So far there are at least five coronaviruses that are responsible for human respiratory infections. The pathogenesis and immunity of coronavirus are the result of interaction between virus and host. The innate immune response will be started when coronaviruses infect cells. Coronaviruses usually encode multifunctional proteins that are critical for viral replication and blocking the innate immune response to viral infection. Human coronavirus NL63 (HCoV-NL63), a new-emerging human coronavirus for human respiratory infections, has two papain-like protease(PLP) core domains, PLP1 and PLP2, in nonstructural protein nsp3. Besides the proteolytic processing activity towards pp1a(1ab), it was previously demonstrated that PLP2 of HCoV-NL63 has in vitro deubiquitinase (DUB) activity. However, the characteristics and functions of DUB activity of NL63 coronavirus PLPs are poorly understood. It was first demonstrated that the core domain of PLP2, but not PLP1, has in vivo DUB activity, and the DUB activity is not dependent on the catalytic residue of D1849. However, the PLP2 DUB activity is significantly reduced when the catalytic sites of C1678 and H1836 were mutated. PLP2 has both in vivo DUB and DeISGylation activity, and PLP2 exhibits DUB activity toward ubiquitinated branched peptides without any specificity for either Lys⁴⁸ linkages or Lys⁶³ linkages. Furthermore, PLP2, but not PLP1, is the only core domain responsible for the inhibition of both RIG-I and TLR3-dependent induction of IFNα/β expression. Mechanism study demonstrated that PLP2 interacts with the key regulation players of RIG-I and ERIS (also called STING/MITA) of IFN induction pathway, and induces the deubiquitination of RIG-I and ERIS. Overall, these results definitely demonstrated that for the two core domains of PLP responsible for proteolytically processing of N-terminal part of pp1a(1ab) of NL63 coronavirus, PLP2 is the only core domain of PLPs that responsible for DUB activity and IFN antagonists. The studies are currently underway to determine the biological significance of DUB of NL63 coronavirus PLP2 in virus replication and pathogenesis.