Summary: While anti-ABO(H) antibodies represent the most common immunological barrier to transfusion and transplantation, variation in anti-ABO(H) antibody levels and overall specificity may play a critical role in dictating the clinical consequence of these naturally occurring antibodies on adverse events following ABO(H) incompatible transfusion and transplantation. However, the factors that regulate this variation, and development of anti-ABO(H) antibodies in general, remain relatively unknown. Given this, our long-term goal is to define key factors that regulate the production and clinical outcome of naturally occurring anti-ABO(H) antibodies. Our central hypothesis is that innate immune factors target blood group expressing microbes, which shapes the specificity and clinical significance of naturally occurring anti-ABO(H) blood group antibodies. Our results also demonstrate that a series of innate immune lectins called galectins possess the ability to specifically bind and kill blood group positive (BG+) microbes through engagement of their carbohydrate BG antigens. Using a new preclinical model that leverages the murine equivalent of ABO(H) blood group antigens, we have also shown that these same microbes can drive the formation of anti-BG antibodies capable of causing ABO(H) incompatible hemolytic transfusion reactions (HTRs). Microbial populations within galectin knockout strains demonstrate significantly increased numbers of BG+ microbes and exposure of recipients to the non-metabolizable pan galectin inhibitor, thiodigalactoside (TDG), likewise results in increased colonization by BG+ microbes. However, galectin absence or inhibition blunts the ability of BG+ microbes to stimulate anti-blood group antibody formation, suggesting that galectin-mediated microbial killing enhances anti-blood group antibody formation. Importantly, we have also shown that galectins do not target all BG+ microbes equally. Thus, a dynamic interplay between the composition of distinct blood group positive microbes in an individual’s microbiota and the ability of galectins to target these microbes exists that may ultimately shape the specificity and overall levels of anti-BG antibodies. As studies to address this intriguing possibility require a unique combination of expertise, I will leverage my current expertise in glycobiology and microbiology with key training in immunology and transfusion medicine, obtained during the K99 phase of this proposal. I will also leverage our new preclinical model and series of recently developed glycan microarrays and build on these tools with establishment of key galectin KO lines and continued microarray expansion to address the following specific aims. Aim 1: Define the role of galectins in shaping anti-blood group antibody levels. Aim 2: Define the consequence of anti-blood group antibody repertoire on HTRs. Completion of these aims will provide unprecedented insight into the fundamental and previously unrecognized role of th...