Targeted therapy to the IL‐2R using diphtheria toxin and caspase‐3 fusion proteins modulates Treg and ameliorates inflammatory colitis

Pathogenic lymphocytes in the enteric wall of inflammatory bowel disease patients display various abnormalities, including reduced sensitivity to apoptosis. We evaluated a therapeutic approach to elimination of cytotoxic cells, using two IL‐2 fusion proteins, a diphtheria toxin (IL2‐DT) and a caspase‐3 (IL2‐cas) conjugate. In models of acute (dextran sodium sulfate and trinitrobenzene sulfonic acid) and chronic (dextran sodium sulfate) toxic colitis, therapeutic doses of the fusion proteins improved survival and prevented colon shortening. While both chimeric proteins eradicated CD4⁺CD25⁺Foxp3⁺ T cells in mesenteric LN, IL2‐DT caused severe lymphopenia. In contrast, IL2‐cas was equally protective and increased fractional expression of Foxp3. Similar effects of the fusion proteins were observed in healthy mice: IL2‐DT caused lymphopenia and IL2‐cas increased fractional expression of FoxP3. The fusion proteins induced apoptosis in CD25⁺ T cells in vitro, with lower toxicity of IL2‐cas to Foxp3⁺ T cells. These data infer that targeted depletion of cells expressing the IL‐2 receptor has therapeutic potential in models of inflammatory colitis, despite depletion of CD25⁺ Treg. The IL2‐cas fusion protein is particularly relevant to inflammatory bowel disease, as direct internalization of toxic moieties overcomes multiple pathways of resistance to apoptosis of colitogenic T cells.