ISG15 decreases the amount of USP18. 3 / 3
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"ISG15 appeared to act in its unconjugated free form, since silencing of UBE1L or of other ISGylation enzymes failed to reduce USP18 levels (XREF_FIG and XREF_FIG) and patients ' cells transduced with wild-type ISG15 or ISG15 (DeltaGG) exhibited attenuated levels of interferon-stimulated-gene transcripts and proteins (XREF_FIG and XREF_FIG)."

"These data indicate that intracellular free ISG15 downregulates the IFN-alpha and beta response by maintaining levels of the negative-feedback regulator USP18."

"We found that ISG15 deficiency led to reduced levels of the negative regulator USP18 because of increased proteolysis due, at least in part, to SKP2 mediated ubiquitination, resulting in stronger responses to IFN-alpha and beta and an ensuing amplification of IFN-alpha and beta-induced responses."