Molecular mechanisms of interleukin -10 -mediated immune regulation.

Abstract

Cellular IL-10s (cIL-10) of human and murine origin have extensive sequence and structural homology to the Epstein-Barr Virus <italic>BCRF I</italic> gene product, known as viral IL-10 (vIL-10). Although these cytokines share many immunosuppressive properties, vIL-10 lacks several of the immunostimulatory activities of cIL-10 on certain cell types. The molecular and cellular basis for this dichotomy is not currently defined. In this thesis, we show that the single amino acid isoleucine at position 87 of cIL-10 is required for its immunostimulatory function. Substitution of isoleucine in cIL-10 with alanine, which corresponds to the vIL-10 residue, abrogates immunostimulatory activity for thymocytes, mast cells and alloantigenic responses, while preserving immunosuppressive activity for inhibition of IFNgamma production and prolongation of cardiac allograft survival. Conversely, substitution of alanine with isoleucine in vIL-10 converts it to a cIL-10 like molecule with immunostimulatory activity. We characterized the receptor binding specificity and affinity for hIL-10 and vIL-10 using CHO cells expressing different IL-10 receptor component(s). IL-10R1 alone binds to hIL-10 or vIL-10, and the binding affinity of hIL-10 is 1000-fold higher than that of vL-10. Although IL-10R2 alone does not bind to IL-10, it is essential for both hIL-10 and vIL-10 mediated signal transduction and immune regulation. Further studies on previously identified single amino acid mutants demonstrated that the amino acid at position 87 of IL-10 plays a critical role in determining its receptor binding affinity, suggesting that receptor binding affinity rather than specificity might determine the spectrum of the biological activities of various IL-10s. The signal transduction initiated by hIL-10 and vIL-10 were compared in B cell and mast cell lines, and we demonstrated that the inability of vIL-10 to stimulate immune responses as compared to hIL-10 is largely due to the inability to initiate signaling. The absent signal transduction is due to low level expression of cell surface IL-10R1 expression, as overexpressing IL-10R1 on these cells allows vIL-10 to initiate hIL-10-like signals and subsequent biological responses. Further more, the intensity of these responses depends on the density of IL-10R1 expression. These data demonstrate that expression of IL-10R1 plays a critical role in determining whether cells respond to IL-10s. Taken together, our results demonstrate that ligand-receptor interactions play a critical role in determining whether certain cells are activated by various IL-10s. Alteration of receptor binding affinity or modulation of cell surface IL-10R1 expression can lead an IL-10 molecule to have stimulatory or suppressive immune responses in vivo.