Partial CD25 Antagonism Enables Dominance of Antigen-Inducible CD25 high FOXP3 + Regulatory T Cells As a Basis for a Regulatory T Cell-Based Adoptive Immunotherapy.

FOXP3 ⁺ regulatory T cells (Tregs) represent a promising platform for effective adoptive immunotherapy of chronic inflammatory disease, including autoimmune diseases such as multiple sclerosis. Successful Treg immunotherapy however requires new technologies to enable long-term expansion of stable, antigen-specific FOXP3 ⁺ Tregs in cell culture. Antigen-specific activation of naïve T cells in the presence of TGF-β elicits the initial differentiation of the FOXP3 ⁺ lineage, but these Treg lines lack phenotypic stability and rapidly transition to a conventional T cell (Tcon) phenotype during in vitro propagation. Because Tregs and Tcons differentially express CD25, we hypothesized that anti-CD25 monoclonal antibodies (mAbs) would only partially block IL-2 signaling in CD25 ^high FOXP3 ⁺ Tregs while completely blocking IL-2 responses of CD25 ^{low-intermediate} Tcons to enable preferential outgrowth of Tregs during in vitro propagation. Indeed, murine TGF-β-induced MOG-specific Treg lines from 2D2 transgenic mice that were maintained in IL-2 with the anti-CD25 PC61 mAb rapidly acquired and indefinitely maintained a FOXP3 ^high phenotype during long-term in vitro propagation (>90% FOXP3 ⁺ Tregs), whereas parallel cultures lacking PC61 rapidly lost FOXP3. These results pertained to TGF-β-inducible "iTregs" because Tregs from 2D2-FIG Rag1 ^{- / -} mice, which lack thymic or natural Tregs, were stabilized by continuous culture in IL-2 and PC61. MOG-specific and polyclonal Tregs upregulated the Treg-associated markers Neuropilin-1 (NRP1) and Helios (IKZF2). Just as PC61 stabilized FOXP3 ⁺ Tregs during expansion in IL-2, TGF-β fully stabilized FOXP3 ⁺ Tregs during cellular activation in the presence of dendritic cells and antigen/mitogen. Adoptive transfer of blastogenic CD25 ^high FOXP3 ⁺ Tregs from MOG35-55-specific 2D2 TCR transgenic mice suppressed experimental autoimmune encephalomyelitis in pretreatment and therapeutic protocols. In conclusion, low IL-2 concentrations coupled with high PC61 concentrations constrained IL-2 signaling to a low-intensity range that enabled dominant stable outgrowth of suppressive CD25 ^high FOXP3 ⁺ Tregs. The ability to indefinitely expand stable Treg lines will provide insight into FOXP3 ⁺ Treg physiology and will be foundational for Treg-based immunotherapy.