The overall goal of this thesis is to use recombinant engineering to improve the therapeutic index of cytokine immunotherapies. The model cytokine that we used was interleukin-12 (IL-12), which had displayed intolerable toxicity in clinical trials when administered in its native form.Chapter 1 introduces the pathophysiological aspects of solid tumors, namely vascular remodeling, abnormal deposition of extracellular matrix and overexpression of proteases. We then discuss the role of the immune system in cancer progression and mention the key players of the immune system that either facilitate or impede tumor growth. We also provide a brief survey of various immunotherapy strategies that are currently employed in the clinic. Last, we introduce cytokines as potential immunotherapeutics, predominantly focusing on IL-12. Chapter 2 describes the collagen-targeting technology that we applied to IL-12. Because collagen is overexpressed in solid tumors and is abberantly exposed to the bloodstream, we decided to engineer a fusion protein consisting of a collagen-binding domain (CBD) and IL-12 to target the tumor matrix. We confirmed that CBD-IL-12 fusion protein was bioactive in vitro and displayed high affinity to collagen. We showed that CBD-IL-12 exerted superior antitumor efficacy compared to unmodified IL-12 in various solid tumor models. We also demonstrated that CBD-IL-12 induces less systemic side effects in tumor-bearing and healthy mice. Finally, we showed that CBD-IL-12 strongly synergizes with checkpoint inhibitor (CPI) antibodies in aggressive transplantable melanoma as well as in autochthonous melanoma. Chapter 3 describes the masking technology that we applied to IL-12. A masking domain, which inhibits the interaction between IL-12 and its endogenous receptor, was fused to IL-12 via a protease-cleavable linker. Upon cleavage by tumor-associated proteases, the masking domain dissociates, yielding bioactive IL-12 which activates the immune cells locally. In the periphery, due to lack of proteases, the masking domain prevents IL-12 from binding to its receptor, eliminating any off-target effects. We first confirmed that masked IL-12 displayed ~80-fold less bioactivity than unmodified IL-12, whereas treatment with recombinant proteases fully restored the bioactivity. We then demonstrated that masked IL-12 induced potent antitumor efficacy in various tumor models and potentiated CPI therapy. Last, we showed that masked IL-12 significantly decreased the incidence of adverse events, boosting the therapeutic index of IL-12. In Chapter 4, we discuss potential limitations of each technology and offer potential strategies to overcome these limitations. We also comment on how IL-12 can be used in the clinic, given its complex biology and restricted expression of its receptor.