Resident commensals shaping immunity

Abstract

All animals coexist with myriad commensal microorganisms in a symbiotic relationship that plays a key role in health and disease. Continuous commensal-host interactions profoundly affect the development and regulation of the host's immune system. The complex interaction of the commensal microbiota with the immune system is a topic of substantial interest. An understanding of these interactions and the mechanisms through which commensal microbes actively shape host immunity may yield new insights into the pathogenesis of many immune-mediated diseases and lead to new prophylactic and therapeutic interventions. This review examines recent advances in this field and their potential implications not just for the colonized tissues but also for the entire immune system.

Figure 1. Commensal microorganisms shape the host immunity by multiple mechanisms

Mammals coevolved with their commensals, comprised of mostly bacteria but also include viruses, fungi, archaea and protozoa. Several factors such as hygienic conditions, antibiotic use, diet, sex and age of the host determines the composition of the microbiota, which in turn interacts with host tissues to orchestrate a finely tuned immune system. A healthy microbiota provides immunomodulatory signals for the development of immune tissues, secretion of antimicrobial molecules by the epithelial barrier and activation of immune cells. Any disturbance in these interactions results in an imbalanced immune system with consequent susceptibility to various diseases.