Concomitant immunity in persistent Leishmania infections: could it represent an evolutionary balance and a vaccine target?

Concomitant immunity (CI) can be viewed as an example of coevolution between the microorganisms and their long-lived hosts. Such an ecological trade-off may be advantageous to both the microbe and the host, as it allows protozoa and helminths to maintain their genetic features while providing the host, particularly mammals, with long-standing protection against reinfection by the same microbe. In Leishmania infection, CI is the mechanism whereby parasites remain at low-level infection in the host, which develops a strong immune reaction that protects against reinfection. Mechanistically, several CD4⁺ T cell populations seem to be involved in such fine immune responses. While immunity against Leishmania, Plasmodium, Taenia, Schistosoma, and Echinococcus is well known, the mechanisms of CI involving these pathogens have been poorly studied. Finally, the phenomenon of CI should be carefully assessed in the design of novel vaccine preparations.