Evidence for a structural motif in toxins and interleukin-2 that may be responsible for binding to endothelial cells and initiating vascular leak syndrome

The dose-limiting toxicity of interleukin-2 (IL-2) and immunotoxin (IT) the rapy in humans is vascular leak syndrome (VLS), VLS has a complex etiology involving damage to vascular endothelial cells (ECs), extravasation of flui ds and proteins, interstitial edema, and organ failure. IL-2 and ITs prepar ed with the catalytic A chain of the plant toxin, ricin (RTA), and other to xins, damage human ECs in vitro and in vivo. Damage to ECs may initiate VLS ; if this damage could be avoided without losing the efficacy of ITs or IL- 2, larger doses could be administered. In this paper, we provide evidence t hat a three amino acid sequence motif, (x)D(y), in toxins and IL-2 damages ECs, Thus, when peptides from RTA or IL-2 containing this sequence motif ar e coupled to mouse IgG, they bind to and damage ECs both in vitro and, in t he case of RTA, in vivo. In contrast, the same peptides with a deleted or m utated sequence do not. Furthermore, the peptide from RTA attached to mouse Ige can block the binding of intact RTA to ECs in vitro and vice versa. In addition, RTA, a fragment of Pseudomonas exotoxin A (PE38-lys), and fibron ectin also block the binding of the mouse IgG-RTA peptide to ECs, suggestin g that an (x)D(y) motif is exposed on all three molecules. Our results sugg est that deletions or mutations in this sequence or the use of nondamaging blocking peptides may increase the therapeutic index of both IL-2, as well as ITs prepared with a variety of plant or bacterial toxins.