2C). context and help determine functional outcomes in B cells. H2O2 is the primary reactive oxygen species (ROS) produced by B cells. H2O2 amplifies BCR signaling by transiently inhibiting BCR-associated protein tyrosine phosphatases (PTPs) (1). H2O2 is also produced as part of innate immune responses to wounds and contamination (2). However, it Tartaric acid is not known what impact H2O2 has on healthy human B-cell signaling responses and whether B cells undergoing GC reactions respond differently to H2O2. Seconds after BCR crosslinking, a network of signaling molecules becomes activated through post-translational modifications. As signaling directs B cells down differentiation pathways, B cells adopt well characterized signatures defined primarily by protein expression (3). Na?ve B cells in humans are defined by expression of CD19, CD20, and IgD. GC B cells are defined as CD19+, CD20hi, CD38+, IgD? B cells. Memory B cells, on the other hand, express CD19, CD20, and CD27. Furthermore, human plasmablasts are defined as CD38hi, CD20lo cells that are in the process of down regulating surface BCR and most other surface antigens. The GC is usually a highly active environment vital for proper functioning of the adaptive immune system. GC B cells Tartaric acid undergo affinity maturation, which involves iterative cycles of clonal expansion, somatic hypermutation, and selection Tartaric acid that result in class-switched memory B cells and antibody-secreting plasma cells (4, 5). How high-affinity B cells are selected in the GC is not entirely clear. Increased antigen capture and presentation leads to increased rates of cell division (5, 6). It is also possible that actively proliferating GC B cells produce unique signals that promote their survival and proliferation. In addition, GC B cell signaling is usually regulated by PTPs (7, 8). For example, cell surface CD22 can recruit phosphatases, such as SHP-1, to attenuate BCR signaling (8, 9). Opposing this activity are NADPH oxidases (NOXs), such as DUOX1, which produce H2O2 and lower BCR signaling thresholds by reversibly inhibiting phosphatases (2). The environment surrounding the BCR simulates NOX, which produces endogenous ROS (10). In turn, ROS oxidize the extracellular compartment and activate COL4A3BP the BCR signaling pathway, creating a positive feedback loop. BCR signaling governs B-cell functions, Tartaric acid and activation and termination of BCR signaling is usually finely tuned by multiple levels of regulation in healthy cells. While the biochemistry of BCR signaling is usually well-understood in model systems, little is known about the quality of BCR signaling in mature, healthy human B cells. Addressing this gap by mapping the influence of ROS on healthy B-cell signaling is important for placing into context the extreme BCR signaling and H2O2 responses observed in B-cell diseases and disorders (11). Here, we used high-dimensional mass cytometry, phospho-specific flow cytometry, and novel computational data analysis tools (12-14) to better understand how ROS regulate BCR signaling within subsets of primary human tonsillar B cells. MATERIALS & METHODS HUMAN SAMPLES Tonsils were obtained from children undergoing routine tonsillectomies in accordance with the Declaration of Helsinki following protocols approved by Vanderbilt University Medical Center (VUMC) Institutional Review Board. Single cell suspensions were prepared and stored in liquid nitrogen. ANTIBODIES Fluorescent antibodies for CD20, IgD, CD38, CD3, CD27, p-SRC, p-SYK, p-PLC, and p-NFB were conjugated to BV421, PerCP Cy5.5, FITC, PE-Cy7, BUV395, BV570, BV605, PE, and AlexaFluor647 (BD Biosciences, Invitrogen, or Biolegend). Mass cytometry antibodies are listed in Supplemental Information (Supplemental Table 1). FLUORESCENT CYTOMETRY Aliquots of cryopreserved single cell tonsillar samples were thawed into 10.