The skin is the first line of defense against pathogens, toxins, and chemical agents and an important site of immune protection and regulation. The skin was first recognized as an important lymphoid tissue in the 1980s when its link to the lymph system was described as skin-associated lymphoid tissue (SALT) and later as the “skin immune system” [60–62]. Skin has two main layers: the epidermis and the dermis. The epidermis has a number of cells types with immune function including keratinocytes, epidermal DCs called Langerhans cells, γδ T cells known as dendritic epidermal T cells (DETCs), and memory αβ CD8⁺ T cells [62–65]. The epidermis is also where Merkel cells, which communicate with the nervous system, and melanocytes, which provide protection from UV radiation, are found. In contrast, the dermis consists mainly of elastin, collagen fibers, and fibroblasts within an extracellular matrix. Cells with immune function in the epidermis include macrophages, MCs, various DC subtypes, innate lymphoid cells (ILCs), γδ T cells, and αβ T cells [64, 66]. The epidermis is also the site of blood vessels and capillary beds as well as the draining lymphatics.

Keratinocytes not only serve as a protective outer layer of the skin but behave like innate immune cells expressing innate TLRs and Nod-like receptors (NLRs) [62, 64, 67]. They recruit immune cells to the skin by release of chemokines and cytokines like TNF, IL-1α/β, IL-6, IL-18, and IL-33. Interestingly, most of these cytokines are associated with activation of the inflammasome [64, 68]. The inflammasome is an innate immune mechanism of defense within host cells that leads to the production of pro-IL-1β and pro-IL-18 via TLR4 signaling that is activated by caspase-1 resulting in release of active IL-1β and IL-18 from the cell [69]. IL-33 and its receptor ST2 are part of the TLR4/IL-1R signaling family. IL-33 can act synergistically with IL-4 or on its own to induce Th2 responses, while IL-18 (originally named IFN-γ-inducing factor) strongly increases Th1 responses [2, 28, 70]. IL-33 is not only a cytokine but is termed a damage-associated molecular pattern (DAMP) also called an “alarmin” and is released when skin is damaged due to infection, toxins, chemicals, or physical trauma [71]. IL-33 is able to activate skin MCs and type 2 ILCs (Th2-like) via the IL-33R ST2 and to recruit eosinophils to the skin [72]. MCs and ILCs are able to drive immune responses in any direction (i.e., Th1, Th2, Th17) that provides the best protective immune response [64, 66, 73].

Innate lymphoid cells have been identified at many tissues sites, but only recently in the skin [64, 73–76]. ILCs include NK cells, γδ T cells, as well as a newly defined subtype of ILCs termed ILC1, ILC2, and ILC3, reminiscent of the Th1, Th2, and Th17 classification. ILC1s express the Th1-associated transcription factor Tbet and release IFNγ. This cell population is considered to protect against viral infections where IFNs are needed for viral clearance, and these cells are driven by the classic Th1 cytokine IL-12 or the inflammasome-derived cytokine IL-18. ILC2 include cell types named “nuocytes” and “natural helper cells” and express the Th2-associated transcription factor GATA-3 and produce the Th2 cytokines IL-5, IL-9, and IL-13. ILC2 are promoted by IL-25 and the inflammasome-associated cytokine IL-33 (IL-33 has the closest structural homology to IL-18 and is part of the TLR4/IL-1R signaling family) [72]. ILC2 responses are needed to protect against helminth infections and perform tissue repair but can also promote asthma and tissue scaring/fibrosis. Lastly, ILC3 express the Th17-associated transcription factor RORγt and produce IL-17A, IL-22, and some IFNγ and respond to IL-23 and the inflammasome-derived cytokine IL-1β. This response is important in protecting against bacterial infections and promotes tissue repair but can also lead to fibrosis. Importantly, IL-33 not only increases classic ILC2 and Th2 responses but has also been found to increase Th1 and Th17 responses [72]. This is likely due to ST2 expression on MCs, which are able to promote Th1, Th2, or Th17 responses [66, 72, 77]. MCs are also important in regulating inflammation by responding to vitamin D3 (VitD) (they express the vitamin D receptor) to produce IL-10, a cytokine that downregulates all types of inflammatory responses. They also release TGFβ which can increase Treg populations and drive macrophages to a regulatory myeloid-derived suppressor cell (MDSC) or alternatively activated M2 phenotype [66, 78].

MCs are typically located at sites where the host first encounters pathogens, toxins, and allergens like the skin, respiratory tract, and mucosa (mouth, nose, and bowel) [66, 79]. MCs are able to protect the skin and mucosa from infections of each class of pathogen including viruses, parasites, and bacteria (Th1, Th2, and Th17, respectively) and are specialized to neutralize toxins through the release of detoxifying enzymes [78, 80–84]. MC release of enzymes and profibrotic cytokines like IL-4, TNF, IL-1β, and TGFβ promotes healthy remodeling but can also lead to fibrosis in the skin and other tissue sites [79, 80, 85–87]. MCs are classified as MC_TC, MC_T, and MC_C subtypes based on the release of certain proteases. MC_TC release tryptase, chymase, carboxypeptidase, and cathepsin G-like proteinase. MC_T cells release only tryptase, while MC_C cells release only chymase and carboxypeptidase but not tryptase [66, 88]. Whereas MC_T cells predominate in the lung and bowel mucosa, MC_TC are the main MC type within the skin [66, 88]. Aside from inactivating toxins and promoting remodeling, tryptase and chymase play a role in recruiting inflammation to the skin – in particular, neutrophils, eosinophils, monocyte/macrophages, and T cells. Importantly, chymase is able to cleave pro-IL-1β and pro-IL-18 to their active cytokines without the help of caspase-1 [89, 90]. MCs are resident in the skin but proliferate in response to infection, trauma, and with chronic inflammatory conditions like psoriasis and basal cell carcinoma [66]. MCs also act as antigen presenting cells (APC), along with resident macrophages and DCs; express MHC class I and II, CD80, and CD86; and respond to complement [78, 80]. Skin MCs express a number of receptors that bind antibody including FcεRI that binds IgE (classic allergy response) and FcγRI and FcγRIIa that bind IgG antibodies and autoantibodies and form ICs [91]. MCs are the key immune cells in the skin that are capable of responding to autoantibodies, which suggests that they may be important in mediating skin-specific autoimmune diseases.

Thus, the most important functions of MCs in promoting autoimmune skin disease include (1) recruiting inflammation to the skin; (2) responding to antibodies, autoantibodies, and ICs; (3) serving as principal cells within the skin involved in TLR4/IL-1R/ST2 signaling and inflammasome activation; and (4) an ability to promote remodeling and fibrosis.

Vitamin D is a fat-soluble prohormone that regulates serum calcium levels and bone homeostasis but also functions as a regulator of immune responses and, equally important for this discussion, as a sex steroid. Although most studies examine its role as a regulator of calcium and immune responses, few studies report their findings by sex or interpret their data based on vitamin D as a sex steroid. Most studies also do not emphasize whether women in their studies were pre- or postmenopausal.

Vitamin D is synthesized by keratinocytes in the skin after exposure to the sun (UVB) or from dietary sources and is hydroxylated in the liver by 25-hydroxylase (Cyp2R1) and carried in the bloodstream by vitamin D-binding protein (DBP) to the kidney where it is converted by 25-OH-d-1α-hydroxylase (Cyp27B1) to the active form of vitamin D (i.e., 1α, 25-dihydroxyvitamin D₃), which binds the vitamin D receptor (VDR) [92–94]. VDR is a member of the steroid nuclear receptor superfamily and expressed on the surface of and/or intracellularly in a wide variety of cells including keratinocytes, fibroblasts, and most immune cells [95]. Many tissues not only express the VDR but also express the key enzyme to convert 25(OH)₂D, the form of vitamin D in the serum, to its active form [96]. This is the form of vitamin D that is used to determine deficiency clinically. Macrophages possess all of the components necessary to import/synthesize cholesterol and convert it to active vitamin D including Cyp2R1 and Cyp27B1, as well as expressing the VDR [97, 98]. Liganded VDR forms a complex with the retinoid X receptor, and this complex has been estimated to regulate around 3 % of the human genome via activation of vitamin D response elements (VDREs) [99]. Many genes that regulate immune function have a VDRE in their promoter, like TNF, for example [97].

Vitamin D via the VDR has important effects on immune cells. In general, vitamin D has been shown in culture studies and animal models to increase Th2 immune responses, IL-10, and Treg and alternatively activate M2 macrophages and TGFβ [97, 100–103]. Additionally, vitamin D is known to mediate protection against infections where it acts on keratinocytes, MCs, macrophages, and DCs to activate innate immune responses. In response to infection, vitamin D upregulates TLR2, TLR4, CD14 (part of TLR4 signaling complex), the inflammasome, IL-1β, TNF, and IFNγ, for example [67, 68, 104–108]. VDRE activation increases many antimicrobial mediators like β-defensin, cathelicidin, and reactive oxygen species. However, the innate responses induced by VDR signaling described here drive Th1 and M1 macrophage responses. This appears contradictory to the ability of vitamin D to upregulate Th2 and regulatory immune responses. Confusion on this topic exists, at least in part, because most studies of vitamin D do not consider their findings in the context of sex. Usually there is no identification of the sex of the cells or animals used to conduct experiments and no interpretation of data taking sex into consideration. Because vitamin D is a sex steroid, sex differences in its effect on immune cells should exist similar to the sex differences observed for estrogens and androgens. A Th2, anti-inflammatory response may be frequently reported for vitamin D because autoimmune diseases that affect mainly women are studied that use female mice. Although few studies examine the issue of sex differences in vitamin D, one study reported that vitamin D administration was only protective in female mice in an animal model of multiple sclerosis, an autoimmune disease that occurs more frequently in women than men [2, 109]. To improve our understanding of the role of vitamin D/VDR on inflammation and autoimmune disease, it is vital that researchers design experiments, analyze data, and report results according to sex. It must also be kept in mind that although many diseases are inhibited by Th2 responses, others like allergy, asthma, and rheumatic autoimmune diseases are promoted by Th2 responses [110, 111].

Deficiency in the active form of vitamin D (i.e., 1α,25-dihydroxyvitamin D₃) is highly prevalent in the United States and worldwide, with roughly 25 % of the population in the United States reported to have inadequate vitamin D levels (<30 ng/mL) while 8 % are at risk for deficiency (Institute of Medicine criterion is <10 ng/mL, but many studies define deficiency as <20 ng/mL) (Table 26.1) [92, 112, 113]. Many but not all epidemiological studies have found an association between low levels of vitamin D and all-cause mortality [114]. Epidemiological studies also provide evidence of a significant association between vitamin D deficiency and an increased risk of autoimmune diseases [115, 116]. However, whether low vitamin D levels cause disease or occur as a result of the disease process is not clear.

Serum vitamin D levels have been found to be deficient or insufficient in patients with SLE [117, 118]. However, SLE patients are recommended to avoid sunlight and use sunscreen, and most SLE occurs in dark-skinned women – factors that can lead to vitamin D deficiency [119, 120]. Additionally, kidney dysfunction is a primary component of SLE [121], a key organ that metabolizes vitamin D to its active form. Administration of active vitamin D to MRL/lpr mice that spontaneously develop a lupus-like disease was found to decrease circulating autoantibodies, proteinuria, and SLE-like skin lesions and improved survival [122, 123], suggesting a relationship between vitamin D levels and the pathogenesis of disease. However, a review of 17 clinical studies found that although vitamin D deficiency is common in SLE, associations between low vitamin D levels and worse disease severity were lacking [118].

Vitamin D deficiency or insufficiency has also been found in RA patients, but in contrast to SLE, lower vitamin D levels are associated with higher disease activity [124, 125]. Additionally, RA patients treated with vitamin D displayed clinical improvement [126], but not all studies agree [118, 127]. Similar to lupus, animal models of RA found that vitamin D administration inhibited disease progression and severity [128, 129]. Psoriasis and psoriatic arthritis are associated with hyperproliferation of keratinocytes, fibrosis, and inflammation involving the skin and joints. Vitamin D supplementation has been used to treat these conditions [118, 130, 131]. Interestingly, a VDR polymorphism has been found that prevents success of vitamin D therapy and is associated with a higher incidence of psoriasis in afflicted individuals [132]. Overall, these data suggest that vitamin D reduces RA and psoriatic arthritis.

What is believed to be an early stage leading to connective tissue disease (CTD) has been termed undifferentiated CTD (UCTD). About 30–40 % of these patients progress to a CTD such as SLE, mixed CTD (MCTD), systemic sclerosis, Sjögren’s syndrome, RA, polymyositis/dermatomyositis, and/or systemic vasculitis [127]. One large cohort study examined seasonal variation in vitamin D levels in UCTD patients and found that vitamin D levels fluctuated with seasons but were always lower in UCTD patients than controls, suggesting that vitamin D insufficiency could contribute to the pathogenesis of CTDs [127, 133]. Interestingly, vitamin D insufficiency correlated positively with the probability of developing dermatological symptoms in these patients.

Scleroderma is a chronic autoimmune disease characterized by diffuse skin fibrosis and vasculopathy. Symptoms of scleroderma may occur as part of MCTD. Low circulating vitamin D levels are frequently observed in scleroderma patients. A number of studies report vitamin D insufficiency in 63–86 % of Sc patients and deficiency in 35–95 % of patients [134–136]. Additionally, patients with vitamin D deficiency have worse disease than those that were vitamin D insufficient, suggesting a role for vitamin D in disease pathogenesis. Dermatologists use the topical form of vitamin D to treat Sc, implicating its importance in reducing disease. TGFβ is a profibrotic cytokine that is believed to be responsible for increasing fibrosis in Sc patients. Vitamin D administered to murine cells has been found to inhibit TGF production [127, 137]. Again, sex of the cells, animals, or patients was not addressed in these studies.

Sjögren’s syndrome is characterized by inflammation, dysfunction, and destruction of the exocrine glands, especially the salivary and lacrimal glands, resulting in dry mouth and eyes. However, the lungs, liver, skeletal muscle, and kidneys are often involved [121]. SS can be termed primary SS if it occurs as the only autoimmune disease, but frequently SS is part of an autoimmune disease spectrum and termed secondary SS. Autoimmune diseases that often overlap with SS include SLE, RA, and scleroderma. Several studies found that vitamin D levels are deficient or insufficient in primary SS patients and low levels correlate with disease type and severity [138, 139]. Low vitamin D was found to remain low over a 2-year period of time. Additionally, low vitamin D levels correlated with higher levels of RF in SS patients, suggesting that vitamin D could protect against SS by reducing rheumatoid factor levels (recall that RF helps drive IC formation). Currently there are no data on vitamin D supplementation in SS patients [127]. It is important to point out that low vitamin D in SS patients may be due in part to liver and kidney damage caused by the disease potentially affecting the body’s ability to synthesize active vitamin D [121].

Epidemiological data indicate that more than 60 % of postmenopausal women have vitamin D insufficiency and 16 % are vitamin D deficient [127, 140]. With aging dermal and epidermal skin thickness reduces [120]. Consequently, cutaneous vitamin D synthesis decreases with age and following menopause because of smaller stores of the precursor 7-dehydroxycholesterol in the skin [141–146]. The elderly may be at additional risk of vitamin D deficiency because of decreased mobility and thereby less sun exposure and potentially due to kidney or liver disease [147].

Thus, it remains unclear whether vitamin D deficiency occurs as a consequence of autoinflammatory damage to the skin and kidney or whether it is caused by low exposure to sunlight (city dwellers and/or geographic location) and the use of sunscreen. What is clear is that vitamin D strongly regulates immune function and particularly immune cells within the skin like MCs, macrophages, and keratinocytes. Even if vitamin D deficiency is caused by autoimmune disease, it could promote disease severity and progression because of its powerful local and systemic effects on inflammation.

Of rheumatic autoimmune diseases, only two typically peak in appearance before menopause, SLE, and scleroderma. SLE typically presents in females during childbearing years (20s and 30s) [2, 174, 175]. One study found that only 16 % of SLE patients presented after age 50, and in another study the female to male ratio was 13:1 before age 50 but only 3:1 after 50 [176, 177]. Scleroderma disease onset peaks in women from age 30 to 40 years while estrogen levels remain high [164, 178].

The remaining three rheumatic autoimmune diseases, RA, dermatomyositis, and Sjögren’s syndrome, peak after menopause suggesting a protective role for estrogen. Little information is available regarding sex differences or effects of menopause on dermatomyositis, so the following discussion will focus on RA and SS.

Autoimmune diseases that affect the skin occur predominantly in women with rheumatic autoimmune diseases such as SLE, RA, Sjögren’s syndrome, scleroderma, and dermatomyositis. Common immune mechanisms drive pathology in all rheumatic diseases and include MC and macrophage activation of the inflammasome, vitamin D deficiency, elevated autoantibodies and IC deposition, and elevated levels of proinflammatory and profibrotic cytokines like TNF and IL-1β. Evidence to date suggests that cutaneous manifestations in SLE and scleroderma are lower following menopause, suggesting that high levels of estrogen that are present during peak childbearing years contribute to disease pathogenesis. The primary pathology in SLE and scleroderma is ICs and fibrosis, respectively. Estrogen is well known to increase the autoantibodies and profibrotic cytokines needed to promote these diseases.

In contrast, Sjögren’s syndrome, RA, and dermatomyositis peak after menopause. Understanding how the menopausal transition affects inflammation in this case is complicated by changes in the immune response that occur with aging. However, low levels of estrogen are able to promote B-cell proliferation and antibody production, and autoantibody levels in women continue to increase with age. Additionally, lower estrogen levels reduce the protective effects of estrogen that were present prior to menopause and allow increased activation of innate immune cells resulting in elevated proinflammatory and profibrotic cytokines. The skin is particularly susceptible to the effects of fluctuating estrogen and vitamin D levels because skin immune cells and keratinocytes are directly influenced by these sex steroids. It is not clear yet whether vitamin D deficiency causes rheumatic autoimmune diseases or occurs as a result of autoimmune damage to the skin and kidneys reducing production of the active form of the hormone. Regardless of when it occurs, low vitamin D levels are likely to affect the pathogenesis of disease because of the potent regulatory effects vitamin D has on immune and skin cells.

Although some aspects of the immune response important in driving autoinflammation in the skin are known, there are many gaps that still need to be addressed (Table 26.2). Published studies need to be reanalyzed according to sex and age and new studies designed that examine whether sex differences exist. All clinical, animal, and culture studies must report the sex used for the study. Vitamin D needs to be understood as a sex steroid that is expected to have different effects on inflammation according to sex, just as estrogens and androgens induce different affects. Additionally, the skin is an organ that is centrally involved in regulating vitamin D levels in the body and should be considered when attempting to understand the effects of vitamin D deficiency or insufficiency on chronic inflammatory diseases. We need a better understanding of the role of MCs in autoimmune skin diseases, since they are the primary immune cells responding to ICs within the skin. We also need studies that examine the role of IL-33/ST2, the inflammasome, and ILCs in the skin during rheumatic autoimmune diseases. And finally, we need to define the relationship of ER, AR, and VDR signaling on immune cells and how differing ratios of estrogen to testosterone, as occurs following menopause and with aging, and differing vitamin D levels affect skin inflammation. This knowledge would improve our ability to treat these diseases with vitamin D supplementation and HRT.