USP44 positively regulates innate immune response to DNA viruses through deubiquitinating MITA

Mediator of IRF3 activation (MITA, also known as stimulator of interferon genes, STING) senses the second messenger cyclic GMP-AMP (cGAMP) which is synthesized upon DNA virus infection and activates innate antiviral immune response. It has been demonstrated that the activity of MITA is delicately regulated by various post-translational modifications including polyubiquitination. In this study, we identified the deubiquitinating enzyme USP44 as a positive regulator of MITA. USP44 is recruited to MITA following DNA virus infection and removes K48-linked polyubiquitin moieties from MITA at K236, therefore prevents MITA from proteasome mediated degradation. USP44-deficiency results in acceleration of HSV-1-induced degradation of MITA and reduced induction of type I interferons (IFNs) and proinflammatory cytokines. Consistently, Usp44(-/-) mice are more susceptible to HSV-1 infection as indicated by higher tissue viral titers, greater tissue damage and lower survival rate. These findings suggest that USP44 plays a specific and critical role in the regulation of innate immune response against DNA viruses. Author summary Cyclic GMP-AMP synthase (cGAS) senses cytosolic dsDNA and initiates signal transductions, leading to activation of innate immune response. MITA is the key adaptor protein downstream of cGAS and plays a critical role in cGAS-mediated signaling. The activity of MITA is tightly regulated by various post-translational modifications including polyubiquitination and deubiquitination. Here we found that the deubiquitinating enzymes USP44 associates with MITA and removes the K48-linked polyubiquitin chains from MITA, therefore maintains the stability of MITA after DNA virus infection. Deficiency of USP44 results in accelerated degradation of MITA, impaired induction of type I IFNs and proinflammatory cytokines, and increased viral replication. These findings suggest that USP44 is a positive regulator of MITA and plays an important role in the regulation of innate immune response against DNA viruses.