Moreover, APEX4v1 polynucleotides and polypeptides of the present invention, including agonists and/or fragments thereof, have uses that include, but are not limited to modulating cell adhesion, particularly in leukocytes, modulating the generation of co-stimulatory signals, enhancing antigen-specific proliferation, enhancing antigen-specific cytokine production (e.g., such as those induced by SLA