Innate immunity: sensing the environment and regulating the regulators.

PURPOSE OF REVIEW: Innate immune cells act as sensors for environmental pathogens and key regulators of pathogen-specific T-cell effector responses. The expression of Toll-like receptors is essential for these functions. Innate immunity is a rapidly evolving field. Great progress has been made in the past year in characterizing the signaling pathways downstream of Toll-like receptors, and the role of Toll-like receptors in the regulation of pathogen responses.

RECENT FINDINGS: The ability to control the activation of regulatory T cells has emerged as a key function of innate immunity. Regulatory T cells are involved in the induction of tolerance and the prevention of harmful immune pathology, particularly T-cell responses directed against self-antigens. Toll-like receptor-mediated interactions between pathogen-stimulated innate immune cells and regulatory T cells result in the release of suppression by regulatory T cells, thus allowing pathogen-specific responses. However, pathogenic microorganisms may exploit this interaction to evade the host's response. In addition, recent studies raise the possibility that regulatory T cells may express some Toll-like receptor family members, thereby responding directly to pathogens.

SUMMARY: A deeper understanding of the complex role of innate immune cells as sensors of the environment and regulators of pathogen responses will probably influence the current models of immune regulation, particularly those centered on the role of the environment in shaping immune responses.