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The innate immune system recognizes invading microbes via conserved pattern recognition receptors and uses inflammatory signals to concert adaptive defense mechanisms. However, microbial and host parameters involved in CD4 Τ cell priming and direction of Th1, Th2, and Th17 differentiation in the context of infections with complex pathogens in vivo are incompletely understood. In this study, we used Legionella pneumophila, which triggers membrane-bound and cytosolic pattern recognition receptors, to study the innate instruction of adaptive immunity. Upon airway infection, Τ cells were primed exclusively in the lungdraining lymph nodes and differentiated into Th1/Th17 effector cells upon arrival in the lung. Although engagement of membrane-bound pattern recognition receptors was sufficient for initial Τ cell activation and proliferation, cytosolic pattern recognition was required for effector Τ cell differentiation. In the absence of cytoplasmic pattern recognition, My D88 was key for Τ cell priming, whereas, in its presence, My D88-mediated signals were crucial for Th17 differentiation. Specifically, cytosolic sensing of Legionella-derived flagellin, inflammasome activation, and IL-1 signaling contributed to Th17 development. In the absence of TLR signaling, a simultaneous Th1/Th2 response developed that was independent of the inflammasome-IL-1 axis. Collectively, these data illustrate the important role for various pattern recognition receptors triggered by complex pathogens and how they each instruct specific differentiation programs in responding CD4 Τ cells. [ABSTRACT FROM AUTHOR]

A response by the authors to a letter to the editor about their study titled "Nonhematopoietic Cells Are Key Players in Innate Control of Bacterial Airway Infection" is presented.

Abs are able to mediate local protection from pulmonary infection with Legionella pneumophila, the causative agent of a severe form of pneumonia known as Legionnaires' disease. L. pneumophila is able to infect alveolar macrophages in the lung and replicates intracellularly in a vacuolar compartment with endoplasmic reticulum-like characteristics. However, Abs opsonize the bacteria and confer an FcR-mediated signal to phagocytic host cells that vetoes the bacterial evasion strategies, thereby efficiently targeting the bacteria to intracellular lysosomal degradation. In this study we analyzed the prevalence of pathogen-specific IgG subclasses present in immunized mice and found that the presence of IgG2c and IgG3 correlated with reduced bacterial titers after intranasal infection. We then isolated different IgG subclasses and compared their differential prophylactic potential in restricting airway L. pneumophila replication. We found that all IgG subclasses were effective in restricting pulmonary airway infection in mice when administered at high and equivalent doses. However, at limiting Ab concentrations we found a superior role of IgG2c in restricting L. pneumophila replication in a prophylactic setting. Furthermore, we assessed the therapeutic efficacy of administering an mAb during an established infection and found that bacterial titers could be reduced very efficiently with such a treatment. Thus, we propose the therapeutic use of Abs for the treatment of intracellular bacterial infections in situations where antibiotics might be ineffective. [ABSTRACT FROM AUTHOR]

Certain virus infections depend on the presence of T cell help for the generation of primary CD8+ T cell responses. However, the mechanisms that render these particular viral infections T cell help dependent is largely unknown. In this study, we compared CD8+ T cell responses elicited by lymphocytic choriomeningitis virus infection, as prototype of a T cell help independent infection, with T cell help dependent CD8+ T cell responses induced by vaccinia virus infection. In this paper, we show that a key parameter decisive for T cell help independence is the ability of an infectious agent to stimulate early and robust production of type I IFN. Experimental provision of type I IFN during VV infection rendered the ensuing CD8+ T cell response completely T cell help independent. Our results support a model in which type I IFN has to be present during the first 3 d of Ag encounter and has to act directly on the responding CD8+ T cells to promote their survival and effector differentiation. We show that type I IFN signaling on responding CD8+ T cells induces profound upregulation of CD25 and increased IL-2 expression; however, neither this nor IL- 15 accounts for the type I IFN effects on responding CD8+ T cells. Thus, type I IFN can effectively replace the requirement of T cell help by directly promoting CD8+ T cell survival and differentiation independent of the type I IFN-induced cytokines IL-2 and IL-15. [ABSTRACT FROM AUTHOR]

Progressive quantitative and qualitative decline of CD4+ T cell responses is one hallmark of HIV-1 infection and likely depends on several factors, including a possible contribution by the HIV-1 envelope glycoprotein gp120, which binds with high affinity to the CD4 receptor. Besides virion-associated and cell-expressed gp120, considerable amounts of soluble gp120 are found in plasma or lymphoid tissue, predominantly in the form of gp120–anti-gp120 immune complexes (ICs). Because the functional consequences of gp120 binding to CD4+ T cells are controversially discussed, we investigated how gp120 affects TCR-mediated activation of human CD4+ T cells by agonistic anti-CD3 mAb or by HLA class II–presented peptide Ags. We show that the spatial orientation of gp120–CD4 receptor binding relative to the site of TCR engagement differentially affects TCR signaling efficiency and hence CD4+ T cell activation. Whereas spatially and temporally linked CD4 and TCR triggering at a defined site promotes CD4+ T cell activation by exceeding local thresholds for signaling propagation, CD4 receptor engagement by gp120-containing ICs all around the CD4+ T cell undermine its capacity in supporting proximal TCR signaling. In vitro, gp120 ICs are efficiently captured by CD4+ T cells and thereby render them hyporesponsive to TCR stimulation. Consistent with these in vitro results we show that CD4+ T cells isolated from HIV+ individuals are covered with ICs, which at least partially contain gp120, and suggest that IC binding to CD4 receptors might contribute to the progressive decline of CD4+ T cell function during HIV-1 infection. [ABSTRACT FROM AUTHOR]

Hyperactivation of T cells, particularly of CD8+ T cells, is a hallmark of chronic HIV 1 (HIV-1) infection. Little is known about the antigenic specificities and the mechanisms by which HIV-1 causes activation of CD8+ T cells during chronic infection. We report that CD8+ T cells were activated during in vivo HIV-1 replication irrespective of their Ag specificity. Cytokines present during untreated HIV-1 infection, most prominently IL-15, triggered proliferation and expression of activation markers in CD8+ T cells, but not CD4+ T cells, in the absence of TCR stimulation. Moreover, LPS or HIV-1-activated dendritic cells (DCs) stimulated CD8+ T cells in an IL-15-dependent but Ag-independent manner, and IL-15 expression was highly increased in DCs isolated from viremic HIV-1 patients, suggesting that CD8+ T cells are activated by inflammatory cytokines in untreated HIV-1 patients independent of Ag specificity. This finding contrasts with CD4+ T cells whose in vivo activation seems biased toward specificities for persistent Ags. These observations explain the higher abundance of activated CD8+ T cells compared with CD4+ T cells in untreated HIV-1 infection. [ABSTRACT FROM AUTHOR]

Abs confer protection from secondary infection with Legionella pneumophila, the causative agent of a severe form of pneumonia known as Legionnaires' disease. In this study, we demonstrate that Ab-mediated protection is effective across L. pneumophila serogroups, suggesting that Abs specific for conserved protein Ags are sufficient to mediate this protective effect. We used two independent methods to identify immunogenic L. pneumophila protein Ags, namely, the screening of a &lgr; phage library representing the complete L. pneumophila genome and two-dimensional gel electrophoresis combined with Western blot analysis and protein spot identification by mass spectrometry. A total of 30 novel L. pneumophila B Cell Ags were identified, the majority of which are located in or associated with the bacterial membrane, where they are accessible for Abs and, therefore, likely to be relevant for Ab-mediated protection against L. pneumophila. Selected B Cell Ags were recombinantly expressed and tested in a vaccination protocol. Mice immunized with either single-protein Ags or an Ag combination showed reduced bacterial titers in bronchoalveolar lavage and lung after L. pneumophila challenge. To determine the clinical relevance of these findings, we tested Legionnaires' disease patient sera for reactivity with the identified L. pneumophila Ags. The recognized Ags were indeed conserved across host species, because Abs specific for all three selected Ags could be detected in patient sera, rendering the identified protein Ags potential vaccine candidates. [ABSTRACT FROM AUTHOR]

Fungal pathogens are a frequent cause of opportunistic infections. They live as commensals in healthy individuals but can cause disease when the immune status of the host is altered. T lymphocytes play a critical role in pathogen control. However, specific Ags determining the activation and function of antifungal T cells remain largely unknown. By using an immunoproteomic approach, we have identified for the first time, to our knowledge, a natural T cell epitope from Candida albicans. Isolation and sequencing of MHC class II-bound ligands from infected dendritic cells revealed a peptide that was recognized by a major population of all Candida-specific Th cells isolated from infected mice. Importantly, human Th cells also responded to stimulation with the peptide in an HLA-dependent manner but without restriction to any particular HLA class II allele. Immunization of mice with the peptide resulted in a population of epitope-specific Th cells that reacted not only with C. albicans but also with other clinically highly relevant species of Candida including the distantly related Candida glabrata. The extent of the reaction to different Candida species correlated with their degree of phylogenetic relationship to C. albicans. Finally, we show that the newly identified peptide acts as an efficient vaccine when used in combination with an adjuvant inducing IL-17A secretion from peptide-specific T cells. Immunized mice were protected from fatal candidiasis. Together, these results uncover a new immune determinant of the host response against Candida ssp. that could be exploited for the development of antifungal vaccines and immunotherapies. [ABSTRACT FROM AUTHOR]

NK cells recognize virus-infected cells with germline-encoded activating and inhibitory receptors that do not undergo genetic recombination or mutation. Accordingly, NK cells are often considered part of the innate immune response. The innate response comprises rapid early defenders that do not form immune memory. However, there is increasing evidence that experienced NK cells provide increased protection to secondary infection, a hallmark of the adaptive response. In this study, we compare the dynamics of the innate and adaptive immune responses by examining the kinetic profiles of the NK and T cell response to murine CMV infection. We find that, unexpectedly, the kinetics of NK cell proliferation is neither earlier nor faster than the CD4 or CD8 T cell response. Furthermore, early NK cell contraction after the peak of the response is slower than that of T cells. Finally, unlike T cells, experienced NK cells do not experience biphasic decay after the response peak, a trait associated with memory formation. Rather, NK cell contraction is continuous, constant, and returns to below endogenous preinfection levels. This indicates that the reason why Ag-experienced NK cells remain detectable for a prolonged period after adoptive transfer and infection is in part due to the high precursor frequency, slow decay rate, and low background levels of Ly49H+ NK cells in recipient DAP12-deficient mice. Thus, the quantitative contribution of Ag-experienced NK cells in an endogenous secondary response, with higher background levels of Ly49H+ NK cells, may be not be as robust as the secondary response observed in T cells. [ABSTRACT FROM AUTHOR]

Airborne pathogens encounter several hurdles during host invasion, including alveolar macrophages (AMs) and airway epithelial cells (AECs) and their products. Although growing evidence indicates pathogen-sensing capacities of epithelial cells, the relative contribution of hematopoietic versus nonhematopoietic cells in the induction of an inflammatory response and their possible interplay is still poorly defined in vivo in the context of infections with pathogenic microorganisms. In this study, we show that nonhematopoietic cells, including AECs, are critical players in the inflammatory process induced upon airway infection with Legionella pneumophila, and that they are essential for control of bacterial infections. Lung parenchymal cells, including AECs, are not infected themselves by L. pneumophila in vivo but rather act as sensors and amplifiers of inflammatory cues delivered by L. pneumophila-infected AM. We identified AM-derived IL-1β as the critical mediator to induce chemokine production in nonhematopoietic cells in the lung, resulting in swift and robust recruitment of infection-controlling neutrophils into the airways. These data add a new level of complexity to the coordination of the innate immune response to L. pneumophila and illustrate how the cross talk between leukocytes and nonhematopoietic cells contributes to efficient host protection. [ABSTRACT FROM AUTHOR]