surface
§ 4. Stimulate inhibitory receptors
§ 5. Disrupt intracellular signalling
