IntroductionDendritic cells (DCs) are potent mediators of T-cell activation and proinflammatory immune responses to foreign antigens and pathogens. 1,2 However, DCs also have an important role in maintaining immune homeostasis and tolerance to self-proteins. [3][4][5][6][7] These 2 opposing functions are believed in part to reflect differences in DC activation, maturation, and/or subset. Tolerogenic DCs typically exhibit an immature phenotype characterized by low cell-surface expression of MHC and costimulatory molecules and do not secrete proinflammatory cytokines. Furthermore, soluble and cellular mediators that inhibit DC activation and maturation can establish a tolerogenic phenotype. For example, binding to and phagocytosis of apoptotic cells (ACs) by immature DCs inhibits activation and maturation induced by various stimuli. 8,9 This inhibitory effect serves an important role because ACs are present in tissues under both homeostatic and inflamed conditions and provide a potential source of self-proteins to mediate autoimmunity. Defective clearance of ACs has been linked to different types of autoimmunity. 10,11 A number of receptors expressed by immature DCs such as the phosphatidylserine (PS) receptor, CD36, ␣ v ␤ 5 integrin, and complement receptor C1qR are involved in AC binding and/or ingestion. [12][13][14][15] However, the relative contribution of these receptors in mediating the immunoregulatory effect(s) of ACs on immature DCs is unclear, and the molecular basis for this inhibition has not been defined in DCs.Recently, the Axl/Mer/Tyro3 receptor tyrosine kinase (RTK) family has been implicated in homeostatic regulation of antigenpresenting cell (APC) activation. 16,17 This family, consisting of Axl, Tyro3, and MerTK, is expressed by a variety of cell types, including macrophages (Ms) and DCs. Mice lacking expression of all 3 RTKs exhibit hyperactivated Ms and DCs, which in turn drive lymphoproliferation and systemic autoimmunity. 16 Similarly, our group has shown that mice lacking MerTK expression (MerT-K KD ) develop lupuslike autoimmunity and are more prone to lipopolysaccharide (LPS)-induced endotoxic shock. [18][19][20] Autoimmunity in MerTK KD mice correlates with a reduced rate of in vivo clearance of ACs, which is consistent with findings that MerTK mediates AC phagocytosis by Ms. 19,20 A ligand for MerTK is growth arrest-specific gene 6 (GAS6), which binds to PS expressed on the inverted plasma membrane of ACs. 21 Recognition of a GAS6-PS complex facilitates binding of ACs and subsequent phagocytosis by Ms. Accordingly, MerTK has been proposed to facilitate phagocytosis of ACs and down-regulate activation in Ms. [17][18][19][20] Whether MerTK functions similarly in DCs has yet to be determined.We and others [22][23][24][25][26][27] have demonstrated a key role for the transcription factor NF-B in regulating gene expression associated with the development, activation, maturation, and APC function of DCs. The NF-B complex consists of homodimers and heterodimers of the structurally related pro...

Objective HIV-1 replication and microbial translocation occur concomitant with systemic immune activation. This study delineates mechanisms of immune activation and CD4 T cell decline in pediatric HIV-1 infection. Design Cross-sectional and longitudinal cellular and soluble plasma markers for inflammation were evaluated in 14 healthy and 33 perinatally HIV-1-infected pediatric subjects prior to and over 96 weeks of protease-inhibitor-containing combination antiretroviral treatment [ART]. All HIV-1-infected subjects reconstituted CD4 T cells either with suppression of viremia or rebound of drug-resistant virus. Methods Systemic immune activation was determined by polychromatic flow cytometry of blood lymphocytes and ELISA for plasma soluble CD27 [sCD27], soluble CD14 [sCD14], and tumor necrosis factor [TNF]. Microbial translocation was evaluated by limulus amebocyte lysate assay to detect bacterial lipopolysaccharide [LPS] and ELISA for anti-endotoxin core antigen IgM antibodies. Immune activation markers were compared to viral load, CD4% and LPS by regression models. Comparisons between healthy and HIV-1 infected or between different viral outcome groups were performed by non-parametric rank sum. Results Microbial translocation was detected in healthy infants but resolved with age (P<0.05). LPS and sCD14 levels were elevated in all HIV-1 infected subjects (P<0.05 and P<0.0001, respectively) and persisted even if CD4 T cells were fully reconstituted, virus optimally suppressed, and lymphocyte activation resolved by ART. Children with CD4 T cell reconstitution but viral rebound following ART continued to display high levels of sCD27. Conclusions Microbial translocation in pediatric HIV-1-infection is associated with persistent monocyte/macrophage activation independent of viral replication or T cell activation.

Self-antigens expressed by apoptotic cells (ACs) may become targets for autoimmunity. Tolerance to these antigens is partly established by an ill-defined capacity of ACs to inhibit antigen-presenting cells such as dendritic cells (DCs). We present evidence that the receptor tyrosine kinase Mer (MerTK) has a key role in mediating AC-induced inhibition of DC activation/maturation. Pretreatment of DCs prepared from nonobese diabetic (NOD) mice with AC blocked secretion of proinflammatory cytokines, up-regulation of costimulatory molecule expression, and T cell activation. The effect of ACs on DCs was dependent on Gas6, which is a MerTK ligand. NOD DCs lacking MerTK expression (NOD.MerTKKD/KD) were resistant to AC-induced inhibition. Notably, autoimmune diabetes was exacerbated in NOD.MerTKKD/KD versus NOD mice expressing the transgenic BDC T cell receptor. In addition, β cell–specific CD4+ T cells adoptively transferred into NOD.MerTKKD/KD mice in which β cell apoptosis was induced with streptozotocin exhibited increased expansion and differentiation into type 1 T cell effectors. In both models, the lack of MerTK expression was associated with an increased frequency of activated pancreatic CD11c+CD8α+ DCs, which exhibited an enhanced T cell stimulatory capacity. These findings demonstrate that MerTK plays a critical role in regulating self-tolerance mediated between ACs, DCs, and T cells.

Sepsis, the systemic inflammatory response to microbial infection, induces changes in both innate and adaptive immunity that presumably lead to increased susceptibility to secondary infections, multi-organ failure and death. Using a model of murine polymicrobial sepsis whose severity approximates human sepsis, we examined outcomes and defined requirements for survival after secondary Pseudomonas aeruginosa pneumonia or disseminated Listeria monocytogenes infection. We demonstrate that early after sepsis, neutrophil numbers and function are decreased, whereas monocyte recruitment through the CCR2/MCP1 pathway and function are enhanced. Consequently, lethality to Pseudomonas pneumonia is increased early but not late after induction of sepsis. In contrast, lethality to listeriosis, whose eradication is dependent upon monocyte/macrophage phagocytosis, is actually decreased both early and late after sepsis. Adaptive immunity plays little role in these secondary infectious responses. This study demonstrates that sepsis promotes selective early, impaired innate immune responses, primarily in neutrophils, that lead to a pathogen-specific, increased susceptibility to secondary infections.

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Adeno-associated virus (AAV) is a replication-deficient parvovirus that is extensively used as a gene therapy vector. CD8 T-cell responses against the AAV capsid protein can, however, affect therapeutic efficacy. Little is known about the in vivo mechanism that leads to the crosspriming of CD8 T cells against the input viral capsid antigen. In this study, we report that the Toll-like receptor 9 (TLR9)-MyD88 pattern-recognition receptor pathway is uniquely capable of initiating this response. By contrast, the absence of TLR2, STING, or the addition of TLR4 agonist has no effect. Surprisingly, both conventional dendritic cells (cDCs) and plasmacytoid DCs (pDCs) are required for the crosspriming of capsid-specific CD8 T cells, whereas other antigen-presenting cells are not involved. TLR9 signaling is specifically essential in pDCs but not in cDCs, indicating that sensing of the viral genome by pDCs activates cDCs in trans to cross-present capsid antigen during CD8 T-cell activation. Cross-presentation and crosspriming depend not only on TLR9, but also on interferon type I signaling, and both mechanisms can be inhibited by administering specific molecules to prevent induction of capsid-specific CD8 T cells. Thus, these outcomes directly point to therapeutic interventions and demonstrate that innate immune blockade can eliminate unwanted immune responses in gene therapy.

The paradigm of tolerogenic/immature versus inflammatory/mature dendritic cells has dominated the recent literature regarding the role of these antigen-presenting cells in mediating immune homeostasis or self-tolerance and response to pathogens, respectively. This issue is further complicated by the identification of distinct subtypes of dendritic cells that exhibit different antigen-presenting cell effector functions. The discovery of pathogen-associated molecular patterns and toll-like receptors provides the mechanistic basis for dendritic cell recognition of specific pathogens and induction of appropriate innate and adaptive immune responses. Only recently has insight been gained into how dendritic cells contribute to establishing and/or maintaining immunological tolerance to self. Soluble and cellular mediators have been reported to effectively regulate the function of dendritic cells by inducing several outcomes ranging from non-inflammatory dendritic cells that lack the ability to induce T lymphocyte activation to dendritic cells that actively suppress T lymphocyte responses. A thorough discussion of these stimuli and their outcomes is essential to understanding the potential for modulating dendritic cell function in the treatment of inflammatory disease conditions.

Prior studies identified HLA class-II and 57 additional loci as contributors to genetic susceptibility for type 1 diabetes (T1D). We hypothesized that race and/or ethnicity would be contextually important for evaluating genetic risk markers previously identified from Caucasian/European cohorts. We determined the capacity for a combined genetic risk score (GRS) to discriminate disease-risk subgroups in a racially and ethnically diverse cohort from the southeastern U.S. including 637 T1D patients, 46 at-risk relatives having two or more T1D-related autoantibodies (≥2AAb+), 790 first-degree relatives (≤1AAb+), 68 second-degree relatives (≤1 AAb+), and 405 controls. GRS was higher among Caucasian T1D and at-risk subjects versus ≤ 1AAb+ relatives or controls (P < 0.001). GRS receiver operating characteristic AUC (AUROC) for T1D versus controls was 0.86 (P < 0.001, specificity = 73.9%, sensitivity = 83.3%) among all Caucasian subjects and 0.90 for Hispanic Caucasians (P < 0.001, specificity = 86.5%, sensitivity = 84.4%). Age-at-diagnosis negatively correlated with GRS (P < 0.001) and associated with HLA-DR3/DR4 diplotype. Conversely, GRS was less robust (AUROC = 0.75) and did not correlate with age-of-diagnosis for African Americans. Our findings confirm GRS should be further used in Caucasian populations to assign T1D risk for clinical trials designed for biomarker identification and development of personalized treatment strategies. We also highlight the need to develop a GRS model that accommodates racial diversity.