Recombinant, ETA′-based CD64 immunotoxins: improved efficacy by increased valency, both in vitro and in vivo in a chronic cutaneous inflammation model in human CD64 transgenic mice

Summary Background Dysregulated, activated macrophages play a pivotal role in chronic inflammatory diseases such as arthritis and atopic dermatitis. These cells display increased expression of the high‐affinity Fcγ receptor (CD64), making them ideal targets for CD64‐specific immunotoxins. We previously showed that a chemically linked immunotoxin, the monoclonal H22‐RicinA, specifically eliminated infiltrating activated macrophages and resolved chronic cutaneous inflammation. However, several disadvantages are associated with classic immunotoxins, and we therefore followed a fusion protein strategy to express the antigen‐binding site alone (scFv H22) fused to a derivative of Pseudomonas exotoxin A (ETA′). Objectives To assess the potential effect of increased valency on efficacy, we produced monovalent [H22(scFv)‐ETA′] and bivalent [H22(scFv)2‐ETA′] versions and evaluated their potential for eliminating activated macrophages both in vitro and in vivo. Methods Both immunotoxins were produced by bacterial fermentation. Binding was assessed by flow cytometry on the monocytic CD64+ cell line U937. Toxicity was analysed by XTT and apoptosis induction by annexin V bioassay. The in vivo effect was tested in a human CD64 transgenic mouse model for cutaneous inflammation. Results The cytotoxic effects of both immunotoxins were clearly due to apoptosis with an IC50 of 140 pmol L−1 for monovalent and only 14 pmol L−1 for the divalent version. In vivo treatment with H22(scFv)‐ETA′ reduced CD64+ activated macrophages to 21% of their initial numbers whereas H22(scFv)2‐ETA′ treatment reduced these cells to 4·8% (P