Marein A. W. P. de Jong

Salp15 binding to DC-SIGN inhibits cytokine expression by impairing both nucleosome remodeling and mRNA stabilization

Ixodes ticks are major vectors for human pathogens, such as Borrelia burgdorferi, the causative agent of Lyme disease. Tick saliva contains immunosuppressive molecules that facilitate tick feeding and B. burgdorferi infection. We here demonstrate, to our knowledge for the first time, that the Ixodes scapularis salivary protein Salp15 inhibits adaptive immune responses by suppressing human dendritic cell (DC) functions. Salp15 inhibits both Toll-like receptor- and B. burgdorferi–induced production of pro-inflammatory cytokines by DCs and DC-induced T cell activation. Salp15 interacts with DC-SIGN on DCs, which results in activation of the serine/threonine kinase Raf-1. Strikingly, Raf-1 activation by Salp15 leads to mitogen-activated protein kinase kinase (MEK)-dependent decrease of IL-6 and TNF-α mRNA stability and impaired nucleosome remodeling at the IL-12p35 promoter. These data demonstrate that Salp15 binding to DC-SIGN triggers a novel Raf-1/MEK-dependent signaling pathway acting at both cytokine transcriptional and post-transcriptional level to modulate Toll-like receptor–induced DC activation, which might be instrumental to tick feeding and B. burgdorferi infection, and an important factor in the pathogenesis of Lyme disease. Insight into the molecular mechanism of immunosuppression by tick salivary proteins might provide innovative strategies to combat Lyme disease and could lead to the development of novel anti-inflammatory or immunosuppressive agents.

TNF-α and TLR agonists increase susceptibility to HIV-1 transmission by human Langerhans cells ex vivo

Genital coinfections increase an individuals risk of becoming infected with HIV-1 by sexual contact. Several mechanisms have been proposed to explain this, such as the presence of ulceration and bleeding caused by the coinfecting pathogen. Here we demonstrate that Langerhans cells (LCs) are involved in the increased susceptibility to HIV-1 in the presence of genital coinfections. Although LCs are a target for HIV-1 infection in genital tissues, we found that immature LCs did not efficiently mediate HIV-1 transmission in an ex vivo human skin explant model. However, the inflammatory stimuli TNF-alpha and Pam3CysSerLys4 (Pam3CSK4), the ligand for the TLR1/TLR2 heterodimer, strongly increased HIV-1 transmission by LCs through distinct mechanisms. TNF-alpha enhanced transmission by increasing HIV-1 replication in LCs, whereas Pam3CSK4 acted by increasing LC capture of HIV-1 and subsequent trans-infection of T cells. Genital infections such as Candida albicans and Neisseria gonorrhea not only triggered TLRs but also induced TNF-alpha production in vaginal and skin explants. Thus, during coinfection, LCs could be directly activated by pathogenic structures and indirectly activated by inflammatory factors, thereby increasing the risk of acquiring HIV-1. Our data demonstrate a decisive role for LCs in HIV-1 transmission during genital coinfections and suggest antiinflammatory therapies as potential strategies to prevent HIV-1 transmission.

C-type lectin Langerin is a beta-glucan receptor on human Langerhans cells that recognizes opportunistic and pathogenic fungi.

Langerhans cells (LCs) lining the stratified epithelia and mucosal tissues are the first antigen presenting cells to encounter invading pathogens, such as viruses, bacteria and fungi. Fungal infections form a health threat especially in immuno-compromised individuals. LCs express C-type lectin Langerin that has specificity for mannose, fucose and GlcNAc structures. Little is known about the role of human Langerin in fungal infections. Our data show that Langerin interacts with both mannan and β-glucan structures, common cell-wall carbohydrate structures of fungi. We have screened a large panel of fungi for recognition by human Langerin and, strikingly, we observed strong binding of Langerin to a variety of Candida and Saccharomyces species and Malassezia furfur, but very weak binding was observed to Cryptococcus gattii and Cryptococcus neoformans. Notably, Langerin is the primary fungal receptor on LCs, since the interaction of LCs with the different fungi was blocked by antibodies against Langerin. Langerin recognizes both mannose and β-glucans present on fungal cell walls and our data demonstrate that Langerin is the major fungal pathogen receptor on human LCs that recognizes pathogenic and commensal fungi. Together these data may provide more insight in the role of LCs in fungal infections.

Human Langerhans cells capture measles virus through Langerin and present viral antigens to CD4+ T cells but are incapable of cross-presentation

Langerhans cells (LCs) are a subset of DCs that reside in the upper respiratory tract and are ideally suited to sense respiratory virus infections. Measles virus (MV) is a highly infectious lymphotropic and myelotropic virus that enters the host via the respiratory tract. Here, we show that human primary LCs are capable of capturing MV through the C‐type lectin Langerin. Both immature and mature LCs presented MV‐derived antigens in the context of HLA class II to MV‐specific CD4+ T cells. Immature LCs were not susceptible to productive infection by MV and did not present endogenous viral antigens in the context of HLA class I. In contrast, mature LCs could be infected by MV and presented de novo synthesized viral antigens to MV‐specific CD8+ T cells. Notably, neither immature nor mature LCs were able to cross‐present exogenous UV‐inactivated MV or MV‐infected apoptotic cells. The lack of direct infection of immature LCs, and the inability of both immature and mature LCs to cross‐present MV antigens, suggest that human LCs may not be directly involved in priming MV‐specific CD8+ T cells. Immune activation of LCs seems a prerequisite for MV infection of LCs and subsequent CD8+ T‐cell priming via the endogenous antigen presentation pathway.

Herpes Simplex Virus Type 2 Enhances HIV-1 Susceptibility by Affecting Langerhans Cell Function

Genital herpes is the most prevalent viral sexually transmitted infection worldwide and is mainly caused by HSV type 2 (HSV-2). HSV-2 infection enhances HIV-1 susceptibility, even in the absence of clinical symptoms. In this study, we investigated the effect of HSV-2 on HIV-1 transmission by mucosal Langerhans cells (LCs). LCs are important in heterosexual transmission because they form a barrier against HIV-1 infection; LCs efficiently capture and degrade HIV-1 through the C-type lectin langerin, thereby preventing HIV-1 transmission. Notably, our data showed that HSV-2 enhanced HIV-1 infection of LCs and subsequent HIV-1 transmission to T cells. HSV-2 interfered with HIV-1 capture by langerin, which allowed efficient HIV-1 infection of LCs. HSV-2 inhibited the antiviral function of langerin at two levels; HSV-2 decreased langerin expression and competed with HIV-1 for langerin binding. HSV-2 replication was not required, because both UV-inactivated HSV-2 and TLR-3 agonist polyinosinic:polycytidylic acid similarly increased HIV-1 transmission by LCs. Therefore, we identified a mechanism by which HSV-2 enhances HIV-1 susceptibility, even in the absence of clinical symptoms. Our data demonstrated that viral coinfections, such as HSV-2, breach the protective function of LCs by abrogating langerin function, which increases HIV-1 susceptibility. These data reinforce the importance of preventing sexually transmitted infections, such as HSV-2, to reduce the transmission of HIV-1.

Fucose-based PAMPs prime dendritic cells for follicular T helper cell polarization via DC-SIGN-dependent IL-27 production

Dendritic cells (DCs) orchestrate antibody-mediated responses to combat extracellular pathogens including parasites by initiating T helper cell differentiation. Here we demonstrate that carbohydrate-specific signalling by DC-SIGN drives follicular T helper cell (TFH) differentiation via IL-27 expression. Fucose, but not mannose, engagement of DC-SIGN results in activation of IKKε, which collaborates with type I IFNR signalling to induce formation and activation of transcription factor ISGF3. Notably, ISGF3 induces expression of IL-27 subunit p28, and subsequent IL-27 secreted by DC-SIGN-primed DCs is pivotal for the induction of Bcl-6(+)CXCR5(+)PD-1(hi)Foxp1(lo) TFH cells, IL-21 secretion by TFH cells and T-cell-dependent IgG production by B cells. Thus, we have identified an essential role for DC-SIGN-induced ISGF3 by fucose-based PAMPs in driving IL-27 and subsequent TFH polarization, which might be harnessed for vaccination design.

Langerhans cells in innate defense against pathogens.

Langerhans cells (LCs) are at the frontline in defense against mucosal infections because they line the mucosal tissues and are ideally situated to intercept pathogens. Recent data suggest that LCs have an innate anti-HIV-1 function. LCs express the LC-specific C-type lectin Langerin that efficiently captures HIV-1, which prevents HIV-1 transmission. However, immune activation of LCs changes these protective cells into HIV-1-transmitting cells, which indicates that the antiviral function of LCs depends on several factors including co-infections. In this review, we discuss the dual role of LCs in innate defense against pathogens, with a focus on HIV-1 dissemination.

Mutz-3-derived Langerhans cells are a model to study HIV-1 transmission and potential inhibitors

Sexual transmission is the primary route of HIV‐1 infection, and DC subsets are thought to be involved in viral dissemination to T cells. In the genital mucosa, two main subsets of DCs are present: epithelial LCs capture and degrade HIV‐1 through C‐type lectin Langerin, whereas subepithelial DCs express DC‐SIGN, which facilitates HIV‐1 transmission to T cells. As there is currently no HIV‐1 vaccine available, microbicides provide an alternative strategy to limit HIV‐1 spread. However, research into the function of LCs is hampered by the low availability and donor differences. Here, we set out to investigate whether LCs derived from the Mutz‐3 cell line (Mu‐LCs) provide a valuable tool to investigate the role of LCs in HIV‐1 transmission and identify suitable potential microbicides. We demonstrate that Mu‐LCs phenotypically resemble human primary LCs; Mu‐LCs do not transmit HIV‐1 efficiently, and inhibition of Langerin enhances HIV‐1 transmission to T cells. We show that carbohydrate structures blocking DC‐SIGN but not Langerin are potential microbicides, as they prevent HIV‐1 transmission by DCs but do not affect the antiviral function of LCs. Therefore, Mu‐LCs are a suitable model to investigate the role of LCs in HIV‐1 transmission and to screen potential microbicides.

Langerhans Cell–Dendritic Cell Cross-Talk via Langerin and Hyaluronic Acid Mediates Antigen Transfer and Cross-Presentation of HIV-1

Human epidermal and mucosal Langerhans cells (LCs) express the C-type lectin receptor langerin that functions as a pattern recognition receptor. LCs are among the first immune cells to interact with HIV-1 during sexual transmission. In this study, we demonstrate that langerin not only functions as a pattern recognition receptor but also as an adhesion receptor mediating clustering between LCs and dendritic cells (DCs). Langerin recognized hyaluronic acid on DCs and removal of these carbohydrate structures partially abrogated LC–DC clustering. Because LCs did not cross-present HIV-1–derived Ags to CD8+ T cells in a cross-presentation model, we investigated whether LCs were able to transfer Ags to DCs. LC–DC clustering led to maturation of DCs and facilitated Ag transfer of HIV-1 to DCs, which subsequently induced activation of CD8+ cells. The rapid transfer of Ags to DCs, in contrast to productive infection of LCs, suggests that this might be an important mechanism for induction of anti–HIV-1 CD8+ T cells. Induction of the enzyme hyaluronidase-2 by DC maturation allowed degradation of hyaluronic acid and abrogated LC–DC interactions. Thus, we have identified an important function of langerin in mediating LC–DC clustering, which allows Ag transfer to induce CTL responses to HIV-1. Furthermore, we showed this interaction is mediated by hyaluronidase-2 upregulation after DC maturation. These data underscore the importance of LCs and DCs in orchestrating adaptive immunity to HIV-1. Novel strategies might be developed to harness this mechanism for vaccination.

Burn injury suppresses human dermal dendritic cell and Langerhans cell function.

Human skin contains epidermal Langerhans cells (LCs) and dermal dendritic cells (DCs) that are key players in induction of adaptive immunity upon infection. After major burn injury, suppressed adaptive immunity has been observed in patients. Here we demonstrate that burn injury affects adaptive immunity by altering both epidermal LC and dermal DC functions. We developed a human ex vivo burn injury model to study the function of DCs in thermally injured skin. No differences were observed in the capacity of both LCs and dermal DCs to migrate out of burned skin compared to unburned skin. Similarly, expression levels of co-stimulatory molecules were unaltered. Notably, we observed a strong reduction of T cell activation induced by antigen presenting cell (APC) subsets that migrated from burned skin through soluble burn factors. Further analyses demonstrated that both epidermal LCs and dermal DCs have a decreased T cell stimulatory capacity after burn injury. Restoring the T cell stimulatory capacity of DC subsets might improve tissue regeneration in patients with burn wounds.