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Influenza A Virus (IAV), Staphylococcus aureus (staphylococci), and Streptococcus pneumoniae (pneumococci) are leading viral and bacterial causes of pneumonia. Dendritic cells (DCs) are present in the lower respiratory tract. They are characterized by low expression of co-stimulatory molecules, including CD80 and CD86 and high capacity of antigen uptake. Subsequently, DCs upregulate co-stimulatory signals and cytokine secretion to effectively induce T-cell priming. Here, we investigated these processes in response to bacterial and viral single as well as coinfections using human monocyte-derived (mo)DCs. Irrespective of single or coinfections, moDCs matured in response to IAV and/or staphylococcal infections, secreted a wide range of cytokines, and activated CD4+, CD8+ as well as double-negative T cells. In contrast, pneumococcal single and coinfections impaired moDC maturation, which was characterized by low expression of CD80 and CD86, downregulated expression of CD40, and a mild cytokine release resulting in abrogated CD4+ T-cell activation. These actions were attributed to the cholesterol-dependent cytotoxin pneumolysin (Ply). Infections with a ply-deficient mutant resulted in restored moDC maturation and exclusive CD4+ T-cell activation. These findings show that Ply has important immunomodulatory functions, supporting further investigations in specific modalities of Ply-DC interplay.
During infections, innate immune cells are crucial for initiating a pro-inflammatory immune response and clearing the invading pathogen. Delay in pathogen clearance or initiation of an immune response due to impaired functionality of immune cells can result in devastating consequences. The cellular compartment of the innate immune system comprises an array of specialized cell types: Macrophages are tissue-resident professional phagocytes that clear cellular debris, pathogens, and foreign objects. Dendritic cells (DCs) are immune sentinels specialized in antigen uptake and subsequent T cell priming. They are primary sources of cytokines in response to infection. Neutrophils are efficient effector cells that respond rapidly to infection and clear bacteria by different mechanisms. If effector mechanisms of these cells are affected by either bacterial or other factors, infections might not be resolved and can spread throughout the host. Cobalt-chromium-molybdenum biomaterial is widely used in arthroplasty. Implant-derived wear particles and ions lead to macrophage-driven adverse local tissue reactions: Such reactions have been linked to an increased risk of periprosthetic joint infection after revision arthroplasty. While metal-induced cytotoxicity is well characterized in human macrophages, direct effects on their functionality remain elusive. In Paper I, we show that local peri-implant tissue is exposed to Co and Cr in situ. Influx of macrophages is also evident. Exposure of isolated human monocytes/macrophages to Cr3+ in vitro had only minor effects. However, exposure of monocytes/macrophages to pathologic concentrations of Co2+ significantly impaired both phenotype and functionality. High concentrations of Co2+ induced loss of surface markers, including CD14 and CD16. Both Co2+ and Cr3+ impaired macrophage responses to Staphylococcus aureus infection. Co2+ -exposed macrophages, in particular, showed decreased phagocytic activity. These findings demonstrate the immunosuppressive effects of locally elevated metal ions on the innate immune response. Streptococcus pyogenes (group A streptococcus, GAS) causes a variety of diseases ranging from mild to severe necrotizing soft tissue infections (NSTIs). In the host environment hypervirulent GAS variants carrying mutations within the genes encoding for control of virulence (Cov)R/S two component system are enriched. This adaptation is associated with loss of SpeB secretion. In Paper II, we show that in vitro infections with hyper-virulent GAS variants harboring dysfunctional CovR/S suppress secretion of IL-8 and IL-18 by human monocytic cells. This phenotype was mediated by a caspase-8 dependent mechanism. Knockout of streptococcal SLO in a GAS strain carrying functional CovR/S even increased secretion of IL1β and IL-18 by moDCs. Of 67 fully sequenced GAS NSTI isolates, 28 contained covS or covR mutations that rendered the TCS dysfunctional. However, no differences in systemic IL-8 and IL-18 were detected in these patients. GAS isolates recovered from patients often display a mixed phenotype, consisting of SpeB positive (SpeB+ ) and SpeB negative (SpeB- ) clones. Irreversible loss of SpeB expression is often caused by loss of function mutations in regulatory components (CovR/S, RopB). Loss of SpeB is often associated with hyper-virulence. In Paper III, we show that the host environment induces transiently abrogated secretion of SpeB by GAS. Tissue inflammation, neutrophil influx, and degranulation correlated with increased frequencies of SpeB- GAS clones. Isolates recovered from tissue expressed but did not secrete SpeB, which was reversible. Neutrophilderived ROS were identified as the main factor responsible for abrogated SpeB secretion. Hyper-virulent SpeB- clones also exhibit better survival within and induce excessive degranulation of neutrophils.