Regulatory T cells in control of the immune response to acute bacterial infection and sepsis

The immune system is evolved to mount a rapid, protective response against infection. At the same time, it is tightly regulated in order that this response does not overshoot so as to cause inflammatory damage. A key way in which such regulation comes about is through the action of a white blood cell population termed regulatory T cells, or Tregs. Relatively little work has previously been done on the role that Tregs might play in regulating the immune response in the context of severe bacterial infection. These infections can involve sepsis, the condition whereby there is an inflammatory response throughout the body, often leading to organ failure or, in the condition termed septic shock , rapid fall in blood pressure. Septic shock leads to death in 40-70% of cases. There is thus an urgent need to gain a better understanding of the immunological events leading to this condition and so design better medicines. A current view suggests that it might be the consequence of an exaggerated, damaging, immune response. Little work has been done on the role of Tregs in controlling this, though we have recently been doing work to find out. The patient needs to mount a rapid and effective immune response to infection, so might this involve some mechanism for taking the brakes off with respect to Treg control, yielding a bigger response? Alternatively, perhaps bacteria have evolved mechanisms to subvert the immune response, producing substances that are able to further enhance Treg function and constrain immunity. A third possibility is that a large amount of control by Tregs is indeed brought to bear, but this is an appropriate response to limit disease damage. We will address these possibilities in a well-defined transgenic mouse model of gram-positive sepsis. The work involves defining the Treg changes that occur at the site of infection and in the whole body after infecting with the gram-positive bacteria, Streptococcus pyogenes. We will then see what happens if the influence of Tregs is removed, either by giving an antibody or by doing the work in knockout mice. These studies should aid our fundamental understanding of Tregs during bacterial infection, as well as offering clues for future therapeutic manipulation of the immune response in septic shock.

Regulatory T cells (Treg) have received much attention with respect to modulation of T cell responses in autoimmunity, transplantation and tumour immunity. Less is known about the control of responses during infection, particularly bacterial infection. We plan to analyse the involvement of Tregs during Gram positive sepsis. The group A strep model represents a particularly pertinent case, as infection is characterised by high mortality, an excessive inflammatory response and the characteristic features of toxic shock triggered by superantigen production in some cases. This provides a unique opportunity to explore the relevance of Treg to the balance between limitation of bacterial infection and an excessive ?cytokine storm? leading to shock. Does the immune system cope with bacterial challenge by damping Treg, so allowing a swifter clearance of infection, or by enhancing Treg function, thereby avoiding excessive inflammation? Alternatively, do Treg orchestrate both type of response at different stages? We will use a well-characterised HLA transgenic model of S. pyogenes septic shock. Our pilot data show that infection is accompanied by an initial increase in FoxP3+ cells and associated transcripts including IL-10, GITR and LAG-3, accompanied by a local increase in TLR2 expression. These changes are seen on infection with a highly invasive, superantigen+ isolate, but not with a poorly-invasive, superantigen- isolate. We will now relate changes in Treg, assessed by alterations in lymph node transcripts, flow cytometry and functional assays to CD4 effector function, bacterial counts and disease outcome. Any proposed role of Tregs will be confirmed in Treg ablation experiments, using both depleting antibody and inducible knockouts. We will investigate changes in TLR expression in different cell-types. Any proposed causal role of TLR changes implicated in Treg modulation will be analysed in crosses to the appropriate TLR knockouts. Since the bacterial superantigens are candidates for the excessive inflammatory stimulation in septic shock, we will compare superantigen+ and superantigen- bacterial isolates as well as investigating Treg control of toxic shock following administration of the superantigens themselves. We will then purify Tregs from lymph nodes at different stages of infection to evaluate whether functional regulation is intact. Adoptive transfer experiments with purified Tregs will address the extent to which shock can be modulated. In summary, these studies should aid our fundamental understanding of Tregs during bacterial infection, as well as offering clues for future therapeutic manipulation of the immune response in septic shock.