Mannoproteins are a ubiquitous component of fungal cell walls composed of protein-bound branched mannose polymers (Figure 1). Mammalian glycoproteins rarely have terminal mannose (57), making mannoproteins good antigens (58, 59). Innate immune recognition of mannoproteins by leukocytes occurs through macrophage mannose receptor (MMR) uptake, followed by presentation of mannoprotein peptides to CD4+ T cells using HLA-D molecules (58–60). As compared to cell walls of Gram-negative bacteria or Saccharomyces cerevisiae (a benign fungus), C. albicans mannoproteins induce strong production of IL-17 through this process (60). The proposed functions of the MMR include the clearance of fungi, bacteria, viruses, and homeostasis of human glycoproteins (61). The MMR is expressed by macrophages in active MS lesions, but not in controls or inactive disease (62). This suggests infectious agents inside the CNS may be contributing to inflammation in MS, or alternatively that human cell debris elicit MMR expression.

Mannose-binding lectin (MBL) is an innate immune defense protein synthesized in the liver, which binds to terminal mannose, N-acetylglucosamine, and fucose on microbial cell walls (63), and initiates the lectin complement pathway by activating mannose-binding protein-associated serine protease 2 (MASP-2) (64). MBL also binds to defective human glycoproteins lacking protective terminal residues such as galactose (57). Functional MBL/MASP-2 complexes are elevated in the plasma of MS cases as compared to controls, but not in the CSF (65). This suggests the immune response against infectious agents, such as bacteria or fungi, outside the CNS may be contributing to MS risk.

IL-17 is a cytokine produced by T cells (especially Th17 cells) and innate immune system cells (66) in response to intracellular and extracellular (67) bacterial and fungal infections (68, 69). For example, chronic Helicobacter pylori infections of the stomach induce IL-17 production (70). Genetic defects in the IL-17 pathway inhibit the clearance of fungal infections such as C. albicans (71).

IL-17 is also elevated in many idiopathic inflammatory diseases which do not have known infectious etiologies, such as psoriasis, ankylosing spondylitis, systemic lupus erythematosus, Crohn’s disease, and MS (72, 73). Thus IL-17 seems to play a role in chronic inflammation where infectious stimuli are lacking. Note that some of these conditions may have an as-yet-undiscovered infectious etiology, as became apparent for gastritis once H. pylori was discovered (74).

Though IL-17 has been implicated in the clearance of both bacterial and fungal infections (67, 75), its production is readily induced by C. albicans mannoproteins as compared to bacterial antigens (60). In a mouse model of arthritis stimulated by the injection of bacterial antigens into knee-joints, the addition of minute amounts of fungal antigens skewed the T cell immune response toward Th17 cells, tripling IL-17 concentration in the joints (76).

Though IL-17 is unambiguously associated with both fungal infections and MS, this association only provides weak circumstantial evidence of a causative link between MS and fungi, as IL-17 is also associated with bacterial infections and many idiopathic inflammatory diseases. Since IL-17 production is very sensitive to fungal antigens (60), even low fungal loads inside the CNS could provoke a Th17 immune response, especially in the presence of other inflammatory stimuli (76).

Chitotriosidase is a protein produced by activated macrophages, which hydrolyzes chitin (77). Chitin is a polysaccharide present in fungal cell walls, yet absent from bacterial and mammalian cells (Figure 1) (77). The exact function of chitotriosidase in the clearance of fungal infections is unclear (77). Macrophage chitotriosidase synthesis is stimulated by chitin, but not by other immunostimulatory compounds such as lipopolysaccharide or zymosan (78), suggesting that it is part of the innate immune response against chitin-bearing microbes (77).

Elevated chitotriosidase levels occur in a variety of diseases, both infectious and non-infectious (79). Chitotriosidase is thought to be a biomarker of macrophage activation, and may have functions unrelated to chitin (79). Elevated chitotriosidase in the CSF is an important biomarker of MS (80–82). This suggests a fungal infection inside the CNS may be contributing to inflammation in MS, or alternatively that macrophages produce chitotriosidase in response to non-chitin stimuli.

Calprotectin is a broad-spectrum antimicrobial protein complex produced mainly by neutrophils, but also by monocytes, macrophages, endothelial, and epithelial cells, which is released during infections or inflammation (83). Fungi are particularly susceptible to calprotectin, with minimum inhibitory concentrations of ~16 μg/ml as compared to 64–256 μg/ml for bacteria (84). Elevated calprotectin has been reported in many autoimmune diseases such as Crohn’s disease and systemic lupus erythematosus (85). Calprotectin levels in the CSF are markedly elevated during MS flare-ups (86), suggesting an infection may be present inside the CNS.

Benign commensal fungi frequently colonize the oral cavity and respiratory system, including Aspergillus species (105). Aspergillus cells are recognized by CD4+ T cells following HLA-D antigen presentation by phagocytes. This is particularly efficient in HLA-DRB1*1501 and HLA-DRB1*1503 carriers, and leads to a population of activated CD4+ T cells recognizing Aspergillus antigens in the lymph nodes of the mouth (Waldeyer’s ring).

Meanwhile, a resting naive B cell near Waldeyer’s ring is infected by an EBV virion: this occurs often in healthy EBV carriers (106). Once infected, this naive B cell migrates to a germinal center within Waldeyer’s ring, performs affinity maturation and emerges as an isotype-switched memory B cell containing a few dormant EBV episomes (106). EBV hides from the immune system within these long-lived memory B cells, maintaining a persistent infection for the rest of the host’s life (106). Eventually these memory B cells reactivate and differentiate into plasma cells in the mouth by finding a cognate antigen which is also recognized by CD4+ T cells (55, 106). EBV virions are then produced in these plasma cells, from which they can infect other hosts and maintain a stable EBV-positive memory B cell population in the same host (106).

Under EBV-free circumstances, a resting naive B cell only activates and goes through affinity maturation if its B cell receptor (BCR) has a decent affinity to an abundant antigen, and if it receives CD4+ T cell help for a peptide present in this antigen (106). When EBV infected, a naive B cell will activate and go through affinity maturation despite filling neither of these two conditions (106): its BCR will be honed to recognize any abundant antigen present in the lymph node (55). Which antigen is selected matters little for EBV, as long as the memory B cell eventually differentiates into a plasma cell in the mouth (55). This means commensal microbe antigens present in Waldeyer’s ring and recognized by CD4+ T cells in this site will be selected (55).

When present, CD4+ T cells recognizing Aspergillus antigens will skew epitope recognition of EBV-infected memory B cells toward fungal antigens by providing CD4+ T cell help during affinity selection in the light zone of the germinal center. Antibodies against the mannose polymer portion of fungal mannoproteins have been associated with many autoimmune diseases (Table 1), so these are good antigen candidates (Figure 1). Mannoproteins provide a high avidity target for memory B cells because of their branched mannose polymer structure: mannose termini attached to the same protein can simultaneously bind to each arm of a BCR (Figure 2). Their protein portion can be recognized by CD4+ T cells, giving memory B cells the CD4+ T cell help necessary to differentiate into a plasma cell in the mouth and shed EBV virions.

After completing affinity maturation, the EBV-infected memory B cell recirculates in peripheral blood and in the lymphatic system (106). During this time, no protein-coding genes from any of the EBV episomes in the memory B cell’s nucleus are expressed: this is essential for EBV’s survival because EBV-infected memory B cells are sought by a large number of T cells: ~3% of CD8+ T cells recognize EBV proteins (107).

In order to maintain long-term immunity, the memory B cell periodically divides—about 2.7% of memory B cells are dividing at any given time (106). During memory B cell division, EBV episomes must be replicated and partitioned between daughter cells. This is accomplished by expressing a single EBV gene (EBNA-1); the EBNA-1 protein is only present during cellular division to reduce the probability of the memory B cell being killed by CD8+ T cells while it is not dividing (106).

Unlike human chromosomes, EBV episome replication and partitioning is not very reliable, and after a few dozen divisions, some daughter cells become EBV-free (108). Once EBV-free, daughter memory B cells are no longer sought by T cells, so their longevity, clonal expansion, and differentiation into plasma cells cease being constrained by immune surveillance of EBV proteins.

When EBV first infects the host, up to half of memory B cells contain EBV episomes (106). This high viral load is due to a lack of T cells recognizing EBV proteins (106). As adaptive immunity is acquired over the following year, the fraction of memory B cells containing EBV episomes drops ten thousand-fold and stabilizes at about half a million memory B cells (or 0.001%) (106). For the rest of the host’s life, a small number of EBV-positive memory B cells are produced in the mouth to replace those eliminated by immune surveillance and other forms of attrition (106). A small subset of the EBV-positive memory B cell pool is expected to target a “forbidden” antigen and survive long enough to become EBV-free (55). However, this population of memory B cells is likely too small to cross the blood–brain barrier and find a cognate antigen in the CNS without first clonally expanding.

To clonally expand, the now EBV-free “forbidden” memory B cell that we have been following from the mouth must find a cognate antigen and an activated CD4+ T cell that recognizes a peptide from this antigen. Lymph nodes exposed to fungal antigens from the gastrointestinal tract and genitals provide the best environment for this chance event to occur—especially when these sites are colonized with Candida. CD4+ T cells which readily recognize fungal antigens should increase the probability of this chance event happening: such cells are associated with idiopathic uveitis (44), though no similar study could be found for MS. Antibodies against Candida species can be used as a proxy for CD4+ T cell sensitivity to fungal antigens: they are strongly associated with MS risk (27–29).

Fungal colonization is thus expected to be an MS risk factor because it allows “forbidden” memory B cells to clonally expand after shedding their EBV episomes. To the best of our knowledge, no study has directly measured fungal colonization in relation to MS. The efficacy of the oral fungicide DMF suggests fungal colonization of the gut may be an important contributor to the clonal expansion of “forbidden” MS-causing memory B cells.

As the now large EBV-free “forbidden” memory B cell population recirculates through the blood and lymphatic system, a small fraction of these cells will reach the CNS. Once in the CNS, these cells will find an antigen cognate to their BCR, enabling the chain of events leading to demyelination. Eliminating these memory B cells should improve MS symptoms, as suggested by the efficacy of anti-CD20 drugs (52). The exact nature of the antigen found by “forbidden” memory B cells in the CNS is not known, though mannose polymers are plausible candidates.

Because fungi are eukaryotes, they have many genes which are similar to human genes: molecular mimicry due to an epitope conserved within opisthokonta could explain why memory B cells recognizing a fungal epitope also recognize a human epitope. Alternatively, there could be an elusive fungal infection in the CNS against which immune tolerance is lost in the presence of these memory B cells. Several histological reports are consistent with a fungus in the human brain, though definite proof is currently lacking (109–117).

The events in the example above account for the two best established MS risk factors: HLA-DRB1*15 and EBV seropositivity, as previously described (55). They also account for the association with antibodies against Candida (and thus female sex), and for the efficacy of DMF and anti-CD20 drugs in MS. However, they do not account for three other risk factors: age at onset (young adulthood), smoking, and herpes simplex virus type 2 (HSV-2) seropositivity (118–121). These three additional risk factors suggest a sexually acquired infection is involved.

While Candida is very common in children (53), vaginal Candida infections are rare in girls (54). The distribution of the age at first vulvovaginal candidiasis episode and MS onset match well (10, 54). Perhaps genital exposure to Candida species can trigger MS. Adult women have higher antibody titers against Candida than men, for which vulvovaginal candidiasis is the simplest explanation (56). This difference may contribute to women’s increased risk of MS as compared to men’s. Because antibodies against Candida are common in children (122), humoral immune responses to genital Candida infections cannot directly explain the very low incidence of MS in children. Only if the humoral immune responses in these sites were different, producing “forbidden” memory B cells in the genitals, but not in the mouth or gut, could vulvovaginal candidiasis explain the age at onset of MS. Perhaps naive B cells in genitalia are simultaneously exposed to EBV virions and Candida antigens, whereas combined exposure is rare in the mouth and gut. This hypothesis would be plausible for Candida in the gut (where EBV virions are generally absent), but not for Candida in the mouth (which is replete with EBV virions). While genital Candida exposure may well be an MS risk factor, it cannot account for the low incidence of MS in children.

Two diseases have low incidence in children, peak incidence in young adults, and moderate associations with smoking and HSV-2: cervical cancer, respectively OR = ~1.8 (123) and OR = ~1.6 (124) and MS, respectively OR = ~1.5 (8) and OR = ~1.6 (121). We now know that cervical cancer is not caused by HSV-2 and smoking—these two risk factors act as surrogates for oncogenic human papilloma virus (HPV) exposure, and their association with cervical cancer disappears after controlling for oncogenic HPV (124–128).

Multiple sclerosis’ association with young adulthood, smoking, and HSV-2 seropositivity remains unexplained. In 2002, Hawkes hypothesized that an as-yet-unrecognized STI was causing MS based on three observations (129). First, the distribution of the age at onset of common STIs approximately matches MS (10, 130). Second, MS “clusters” have been reported on small islands where soldiers were posted during World War II (129). Third, many known STIs disseminate throughout the body and cause chronic neurological symptoms through demyelination, such as syphilis (131), human T-lymphotropic virus (132), and human immunodeficiency virus (133). A fourth observation was added by Hawkes in 2006: HSV-2 seropositivity is associated with MS (121). We add a fifth observation: smoking increases MS risk, possibly by acting as a surrogate for an STI.

Though the existence of an as-yet-unrecognized STI may seem unlikely, this hypothesis has been proposed to explain the sexual risk factors of prostate cancer (134–136) (currently no strong candidate) (137) and reactive arthritis (138) (strongest candidate is Chlamydia trachomatis, found in ~13% of cases and ~2.4% of controls) (139). Prostate cancer has recently been linked to an intracellular fungal infection (35, 36). Antibodies against fungi have been linked to reactive arthritis symptoms, especially uveitis (21). Older studies have found antibodies against a component of the prostate and prostatic inflammation in reactive arthritis (140–143) and uveitis (46, 47, 144) patients. Finally, HLA-DRB1*15 and antibodies against a component of the prostate have been linked to granulomatous prostatitis (42, 145).

Together, these studies suggest genital exposure to fungi may play an important etiological role in prostate disease, reactive arthritis, and MS. Could the unexplained histological evidence of fungal cells in the CNS (110, 111, 113, 117) and in the prostate (146, 147) be caused by a fungal infection spreading from the genitals after sexual debut? If so, loss of immune tolerance to this putative infection could be a plausible cause of prostate disease, reactive arthritis, uveitis, and MS.