A single amino acid determines the immunostimulatory activity of interleukin 10

Cellular interleukin 10s (cIL-10s) of human and murine origin have extensiv e sequence and structural homology to the Epstein-Barr virus BCRF-I gene pr oduct, known as viral IL-10 (vIL-10). Although these cytokines share many i mmunosuppressive properties, vIL-10 lacks several of the immunostimulatory activities of cIL-10 on certain cell types, The molecular and cellular base s for this dichotomy are not currently defined. Here, we show that the sing le amino acid isoleucine at position 87 of cIL-10 is required for its immun ostimulatory function. Substitution of isoleucine in cIL-10 with alanine, w hich corresponds to the vIL-10 residue, abrogates immunostimulatory activit y for thymocytes, mast cells, and alloantigenic responses while preserving immunosuppressive activity for inhibition of interferon gamma production an d prolongation of cardiac allograft survival. Conversely, substitution of a lanine with isoleucine in vIL-10 converts it to a cIL-10-like molecule with immunostimulatory activity. This single con servative residue alteration s ignificantly affects ligand affinity for receptor; however, affinity change s do not necessarily alter specific activities for biologic responses in a predictable fashion. These results suggest complex regulation of IL-10 rece ptor-ligand interactions and subsequent biological responses. These results demonstrate that vIL-10 may represent a captured and selectively mutated c IL-10 gene that benefits viral pathogenesis by leading to ineffective host immune responses. The ability to manipulate the activity of IL-10 in either a stimulatory or suppressive direction may be of practical value for regul ating immune responses for disease therapy, and of theoretical value for de termining what aspects of IL-10 activity are important for normal T cell re sponses.