Summary Oxford University FHS revision notes: Coevolution of Human Pathogens and Human Immunity
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Course
Genetics and Evolution
Institution
Oxford University (OX)
My notes for the FHS exam in Genetics and Evolution. Useful for Biology, Biomedical Sciences and Human Sciences. I achieved a first and multiple academic prizes. Includes descriptions of concepts and key references/experiments.
How have human pathogens and human immunity coevolved?
Discuss the evolutionary ‘arms race’ between humans and their pathogens.
How do we know humans have adapted to infectious diseases?
Background
Pathogens
A pathogen is a biological agent that causes illness or disease in its host
Doesn’t mean microorganism
5 types of pathogen
1) Bacteria (unicellular, lacking organelles)
2) Viruses (nucleic acid in a protein coat)
3) Parasites (protozoa, helminths, ectoparasites)
4) Fungi (eukaryotic; yeasts, moulds)
5) Prions (misfolded protein)
a. Debate over whether prions should be included (don’t contain nucleic acid)
Arms of the immune system
1) Innate immunity
a. First line of defence
b. Rapid
c. Generalised (not specific to certain pathogens)
d. Physical barriers to protect against invading pathogens
i. Enzymes in mucus, tears and saliva
ii. Coughing and sneezing
iii. Cilia in respiratory tract trap foreign material
iv. Intact skin
v. Mucus and intact mucous membranes (throat and lungs)
vi. Acid in sweat
vii. Acid in stomach
viii. Anti-bacterial proteins and zinc in semen
ix. Competition from commensal bacteria in gut and genital tract
e. Soluble mediators (chemokines, cytokines, complement)
f. Cellular mediators (phagocytes, dendritic cells, NK cells)
g. Antimicrobial peptides
2) Adaptive immunity
a. Second line of defence
b. Delayed
c. Specific immunological memory
i. Humoral immunity (B cells, antibodies)
ii. Cellular immunity (T cells)
Hosts and pathogens: a coevolutionary arms race
Pathogens consistently one of the strongest selective pressures on human genomes.
, Introduction of agriculture in the Neolithic period 12,000 years ago and the associated lifestyle and dietary
changes put additional strain on our immune systems as our interactions with pathogens changed
significantly.
Pathogens that have affected us for the longest time will have had the most significant impact on our
immune systems e.g. cholera, malaria, and leprosy.
Adaptations to pathogens likely to be polygenic.
Host-pathogen arms race in a population
Time: generations
Variables: pathogen virulence and host resistance
Selection: at the level of the germline e.g. innate immune loci, MHC
Host-pathogen arms race in an individual
Time: lifespan
Variables: pathogen virulence and host resistance
Selection: at the level of the germline (susceptible or protected); at the level of somatic diversity
(TCRs and BCRs)
WHAT ARE THE KINDS OF SELECTIVE PRESSURES THAT AFFECT HOST-PATHOGEN COEVOLUTION IN
HUMANS?
Positive (directional) selection
Prerequisite: genetic factors in host affect susceptibility to a pathogen.
If pathogen virulence is significant enough to have a detrimental impact on host fitness, selection will
favour genetic variants that are resistant to the disease.
Phenotypes that increase fitness undergo selection and thus become more common in a population
This can lead to coevolution of host and pathogen.
Example: malaria and Duffy negative blood group
Duffy antigens:
Human blood group system based on the presence of glycoproteins (Fy antigens)
Receptors for chemokines (proinflammatory cytokines)
Found on the surface of red blood cells
4 possible phenotypes: Fya+b+, Fya+b−, Fya−b+, and Fya−b−
Duffy antigens not expressed in Fya−b− phenotype (known as Duffy negative)
Duffy antigens also used by P. vivax to enter red blood cells
-> Duffy negativity associated with resistance to malaria caused by P. vivax
Duffy negative genotype under positive selection and has reached fixation in some areas with high
prevalence of P. vivax
Example: TB
BRITES AND GAGNEUX (2015): coevolution of Mycobacterium tuberculosis (MTB) and human immune
responses
Bacteria can cause the pulmonary disease tuberculosis
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