Project Summary Our long-term goal is to study the ubiquitin system in immune regulation. We initiated a new study of the deubiquitinating enzyme CYLD and found that deficiency of CYLD in T regulatory cells (Treg) caused chronic lung inflammation by increasing IL-4 production. These preliminary studies pointed to new mechanisms of CYLD regulation of Tregs, via modulating the cytokine production, and highlighted potential tissue-specific aspects of Treg function. Recently, we found that CYLD acts as a positive regulator in the differentiation of Treg to T follicular regulatory T cells (Tfr) in lymphoid tissues. These preliminary studies thus provide us with a solid basis for testing our central hypothesis that the ubiquitination/deubiquitination system plays a critical role in immune regulation via modulating different T cell subsets in tissue context-dependent manner. Here we plan to test this hypothesis by proposing two Specific Aims: Aim 1, to study CYLD in Treg regulation; and Aim 2, to study the role of CYLD in Tfr. The expected results will significantly advance our basic knowledge on the protein deubiquitination pathway in immune regulation, and will provide clues to therapeutic intervention of human diseases.