Sebastián A. Riquelme

Mechanisms used by virulent Salmonella to impair dendritic cell function and evade adaptive immunity

Innate and adaptive immunity are inter‐related by dendritic cells (DCs), which directly recognize bacteria through the binding of pathogen‐associated molecular patterns (PAMPs) to specialized receptors on their surface. After capturing and degrading bacteria, DCs present their antigens as small peptides bound to MHC molecules and prime naive bacteria‐specific T cells. In response to PAMP recognition DCs undergo maturation, which is a phenotypic change that increases their immunogenicity and promotes the activation of naive T cells. As a result, a specific immune response that targets bacteria‐derived antigens is initiated. Therefore, the characterization of DC–bacteria interactions is important to understand the mechanisms used by virulent bacteria to avoid adaptive immunity. Furthermore, any impairment of DC function might contribute to bacterial survival and dissemination inside the host. An example of a bacterial pathogen capable of interfering with DC function is Salmonella enterica serovar Typhimurium (S. Typhimurium). Virulent strains of this bacterium are able to differentially modulate the entrance to DCs, avoid lysosomal degradation and prevent antigen presentation on MHC molecules. These features of virulent S. Typhimurium are controlled by virulence proteins, which are encoded by pathogenicity islands. Modulation of DC functions by these gene products is supported by several studies showing that pathogenesis might depend on this attribute of virulent S. Typhimurium. Here we discuss some of the recent data reported by the literature showing that several virulence proteins from Salmonella are required to modulate DC function and the activation of host adaptive immunity.

Impaired learning resulting from Respiratory Syncytial Virus infection

Respiratory syncytial virus (RSV) is the major cause of respiratory illness in infants worldwide. Neurologic alterations, such as seizures and ataxia, have been associated with RSV infection. We demonstrate the presence of RSV proteins and RNA in zones of the brain—such as the hippocampus, ventromedial hypothalamic nucleus, and brainstem—of infected mice. One month after disease resolution, rodents showed behavioral and cognitive impairment in marble burying (MB) and Morris water maze (MWM) tests. Our data indicate that the learning impairment caused by RSV is a result of a deficient induction of long-term potentiation in the hippocampus of infected animals. In addition, immunization with recombinant bacillus Calmette–Guérin (BCG) expressing RSV nucleoprotein prevented behavioral disorders, corroborating the specific effect of RSV infection over the central nervous system. Our findings provide evidence that RSV can spread from the airways to the central nervous system and cause functional alterations to the brain, both of which can be prevented by proper immunization against RSV.

Salmonella pathogenicity island 1 differentially modulates bacterial entry to dendritic and non-phagocytic cells

Salmonella enterica serovar Typhimurium can enter non‐phagocytic cells, such as intestinal epithelial cells, by virtue of a Type Three Secretion System (TTSS) encoded in the Salmonella Pathogenicity Island 1 (SPI‐1), which translocates bacterial effector molecules into the host cell. Salmonella can also be taken up by dendritic cells (DCs). Although the role of SPI‐1 in non‐phagocytic cell invasion is well established, its contribution to invasion of phagocytic cells has not been evaluated. Here, we have tested the invasive capacity of a S. Typhimurium strain lacking a key component of its TTSS‐1 (ΔInvC) leading to defective translocation of SPI‐1‐encoded effectors. Whereas this mutant Salmonella strain was impaired for invasion of non‐phagocytic cells, it was taken up by DCs at a significantly higher rate than wild‐type Salmonella. Similar to wild‐type Salmonella, the ΔInvC mutant strain retained the capacity to avoid antigen presentation to T cells. However, mice infected with the ΔInvC mutant strain showed higher survival rate and reduced organ colonization. Our data suggest that, besides promoting phagocytosis by non‐phagocytic cells, SPI‐1 modulates the number of bacteria that enters DCs. The SPI‐1 could be considered not only as an inducer of epithelial cell invasion but as a controller of DC entry.

Surface expression of the hRSV nucleoprotein impairs immunological synapse formation with T cells

Significance Human respiratory syncytial virus (hRSV) is the leading cause of bronchiolitis and pneumonia in children worldwide. The induction of poor T-cell immunological memory causes a high susceptibility to reinfections, which contributes to hRSV spread. Previously, we showed that hRSV inhibits T-cell activation by impairing the assembly of the dendritic cell (DC)−T-cell immunological synapse (IS). Here, we show that the nucleoprotein (N) of hRSV—a canonical cytosolic protein—is expressed on the surface of infected DCs. Further, using the supported-lipid-bilayer system (that mimics the DC/ antigen-presenting cells-membrane composition), we observed that the hRSV N interfered with pMHC−T-cell receptor interactions and inhibited IS assembly. We conclude that hRSV N may therefore be instrumental in impairing the host immune response during infection with this virus. Human respiratory syncytial virus (hRSV) is the leading cause of bronchiolitis and pneumonia in young children worldwide. The recurrent hRSV outbreaks and reinfections are the cause of a significant public health burden and associate with an inefficient antiviral immunity, even after disease resolution. Although several mouse- and human cell-based studies have shown that hRSV infection prevents naïve T-cell activation by antigen-presenting cells, the mechanism underlying such inhibition remains unknown. Here, we show that the hRSV nucleoprotein (N) could be at least partially responsible for inhibiting T-cell activation during infection by this virus. Early after infection, the N protein was expressed on the surface of epithelial and dendritic cells, after interacting with trans-Golgi and lysosomal compartments. Further, experiments on supported lipid bilayers loaded with peptide-MHC (pMHC) complexes showed that surface-anchored N protein prevented immunological synapse assembly by naive CD4+ T cells and, to a lesser extent, by antigen-experienced T-cell blasts. Synapse assembly inhibition was in part due to reduced T-cell receptor (TCR) signaling and pMHC clustering at the T-cell−bilayer interface, suggesting that N protein interferes with pMHC−TCR interactions. Moreover, N protein colocalized with the TCR independently of pMHC, consistent with a possible interaction with TCR complex components. Based on these data, we conclude that hRSV N protein expression at the surface of infected cells inhibits T-cell activation. Our study defines this protein as a major virulence factor that contributes to impairing acquired immunity and enhances susceptibility to reinfection by hRSV.

Carbon monoxide decreases endosome–lysosome fusion and inhibits soluble antigen presentation by dendritic cells to T cells.

Heme oxygenase‐1 (HO‐1) inhibits immune responses and inflammatory reactions via the catabolism of heme into carbon monoxide (CO), Fe2+, and biliverdin. We have previously shown that either induction of HO‐1 or treatment with exogenous CO inhibits LPS‐induced maturation of dendritic cells (DCs) and protects in vivo and in vitro antigen‐specific inflammation. Here, we evaluated the capacity of HO‐1 and CO to regulate antigen presentation on MHC class I and MHC class II molecules by LPS‐treated DCs. We observed that HO‐1 and CO treatment significantly inhibited the capacity of DCs to present soluble antigens to T cells. Inhibition was restricted to soluble OVA protein, as no inhibition was observed for antigenic OVA‐derived peptides, bead‐bound OVA protein, or OVA as an endogenous antigen. Inhibition of soluble antigen presentation was not due to reduced antigen uptake by DCs, as endocytosis remained functional after HO‐1 induction and CO treatment. On the contrary, CO significantly reduced the efficiency of fusion between late endosomes and lysosomes and not by phagosomes and lysosomes. These data suggest that HO‐1 and CO can inhibit the ability of LPS‐treated DCs to present exogenous soluble antigens to naïve T cells by blocking antigen trafficking at the level of late endosome–lysosome fusion.

Gap Junction Intercellular Communications Regulate NK Cell Activation and Modulate NK Cytotoxic Capacity

Gap junctions (GJs) mediate intercellular communication between adjacent cells. Previously, we showed that connexin 43 (Cx43), the main GJ protein in the immune system, mediates Ag transfer between human dendritic cells (DCs) and is recruited to the immunological synapse during T cell priming. This crosstalk contributed to T cell activation, intracellular Ca2+ responses, and cytokine release. However, the role of GJs in NK cell activation by DCs and NK cell–mediated cytotoxicity against tumor cells remains unknown. In this study, we found polarization of Cx43 at the NK/DC and NK/tumor cell-contact sites, accompanied by the formation of functional GJs between NK/DCs and NK/tumor cells, respectively. Cx43–GJ-mediated intercellular communication (GJIC) between human NK and DCs was bidirectional. Blockage of Cx43-GJIC inhibited NK cell activation, though it affected neither the phenotype nor the function of DCs. Cx43 knockdown or inhibition using mimetic peptides greatly reduced CD69 and CD25 expression and IFN-γ release by DC-stimulated NK cells. Moreover, blocking Cx43 strongly inhibited the NK cell–mediated tumor cell lysis associated with inhibition of granzyme B activity and Ca2+ influx. Our data identify a novel and active role for Cx43-GJIC in human NK cell activation and antitumor effector functions that may be important for the design of new immune therapeutic strategies.

Carbon monoxide down-modulates Toll-like receptor 4/MD2 expression on innate immune cells and reduces endotoxic shock susceptibility

Carbon monoxide (CO) has been recently reported as the main anti‐inflammatory mediator of the haem‐degrading enzyme haem‐oxygenase 1 (HO‐1). It has been shown that either HO‐1 induction or CO treatment reduces the ability of monocytes to respond to inflammatory stimuli, such as lipopolysaccharide (LPS), due to an inhibition of the signalling pathways leading to nuclear factor‐κB, mitogen‐activated protein kinases and interferon regulatory factor 3 activation. Hence, it has been suggested that CO impairs the stimulation of the Toll‐like receptor 4 (TLR4)/myeloid differentiation factor‐2 (MD2) complex located on the surface of immune cells. However, whether CO can negatively modulate the surface expression of the TLR4/MD2 complex in immune cells remains unknown. Here we report that either HO‐1 induction or treatment with CO decreases the surface expression of TLR4/MD2 in dendritic cells (DC) and neutrophils. In addition, in a septic shock model of mice intraperitoneally injected with lipopolysaccharide (LPS), prophylactic treatment with CO protected animals from hypothermia, weight loss, mobility loss and death. Further, mice pre‐treated with CO and challenged with LPS showed reduced recruitment of DC and neutrophils to peripheral blood, suggesting that this gas causes a systemic tolerance to endotoxin challenge. No differences in the amount of innate cells in lymphoid tissues were observed in CO‐treated mice. Our results suggest that CO treatment reduces the expression of the TLR4/MD2 complex on the surface of myeloid cells, which renders them resistant to LPS priming in vitro, as well as in vivo in a model of endotoxic shock.

Carbon monoxide exposure improves immune function in lupus‐prone mice

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by multiple alterations affecting the normal function of immune cells, such as lymphocytes, dendritic cells (DCs) and monocytes. Although the understanding of autoimmunity has significantly increased, the breakthrough in effective therapies has been modest, making necessary the development of new therapeutic strategies. Here we propose that a new potential target for therapy is haem oxygenase‐1 (HO‐1), an enzyme that catalyses the degradation of the haem group into biliverdin, carbon monoxide (CO) and Fe2+. These products exhibit immunosuppressive and anti‐inflammatory effects, which can contribute to improving tolerance during organ transplantation. Because HO‐1 is highly expressed by immune cells involved in SLE pathogenesis, such as monocytes and DCs, we evaluated whether induction of HO‐1 expression or the administration of CO could ameliorate disease in the FcγRIIb knockout (KO) mouse model for SLE. We found that CO administration decreased the expansion of CD11b+ cells, prevented the decline of regulatory T cells and reduced anti‐histone antibodies observed in untreated FcγRIIb KO mice. Furthermore, CO‐treated animals and HO‐1 induction showed less kidney damage compared with untreated mice. These data suggest that HO‐1 modulation and CO administration can ameliorate autoimmunity and prevent the lupus symptoms shown by FcγRIIb KO mice, highlighting HO‐1 as a potential new target for autoimmune therapy.

Modulation of antigen processing by haem-oxygenase 1. Implications on inflammation and tolerance.

Haem‐oxygenase‐1 (HO‐1) is an enzyme responsible for the degradation of haem that can suppress inflammation, through the production of carbon monoxide (CO). It has been shown in several experimental models that genetic and pharmacological induction of HO‐1, as well as non‐toxic administration of CO, can reduce inflammatory diseases, such as endotoxic shock, type 1 diabetes and graft rejection. Recently, it was shown that the HO‐1/CO system can alter the function of antigen‐presenting cells (APCs) and reduce T‐cell priming, which can be beneficial during immune‐driven inflammatory diseases. The molecular mechanisms by which the HO‐1 and CO reduce both APC‐ and T‐cell‐driven immunity are just beginning to be elucidated. In this article we discuss recent findings related to the immune regulatory capacity of HO‐1 and CO at the level of recognition of pathogen‐associated molecular patterns and T‐cell priming by APCs. Finally, we propose a possible regulatory role for HO‐1 and CO over the recently described mitochondria‐dependent immunity. These concepts could contribute to the design of new therapeutic tools for inflammation‐based diseases.

Contribution of Fcγ receptors to human respiratory syncytial virus pathogenesis and the impairment of T-cell activation by dendritic cells.

Human respiratory syncytial virus (hRSV) is the leading cause of infant hospitalization related to respiratory disease. Infection with hRSV produces abundant infiltration of immune cells into the airways, which combined with an exacerbated pro‐inflammatory immune response can lead to significant damage to the lungs. Human RSV re‐infection is extremely frequent, suggesting that this virus may have evolved molecular mechanisms that interfere with host adaptive immunity. Infection with hRSV can be reduced by administering a humanized neutralizing antibody against the virus fusion protein in high‐risk infants. Although neutralizing antibodies against hRSV effectively block the infection of airway epithelial cells, here we show that both, bone marrow‐derived dendritic cells (DCs) and lung DCs undergo infection with IgG‐coated virus (hRSV‐IC), albeit abortive. Yet, this is enough to negatively modulate DC function. We observed that such a process is mediated by Fcγ receptors (FcγRs) expressed on the surface of DCs. Remarkably, we also observed that in the absence of hRSV‐specific antibodies FcγRIII knockout mice displayed significantly less cellular infiltration in the lungs after hRSV infection, compared with wild‐type mice, suggesting a potentially harmful, IgG‐independent role for this receptor in hRSV disease. Our findings support the notion that FcγRs can contribute significantly to the modulation of DC function by hRSV and hRSV‐IC. Further, we provide evidence for an involvement of FcγRIII in the development of hRSV pathogenesis.