Foxp3(+) regulatory T cells, immune stimulation and host defence against infection

Abstract

The immune system is intricately regulated allowing potent effectors to expand and become rapidly mobilized after infection, while simultaneously silencing potentially detrimental responses that averts immune-mediated damage to host tissues. This relies in large part on the delicate interplay between immune suppressive regulatory CD4(+) T (Treg) cells and immune effectors that without active suppression by Treg cells cause systemic and organ-specific autoimmunity. Although these beneficial roles have been classically described as counterbalanced by impaired host defence against infection, newfound protective roles for Treg cells against specific viral pathogens (e.g. herpes simplex virus 2, lymphocytic choriomeningitis virus, West Nile virus) have been uncovered using transgenic mice that allow in vivo Treg-cell ablation based on Foxp3 expression. In turn, Foxp3(+) Treg cells also provide protection against some parasitic (Plasmodium sp., Toxoplasma gondii) and fungal (Candida albicans) pathogens. By contrast, for bacterial and mycobacterial infections (e.g. Listeria monocytogenes, Salmonella enterica, Mycobacterium tuberculosis), experimental manipulation of Foxp3(+) cells continues to indicate detrimental roles for Treg cells in host defence. This variance is probably related to functional plasticity in Treg cell suppression that shifts discordantly following infection with different types of pathogens. Furthermore, the efficiency whereby Treg cells silence immune activation coupled with the plasticity in Foxp3(+) cell activity suggest that overriding Treg-mediated suppression represents a prerequisite 'signal zero' that together with other stimulation signals [T-cell receptor (signal 1), co-stimulation (signal 2), inflammatory cytokines (signal 3)] are essential for T-cell activation in vivo. Herein, the importance of Foxp3(+) Treg cells in host defence against infection, and the significance of infection-induced shifts in Treg-cell suppression are summarized.

Figure 1

Model whereby overriding Foxp3⁺ regulatory T (Treg) cell-mediated immune suppression represents a prerequisite ‘signal zero’ for effector T (Teff) cell activation in vivo. During immune homeostasis, Treg cells actively suppress effector cell activation. Following immune stimulation, Treg-cell suppression is blunted allowing effector T-cell activation through previously described cell intrinsic stimulation signals [T-cell receptor (signal 1), co-stimulation (signal 2), inflammatory cytokines (signal 3)].