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Allergic eye disease, as in most forms of atopy, ranges in severity among individuals from immediate hypersensitivity to a severe and debilitating chronic disease. Dendritic cells play a key role in stimulating pathogenic T cells in allergen re-exposure, or secondary responses. However, molecular cues by dendritic cells underpinning allergic T cell response levels and the impact that this control has on consequent severity of allergic disease are poorly understood. Here, we show that a deficiency in thrombospondin-1, a matricellular protein known to affect immune function, has subsequent effects on downstream T cell responses during allergy, as revealed in an established mouse model of allergic eye disease. More specifically, we demonstrate that a thrombospondin-1 deficiency specific to dendritic cells leads to heightened secondary T cell responses and consequent clinical disease. Interestingly, whereas thrombospondin-1-deficient dendritic cells augmented activity of allergen-primed T cells, this increase was not recapitulated with naïve T cells in vitro. The role of dendritic cell-derived thrombospondin-1 in regulating secondary allergic T cell responses was confirmed in vivo, as local transfer of thrombospondin-1-sufficient dendritic cells to the ocular mucosa of thrombospondin-1 null hosts prevented the development of augmented secondary T cell responses and heightened allergic eye disease clinical responses. Finally, we demonstrate that topical instillation of thrombospondin-1-derived peptide reduces T cell activity and clinical progression of allergic eye disease. Taken together, this study reveals an important modulatory role of dendritic cell-derived thrombospondin-1 on secondary allergic T cell responses and suggests the possible dysregulation of dendritic cell-derived thrombospondin-1 expression as a factor in allergic eye disease severity., (© Society for Leukocyte Biology.)

Purpose of Review: Here, we explore an emerging theme in the literature, which is the role of dendritic cells in the causation of fibrosis. To fully appreciate this pathway to disease, we also review the most recent literature regarding dendritic cell biology as it pertains to ocular surface tissues. On the basis of this information, we propose a unifying hypothesis for how dendritic cells may cause conjunctival fibrosis in the allergy setting., Recent Findings: Work in models of airway remodeling and liver fibrosis has pointed to a potentially central role for dendritic cells in the pathobiology of fibrosis. Indeed, these cells are recognized as the most potent antigen-presenting cells, and as such activate T lymphocytes that are profibrotic under certain conditions. However, recent findings suggest a more direct role for dendritic cells, which opens up the possibility that a similar pathway may be relevant in the causation of conjunctival fibrosis, particularly in allergic eye disease., Summary: Conjunctival fibrosis is a serious clinical concern and is associated with chronic inflammation of the ocular surface tissue, such as in allergic eye disease. Dendritic cells are required in mediating allergic disease by activating pathologic T lymphocytes. Recent findings pointing to a central role for dendritic cell in fibrosis may, however, mean that these cells could also be contributing directly to conjunctival fibrosis. If so, furthering our understanding of dendritic cells could lead to the identification of novel and more effective therapeutic strategies to treat this disease.

Dendritic cells (DCs) are highly potent stimulators of the immune system, and their contribution as such to the pathogenesis of corneal and ocular surface inflammatory disease has been well established. These vigorous antigen-presenting cells are reliant upon their effective migration from peripheral tissues (e.g., those of the ocular surface) to the lymphoid organs, where immune responses are triggered and can then cause disease. The chemokine receptor CCR7 expressed on DCs has emerged as the master mediator of this highly complex migratory process, and thus it is important in causing corneal and ocular surface inflammation. Furthermore, CCR7 has received considerable attention as a potential therapeutic target, as topically instilled antagonists of this receptor are quite effective therapeutically in a mouse model of ocular allergy. These findings and more are reviewed in the current article. In addition, the understanding regarding CCR7 function in mice and humans, and the biology of DCs that populate the ocular surface are also detailed herein. The involvement of DCs and their expression of CCR7 in corneal and ocular surface diseases such as in ocular allergy, dry eye disease, immune rejection and more, are also reviewed here., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Steady state dendritic cells (DC) found in non-lymphoid tissue sites under normal physiologic conditions play a pivotal role in triggering T cell responses upon immune provocation. CD11b+ and CD103+ DC have received considerable attention in this regard. However, still unknown is whether such CD11b+ and CD103+ DC even exist in the ocular mucosa, and if so, what functions they have in shaping immune responses. We herein identified in the ocular mucosa of normal wild-type (WT) and Flt3-/- mice the presence of a CD11b+ DC (i.e., CD11c+ MHCII+ CD11b+ CD103- F4/80+ Sirp-a+). CD103+ DC (i.e. CD11c+ MHCII+ CD11b low CD103+ CD8a+ DEC205+ Langerin+) were also present in WT, but not in Flt3-/- mice. These CD103+ DC expressed high levels of Id2 and Flt3 mRNA; whereas CD11b+ DC expressed high Irf4, Csfr, and Cx3cr1 mRNA. Additionally, the functions of these DC differed in response to allergic immune provocation. This was assessed utilizing a previously validated model, which includes transferring specific populations of exogenous DC into the ocular mucosa of ovalbumin (OVA)/alum-primed mice. Interestingly, in such mice, topical OVA instillation following engraftment of exogenous CD11b+ DC led to dominant allergic T cell responses and clinical signs of ocular allergy relative to those engrafted with CD103+ DC. Thus, although CD11b+ and CD103+ DC are both present in the normal ocular mucosa, the CD11b+ DC subset plays a dominant role in a mouse model of ocular allergy.

CCR7 plays a key role in mobilizing tissue dendritic cells (DCs) to the lymphoid compartment for consequent elicitation of adaptive immunity. Interfering with CCR7 function therapeutically would therefore be anticipated to inhibit the progression of atopic conditions, for example, allergic conjunctivitis (AC). However, the CCR7-CCL19/CCL21 system in the ocular surface is poorly understood as is the precise role of DCs in AC immunopathogenesis. T cells from ovalbumin (OVA)-primed mice were adoptively transferred into wild-type (WT) hosts. Exogenous WT (eGFP(+)) versus CCR7(-/-) DCs were engrafted subconjunctivally (SCJ), and hosts were challenged with OVA (Texas-Red+) eye drops. AC immunopathogenesis was evaluated via clinical examinations, infiltration of mast cells and eosinophils, Th2 reactivity, and serum IgE levels. AC was also assessed in actively immunized mice challenged with OVA eye drops containing 1% anti-CCR7 antibody or isotype control. In eye-draining lymph nodes (LNs), OVA(+) SCJ engrafted WT DCs conferred upregulated CCR7 and caused augmentation of clinical signs. This result was corroborated by increased conjunctival infiltration, Th2 cytokines in LNs, and serum OVA-specific IgE. Strikingly, this was completely reversed with SCJ engrafted CCR7(-/-) DCs in all parameters tested. Furthermore, topical antibody blockade of CCR7 in actively immunized mice significantly inhibited AC. Ocular surface DCs via CCR7 expression contribute to the immunopathogenesis of AC, thereby allowing significant inhibition of this experimental condition via topical CCR7 antibody blockade., (Copyright © 2012 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.)

Significant interest has been focused on the use of ex vivo-manipulated DCs to optimally induce transplant tolerance and promote allograft survival. Although it is understood that donor-derived, tolerogenic DCs suppress the direct pathway of allosensitization, whether such DCs can similarly suppress the indirect pathway remains unclear. We therefore used the murine model of corneal transplantation to address this, as these allografts are rejected in an indirect pathway-dominant manner. Interestingly, recipients administered with donor bone marrow-derived DCregs, generated via culturing with GM-CSF, IL-10, and TGF-β1, significantly prolonged survival of corneal allografts. Correspondingly, these recipients demonstrated a potent reduction in the frequency of indirectly allosensitized T cells, as determined by ELISPOT. Examination of DCregs relative to mDCs or iDCs showed a resistance to up-regulation of MHC-II and costimulatory molecules, as well as an impaired capacity to stimulate MLRs. In vivo, DCreg administration in corneal-allografted recipients led to inhibition of CD4(+)IFN-γ(+) T cell frequencies and an associated increase in Foxp3 expression in the Treg compartment. We conclude that donor-derived, tolerogenic DCs significantly suppress the indirect pathway, thereby identifying a novel regulatory mechanism for these cells in transplantation.

Purpose: In addition to Langerhans cells (LCs), other dendritic cells (CD11c(+)) have recently been shown to express Langerin (c-type lectin). In skin, (non-LC) Langerin+ dendritic cells initiate adaptive immunity. However, whether such dendritic cells (DC) reside in the cornea, an immune-privileged tissue, is unknown., Methods: Normal C57BL/6 corneas were harvested for qRT-PCR analyses of Langerin expression in the epithelium versus stroma. Immunohistochemistry for Langerin was also performed. Single-cell preparations of epithelium versus stroma were FACS analyzed for CD11c, CD11b, and CD103 expression. Fluorescence microscopy of corneas from muLangerin-eGFP mice (in which all CD11c(+) Langerin+ cells express eGFP), huLangerin-DTA mice (only LCs are constitutively deleted), and huLangerin-Cre eYFP-flox (only LCs express eYFP) was performed., Results: qRT-PCR, immunohistochemistry, and FACS analysis identified CD11c(+) Langerin+ cells in the epithelium and stroma. Similarly, corneas of muLangerin-eGFP mice contained eGFP+ cells in the epithelium and stroma. However, FACS analysis indicated phenotypically differing CD11c(+) Langerin+ populations in the epithelium (CD11b(low)CD103(low)) versus stroma (CD11b(+)CD103(low)). Additionally, corneas from huLangerin-DTA mice were devoid of Langerin+ cells in the epithelium but were detectable in the stroma. In corneas from huLangerin-Cre eYFP-flox, eYFP+ cells were detectable in the epithelium but not in the stroma., Conclusions: The normal corneal epithelium is endowed with CD11c(+) Langerin+ cells that are LCs, whereas the stroma is endowed with a separate population of (non-LC) Langerin+ DCs. These findings should henceforth facilitate the examination of Langerin-expressing DC subsets in the immunopathogeneses of conditions such as keratoconjunctivitis sicca, allergic keratoconjunctivitis, and corneal allograft rejection.

Purpose: Chemokine receptor 7 (CCR7) is a key homing molecule for immune cell trafficking, including corneal antigen-presenting cell (APC) migration from the inflamed cornea to draining lymph nodes (LNs). Here, the authors investigated the effect of CCR7-facilitated donor APC trafficking on allosensitization, regulatory T-cell (Treg) function, and graft survival in corneal transplantation., Methods: CCR7(-/-) or wild-type (WT) allogeneic corneal grafts were transplanted onto the neovascularized high-risk recipient beds. Two weeks later, the frequency of directly alloprimed host T cells was measured by the IFN-gamma ELISPOT assay. Treg function was tested by a coculture suppression assay and an IFN-gamma ELISPOT assay. Kaplan-Meier analysis was performed to evaluate graft survival., Results: The recipients of CCR7(-/-) grafts had fewer migrated donor APCs and lower frequency of IFN-gamma-producing T cells in the draining LNs. However, there was no statistically significant difference in transplant survival between recipients of CCR7(-/-) and those of WT grafts. Tregs from the CCR7(-/-) graft recipient group showed reduced regulatory potential for the suppression of proliferation of naive T cells and direct alloprimed T cells and expressed lower Foxp3 levels. In vitro studies confirmed that mature CCR7(+) major histocompatibility complex class II(+) CD86(+) graft-derived dendritic cells were critical for Treg function., Conclusions: Not only is CCR7-mediated donor-derived APC trafficking to the draining LNs important in the initiation of host T-cell priming, it is crucial for Treg-mediated tolerance.