The innate immune system releases multiple inflammatory cytokines in response to pathogens and their toxins to combat infection. Sepsis and septic shock develop when such inflammatory response becomes overwhelming and uncontrolled, resulting in significant capillary leakage, tissue edema, organ failure and death. The vascular endothelial barrier dysfunction resulting from massive endothelial cell (EC) death or disassembly of adherence junctions (AJs) is a primary pathological feature of sepsis. Restoration of the endothelial barrier by inhibiting inflammation-induced AJs disassembly and wide-spread EC apoptosis is essential for resolving endothelial hyper-permeability, thus represents a promising therapeutic approach. We have obtained strong preliminary data showing that inhibition of TNF? (the principal mediator of inflammatory effects during sepsis) by a single domain VHH nanoantibody (nAb) we identified from phage display library significantly strengthened endothelial barrier function and greatly promoted survival in two sepsis mouse models. More importantly, the therapeutic effect of the engineered nAb multimers and fusions with another nAb for longer in vivo half-life were even more promising in sepsis models and clearly superior to that of conventional monoclonal anti-TNF? antibody Humira, which is widely used for the treatment of autoimmune diseases and previously tested for treating sepsis. Based on our supporting data, we posit that the new generation of engineered anti-TNF? nAbs in our hands have very good potential to be developed into a novel therapeutics for treating sepsis. In the proposed Phase I SBIR studies, we will apply a multidisciplinary approach to further define the efficacy of the main choice of our anti-TNF? nAb and it?s humanized, multivalent version in promoting survival after sepsis induction in mice, understand its functions in strengthening endothelial barrier function and the resultant improved multiple organ function, and subject it for commercial potential assessment aimed at developing a clinical applicable new medicine to treat sepsis.