Scientists have generated a structurally modified form of the cytokine interleukin-2 (IL-2) that they claim triggers far greater antitumor responses than native IL-2 in vivo but with lower toxicity. The Stanford University School of Medicine-led team used an in vitro evolution technique to tweak the IL-2 structure in order to bypass the requirement for signaling through a high affinity receptor complex on activated T cells.
In vivo tests showed that when compared with IL-2, the IL-2 variant, which they termed IL-2 superkine (or Super-2), induced superior expansion of cytotoxic T cells, proportionally less expansion of T regulatory cells, and reduced pulmonary edema. K. Christopher Garcia, M.D., and colleagues, report their results in Nature in a paper titled “Exploiting a natural conformational switch to engineer an interleukin-2 ‘superkine.’” Dr. Garcia says the prospective clinical utility of Super-2 has already generated significant interest from potential licensing partners, and NIH researchers are currently carrying out tests in a range of tumor models to help fast-track its development.
The principle functional effect of IL-2 is to promote T-cell proliferation and particularly for naive T cells. IL-2 signaling on activated T cells is effected through a quaternary high-affinity receptor complex consisting of IL-2, IL-2Rα (CD25), IL-2Rβ, and IL-2Rγ, the researchers explain. However, naive T cells are actually relatively insensitive to IL-2 as they only express small amounts of IL-2Rβ and IL-2Rγ. They only acquire sensitivity after CD25 expression, which captures the cytokine and presents it to the IL-2Rβ and IL-2Rγ receptors.
Crystallographic analysis of the modified Super-2 developed by the Stanford team showed the mutations were primarily in the molecule’s core and acted to both stabilize the structure and give it a receptor-binding conformation that mimics that of native IL-2 bound to CD25, effectively eliminating the functional requirement of IL-2 for CD25 expression and directly triggering proliferation of T cells, they claim.
In vitro experiments demonstrated the potency of Super-2 in terms of the expansion of naive and experienced CD4+ T cells and CD8+ T cells that expressed even low levels of CD25. “The evolved mutations in the IL-2 superkine recapitulated the functional role of CD25 by eliciting potent phosphorylation of STAT5 and vigorous proliferation of T cells irrespective of CD25 expression,” they state.
Initial in vivo experiments in mice confirmed that compared with IL-2, Super-2 was significantly more stimulatory for cytotoxic CD8+ T cells but caused significantly less pulmonary edema. Subsequent tests were then carried out on specific mouse melanoma, colon, and lung tumor models, selected because they had previously been shown to demonstrate tumor regression after high-dose IL-2 administration.
In these animals Super-2 treatment resulted in up to 70% greater tumor regression than IL-2 therapy. “The practical implications are that this conformational nuance in IL-2 can be exploited for therapy,” the authors conclude. “The IL-2 superkine robustly activates cytotoxic CD8+ T cells and NK cells for potent antitumor immune responses, yet it elicits minimal toxicity, suggesting that the IL-2 superkine could warrant reconsideration for clinical applications of IL-2.”