The evolution of antigen drift and switching: Continuously evading pathogens.

Abstract

Following infection to a host, some pathogens repeatedly alter their antigen expression to escape the immune defense (antigenic drift/switching). Assuming that most mutations are deleterious but a minor fraction of which can alter the antigenic property of the pathogen, I examine the evolutionarily stable mutation rate, μESS, of pathogens which maximizes the stationary pathogen density in a host. The model reveals that: (1) If the mutation rate is higher than a threshold μc, pathogens cannot maintain themselves because too much progenies are lost by lethal mutations. (2) If the mutaiton rate lies between zero and μc, the system converges to a traveling wave of antigen variants with a constant wave speed. (3) The μESS is unexpectedly high: more than 0.25 per genome per replication even if most mutations are lethal. I also examine the optimal schedule of pathogen growth in a host.