Disclosed herein are methods of using 3-(5-amino-2-methyl-4-oxoquinazolin-3(4H)-yl)piperidine-2,6-dione (compound 1) for modulating lymphocytic activity, including activity of B cells and/or T cells, and in immune-related diseases or inflammatory diseases, including systemic lupus erythematosus and scleroderma. Disclosed are pharmaceutical compositions and dosing regimens for use in the methods provided.