@adreno
Here is an analogy I made up for friends who had difficulty understudying the role of Tregs.
Imagen a socker game on a playing field.
So one team are players from the T- cells team. Some players are the first line of defenders, for example NK cells, than there are effector T- cells which you could compare with stormers and than you have T -helper cells and so forth. There is a subset on this whole field which are very important and those are the T- regulatory cells -- Tregs. They are the referees.
They control the other cells and check that the game is not getting out of control. Meaning, if there is a pathogen invading the field, all immune cells go into full action. If this action gets to much, the game could get out of hand and the immune cells go basically into overkill mode. That would cause damage to the host, could kill us, and it can happen that the effector immune cells could attack the bodies own tissue-- that is what is commonly understood as autoimmune reaction (auto reactive T cells). So Tregs control that by damping (regulating) the immune response of other cells.
That is a very simplified explanation, because of course the immune system is very complex with lots of more players at different times and in different settings.
So the continued expression of increased Treg do not necessarily indicate a weaker immune response. But as I said elsewhere, in combination with other markers, for example low NKcell activity and a decrease CD8+CD28+ cells, increased CD8+CD28- would indicate a continued long term antigen stimulation and immune exhaustion. Hence, chronic infection which leads over time to immune exhaustion.