Diversifying selection and functional analysis of interleukin-4 suggests antagonism-driven evolution at receptor-binding interfaces.
BACKGROUND
Interleukin-4 (IL4) is a secreted immunoregulatory cytokine critically involved in host protection from parasitic helminths 1. Reasoning that helminths may have evolved mechanisms to antagonize IL4 to maximize their dispersal, we explored mammalian IL4 evolution.


RESULTS
This analysis revealed evidence of diversifying selection at 15 residues, clustered in epitopes responsible for IL4 binding to its Type I and Type II receptors. Such a striking signature of selective pressure suggested either recurrent episodes of pathogen antagonism or ligand/receptor co-evolution. To test the latter possibility, we performed detailed functional analysis of IL4 allotypes expressed by Mus musculus musculus and Mus musculus castaneus, which happen to differ at 5 residues (including three at positively selected sites) in and adjacent to the site 1 epitope that binds the IL4Ralpha subunit shared by the Type I and Type II IL4 receptors. We show that this intra-species variation affects the ability of IL4 neither to bind IL4 receptor alpha (IL4Ralpha) nor to signal biological responses through its Type I receptor.


CONCLUSIONS
Our results - reminiscent of clustered positively selected sites revealing functionally important residues at host-virus interaction interfaces - are consistent with IL4 having evolved to avoid recurrent pathogen antagonism, while maintaining the capacity to bind and signal through its cognate receptor. This work exposes what may be a general feature of evolutionary conflicts fought by pathogen antagonists at host protein-protein interaction interfaces involved in immune signaling: the emergence of receptor-binding ligand epitopes capable of buffering amino acid variation.
Koyanagi
Madoka
M
Department of Immunology, St. Jude Children's Research Hospital, 262 Danny Thomas Place, Memphis, TN 38105, USA.
Kerns
Julie A
JA
Chung
Linda
L
Zhang
Yan
Y
Brown
Scott
S
Moldoveanu
Tudor
T
Malik
Harmit S
HS
Bix
Mark
M
D000595:Amino Acid Sequence; D000818:Animals; D019143:Evolution, Molecular; D014644:Genetic Variation; D006801:Humans; D015847:Interleukin-4 / Q000235:genetics*; D016013:Likelihood Functions; D008322:Mammals / Q000235:genetics; D051379:Mice / Q000235:genetics*; D008807:Mice, Inbred BALB C; D008969:Molecular Sequence Data; D010802:Phylogeny; D011485:Protein Binding; D017433:Protein Structure, Secondary; D019948:Receptors, Interleukin-4 / Q000235:genetics*; D012641:Selection, Genetic; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D013045:Species Specificity
D019948:Receptors, Interleukin-4; D015847:Interleukin-4