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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+CD56dim NK cells. Functionally, CD56dim 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. [ABSTRACT FROM AUTHOR]

NK cells play an important role in the defense against viral infections. However, little is known about the regulation of NK cell responses during the first days of acute viral infections in humans. In this study, we used the live attenuated yellow fever virus (YFV) vaccine 17D as a human in vivo model to study the temporal dynamics and regulation of NK cell responses in an acute viral infection. YFV induced a robust NK cell response in vivo, with an early activation and peak in NK cell function at day 6, followed by a delayed peak in Ki67 expression, which was indicative of proliferation, at day 10. The in vivo NK cell response correlated positively with plasma type I/III IFN levels at day 6, as well as with the viral load. YFV induced an increased functional responsiveness to IL-12 and IL-18, as well as to K562 cells, indicating that the NK cells were primed in vivo. The NK cell responses were associated primarily with the stage of differentiation, because the magnitude of induced Ki67 and CD69 expression was distinctly higher in CD57- NK cells. In contrast, NK cells expressing self- and nonself-HLA class I-binding inhibitory killer cell Ig-like receptors contributed, to a similar degree, to the response. Taken together, our results indicate that NK cells are primed by type I/III IFN in vivo early after YFV infection and that their response is governed primarily by the differentiation stage, independently of killer cell Ig-like receptor/HLA class I-mediated inhibition or education. [ABSTRACT FROM AUTHOR]

Amniotic fluid (AF) surrounds the growing fetus, and cells derived from AF are commonly used for diagnosis of genetic diseases. Intra-amniotic infections are strongly linked to preterm birth, which is the leading cause of perinatal mortality worldwide. Surprisingly little is known, however, about mature hematopoietic cells in AF, which could potentially be involved in immune responses during pregnancy. In this study, we show that the dominating population of viable CD45 + cells in AF is represented by a subset of fetal CD103 + group 3 innate lymphoid cells (ILCs) producing high levels of IL-17 and TNF. Fetal CD103 + ILC3s could also be detected at high frequency in second-trimester mucosal tissues (e.g., the intestine and lung). Taken together, our data indicate that CD103 + ILC3s accumulate with gestation in the fetal intestine and subsequently egress to the AF. The dominance of ILC3s producing IL-17 and TNF in AF suggests that they could be involved in controlling intra-amniotic infections and inflammation and as such could be important players in regulating subsequent premature birth., (Copyright © 2016 by The American Association of Immunologists, Inc.)

Although NK cells are considered innate, recent studies in mice revealed the existence of a unique lineage of hepatic CD49a(+)DX5(-) NK cells with adaptive-like features. Development of this NK cell lineage is, in contrast to conventional NK cells, dependent on T-bet but not Eomes. In this study, we describe the identification of a T-bet(+)Eomes(-)CD49a(+) NK cell subset readily detectable in the human liver, but not in afferent or efferent hepatic venous or peripheral blood. Human intrahepatic CD49a(+) NK cells express killer cell Ig-like receptor and NKG2C, indicative of having undergone clonal-like expansion, are CD56(bright), and express low levels of CD16, CD57, and perforin. After stimulation, CD49a(+) NK cells express high levels of inflammatory cytokines but degranulate poorly. CD49a(+) NK cells retain their phenotype after expansion in long-term in vitro cultures. These results demonstrate the presence of a likely human counterpart of mouse intrahepatic NK cells with adaptive-like features., (Copyright © 2015 by The American Association of Immunologists, Inc.)

The live attenuated yellow fever virus (YFV) 17D vaccine provides a good model to study immune responses to an acute viral infection in humans. We studied the temporal dynamics, composition, and character of the primary human T cell response to YFV. The acute YFV-specific effector CD8 T cell response was broad and complex; it was composed of dominant responses that persisted into the memory population, as well as of transient subdominant responses that were not detected at the memory stage. Furthermore, HLA-A2- and HLA-B7-restricted YFV epitope-specific effector cells predominantly displayed a CD45RA(-)CCR7(-)PD-1(+)CD27(high) phenotype, which transitioned into a CD45RA(+)CCR7(-)PD-1(-)CD27(low) memory population phenotype. The functional profile of the YFV-specific CD8 T cell response changed in composition as it matured from an effector- to a memory-type response, and it tended to become less polyfunctional during the course of this transition. Interestingly, activation of CD4 T cells, as well as FOXP3(+) T regulatory cells, in response to YFV vaccination preceded the kinetics of the CD8 T cell response. The present results contribute to our understanding of how immunodominance patterns develop, as well as the phenotypic and functional characteristics of the primary human T cell response to a viral infection as it evolves and matures into memory.

Stem cell transplantation across HLA barriers may trigger NK cell-mediated graft-vs-leukemia effects leading to improved survival for patients with hematological malignancies. However, the genetic algorithm based on killer cell Ig-like receptor (KIR) and HLA genes used to predict NK cell alloreactivity have yielded discrepant results. Accordingly, it has been difficult to define transplantation settings that favor NK cell alloreactivity. In this study, we have used multiparameter flow cytometry to simultaneously analyze the cell surface expression of all four major inhibitory KIR and CD94/NKG2A to determine the size of the alloreactive NK cell repertoires in 31 individuals homozygous for the group A KIR haplotype. We observed a vast variability in the frequencies of cells with an alloreactive potential, ranging from 0 to 62% of the total NK cell population depending on which, and how many, KIR ligands were missing in theoretical recipients. This analysis required a functional examination of KIR3DL2-single positive NK cells, showing that this subset was hyporesponsive in individuals harboring the cognate ligands HLA-A3/A11. The results provide new insights into the variability of the functional alloreactive NK cell repertoire and have implications for donor selection in hematopoietic stem cell transplantation and adoptive NK cell-based immunotherapy.

NK cells play an integral role in the innate immune response by targeting virally infected and transformed cells with direct killing and providing help to adaptive responses through cytokine secretion. Whereas recent studies have focused on NK cells in HIV-1-infected adults, the role of NK cells in perinatally HIV-1-infected children is less studied. Using multiparametric flow cytometric analysis, we assessed the number, phenotype, and function of NK cell subsets in the peripheral blood of perinatally HIV-1-infected children on highly active antiretroviral therapy and compared them to perinatally exposed but uninfected children. We observed an increased frequency of NK cells expressing inhibitory killer Ig-like receptors in infected children. This difference existed despite comparable levels of total NK cells and NK cell subpopulations between the two groups. Additionally, NK cell subsets from infected children expressed, with and without stimulation, significantly lower levels of the degranulation marker CD107, which correlates with NK cell cytotoxicity. Lastly, increased expression of KIR2DL3, NKG2C, and NKp46 on NK cells correlated with decreased CD4+ T-lymphocyte percentage, an indicator of disease severity in HIV-1- infected children. Taken together, these results show that HIV-1-infected children retain a large population of cytotoxically dysfunctional NK cells relative to perinatally exposed uninfected children. This reduced function appears concurrently with distinct NK cell surface receptor expression and is associated with a loss of CD4+ T cells. This finding suggests that NK cells may have an important role in HIV-1 disease pathogenesis in HIV-1-infected children.

Although human T cells enter the peripheral lymphoid tissues early during fetal development, the adaptive immune system in the fetus has largely been regarded as functionally immature and unresponsive to stimulation. In this study, we show that depletion of fetal CD4+CD25(high) T regulatory (T(Reg)) cells, which are present at high frequency in fetal lymphoid tissues, results in vigorous T cell proliferation and cytokine production in vitro, even in the absence of exogenous stimulation. Analysis of CD4+ and CD8(+) T cell populations revealed a large subset of cells that expressed the early activation Ag, CD69. We show that this population represents a subset of highly reactive fetal T cells actively suppressed by fetal CD4+CD25(high) T(Reg) cells during development. These findings indicate that fetal T cells are, in the absence of CD4+CD25(high) T(Reg) cells, highly responsive to stimulation and provide evidence for an important role for CD4+CD25(high) T(Reg) cells in controlling T cell responses in utero.

Lymphocytic choriomeningitis virus infection of H-2(b) mice generates a strong CD8(+) CTL response mainly directed toward three immunodominant epitopes, one of which, gp33, is presented by both H-2D(b) and H-2K(b) MHC class I molecules. This CTL response acts as a selective agent for the emergence of viral escape variants. These variants generate altered peptide ligands (APLs) that, when presented by class I MHC molecules, antagonize CTL recognition and ultimately allow the virus to evade the cellular immune response. The emergence of APLs of the gp33 epitope is particularly advantageous for LCMV, as it allows viral escape in the context of both H-2D(b) and H-2K(b) MHC class I molecules. We have determined crystal structures of three different APLs of gp33 in complex with both H-2D(b) and H-2K(b). Comparison between these APL/MHC structures and those of the index gp33 peptide/MHC reveals the structural basis for three different strategies used by LCMV viral escape mutations: 1) conformational changes in peptide and MHC residues that are potential TCR contacts, 2) impairment of APL binding to the MHC peptide binding cleft, and 3) introduction of subtle changes at the TCR/pMHC interface, such as the removal of a single hydroxyl group.

Mouse NK cells express Ly-49 receptors specific for classical MHC class I molecules. Several of the Ly-49 receptors have been characterized in terms of function and ligand specificity. However, the only Ly-49 receptor-ligand interaction previously described in detail is that between Ly-49A and H-2D(d), as studied by point mutations in the ligand and the crystal structure of the co-complex of these molecules. It is not known whether other Ly-49 receptors bind MHC class I in a similar manner as Ly-49A. Here we have studied the effect of mutations in Ly-49C on binding to the MHC class I molecules H-2K(b), H-2D(b), and H-2D(d). The MHC class I molecules were used as soluble tetramers to stain transiently transfected 293T cells expressing the mutated Ly-49C receptors. Three of nine mutations in Ly-49C led to loss of MHC class I binding. The three Ly-49C mutations that affected MHC binding correspond to Ly-49A residues that are in contact or close to H-2D(d) in the co-crystal, demonstrating that MHC class I binding by Ly-49C is dependent on residues in the same area as that used by Ly-49A for ligand contacts.