Try to find this article: 

Earle KE, Tang Q, Zhou X, Liu W, Zhu S, Bonyhadi ML, Bluestone JA. In vitro expanded human CD4+CD25+ regulatory T cells suppress effector T cell proliferation. Clin Immunol. 2005;115:3–9.

Abstract

Regulatory T cells (Tregs) have been shown to be critical in the balance between autoimmunity and tolerance and have been implicated in several human autoimmune diseases. However, the small number of Tregs in peripheral blood limits their therapeutic potential. Therefore, we developed a protocol that would allow for the expansion of Tregs while retaining their suppressive activity. We isolated CD4+CD25 hi cells from human peripheral blood and expanded them in vitro in the presence of anti-CD3 and anti-CD28 magnetic Xcyte™Dynabeads® and high concentrations of exogenous Interleukin (IL)-2. Tregs were effectively expanded up to 200-fold while maintaining surface expression of CD25 and other markers of Tregs: CD62L, HLA-DR, CCR6, and FOXP3. The expanded Tregs suppressed proliferation and cytokine secretion of responder PBMCs in co-cultures stimulated with anti-CD3 or alloantigen. Treg expansion is a critical first step before consideration of Tregs as a therapeutic intervention in patients with autoimmune or graft-versus-host disease.

Unfortunately, I don't have full-text of it. Good luck!

Also: 

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T cell proliferation assay

A total of 1 × 103–1 × 106 CD11b+/Gr-1+ cells isolated from control or 24-h traumatic stress mice were cultured in the upper chamber of a Transwell (6-well plates), which has 0.4-μm pores (Falcon/BD Biosciences) for 24 h using RPMI 1640 medium containing 150 μM L-arginine (physiological levels). In parallel, 1 × 106 normal splenic T cells were stimulated with 1 μg/ml anti-CD3 plus 1 μg/ml anti-CD28 (BD Pharmingen) in the absence of L-arginine for 24 h. The stimulated T cells were then cultured in the bottom chamber of a Transwell system at 37°C in a humidified 5% CO2 atmosphere for 72 h. Nontreated T cells served as controls. Each well was pulsed with 1 μCi [3H]methyl-thymidine (DuPont/NEN) for the final 18 h of incubation. Ten micromoles of nor-NOHA (Calbiochem) was added to block arginase, as well as medium containing 1.2 mM L-arginine was used. Cells were harvested onto filtermates (Wallac), and isotope incorporation was measured by 1450 MicroBeta TRILUX liquid scintillation counter. Data are expressed as cpm ± SEM.

IL-2 production (ELISA)

To evaluate the effect of CD11b+/Gr-1+ cells on IL-2 production by Th1 cells, the level of IL-2 in cell-free supernatants was measured by ELISA using the Mouse IL-2 Immunoassay Quantikine kit (R&D Systems). T cells (106 T cells per 0.5 ml of medium per well) were placed in the bottom chamber of a Transwell (24-well plates) and stimulated with 1 μg/ml plate-bound anti-CD3 Ab and 1 μg/ml soluble anti-CD28 Ab (BD Pharmingen). Control or traumatic stress-induced CD11b+/Gr-1+ cells (1 × 106) were placed in 0.5 ml of medium in the upper chamber. Cell-free supernatants were collected in 12 h and stored at −80°C.

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- replace [CD11b+Gr1+] (= another immune-suppressive cells) with Tregs, and this is the protocol.

Thank you so much.

there is at least one lab in oxford doing T reg stuff (http://www.kennedy.ox.ac.uk/research/group/fpowrie) if you want further local expertise...cheers, a

You can find a suppressor assay protocol I used in this paper below..

Good luck..

http://www.nature.com/mi/journal/v8/n2/full/mi201464a.html

The classic Treg suppression assay is by Thornton and Shevach (J Exp Med 2005).  There are many variations on the theme, depending on the source of your Treg and what aspect of function you wish to test.  The protocols for many of these assays are in the Nature paper in Ghaith's response and in protocols by Collison and Vignali (Methods Mol Biol. 2011; avail for free download here: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3043080/pdf/nihms-272907.pdf )

The assays take a little bit of time to set up, but in general are pretty robust once you have them going.  We have had relatively new students run them successfully.  

Best of luck.    

Treg suppression assay. Check out Treg Suppression Inspector - Miltenyi biotec for detailed assay set up. Best!