Marlowe S. Tessmer

Cutting edge: Priming of NK cells by IL-18.

Recent evidence suggests that NK cells require priming to display full effector activity. In this study, we demonstrate that IL-18 contributed to this phenomenon. IL-18 signaling-deficient NK cells were found to be unable to secrete IFN-γ in response to ex vivo stimulation with IL-12. This was not due to a costimulatory role of IL-18, because blocking IL-18 signaling during the ex vivo stimulation with IL-12 did not alter IFN-γ production by wild-type NK cells. Rather, we demonstrate that IL-18 primes NK cells in vivo to produce IFN-γ upon subsequent stimulation with IL-12. Importantly, IL-12-induced IFN-γ transcription by NK cells was comparable in IL-18 signaling-deficient and -sufficient NK cells. This suggests that priming by IL-18 leads to an improved translation of IFN-γ mRNA. These results reveal a novel type of cooperation between IL-12 and IL-18 that requires the sequential action of these cytokines.

Expansion and Contraction of the NK Cell Compartment in Response to Murine Cytomegalovirus Infection

NK cells are capable of responding quickly to infectious challenge and contribute to the early defense against a wide variety of pathogens. Although the innate NK cell response to murine CMV (MCMV) has been extensively characterized, its resolution and the fate of the activated NK cell population remains unexplored. Herein, we characterize both the expansion and contraction phases of the NK cell response to MCMV. We demonstrate that NK cell recruitment into the immune response to MCMV infection is restricted to the first 3 days of infection and as the peripheral NK cell compartment expands, NK cells undergo accelerated phenotypic maturation. During the resolution of the immune response, NK cell compartmental contraction is marked by the selective death of responding NK cells. Additionally, throughout the infection, a naive NK cell pool that remains responsive to additional stimuli is actively maintained. These findings illustrate the plasticity of the NK cell compartment in response to pathogens and underscore the homeostatic maintenance of the resting peripheral NK cell pool.

NK Cell–Like Behavior of Vα14i NK T Cells during MCMV Infection

Immunity to the murine cytomegalovirus (MCMV) is critically dependent on the innate response for initial containment of viral replication, resolution of active infection, and proper induction of the adaptive phase of the anti-viral response. In contrast to NK cells, the Vα14 invariant natural killer T cell response to MCMV has not been examined. We found that Vα14i NK T cells become activated and produce significant levels of IFN-γ, but do not proliferate or produce IL-4 following MCMV infection. In vivo treatment with an anti-CD1d mAb and adoptive transfer of Vα14i NK T cells into MCMV-infected CD1d−/− mice demonstrate that CD1d is dispensable for Vα14i NK T cell activation. In contrast, both IFN-α/β and IL-12 are required for optimal activation. Vα14i NK T cell–derived IFN-γ is partially dependent on IFN-α/β but highly dependent on IL-12. Vα14i NK T cells contribute to the immune response to MCMV and amplify NK cell–derived IFN-γ. Importantly, mortality is increased in CD1d−/− mice in response to high dose MCMV infection when compared to heterozygote littermate controls. Collectively, these findings illustrate the plasticity of Vα14i NK T cells that act as effector T cells during bacterial infection, but have NK cell–like behavior during the innate immune response to MCMV infection.

NKT cell immune responses to viral infection.

Background: Natural killer T (NKT) cells are a heterogeneous population of innate T cells that have attracted interest because of their potential to regulate immune responses to a variety of pathogens. The most widely studied NKT cell subset is the invariant (i)NKT cells that recognize glycolipids in the context of the CD1d molecule. The multifaceted methods of activation iNKT cells possess and their ability to produce regulatory cytokines has made them a primary target for studies. Objective/methods: To give insights into the roles of iNKT cells during infectious diseases, particularly viral infections. We also highlight mechanisms leading to iNKT cell activation in response to pathogens. Conclusions: iNKT cells versatility allows them to detect and respond to several viruses. Therapeutic approaches to specifically target iNKT cells will require additional research. Notably, the roles of non-invariant NKT cells in response to pathogens warrant further investigation.

Direct effects of T-bet and MHC class I expression, but not STAT1, on peripheral NK cell maturation.

The homeostatic maturation of NK cells is severely impaired in mice lacking the transcription factor T‐bet, and the expression of the NK cell maturation marker killer cell lectin‐like receptor G1 (KLRG1) has been shown to be dependent on MHC class I molecules. Interferon (IFN)‐γ signaling via the signal transducer and activator of transcription (STAT)1 is vital for T‐bet and MHC class I induction. Here we investigated the relationship between STAT1, T‐bet, and MHC class I molecules with regard to the phenotypic maturation of peripheral NK cells. We demonstrate that, to varying degrees, the maturation status of peripheral NK cells is impaired in naive mice with deficiencies in STAT1, T‐bet, or MHC class I molecules. We find that in naive animals, the expression of wild‐type levels of MHC class I molecules in trans is sufficient to restore the maturation profiles of STAT1–/– NK cells in vivo. In contrast, expression of T‐bet is required in cis for normal NK cell maturation to occur. Additionally, we demonstrate that the activation‐induced maturation of NK cells during the course of murine cytomegalovirus (MCMV) infection does not require expression of MHC class I molecules or STAT1 but is severely delayed in the absence of T‐bet.

Salivary gland NK cells are phenotypically and functionally unique.

Natural killer (NK) cells and CD8+ T cells play vital roles in containing and eliminating systemic cytomegalovirus (CMV). However, CMV has a tropism for the salivary gland acinar epithelial cells and persists in this organ for several weeks after primary infection. Here we characterize a distinct NK cell population that resides in the salivary gland, uncommon to any described to date, expressing both mature and immature NK cell markers. Using RORγt reporter mice and nude mice, we also show that the salivary gland NK cells are not lymphoid tissue inducer NK-like cells and are not thymic derived. During the course of murine cytomegalovirus (MCMV) infection, we found that salivary gland NK cells detect the infection and acquire activation markers, but have limited capacity to produce IFN-γ and degranulate. Salivary gland NK cell effector functions are not regulated by iNKT or Treg cells, which are mostly absent in the salivary gland. Additionally, we demonstrate that peripheral NK cells are not recruited to this organ even after the systemic infection has been controlled. Altogether, these results indicate that viral persistence and latency in the salivary glands may be due in part to the presence of unfit NK cells and the lack of recruitment of peripheral NK cells.

A Y Chromosome-Linked Factor Impairs NK T Development

Vα14 invariant (Vα14i) NK T cell development is unique from mainstream T cell selection, and the polygenic factors that influence NK T cell ontogeny are still unclear. In this study, we report the absence of Vα14i NK T cells in B6.IFN-αβR1−/− male mice, whereas both the conventional T and NK cell populations are relatively unaffected. The lack of Vα14i NK T cells in the B6.IFN-αβR1−/− males is not due to an insufficient level of CD1d1 or a defect in CD1d1-Ag presentation, but it is intrinsic to the male Vα14i NK T cells. This surprising defect displays ≥99% penetrance in the male population, whereas female mice remain unaffected, indicating the deficiency is not X linked. Analysis of the Vα14i NK T cell compartment in B6.Tyk2−/−, B6.STAT1−/−, 129.IFN-αβR1−/−, and B6.IFN-αβR1−/+ mice demonstrate that the deficiency is linked to the Y chromosome, but independent of IFN-αβ. This is the first study demonstrating that Y-linked genes can exclusively impact Vα14i NK T development and further highlight the unique ontogeny of these innate T cells.