The present invention provides nucleic acids encoding B7-related factors that modulate the activation of immune or inflammatory response cells, such as T-cells. Also provided are expression vectors and fusion constructs comprising nucleic acids encoding B7-related polypeptides, including BSL1, BSL2, and BSL3. The present invention further provides isolated B7-related polypeptides, isolated fusion proteins comprising B7-related polypeptides, and antibodies that are specifically reactive with B7-related polypeptides, or portions thereof. In addition, the present invention provides assays utilizing B7-related nucleic acids, polypeptides, or peptides. The present invention further provides compositions of B7-related nucleic acids, polypeptides, fusion proteins, or antibodies that are useful for the immunomodulation of a human or animal subject.