TLR3/TRIF pathway confers protection against herpes simplex encephalitis through NK cell activation mediated by a loop of type I IFN and IL-15 from epithelial and dendritic cells

Autosomal recessive (AR) and dominant (AD) deficiencies of TLR3 and TRIF are believed to be crucial genetic causes of herpes simplex encephalitis (HSE), which is a fatal disease causing focal or global cerebral dysfunction following infection with herpes simplex virus type 1 (HSV-1). However, few studies have been conducted on the immunopathological networks of HSE in the context of TLR3 and TRIF defects at the cellular and molecular levels. In this work, we deciphered the crosstalk between type I IFN (IFN-I)-producing epithelial layer and IL-15-producing dendritic cells (DC) to activate NK cells for the protective role of TLR3/TRIF pathway in HSE progression after vaginal HSV-1 infection. TLR3- and TRIF-ablated mice showed enhanced susceptibility to HSE progression, along with high HSV-1 burden in vaginal tract, lymphoid tissues and CNS. The increased HSV-1 burden in TLR3- and TRIF-ablated mice did not correlate with increased infiltration of Ly-6C⁺ monocytes, but it was closely associated with impaired NK cell activation in vaginal tract. Furthermore, using delicate ex vivo experiments and bone marrow transplantation, TRIF deficiency in tissue-resident cells, such as epithelial cells in vaginal tract, was found to cause impaired NK cell activation by means of low IFN-I production, whereas IFN-I receptor in DC was required for NK cell activation via IL-15 production in response to IFN-I produced from epithelial layer. These results provide new information about IFN-I- and IL-15-mediated crosstalk between epithelial cells and DC at the primary infection site, which suppresses HSE progression in a TLR3- and TRIF-dependent manner.