The role of mast cells in CD8+ T cell-mediated immune responses

A precisely regulated crosstalk between innate and adaptive immunity is a prerequisite for an optimal immune response and successful survival strategy. Important players of innate immunity are the mast cells (MCs). These are long-living cells at sites of host-environment interface and important effector players during allergic responses. Recently, MCs have been described as central regulatory cells not only in innate but also in adaptive immune responses. MCs interact with cells of the adaptive immune system and recruit CD8+ T cells upon different stimuli. The purpose of this study was to investigate the interaction between MCs and CD8+ T cells, identify the factors that modulate this interaction and examine its downstream effects. By using murine bone marrow-derived MCs, this study demonstrated that MCs promote the survival of naïve, primary CD8+ T cells in an antigen-independent and cell-cell contact-dependent manner. The investigation of the antigen-dependent interaction between MCs and CD8+ T cells showed that MCs induce antigen-specific activation, proliferation and cytokine secretion by TCR-transgenic CD8+ T cells in vitro. Furthermore, the increased intracellular content of granzyme B and enhanced CD8+ T cell degranulation indicated an increase in the cytotoxic potential of CD8+ T cells. This antigen-driven communication between MCs and CD8+ T cells required both direct cell-cell contact and the release of soluble factors by MCs. TLR-mediated activation of MCs augmented their capacity to activate CD8+ T cells, partially due to enhanced surface expression of MHC class I molecules. Remarkably, the adoptive transfer of antigen-pulsed MCs induced proliferation of antigen-specific CD8+ T cells in vivo, in wild-type as well as in 2-microglobulin-deficient mice, which lack functional MHC class I expression. Thus, MCs promote CD8+ T cell responses, inducing effector CD8+ T cells in vitro and in vivo. Furthermore, CD8+ T cells enhanced the expression of several genes in MCs, in an antigen-dependent as well as antigen-independent manner, as demonstrated by differential gene expression analysis of MCs. Many of these genes are implicated in the signal transduction pathway of interferons, suggesting that the MC-CD8+ T cell interaction may contribute significantly to host defense mechanisms. Additionally, upregulation of major histocompatibility complex-related molecules and of the co-stimulatory molecule 4-1BB suggests that the contact with CD8+ T cells enhances the potential of MCs to modulate adaptive immune responses. In conclusion, this study adds new insights into the physiological role of MCs in the context of adaptive immune responses, such as a CD8+ T cell-driven antiviral immune response. This novel understanding of MC biology foresees new promising approaches for a therapeutic manipulation of antiviral immunity.Ein exakt regulierter Dialog zwischen angeborener und adaptiver Immunität ist eine wesentliche Voraussetzung für eine optimale Immunantwort und somit für eine erfolgreiche Überlebensstrategie. Zu den bedeutenden Zellen der angeborenen Immunität zählen unter anderen die Mastzellen (MZ). MZ sind langlebige Zellen, welche überwiegend an Umwelt-exponierten Körperflächen lokalisiert sind und wichtige Effektorzellen während einer allergischen Reaktion darstellen. Kürzlich wurden MZ als zentrale, regulatorische Zellen sowohl der angeborenen, als auch innerhalb der adaptiven Immunantwort beschrieben. MZ interagieren mit Zellen des angeborenen Immunsystems und vermögen nach unterschiedlicher Stimulation CD8+ T-Zellen zu rekrutieren. Absicht der vorliegenden Studie war es, die Interaktion zwischen MZ und CD8+ T-Zellen zu untersuchen, die diese Interaktion modulierenden Faktoren zu identifizieren und deren weiterführende Auswirkungen zu bestimmen. Unter Verwendung aus murinem Knochenmark generierter MZ zeigte diese Studie, dass MZ das Überleben naiver Primär-CD8+ T-Zellen Antigen-unabhängig und Zell-Zell-Kontakt-abhängig unterstützen. Untersuchungen der Antigen-abhängigen Interaktionen zwischen MZ und CD8+ T-Zellen zeigten, dass MZ eine Antigen-spezifische Aktivierung, Proliferation und Zytokinproduktion TCR-transgener CD8+ T-Zellen in vitro induzieren. Desweiteren deuten ein erhöhter intrazellulärer Gehalt an Granzym B und ein Anstieg der CD8+ T-Zell-Degranulation auf ein gesteigertes zytotoxisches Potential der CD8+ T-Zellen hin. Diese Antigen-gesteuerte Kommunikation zwischen MZ und CD8+ T-Zellen benötigte sowohl Zell-Zell-Kontakt als auch die Freisetzung löslicher Faktoren durch Mastzellen. Eine Aktivierung der MZ durch die Toll-like-Rezeptoren erhöhte deren Fähigkeit CD8+ T-Zellen zu aktivieren, teilweise vermittelt durch eine gesteigerte Zelloberflächenexpression der MHC Klasse I Molküle. Bemerkenswerter Weise induzierte der direkte Transfer Antigen-stimulierter MZ die Proliferation Antigen-spezifischer CD8+ T-Zellen in vivo, sowohl in wildtypischen als auch in 2-Mikroglobulin-defizienten Mäusen, welchen eine funktionale MHC Klasse I Expression fehlt. Somit wurde deutlich, dass MZ die CD8+ T-Zelle-Antwort fördern und dabei Effektor-CD8+ T-Zellen in vitro und in vivo induzieren. Zudem waren CD8+ T-Zellen in der Lage die Expression verschiedener Gene in MZ Antigen-abhängig als auch Antigen-unabhängig deutlich zu verstärken, wie mittels differentieller Genexpressionsanalyse gezeigt werden konnte. Viele dieser Gene haben eine wichtige Funktion innerhalb der Signaltransduktionswege von Interferonen, was zu der Annahme führte, dass die MZ-CD8+ T-Zell-Interaktion wesentlich zu Abwehrmechanismen beitragen könnte. Zusätzlich lässt die Hochregulation des Major Histocompatibility Complex-verwandten Moleküls 4—1BB vermuten, dass der Kontakt mit CD8+ T-Zellen das Potential der MZ, die adaptive Immunantwort zu modulieren, erhöhen könnte. Zusammenfassend ist zu sagen, dass diese Studie neue Einsichten in die physiologische Rolle der MZ im Kontext der adaptiven, wie einer CD8+ T-Zelle-gesteuerten anti-viralen Immunantwort gibt. Dieses neue Verständnis der MZ-Biologie birgt vielversprechende Ansätze für eine Manipulation der anti-viralen Immunantwort im therapeutischen Sinne

Trimellitic anhydride-conjugated serum albumin activates rat alveolar macrophages in vitro

BACKGROUND: Occupational exposure to airborne low molecular weight chemicals, like trimellitic anhydride (TMA), can result in occupational asthma. Alveolar macrophages (AMs) are among the first cells to encounter these inhaled compounds and were previously shown to influence TMA-induced asthma-like symptoms in the Brown Norway rat. TMA is a hapten that will bind to endogenous proteins upon entrance of the body. Therefore, in the present study we determined if TMA and TMA conjugated to serum albumin induced the production of the macrophage mediators nitric oxide (NO), tumour necrosis factor (TNF), and interleukin 6 (IL-6) in vitro using the rat AM cell line NR8383 and primary AMs derived from TMA-sensitized and naïve Brown Norway rats. METHODS: Cells were incubated with different concentrations of TMA, TMA conjugated to bovine serum albumin (BSA), and BSA as a control for 24 h and the culture supernatant was analyzed for mediator content. RESULTS: TMA alone was not able to induce the production of mediators by NR8383 cells and primary AMs from sensitized and sham-treated rats. TMA-BSA, on the contrary, dose-dependently stimulated the production of NO, TNF, and IL-6 by NR8383 cells and of NO and TNF, but not IL-6, by primary AMs independent of sensitization. CONCLUSION: Results suggest that although TMA is a highly reactive compound, conjugation to a suitable protein is necessary to induce mediator production by AMs. Furthermore, the observation that effects of TMA-BSA were independent of sensitization suggests involvement of an immunologically non-specific receptor. In the discussion it is argued that a macrophage scavenger receptor is a likely candidate

Bystander hyperactivation of preimmune CD8+ T cells in chronic HCV patients

Chronic infection perturbs immune homeostasis. While prior studies have reported dysregulation of effector and memory cells, little is known about the effects on naïve T cell populations. We performed a cross-sectional study of chronic hepatitis C (cHCV) patients using tetramer-associated magnetic enrichment to study antigen-specific inexperienced CD8(+) T cells (i.e., tumor or unrelated virus-specific populations in tumor-free and sero-negative individuals). cHCV showed normal precursor frequencies, but increased proportions of memory-phenotype inexperienced cells, as compared to healthy donors or cured HCV patients. These observations could be explained by low surface expression of CD5, a negative regulator of TCR signaling. Accordingly, we demonstrated TCR hyperactivation and generation of potent CD8(+) T cell responses from the altered T cell repertoire of cHCV patients. In sum, we provide the first evidence that naïve CD8(+) T cells are dysregulated during cHCV infection, and establish a new mechanism of immune perturbation secondary to chronic infection

Long-Term Persistence of Exhausted CD8 T Cells in Chronic Infection Is Regulated by MicroRNA-155

Persistent viral infections and tumors drive development of exhausted T (TEX) cells. In these settings, TEX cells establish an important host-pathogen or host-tumor stalemate. However, TEX cells erode over time, leading to loss of pathogen or cancer containment. We identified microRNA (miR)-155 as a key regulator of sustained TEX cell responses during chronic lymphocytic choriomeningitis virus (LCMV) infection. Genetic deficiency of miR-155 ablated CD8 T cell responses during chronic infection. Conversely, enhanced miR-155 expression promoted expansion and long-term persistence of TEX cells. However, rather than strictly antagonizing exhaustion, miR-155 promoted a terminal TEX cell subset. Transcriptional profiling identified coordinated control of cell signaling and transcription factor pathways, including the key AP-1 family member Fosl2. Overexpression of Fosl2 reversed the miR-155 effects, identifying a link between miR-155 and the AP-1 transcriptional program in regulating TEX cells. Thus, we identify a mechanism of miR-155 regulation of TEX cells and a key role for Fosl2 in T cell exhaustion. During persistent viral infections, exhausted T cells (TEX) erode quantitatively and qualitatively and therefore fail to provide protection. Stelekati et al. identified microRNA (miR)-155 as a key molecule that can enhance and sustain TEX responses long-term during chronic viral infection

Mast Cells in Allergic Asthma and Beyond

Mast cells have been regarded for a long time as effector cells in IgE mediated type I reactions and in host defence against parasites. However, they are resident in all environmental exposed tissues and express a wide variety of receptors, suggesting that these cells can also function as sentinels in innate immune responses. Indeed, studies have demonstrated an important role of mast cells during the induction of life-saving antibacterial responses. Furthermore, recent findings have shown that mast cells promote and modulate the development of adaptive immune responses, making them an important hinge of innate and acquired immunity. In addition, mast cells and several mast cell-produced mediators have been shown to be important during the development of allergic airway diseases. In the present review, we will summarize findings on the role of mast cells during the development of adaptive immune responses and highlight their function, especially during the development of allergic asthma

Egr2 and 3 control adaptive immune responses by temporally uncoupling expansion from T cell differentiation

Egr2 and 3 are important for maintaining immune homeostasis. Here we define a fundamental function of Egr2 and 3 operating as a checkpoint that controls the transition between clonal expansion and differentiation of effector T cells. Egr2 and 3 deficiency resulted in defective clonal expansion, but hyper-activation and excessive differentiation of T cells in response to viral infection. Conversely, sustained Egr2 expression enhanced expansion, but severely impaired effector differentiation. Egr2 bound to and controlled the expression of genes regulating proliferation (Myc, Myb) and differentiation repressors (Bcl6, Id3), while repressing transcription factors required for effector function (Zeb2, RORa, RORc, Bhlhe40). Egr2 and 3 expression in T cells was regulated reciprocally by antigen and IFNγ providing a mechanism for adjusting proliferation and differentiation of individual T cells. Thus, Egr2 and 3 are upstream regulators of effector CD4 and CD8 T cells that are essential for optimal responses with limited immunopathology

Cytomegalovirus generates long-lived antigen-specific NK cells with diminished bystander activation to heterologous infection

Natural killer (NK) cells play a key role in the host response to cytomegalovirus (CMV) and can mediate an enhanced response to secondary challenge with CMV. We assessed the ability of mouse CMV (MCMV)–induced memory Ly49H(+) NK cells to respond to challenges with influenza, an acute viral infection localized to the lung, and Listeria monocytogenes, a systemic bacterial infection. MCMV-memory NK cells did not display enhanced activation or proliferation after infection with influenza or Listeria, as compared with naive Ly49H(+) or Ly49H(−) NK cells. Memory NK cells also showed impaired activation compared with naive cells when challenged with a mutant MCMV lacking m157, highlighting their antigen-specific response. Ex vivo, MCMV-memory NK cells displayed reduced phosphorylation of STAT4 and STAT1 in response to stimulation by IL-12 and type I interferon (IFN), respectively, and IFN-γ production was reduced in response to IL-12 + IL-18 compared with naive NK cells. However, costimulation of MCMV-memory NK cells with IL-12 and m157 antigen rescues their impaired response compared with cytokines alone. These findings reveal that MCMV-primed memory NK cells are diminished in their response to cytokine-driven bystander responses to heterologous infections as they become specialized and antigen-specific for the control of MCMV upon rechallenge