Disclosed are TALL-1 and TALL-1 receptor protein homologues (agonists and antagonists) designed based on the three-dimensional structure of sTALL-1, eBCMA and eBAFF-R; agonist homologues of APRIL; methods of using wild-type APRIL to inhibit the activity of TALL-1; compositions comprising such homologues, nucleic acid molecules encoding such homologues, and therapeutic methods of using such compounds and compositions. Also disclosed are crystalline complexes of sTALL-1 and sTALL-1 in complex with either BCMA or BAFF-R; models of three-dimensional structures of such crystalline complexes and related structures, methods of drug design using any portion of such structures; methods of design and/or identification of regulatory peptides derived from the such structures; compounds identified by drug design using such structures; and the use of such compounds in therapeutic compositions and methods.