Suppression by T regulatory cells (Tregs) is a major mechanism by which the immune
system controls responses to self and innocuous foreign proteins. Although there are many
different types of Treg cells, the best characterized are those that constitutively express cellsurface
IL-2 R (CD25). The aim of this research was to; increase understanding of the
molecular phenotype of human CD4[sup +]CD25[sup +] Tregs, and was addressed using 3 different
approaches. I) Certain pathogens are known to be capable of modulating the function of
CD4[sup +]CD25[sup +] Tregs, but it was unclear i f this was via a direct or indirect mechanism. The
expression of TLR5 on human CD4[sup +]T cells was studied, and it was observed that both
CD4[sup +]CD25[sup +] effectors and CD4[sup +]CD25[sup +] Tregs express TLR5 at functionally significant
levels. Further, the TLR5 ligand flagellin is a co-stimulatory molecule for CD4[sup +]CD25[sup +] effector cells, and flagellin can increase the suppressive capacity of CD4[sup +]CD25[sup +] Tregs in the
absence o f antigen presenting cells. II) In an effort to understand the mechanism underlying
the unique functional phenotype of CD4[sup +]CD25[sup +] Tregs, including hypo-responsiveness to T
cell receptor (TCR) mediated activation and lack o f cytokine production, TCR-mediated
signaling in pure populations of ex vivo human CD4[sup +]CD25[sup +] Tregs was investigated. A
consistent defect in AKT activation in CD4[sup +]CD25[sup +] Tregs was observed, which when
reversed, abrogated CD4[sup +]CD25[sup +] Treg suppressive capacity. Ill) Clinical applications and
experimental manipulations have been hampered by the lack of a unique cell surface marker
for CD4[sup +]CD25[sup +] Tregs that is not also an activation marker for CD4[sup +]CD25[sup +] T effector cells.
Microarray analysis on single cell-derived Treg and T effector clones was performed as an
initial screen, followed by quantitative R T - PCR validation on polyclonal populations of
human CD4[sup +]CD25[sup +] Tregs. It was observed that human CD4[sup +]CD25[sup +] Tregs specificallyexpress the membrane channel protein Aquaporin 9. In summary, the research described
within is of considerable import to the study of human CD4[sup +]CD25[sup +] Tregs and their control of
immune homeostasis, and represents a significant contribution to the field of immunology.Medicine, Faculty ofMedicine, Department ofExperimental Medicine, Division ofGraduat
