Upstream Bio, Inc. is a clinical-stage biotechnology company developing treatments for inflammatory diseases, with an initial focus on severe respiratory disorders. The Company is developing verekitug, the only known antagonist in clinical development, that targets the receptor for thymic stromal lymphopoietin (TSLP), a cytokine which is a clinically validated driver of inflammatory response positioned upstream of multiple signaling cascades that affect a variety of immune mediated diseases. It has advanced this highly potent monoclonal antibody into separate phase II trials for the treatment of severe asthma and chronic rhinosinusitis with nasal polyps and plans to initiate development in chronic obstructive pulmonary disease. TSLP is a member of a class of epithelial cytokines, also including IL-25 and IL-33, commonly referred to as alarmins. TSLP is primarily produced by epithelial cells, especially in the lung, gastrointestinal tract and skin.