This paper demonstrates for the first time DAMD. This finding shows that dupilumab blocks IL-13 and a known correlation between IL-13 and mucin levels exists. Persons with DAMD displayed a constellation of ocular symptoms including varying degrees of conjunctivitis, blepharitis. and keratitis. Most persons with DAMD had bilateral mild-to-moderate conjunctivitis with limbal hyperemia. Compared with control, ocular Muc5AC levels normalized to total tear protein was statistically significantly lower. The average Muc5AC levels for persons on dupilumab was 1.54 ± 0.58 ng/mg and that of controls was 7.99 ± 1.16 ng/mg. Persons on dupilumab reported a statistically increased occurrence of ocular fatigue/eye strain, uncomfortable sensation, pain, red eye, and itching.
In addition to mucin deficiency, histological analysis of conjunctival biopsies from patients on dupilumab has shown marked decrease and, in certain patients, complete absence of goblet cells.
8 The median conjunctival intraepithelial goblet cell density was 33 cells/mm
2 in persons on dupilumab versus 323 cells/mm
2 in healthy controls. In addition to a paucity of goblet cells, persons on dupilumab had a multicellular immune cell stromal infiltrate. This infiltrate comprised primarily T cells (CD3+/CD4+) and eosinophils. In regard to treatment for these patients, would a recombinant topical mucin be sufficient to abrogate dupilumab associated conjunctivitis? The authors suspect the answer may not be so straightforward.
In addition to demonstrating a link between IL-13 and goblet cell and mucin production, Pflugfelder and colleagues have also explored the role of goblet cells in promoting antigen-presenting cell tolerance, thereby suppressing IL-12 production and TH1 polarization. With this in mind, DAMD may be just the tip of the iceberg with goblet cell loss disturbing ocular immune homeostasis. Further studies are critical to not only benefit patients on dupilumab but also to inform the understanding of mucin and goblet cell function in ocular surface disease.
Dupilumab binds to the alpha subunit of the IL-4 receptor, thereby blocking IL-4 and IL-13 signaling.
20 In addition to the effect on goblet cells and mucin production, it is feasible some degree of dupilumab's ocular side effects are due to perturbation of these signaling pathways separate and apart from goblet cell homeostasis. For example, a recent paper outside the ocular literature has demonstrated selective suppression of IL-23 by IL-4 and IL-13, which in turn reduces Th17 function.
21 From the work of Reza Dana's group, it is known that interferon-γ-expressing Th17 cells are present and are potentially even requisite for severe ocular surface autoimmunity.
22
Other hypothesized mechanisms for dupilumab's ocular side effects include worsening of allergic conjunctivitis. One such hypothesis suggests IL-4 and IL-13 signaling inhibition results in increased activity of OX40L and other ligands known to worsen allergic conjunctivitis.
5 Another hypothesis is based on the documented transient increase in eosinophils found upon the initiation of dupilumab. It has been hypothesized these increased eosinophils are responsible for ocular side effects.
1 These hypotheses may not fully explain the pathogenesis because the ocular side effects of dupilumab tend to not appear acutely when starting the medication, but over a period of weeks or sometimes a month. Further, the phenotype of DAMD patients, at least in the clinical judgment of the authors, appears separate from that of allergic conjunctivitis. In fact, it is this paradox of improving atopic disease and allergic conjunctivitis findings and worsening of a distinct form of ocular inflammation that prompted this study.
The demonstration of goblet cell loss and decreased mucin may be a fruitful area for exploration as the primary cause of the deleterious effects of dupilumab on the ocular surface. Goblet cells have been shown to play a fundamental role in decreasing ocular inflammation, reducing corneal permeability, and decreasing evaporative tear loss.
23,24 Goblet cell loss has been implicated in a variety of well-studied ocular surface diseases, including allergic conjunctivitis, ocular cicatricial pemphigoid, and Sjogren syndrome.
23,25–29 In seminal work by Argueso et al., it was found that patients with Sjogren syndrome have a concomitant loss of goblet cells and Muc5AC and SPEDF messenger RNA are substantially decreased in the conjunctiva of such patients.
28 Moreover, Argueso's group demonstrated that Spdef null mice lack conjunctival goblet cells.
26 The patients in this study with DAMD have an ocular surface phenotype that is reminiscent of that in Spdef null mice. Goblet cells are also known to respond to transforming growth factor-beta signaling and indeed appear sensitive to a variety of external and inflammatory stimuli. Accordingly, disruption of transforming growth factor-beta signaling has been shown to improve ocular surface disease symptoms in an experimental model of autoimmune keratoconjunctivitis sicca.
30 Another example of goblet cells central role in ocular surface homeostasis is demonstrated by the finding that goblet cell secretion is modulated by leukotrienes and is reduced by resolvins D1 and E1.
31 Pflugfelder and colleagues have also demonstrate that neutralization of interferon-gamma decreases loss of goblet cells in an experimental dry eye model.
32
Taken together, it appears that the various inflammatory pathways regulate both the number of goblet cells but also function in various pathogenic conditions. Importantly, this relationship appears to be two-way, with goblet cells proving immunomodulatory crosstalk with other cellular mediators. Goblet cells have been shown to act as antigen-presenting cells to dendritic cells in the conjunctival stroma.
33 In the same way various inflammatory cytokines control goblet cell number, goblet cells have been shown to maintain dendritic cells in an immature state.
34 In the small intestine, goblet cells have been shown to bridge the gap from lumen to the underlying dendritic cells. By sampling the luminal environment and carrying that information to dendritic cells in the underlying epithelium, dendritic cells can sense environmental insults without a break in barrier integrity.
6,7 In the authors’ opinion, it is likely that much like goblet cells in the gut, goblet cells in the conjunctiva will emerge as major players in ocular surface immunology. Accordingly, it is conceivable that dupilumab's effects on the ocular surface are entirely the result of mucin and goblet cell deficiency.
Despite the pioneering work of many of the groups described previously, therapeutic options in DAMD are limited because of the poorly understood mechanisms regulating goblet cell differentiation in the conjunctiva. For the majority of patients who have been plagued by atopic disease their entire life, the ocular side effects are outweighed by the systemic benefits and thus they may be DAMD if they do. Lifitegrast ophthalmic solution (Xiidra) 5%, cyclosporine ophthalmic emulsion (Restasis) 0.05%, tacrolimus, topical steroids, and topical lubricants have been reported to be effective in this condition.
5,8,14,35–38 Tellingly, mast cell stabilizers and antihistamines appear to be of minimal benefit. If primarily a mucin deficiency, it stands to reason that treatment of DAMD should involve mucin replacement. Currently, a recombinant form of mucin is not available. A few therapeutics whose primary mechanism is thought to be increased mucin production are approved outside the United States, including rebamipide in Japan.
This study has a number of limitations. First, the patients referred to the ophthalmology clinic on dupilumab had ocular side effects and thus no comment on prevalence or normal degree of symptoms can be ascertained. As one would expect based upon the symptomatic nature of our patients, the degree of signs and symptoms appeared more than that previously reported in all comers on dupilumab in the dermatology literature. In addition, the sample size in this study was small. Nevertheless, it is telling that in this enriched population, even with small sample size, a significant difference in Muc5AC levels as well as reported ocular surface disease symptoms was found between subjects and controls.
Another significant limitation of this study was the single timepoint when patients were analyzed. Although patients were asked to refrain from topical ocular medications and lubrication on the day of tear collection, the use of topical ocular medication before the day of tear collection was not eliminated as a variable. Because only one healthy control used any ocular medication/lubrication, artificial tears in the one case using drops, the subjects were on a variety of topical ocular medications and lubricants. Because patients were found to clinically and symptomatically improve on these various therapies, it is unlikely that the use of these medications was the cause of the observed decline in tear mucin levels. Nevertheless, a prospective study looking at all patients started on dupilumab with monitoring of Muc5AC as well as corresponding ocular irritation and inflammation would be illustrative.
Although our study does prove that patients on dupilumab with ocular symptoms do have decreased mucin levels, it fails to address three critical questions. Do patients with ocular symptoms from dupilumab introduction already have low Muc5AC levels before starting the medication and dupilumab destroys any Muc5AC reserve? Do all patients that start dupilumab have a decline in Muc5AC levels and, if so, is this percentage decline relatively stable from person to person? Clearly, a prospective study in which Muc5AC and goblet cell density is measured before initiation of dupilumab and followed over time would be of great benefit. Similarly, the evaluation of mucin levels longitudinally after the introduction of topical steroids, cyclosporin, Lifitegrast, rebamipide, or other agents thought to be helpful in DAMD as well as the correlation to clinical signs and ocular surface disease symptoms would be very illustrative. Nevertheless, this study represents an important direction to explore in the better understanding of dupilumab induced ocular surface disease. Moreover, this study further highlights what appears to be a critical role of mucin and goblet cells in a healthy ocular surface.
In conclusion, this study demonstrates for the first time, a relative deficiency of Muc5AC in patients on dupilumab. These results support the previously reported role of IL-13 in increasing goblet cell density and associated Muc5AC production. Persons with DAMD displayed a constellation of ocular symptoms including varying degrees of conjunctivitis, blepharitis, and keratitis. Most persons with DAMD had bilateral, moderate conjunctivitis with limbal hyperemia. Persons on dupilumab reported a statistically increased occurrence of ocular fatigue/eye strain, uncomfortable sensation, pain, red eye, and itching. Further efforts are under way to better understand the relative contribution of Muc5AC deficiency in the overall presentation of ocular surface disease associated with dupilumab use.