Host-Microbiota Co-immunity: An Intimate Relationship That Goes Beyond Protection

Resident microorganisms, known as the microbiota, are essential for many physiological functions including protection against pathogens. Microbiota is indeed required for proper immune system development and function, and can also host-independently protect against infections. Thus, a co-constructed view of host protection involving both host and microbiota, named “co-immunity,” has been proposed, and the idea of an “immunological holobiont” has been suggested. Yet this view of co-immunity might be too limited, as experimental work has shown that the immune system is involved in functions other than protection, essentially development and repair. Microbiota, through co-immunity, is thereby most likely involved in these functions, although strong evidence is currently lacking. Moreover, as our point of view is mainly host-centered, we may miss the implications of co-immunity at broader scales, including cellular and population levels. Intriguingly, co-immunity effects could be beneficial for one function and/or one level, while being detrimental for others. All these elements should be taken into consideration for microbiota manipulation, in order to avoid potentially harmful side effects.