NK Cell Responses to Human Tick-Borne Encephalitis Virus Infection.

Tick-borne encephalitis virus (TBEV) is a flavivirus that is transferred to humans by infected ticks. The virus causes tick-borne encephalitis, a severe infection of the CNS with a high risk for long-lasting sequelae. Currently, no treatment exists for the disease. Understanding the cellular immune response to this infection is important to gain further understanding into the pathogenesis, treatment, and prevention of the disease. NK cells are known to participate in the control of viral infections. We performed a longitudinal analysis of the human NK cell response to TBEV infection in a cohort of infected individuals from the onset of severe clinical symptoms to the convalescence phase. NK cell activation, as measured by expression of Ki67, was apparent at the time of hospitalization. By 3 wk after hospitalization, it decreased to levels seen in healthy controls. Concomitant with the increase in NK cell activation, augmented levels of IL-12, IL-15, IL-18, IFN-γ, and TNF were detected in patient plasma. This TBEV-induced NK cell activation was restricted predominantly to differentiated CD57(+)CD56(dim) NK cells. Functionally, CD56(dim) NK cells responded poorly to target cells at the time of hospitalization, but they recovered functional capacity to control levels during the convalescent phase. In contrast, the responsiveness of NK cells to cytokine stimulation remained intact throughout the disease. These findings demonstrate that NK cells respond to TBEV infection with characteristics that are distinct from those of other human viral infections and provide insights into the NK cell response to clinical TBEV infection.

Specificity and dynamics of effector and memory CD8 T cell responses in human tick-borne encephalitis virus infection.

Tick-borne encephalitis virus (TBEV) is transferred to humans by ticks. The virus causes tick-borne encephalitis (TBE) with symptoms such as meningitis and meningoencephalitis. About one third of the patients suffer from long-lasting sequelae after clearance of the infection. Studies of the immune response during TBEV-infection are essential to the understanding of host responses to TBEV-infection and for the development of therapeutics. Here, we studied in detail the primary CD8 T cell response to TBEV in patients with acute TBE. Peripheral blood CD8 T cells mounted a considerable response to TBEV-infection as assessed by Ki67 and CD38 co-expression. These activated cells showed a CD45RA-CCR7-CD127- phenotype at day 7 after hospitalization, phenotypically defining them as effector cells. An immunodominant HLA-A2-restricted TBEV epitope was identified and utilized to study the characteristics and temporal dynamics of the antigen-specific response. The functional profile of TBEV-specific CD8 T cells was dominated by variants of mono-functional cells as the effector response matured. Antigen-specific CD8 T cells predominantly displayed a distinct Eomes+Ki67+T-bet+ effector phenotype at the peak of the response, which transitioned to an Eomes-Ki67-T-bet+ phenotype as the infection resolved and memory was established. These transcription factors thus characterize and discriminate stages of the antigen-specific T cell response during acute TBEV-infection. Altogether, CD8 T cells responded strongly to acute TBEV infection and passed through an effector phase, prior to gradual differentiation into memory cells with distinct transcription factor expression-patterns throughout the different phases.

Breadth and Dynamics of HLA-A2- and HLA-B7-Restricted CD8+ T Cell Responses against Nonstructural Viral Proteins in Acute Human Tick-Borne Encephalitis Virus Infection.

Tick-borne encephalitis virus (TBEV) is a leading cause of viral meningoencephalitis in many parts of Europe and eastwards in Asia, with high morbidity and often long-term neurologic sequelae. With no treatment available, studies of the immune response to TBEV are essential for the understanding of the immunopathogenesis of tick-borne encephalitis and for the development of therapeutics. We have previously demonstrated that CD8+ T cell responses in peripheral blood in patients with acute TBEV peak at around 7 d after hospitalization in the neuroinvasive phase of the disease. In this study, we identified six novel TBEV HLA-A2- and HLA-B7-restricted epitopes, all derived from the nonstructural proteins of TBEV. This identification allowed for a comprehensive phenotypic and temporal analysis of the HLA-A2- and HLA-B7-restricted Ag-specific CD8+ T cell response during the acute stages of human TBEV infection. HLA-A2- and HLA-B7-restricted TBEV epitope-specific effector cells predominantly displayed a CD45RA-CCR7-CD27+CD57- phenotype at day 7, which transitioned into separate distinct phenotypes for HLA-A2- and HLA-B7-restricted TBEV-specific CD8+ T cells, respectively. At day 21, the most prevalent phenotype in the HLA-A2-restricted CD8+ T cell populations was CD45RA-CCR7-CD27+CD57+, whereas the HLA-B7-restricted CD8+ T cell population was predominantly CD45RA+CCR7-CD27+CD57+ Almost all TBEV epitope-specific CD8+ T cells expressed α4 and ß1 integrins at days 7 and 21, whereas the bulk CD8+ T cells expressed lower integrin levels. Taken together, human TBEV infection elicits broad responses to multiple epitopes, predominantly derived from the nonstructural part of the virus, establishing distinct maturation patterns for HLA-A2- and HLA-B7-restricted TBEV epitope-specific CD8+ T cells.