The present invention relates to novel regulatory T cell proteins. One protein, designated PD-L3, resembles members of the PD-L1 family, and co-stimulates αCD3 proliferation of T cells in vitro. A second, TNF-like, protein has also been identified as being upregulated upon αCD3/αGITR stimulation. This protein has been designated Treg-sTNF. Proteins, antibodies, activated T cells and methods for using the same are disclosed.In particular methods of using these proteins and compounds, preferably antibodies, which bind or modulate (agonize or antagonize) the activity of these proteins, as immune modulators and for the treatment of cancer, autoimmune disease, allergy, infection and inflammatory conditions, e.g. multiple sclerosis is disclosed.