Abstract: In order to verify whether p-nitrophenylalanine-containing BAFF vaccine can be used as a candidate molecule for the treatment of autoimmune diseases with BAFF over-expression, a soluble mutant of B cell activating factor belonging to the TNF Family(smBAFF)and its pNO₂Phe mutant(pNO₂Phe⁶⁵smBAFF), which site specific incorporated pNO₂Phe at position 65 of smBAFF, were expressed and purified. In order to evaluate the feasibility of using pNO₂Phe⁶⁵smBAFF to treat BAFF-over-expressed autoimmune diseases, we investigate its Lymphocyte-stimulating capacity, immunogenicity and inhibitory effect of serum on biological activity of natural BAFF. The pharmacological activity of pNO₂Phe⁶⁵smBAFF was evaluated using a cGVHD(graft-versus-host disease)induced SLE mouse model. Results indicated that pNO₂Phe⁶⁵smBAFF, could bind to mouse lymphocytes but could not promote the proliferation of mouse lymphocytes. Moreover, the incorporation of pNO₂Phe significantly increased the immunogenicity and induced cross-antibody, which can inhibit the biological activity of natural BAFF. In cGVHD induced SLE mouse model, pNO₂Phe⁶⁵smBAFF can significantly reduce the symptoms of the disease and play a therapeutic role. Therefore, pNO₂Phe⁶⁵smBAFF can be used as a candidate molecule for the treatment of autoimmune diseases with BAFF over-expression.