CD22, a surface molecule expressed on B cells, negatively regulates activation, in part by attenuating calcium signals. Its absence is reported to increase autoantibody formation. CD22 is a member of the sialic acid binding immunoglobulin domain containing lectin (siglec) family of proteins that are expressed by hematopoietic cells. Like CD22, many siglecs have ITIMs and act as negative regulators of cell activation. As CD22 is currently the best studied member of the siglec family, an understanding of its function might be expected to provide insight into mechanisms that are widely used in regulating hematopoietic cell functions. We have evidence that CD22 can augment Ca2+ extrusion following B cell activation. Regulation of Ca2+ extrusion of lymphocytes has not been previously described. We will characterize the mechanism for this regulation and see if impacts B cell function in vivo. We have recently established that CD22 requires its lectin activity for full function. We will ask if lectin function plays a role in the association of CD22 with the B cell receptor. We have discovered that the conserved cytoplasmic juxtamembrane region of CD22 contributes to its function, and we will study the basis for this, including a test of the hypothesis that it contributes to association with the BCR. While the importance of ITIMs for negative regulation is well established, we have discovered that CD22 acts as a negative regulator in the absence of ITIMs. We ask how this ITIM-independent negative regulation pathway functions. Overall, our goal is to understand how the several components of CD22, particularly the lectin binding region (specific for (2,6 sialic acids), the juxtamembrane portion of the cytoplasmic tail and the six cytoplasmic tyrosines (including three ITIMs) serve the molecule to diminish calcium signals and to prevent autoantibody formation.