Plasmacytoid dendritic cells control dengue and Chikungunya virus infections via IRF7-regulated interferon responses.

Type I interferon (IFN-I) responses are critical for the control of RNA virus infections, however, many viruses, including Dengue (DENV) and Chikungunya (CHIKV) virus, do not directly activate plasmacytoid dendritic cells (pDCs), robust IFN-I producing cells. Herein, we demonstrated that DENV and CHIKV infected cells are sensed by pDCs, indirectly, resulting in selective IRF7 activation and IFN-I production, in the absence of other inflammatory cytokine responses. To elucidate pDC immunomodulatory functions, we developed a mouse model in which IRF7 signaling is restricted to pDC. Despite undetectable levels of IFN-I protein, pDC-restricted IRF7 signaling controlled both viruses and was sufficient to protect mice from lethal CHIKV infection. Early pDC IRF7-signaling resulted in amplification of downstream antiviral responses, including an accelerated natural killer (NK) cell-mediated type II IFN response. These studies revealed the dominant, yet indirect role of pDC IRF7-signaling in directing both type I and II IFN responses during arbovirus infections.

Tracking antigen-specific CD8⁺ T cells using MHC class I multimers.

The tracking of epitope-specific T cells is a useful approach for the study of adaptive immune responses. This protocol describes how Major Histocompatibility Complex Class I (MHC-I) multimers can be used to stain, enrich, and enumerate (rare) populations of CD8(+) T cells specific for a given antigen. It provides the detailed steps for multimer labeling, magnetic enrichment, and cytometric analysis. Additionally, it provides informations for multiplexing experiments in order to achieve simultaneous detection of multiple antigenic specificities, and strategies for coupling the protocol with functional assays (e.g., intracellular cytokine staining). Future developments in cytometric systems (e.g., mass spectroscopy-based cytometry) and gene expression studies (e.g., single cell PCR) will extend these approaches and provide an unprecedented assessment of the immune repertoire.

Dissecting T cell contraction in vivo using a genetically encoded reporter of apoptosis.

Contraction is a critical phase of immunity whereby the vast majority of effector T cells die by apoptosis, sparing a population of long-lived memory cells. Where, when, and why contraction occurs has been difficult to address directly due in large part to the rapid clearance of apoptotic T cells in vivo. To circumvent this issue, we introduced a genetically encoded reporter for caspase-3 activity into naive T cells to identify cells entering the contraction phase. Using two-photon imaging, we found that caspase-3 activity in T cells was maximal at the peak of the response and was associated with loss of motility followed minutes later by cell death. We demonstrated that contraction is a widespread process occurring uniformly in all organs tested and targeting phenotypically diverse T cells. Importantly, we identified a critical window of time during which antigen encounters act to antagonize T cell apoptosis, supporting a causal link between antigen clearance and T cell contraction. Our results offer insight into a poorly explored phase of immunity and provide a versatile methodology to study apoptosis during the development or function of a variety of immune cells in vivo.

Cutting Edge: A dual role for type I IFNs during polyinosinic-polycytidylic acid-induced NK cell activation.

NK cells are cytotoxic lymphocytes that are most efficient at fulfilling their functions after a phase of priming provided by cytokines and/or accessory cells. Although type I IFNs are known to be important in this process, it remains unclear whether they act directly on NK cells or indirectly on accessory cells. We used adoptive transfer experiments and mixed bone marrow chimeras to dissect the requirement for type I IFN signaling in response to the dsRNA analog polyinosinic-polycytidylic acid. We demonstrate that optimal NK cell priming requires type I IFNs to signal on both NK cells and accessory cells. In the absence of IL-15, the residual NK cell activation was strictly dependent on cell-intrinsic IFNAR signaling in NK cells. Our results suggest that type I IFNs produced following viral infection simultaneously target accessory cells for IL-15 transpresentation and NK cells themselves and that these two pathways cooperate for NK cell priming.

Immunization route dictates cross-priming efficiency and impacts the optimal timing of adjuvant delivery.

Delivery of cell-associated antigen represents an important strategy for vaccination. While many experimental models have been developed in order to define the critical parameters for efficient cross-priming, few have utilized quantitative methods that permit the study of the endogenous repertoire. Comparing different strategies of immunization, we report that local delivery of cell-associated antigen results in delayed T cell cross-priming due to the increased time required for antigen capture and presentation. In comparison, delivery of disseminated antigen resulted in rapid T cell priming. Surprisingly, local injection of cell-associated antigen, while slower, resulted in the differentiation of a more robust, polyfunctional, effector response. We also evaluated the combination of cell-associated antigen with poly I:C delivery and observed an immunization route-specific effect regarding the optimal timing of innate immune stimulation. These studies highlight the importance of considering the timing and persistence of antigen presentation, and suggest that intradermal injection with delayed adjuvant delivery is the optimal strategy for achieving CD8⁺ T cell cross-priming.

Visualizing the functional diversification of CD8+ T cell responses in lymph nodes.

CD8(+) T cell responses generate effector cells endowed with distinct functional potentials but the contribution of early events in this process is unclear. Here, we have imaged T cells expressing a fluorescent reporter for the activation of the interferon-γ (IFN-γ) locus during priming in lymph nodes. We have demonstrated marked differences in the efficiency of gene activation during stable T cell-dentritic cell (DC) contacts, influenced in part by signal strength. Imaging the first cell division, we have demonstrated that heterogeneity in T cell functional potential was largely apparent as T cells initiated clonal expansion. Moreover, by analyzing the fate of single activated T cells ex vivo, we have provided evidence that these early differences resulted in clonal progenies with distinct functional properties. Thus, the early set of T cell-DC interactions in lymph nodes largely contribute to the heterogeneity of T cell responses through the generation of functionally divergent clonal progenies.

Osmoregulation in Lactococcus lactis: BusR, a transcriptional repressor of the glycine betaine uptake system BusA.

The busA (opuA) locus of Lactococcus lactis encodes a glycine betaine uptake system. Transcription of busA is osmotically inducible and its induction after an osmotic stress is reduced in the presence of glycine betaine. Using a genetic screen in CLG802, an Escherichia coli strain carrying a lacZ transcriptional fusion expressed under the control of the busA promoter, we isolated a genomic fragment from the L. lactis subsp. cremoris strain MG1363, which represses transcription from busAp. The cloned locus responsible for this repression was identified as a gene present upstream from the busA operon, encoding a putative DNA binding protein. This gene was named busR. Electrophoretic mobility shift and footprinting experiments showed that BusR is able to bind a site that overlaps the busA promoter. Overexpression of busR in L. lactis reduced expression of busA. Its disruption led to increased and essentially constitutive transcription of busA at low osmolarity. Therefore, BusR is a major actor of the osmotic regulation of busA in L. lactis.