Infectious (Non)tolerance--frustrated commensalism gone awry?

Despite advances in medicine, infectious diseases remain major causes of death and disability worldwide. Acute or chronic infectious agents mediate host tissue damage and cause a spectrum of disease as diverse as overwhelming sepsis and shock within hours to persistent tissue inflammation causing organ failure or even cancer over years. Although pathogen exposure can cause disease via host-derived inflammation, pathogens share recognized elements with harmless human commensals. Mouse models and organisms with simpler flora are revealing the dialogue between multicellular hosts and commensal flora. In some instances the persistent inflammation associated with pathogens can be interpreted within a framework of frustrated commensalism in which the host and pathogen cannot complete the requisite dialogue that establishes homeostasis. In contrast, coevolved commensals interact cooperatively with the host immune system, resulting in immunotolerance. Attempts to more thoroughly understand the molecular nature of the dialogue may uncover novel approaches to the control of inflammation and tissue damage.