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Uveitis describes a heterogeneous group of inflammatory eye diseases characterized by infiltration of leukocytes into the uveal tissues. Uveitis associated with the HLA haplotype B27 (HLA-B27) is a common subtype of uveitis and a prototypical ocular immune-mediated disease. Local immune mechanisms driving human uveitis are poorly characterized mainly due to the limited available biomaterial and subsequent technical limitations. Here, we provide the first high-resolution characterization of intraocular leukocytes in HLA-B27-positive (n = 4) and -negative (n = 2) anterior uveitis and an infectious endophthalmitis control (n = 1) by combining single-cell RNA-sequencing with flow cytometry and protein analysis. Ocular cell infiltrates consisted primarily of lymphocytes in both subtypes of uveitis and of myeloid cells in infectious endophthalmitis. HLA-B27-positive uveitis exclusively featured a plasmacytoid and classical dendritic cell (cDC) infiltrate. Moreover, cDCs were central in predicted local cell-cell communication. This suggests a unique pattern of ocular leukocyte infiltration in HLA-B27-positive uveitis with relevance to DCs., Competing Interests: MK, MH, DS, XL, TL, MM, DB, KW, HW, KL, AH, GM No competing interests declared, (© 2021, Kasper et al.)

The neuropeptide α-melanocyte-stimulating hormone (α-MSH) is a well-known mediator of skin pigmentation. More recently, it has been shown that α-MSH also exerts a strong anti-inflammatory and immunosuppressive activity. To elucidate the mechanisms underlying α-MSH-induced immunosuppression, we investigated whether α-MSH affects dendritic cell/T cell communication, as especially this interaction has an important role in the regulation of immune responses. Here, we show that α-MSH, by binding to MC-1R, induced tolerogenic dendritic cells, which were capable of expanding CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) in vitro, as well as in vivo. Notably, those α-MSH-induced Tregs were functional as they efficiently inhibited cutaneous contact allergy and ongoing psoriasis-like skin inflammation in mice. Furthermore, α-MSH induced tolerogenic dendritic cells capable of generating functional Tregs in human blood. Interestingly, human Tregs expanded via α-MSH-stimulated dendritic cells suppressed the proliferation and cytokine secretion of pathogenic T-helper-17 (Th17) cells from individuals with psoriasis. Taken together, these data indicate that α-MSH induced immunosuppressive Tregs in vitro and in vivo, which inhibited disease progression in a mouse model of psoriasis-like skin inflammation and suppressed the activation and proliferation of effector T cells from subjects with psoriasis.

Despite as yet limited clinical effectiveness, dendritic cell (DC)-based immunotherapy remains a promising approach for the treatment of cancer, but requires further improvement in its immunostimulatory effectiveness. Potent anti-tumor immunity often depends on the induction of type 1 (T(H)1) immune responses. Therefore, we combined different DC maturation stimuli that are known to induce T(H)1 immunity [anti-CD40, interleukin (IL)-12, IL-18], with the aim to trigger a T(H)1 driven anti-tumor CTL response. When compared with untreated DC or DC treated with anti-CD40 alone, DC matured with anti-CD40 plus IL-12 and IL-18 expressed significantly more IFN-gamma and IL-12, induced enhanced CD8(+) T-cell proliferation, prolonged synaptic interaction with T cells and increased CD8(+) T-cell-mediated cytotoxicity. To analyse if these DC are able to induce efficient anti-tumor immunity, mice carrying a B16-OVA tumor were treated with tumor antigen (TA)-loaded DC that had been exposed to anti-CD40 or to anti-CD40 plus IL-12 and IL-18. Our data show that anti-CD40 plus IL-12 and IL-18 matured DC are superior to controls in retarding tumor growth. These data indicate that maturation of DC with anti-CD40 plus IL-12 and IL-18 potently stimulates the generation of an anti-tumor immune response and may lead to improved immunotherapeutic capacity of DC vaccination.

Retroviruses have developed immunmodulatory mechanisms to avoid being attacked by the immune system. The mechanisms of this retrovirus-associated immune suppression are far from clarified. Dendritic cells (DCs) have been attributed a decisive role in these pathogenic processes. We have used the Friend retrovirus (FV) mouse model in order to acquire further knowledge about the role of infection of DCs in virus-induced immunosuppression. About 20% of the myeloid DCs that were generated from the bone marrow of FV-infected mice carried FV proteins. The infection was productive, and infected DCs transmitted the virus in cell culture and in vivo. FV infection of DCs led to a defect in DC maturation, as infected cells expressed very little costimulatory molecules. Live imaging analysis of the cell contact between DCs and T cells revealed prolonged contacts of T cells with infected DCs compared with uninfected DCs. Although naive T cells were still activated by FV-infected DCs, this activation did not result in antigen-specific T-cell proliferation. Interestingly, infected DCs expanded a population of Foxp3(+) regulatory T cells with immunosuppressive potential, suggesting that the contact between naive T cells and retrovirus-infected DCs results in tolerance rather than immunity. Thus, retroviral infection of DCs leads to an expansion of regulatory T cells, which might serve as an immune escape mechanism of the virus.

DCs are a complex cell population in the skin consisting of epidermal LCs and dermal DCs, which differ in their anatomic location, antigen recognition, processing machinery, and migratory capacity. Cutaneous DCs (LCs as well as dermal DCs) function as sentinels that survey invading agents and transmit the information into immune responses by taking up exogenous antigens, migrating to draining LNs, and presenting the processed antigens to T cells resulting in T-cell differentiation and activation. Indeed, further studies are needed to clarify the specific contribution of each cutaneous DC subpopulation to antigen presentation and induction of cutaneous immune responses. Recent results suggested to revisit the "paradigm" that exclusively immigrant LCs present skin-acquired antigens to T cells upon reaching the draining LNs, because viral antigens, for instance, are presented by a DC subpopulation other than LCs after infection of mouse epidermis with herpes simplex virus [153]. Additionally, different DC subpopulations may sequentially present skin-acquired antigens, possibly serving as a regulatory mechanism of cell-mediated immunity and adding further complexity to established concepts. Nevertheless, cutaneous DCs are involved in several pathologies (including infections, inflammatory disorders, or skin cancers) and play a pivotal role in regulating the balance between immunity and peripheral tolerance. However, it is widely accepted that (cutaneous) DC in an immature state may have tolerogenic properties resulting in the induction or expansion of Tregs. CD4+CD25+ Tregs are essential for the control of immune responses in inflammatory, autoimmune, or cancer diseases, and it is well established that in particular the lineage-specific transcription factor Foxp3, as well as cytokines (including IL-2, IL-10, and TGF-beta), characteristic surface markers such as CTLA-4, and members of the TNF superfamily (e.g., RANKL) are critically involved in the thymic development, peripheral maintenance, and suppressive activity of CD4+CD25+ Tregs. Recently, new methods for generating and expanding Tregs in vitro have emerged and supported the use of CD4+CD25+ T cells as a novel strategy for the treatment of patients suffering from autoimmune diseases. In the future, a better understanding of Treg function in vivo and the interactions of Tregs and pathogenic effector T cells in autoimmune disorders may help to improve the design of Treg-based therapies.

Regulatory CD4(+)CD25(+) T cells are important in suppressing immune responses. The requirements for the maintenance of peripheral CD4(+)CD25(+) T cells remain incompletely understood. Receptor activator of NF-kappaB (RANK) and its ligand (RANKL; also known as CD254, OPGL and TRANCE) are key regulators of bone remodeling, mammary gland formation, lymph node development and T-cell/dendritic cell communication. Here we report that RANKL is expressed in keratinocytes of the inflamed skin. RANKL overexpression in keratinocytes resulted in functional alterations of epidermal dendritic cells and systemic increases of regulatory CD4(+)CD25(+) T cells. Thus, epidermal RANKL expression can change dendritic cell functions to maintain the number of peripheral CD4(+)CD25(+) regulatory T cells. Epidermal RANKL mediated ultraviolet-induced immunosuppression and overexpression of epidermal RANKL suppressed allergic contact hypersensitivity responses and the development of systemic autoimmunity. Therefore, environmental stimuli at the skin can rewire the local and systemic immune system by means of RANKL.

The adaptive immune response is initiated when naive T cells interact with dendritic cells (DC). However, the physicodynamics as well as the molecules that constitute the contact plane (immunological synapse) between DC and T cells are not well understood. We show here that for the formation of stable conjugates, T cells need to be preactivated by DC in a CD80/86- and antigen dose-dependent manner. When activated, T cells induce cytoskeletal reorganization within DC via CD40-CD40L signaling. Polarization of the actin and fascin cytoskeleton in DC is associated with sustained DC-T cell contacts, strong T cell proliferation and a Th1 response. Organized contact planes with clearly separated patches containing TCR or CD11a are also formed. Thus, DC-T cell interactions take place in a sequential, interdependent fashion: first, DC "license" naive T cells to engage DC in an antigen dose- and CD80/86-dependent fashion. Then, these preactivated T cells induce cytoskeletal reorientation in DC, resulting in sustained DC-T cell contacts and subsequent T cell activation. These results demonstrate that T cells control the mode of interaction based on information gathered from DC.

Immature dendritic cells (DC) reside in tissues where they initiate immune responses by taking up foreign antigens. Since DC have a limited tissue half-life, the DC pool in tissues has to be replenished constantly. This implies that precursor/immature DC must be able to cross non-activated endothelium using as yet unknown mechanisms. Here we show that immature, but not mature bone marrow-derived murine DC migrate across resting endothelial monolayers in vitro. We find that endothelial intercellular adhesion molecule-2 (ICAM-2) is a major player in transendothelial migration (TEM) of immature DC, accounting for at least 41% of TEM. Surprisingly, the ICAM-2-mediated TEM was independent of beta2-integrins, the known ICAM-2 ligands, since neither blocking of beta2-integrins with antibodies nor the use of CD18-deficient DC affected the ICAM-2-specific TEM. In humans, the C-type lectin DC-specific ICAM-3-grabbing nonintegrin (DC-SIGN) was shown to interact with ICAM-2, suggesting a similar role in mice. However, we find that none of the murine DC-SIGN homologues mDC-SIGN, murine DC-SIGN-related molecule-1 (mSIGN-R1) and mSIGN-R3 is expressed on the surface of bone marrow-derived mouse DC. Taken together, this study shows that ICAM-2 strongly supports transmigration of immature DC across resting endothelium by interacting with ligands that are distinct from beta2-integrins and DC-SIGN homologues.

Herpes simplex virus-type 1 (HSV-1) causes the majority of cutaneous viral infections. Viral infections are controlled by the immune system, and CD8+ cytotoxic T-lymphocytes (CTLs) have been shown to be crucial during the clearance of HSV-1 infections. Although epidermal Langerhans cells (LCs) are the first dendritic cells (DCs) to come into contact with the virus, it has been shown that the processing of viral antigens and the differentiation of antiviral CTLs are mediated by migratory CD103+ dermal DCs and CD8α+ lymph node-resident DCs. In vivo regulatory T-cells (Tregs) are implicated in the regulation of antiviral immunity and we have shown that signaling via the receptor activator of NF-κB (RANK) and its ligand RANKL mediates the peripheral expansion of Tregs. However, in addition to expanding Tregs, RANK-RANKL interactions are involved in the control of antimicrobial immunity by upregulating the priming of CD4+ effector T cells in LCMV infection or by the generation of parasite-specific CD8+ T cells in Trypanosoma cruzi infection. Here, we demonstrate that cutaneous RANK-RANKL signaling is critical for the induction of CD8-mediated antiviral immune responses during HSV-1 infection of the skin by preventing virus-induced LC apoptosis, improving antigen transport to regional lymph nodes, and increasing the CTL priming capacity of lymph node DCs. [ABSTRACT FROM AUTHOR]