TEACHING AN OLD DOG NEW TRICKS: ACTIVITY-ON-TARGET INTERFERON TO TREAT T-ALL

Background Type 1 interferon (IFN) has a long history in the treatment of cancer, including hematological malignancies. The anti-cancer effects induced by IFN result from a combination of 1) direct cancer cell growth inhibition by cell cycle arrest, apoptosis, or differentiation and 2) the activation of the immune system involving antigen presentation by Clec9A+ dendritic cells and priming of cytotoxic CD8+ T-cells. However, IFN therapy experienced variable and unpredictable success in the clinic. Its clinical application is severely impeded by a complex pattern of adverse side-effects, due to the multifaceted activity pattern of IFN. Therefore, safe exploitation of the anti-cancer potential of IFN requires strategies to direct their activity to selected target cells, avoiding systemic toxicity. Aims Safe exploitation of the anti-cancer potential of IFN requires strategies to direct their activity to selected target cells, avoiding systemic toxicity. Methods To improve the therapeutic index of IFN, we have developed AcTaferons (Activity-on-Target Interferon), optimized (mutant) immunocytokines. Mutated IFNa2Q124R, with a strongly reduced affinity for its receptor complex, was fused to single domain antibodies targeting cell-specific domains, which selectively restores the AcTaferon (AFN) activity in a cell-type specific manner. As such, CD8-AFN and Clec9A-AFN were generated which selectively target either CD8+ T(-ALL) cells or Clec9A+ dendritic cells. Results Using CD8- and CD8+ mouse T-ALL cell lines, we evaluated the direct and indirect anti-leukemic effects of our novel AFNs, in vitro and in vivo upon transplantation in immunocompromised and immunocompetent syngeneic hosts. A significant reduction in the leukemic burden was observed. These anti-cancer effects of AFN were similar as observed for the wildtype IFN, but in a cell-type specific manner and with drastically reduced adverse side-effects. Monotherapy was even sufficient to completely cure a proportion of leukemic mice, which highlights the strong anti-leukemic power of these optimized immunocytokines. Strinkingly only the direct effect CD8-AFN on in vivo CD8+ T-ALL was synergistic with asparaginase treatment. No synergism was observed between asparaginase and the indirect immune-mediated Clec9A-AFN anti-leukemic effect. Conclusion In conclusion, we have developed novel optimized immunocytokines as an off-the-shelve targeted immunotherapy for T-ALL.