Olfactory immunity: defending the neural-mucosal barrier

Abstract

Olfaction, or the sense of smell, is the ability to detect airborne chemicals that transmit environmental information. Evolutionarily, this sense is essential for finding and judging the safety of food, mediating social relationships, marking territory, and assessing danger. In humans, olfaction is commonly thought to have a unique position in evoking emotional autobiographical experiences. As a critical sense, the olfactory anatomical organization has been highly conserved across mammals. While familiar to everyone, the olfactory system is generally overlooked by immunologists even though it is a mucosal surface with unique connectivity into the central nervous system (CNS). In this review we highlight the basic structures of the olfactory system, pathogens that infect this system, and highlight our expanding knowledge of local immune mechanisms.

Structure and cell types of the peripheral olfactory system

The olfactory system is comprised of the peripheral olfactory mucosa in the nose and the olfactory bulb in the brain. The main olfactory mucosa (OM) is distinguished from surrounding nasal respiratory mucosa (RM) within the nasal turbinates by the defining cell type, the olfactory sensory neuron (OSN). In many terrestrial vertebrates, the nasal cavity also contains the chemosensory vomeronasal organ that is similar but separated from the main olfactory mucosa at the base of the nasal septum.^1,2 Unlike other mucosal surfaces lining the lower respiratory tract, genital tract, or gut, the olfactory mucosa is unique in that it is dedicated to neuronal and chemosensory function.^3–5

The OM primarily lines the posterosuperior aspect of the nasal cavity and is divided into an overlying avascular neuroepithelium above a basal lamina and underlying lamina propria.^6,7 In most mammals a large OM surface area decorates an extensive and complex series of nasal turbinates; however, in humans, this structure resides primarily in the olfactory cleft at the most superior point of the nasal passages. The neuroepithelium contains OSN cell bodies, the cell type responsible for detecting volatile odorant molecules via olfactory receptors expressed on their nonmotile cilia.⁸ These bipolar cells extend their cilia across the airway surface and simultaneously project their axons through the underlying lamina propria and across the skull’s cribriform plate to synapse directly onto the olfactory bulb of the brain^6,7 (Fig. 1). This anatomy facilitates rapid transmission of odorant information, but by straddling the external environment and the brain, OSNs present a tissue-specific vulnerability where airway microbes find only a single cell separating them from the central nervous system (CNS).

Figure 1.

Structure of murine olfactory mucosa. The olfactory mucosa is comprised of a neuroepithelium above a basement membrane and lamina propria that sits on turbinate bone. The neuroepithelium is protected by a mucus layer and comprised of stem cells including horizontal basal cells and globose basal cells that proliferate to replenish the olfactory sensory neuron population and sustentacular support cells. Bowman’s gland cells make and secrete airway mucus. The lamina propria contains immune cells, support cells, and blood and lymphatic vessels. Axon tracts travel within the lamina propria and are surrounded by olfactory ensheathing cells.

In addition to OSNs, the pseudostratified olfactory neuroepithelium contains supportive sustentacular cells, heterogeneous microvillar cells (including tuft cells),^9,10 Bowman’s gland and ducts,^6,11 as well as a distinct class of resident macrophages.¹² Adjacent to the basement membrane lie stem cell populations including reserve dormant horizontal basal cells (HBC) and proliferating globose basal cells (GBCs) that repopulate the neuroepithelium.^13,14 As OSN axons traverse the basement membrane they collect together in fascicular bundles surrounded by olfactory ensheathing cells (OECs).^11,15 These axon bundles comprise cranial nerve I. A lamina propria composed of numerous immune cells and fibroblasts^7,11,16 surrounds these axon fascicles. Also within the lamina propria are endothelial vessels,^16,17 including lymphatic vessels traveling basally against the underlying turbinate bone (Fig. 1).^18,19 In the nasal cavity, lymphatic vessels are unique, heterogenous, and understudied. While positive for Prox1, some are Lyve1⁻ and contain unique junctions.¹⁹

Regenerative ability of olfactory mucosa

Like other epithelia, but unlike other adult neuronal tissue, the olfactory mucosa retains lifelong regenerative capacity.^14,20 Sampling airborne odorant molecules requires that OSNs be constantly exposed to the external environment, including the microbiome, potential airborne pathogens, or toxins. Consequently, ongoing adult neurogenesis allows continuous maintenance of olfaction through sensory neuron renewal.^21,22 OSNs turnover every 30-60 d at homeostasis²¹ and are replenished by proliferating globose basal cells (GBC), the major homeostatic proliferating population that produce transit amplifying progenitors that contribute both OSNs and non-neuronal cells like sustentacular cells and Bowman’s glands.^22–27 Beneath the GBC layer are horizontal basal cells (HBC) that lay flat against the basal lamina. These ICAM1^hi cells are typically a quiescent, reserve population of long-lived progenitors that can respond to severe epithelial damage to regenerate all neuroepithelial cell types.^28–33

Olfactory mucosal vulnerabilities

Throughout life, the olfactory mucosa experiences a barrage of insults, including pathogen exposures. However, unlike most barrier tissues, the olfactory mucosa’s must balance its primary function, chemosensation, with barrier function. Nevertheless, as a barrier tissue, it must coordinate and mount an immune response for proper defense and repair, a process that inevitably necessitates some “inflammation.”^34,35 Not only is the OM itself vulnerable to local damage like any other mucosal tissue, but the OM also must serve as gatekeeper for neuroinvasive pathogens that might seek to gain access to the CNS either by moving within or along OSNs. Certainly, direct neuroinvasion has dire consequences including encephalitis, meningitis, and death;^36,37 however, even olfactory replication of non-neuroinvasive infections, like severe acute respiratory syndrome coronavirus 2 (SARS-CoV2), can lead inflammatory signals to radiate along these olfactory links to the CNS.^38–40 As such, proper defense against pathogen olfactory replication can not only prevent airway and CNS inflammation, but reduce the reservoir for respiratory transmission.

The specific impact many airborne pathogens have on the olfactory mucosa, distinct from respiratory mucosa, is under described, in part due to difficulties in sampling the human superior nasal turbinates to measure microbial replication and inflammation, as nasal swab samples only reach the lower turbinates. Thus, the general ability of airway pathogens to infect human olfactory mucosa is an area that requires additional study. However, there are known pathogens that infect the olfactory mucosa, these we refer to as “olfactotropic.” Of those that are able to infect the olfactory mucosa, only a subset are known to exploit the direct connection to the CNS and invade the brain, these are referred to as “neurotropic.”^{36,37,41–43} Airway diseases may be biased toward impacting olfactory versus respiratory areas, therefore defining and distinguishing infections that affect olfactory tissue is important to understand immune responses and consequences of infection in this brain barrier site. OSNs and non-neuronal OM cells exhibit inherent vulnerabilities to infection by some intracellular pathogens, and further work to characterize their intrinsic antimicrobial programs is critical to contextualizing the immune mechanisms at play within the olfactory mucosa.

Structural, epithelial, and stromal barrier

Olfactory sensory neurons

As noted above, unlike most neurons, OSNs turn over at a substantial rate throughout life, and this individual dispensability may imply that they can respond differently to pathogen and immune pressures when compared to CNS neurons. As an example, olfactory infections by influenza viruses can have strikingly different outcomes. The commonly used mouse model of Influenza A/Puerto Rico/8/34 (H1N1) doesn’t replicate in the olfactory mucosa following intranasal infection⁴⁴ and therefore doesn’t invade the CNS, while Influenza B/Malaysia/2506/2004 (Mal/04) can directly infect OSNs. However, OSNs do not necessarily die from infection and indeed many are able to intrinsically clear Mal/04 virus.⁴⁵ In contrast, A/WSN/33 (H1N1) can infect OSNs and spread to the CNS in mice^46,47 and highly pathogenic H5N1 strains often cause olfactory neuroinvasion in animals.^48,49 The highly disparate outcomes of olfactory influenza infection suggest important differences within viruses. Additionally, work from our lab suggests that OSNs can utilize antiviral signaling pathways to intrinsically clear influenza infections, but the mediators remain to be determined.

OSN are readily infected with VSV, a type I interferon sensitive virus,⁵⁰ suggesting possible vulnerabilities in the type I interferon response- which is demonstrated to be produced in the OM following intranasal VSV infection.⁵¹ Other viruses quickly induce OSN apoptosis, that may subsequently prevent viral CNS invasion.⁴⁷ In fish, viral infection of the olfactory organ induces rapid antiviral programs including chemokine upregulation and caspase 3 mediated apoptosis⁵² that can impede neuroinvasion. Type III interferons are canonical components of mucosal antiviral response, and interferon λ (IFN-λ) restrains murid herpesvirus infection in the olfactory mucosa.⁵³

In addition to infection, there are many additional causes of inflammation and collateral damage to OSNs. While the olfactory mucosa and in particular OSNs retain lifelong regenerative capacity, aging,^54–57 and other insults including viral-mediated damage,⁵⁸ chemical exposures, smoke⁵⁹ and chronic rhinosinusitis can lead to reductions in olfactory surface area within the nasal turbinates.^60–64 This suggests that regeneration diminishes throughout adulthood, potentially exacerbated by recurrent inflammation. Innate inflammation, particularly driven by excess tumor necrosis factor α (TNF-α) and Il-13, can lead to olfactory loss, reduced OSN number, and consequentially reduced neurogenesis in mouse models.^60,63,65,66 When excess cytokine is removed, tissue recovers with increases in OSN numbers.⁶⁰ Interferon-ɣ may also impact the olfactory epithelium and contribute to smell loss, without OSN loss.^67,68 Inflammation is an essential component of protective responses to pathogens and critical for initiating reparative processes,⁶⁹ yet how the olfactory mucosa is able to balance dual functions as a mucosal barrier and sensory organ remains incompletely understood. More work is needed to understand the driving forces behind how these exposures lead to olfactory damage.

Other epithelial cells

Epithelial cells play a crucial role in maintaining a mucosal barrier. Tight junction proteins like ZO-1, Occludins, and Claudins are expressed between epithelial cells in the olfactory mucosa,⁷⁰ suggesting barrier functionality, however the restrictive nature and ultrastructure between epithelial cells is underdescribed. Sustentacular cells are the major support cell population and are considered peripheral glia.⁷¹ In the human OM, sustentacular cells express the SARS-CoV-2 receptor angiotensin converting enzyme 2 (ACE2).^72–76 Sustentacular cell infection by SARS-CoV-2 results in structural damage and subsequent disruptions to OSN nuclear and ciliary architecture^77,78 or OSN death, all contributing to acute onset smell loss.^{66,72,77–79} A review of studies claiming olfactory neuroinvasion by SARS-CoV-2 suggests direct olfactory neuron infection rarely occurs in humans and neuroinvasion is unlikely.⁷⁹ Sustentacular cells have been shown to respond in inflammatory conditions by producing antimicrobial compounds like Ym2 in mouse models of chronic rhinosinusitis.⁶⁶ These cells also express the chemokine receptor Ackr3, a scavenger for the chemokine CXCL12. Deletion of sustentacular cell Ackr3 led to increased mucus production and increased immune cell infiltration into the olfactory mucosa, suggesting sustentacular cell loss enhances inflammatory recruitment into the olfactory epithelium.⁷¹ Studies also indicate that sustentacular cells play a role in clearing dying neurons by phagocytosis, limiting inflammation from cell debris,⁸⁰ however as with most other cells of the olfactory mucosa, very little is understood about their intrinsic and extrinsic immune functions.

Bowman’s gland cells, along with sustentacular cells, contribute to mucus production, which in addition to providing proper composition for odorant binding for scent detection, protects against airway microbes.^81,82 These cells also express ACE2, and can be infected by SARS-CoV-2.⁷⁴ Tuft-like microvillar cells that express interleukin (IL)-25 and a subset express Trpm5, can direct inflammation, and drive olfactory stem cell proliferation in response to airway allergens (eg Alternaria or house dust mite).^10,83 These microvillar cells may also enhance sustentacular cell ability to phagocytose via modulation of calcium release,⁸⁴ and potentially play critical roles in antiviral defense.⁸⁵ Mice lacking tuft cells appear to have impaired olfaction,⁸⁶ potentially due to their impact on stem cell activity. The other subset of microvillous cells are Trpm5^-Itpr3⁺ and transcriptionally map to ionocytes.^10,87 Following epithelial damage and increased ATP, Trpm5^-Itpr3⁺ cells may respond by releasing NPY which plays a role in directing stem cells to form immature neurons.⁸⁸

Olfactory ensheathing cells

Olfactory ensheathing cells (OEC) surround the axon bundles of⁸⁸ neurons as they travel within the lamina propria below the neuroepithelium (Fig. 1), and are considered non-myelinating peripheral glia distinct from Schwann cells and astrocytes.⁸⁹ Studies have demonstrated that OEC contribute to barrier functionality by protecting axons^70,90 via expression of adherens junctions between cells surrounding axon bundles. Additionally, OEC exhibit immune functions including phagocytosis of neuronal apoptotic debris,^91–93 cytokine and chemokine expression following exposure to bacterial PAMPs,⁹⁴ and nitric oxide synthase production following damage to OSN that facilitates bacterial entry.⁹⁵ Olfactory ensheathing cells are critical for nerve repair after damage and cell death⁹¹ and transplantation of these cells following spinal cord injury has been studied as a possible therapeutic, although efficacy remains controversial.⁹⁶ Very little is known about the role of these cells during and after infection; however, they have been demonstrated to be directly infected by Zika virus in vitro.⁹⁷

Endothelial barriers

Blood olfactory barrier

Our lab has recently described a vascular level barrier—termed the blood-olfactory barrier (BOB)—that prevents circulating blood-borne proteins from reaching the olfactory mucosal surface, unlike neighboring respiratory tissue.⁹⁸ The BOB has profound implications for both mucosal and CNS protection as this barrier restricts large circulating molecules including antibodies or complement proteins from accessing the OM. Prior studies assessing vascular permeability have concluded that some lower molecular weight compounds (<10 KD) are able to enter the OM from the blood, however these studies have lacked resolution and granularity to assess heterogeneity within the OM vascular compartment.^90,99,100 Ongoing work seeks to identify the precise nature of BOB regulation, but we expect that a subset of blood vessels (Fig. 1) confer these tight barrier properties. We have demonstrated the inability of circulating antibodies to protect the OM from viral infection, such that robust neutralizing circulating antibody titers fail to protect olfactory mucosa from VSV or influenza infection.⁹⁸

While the evolutionary reason for vascular specialization in the olfactory mucosa is not entirely clear, it nevertheless serves to protect the olfactory neuroepithelium from any damaging substances within circulation, as well as protect against substances that would potentially travel along olfactory axons into the brain. As such, the BOB seems to serve as an extension of the blood brain barrier (BBB). Endothelial cells within the nasal cavity are distinct and tissue specific, as demonstrated by a recent study describing a plexus of noncanonical venous sinusoids and atypical lymphatic vessels lining the upper respiratory mucosa.^17,19 While this study describes heterogeneity within the vasculature, the focus of this work was predominantly in respiratory mucosa and not olfactory or ethmoid regions. Much remains to be explored with regard to how vasculature may regulate OM immunity.

Lymphatic vessels

The recent surge in mucosal vaccination interest has revealed how little is known in general about airway antigen handling upon administration. How antigens from within the olfactory mucosa are trafficked to draining lymph nodes is wholly unknown. Lymphatic vessels (Fig. 1) in the olfactory mucosa do not express the typical lymphatic marker Lyve1, but do express Prox1 and a subset express Vegfr3.^17,101 While airway antigen capture and delivery remains unclear, several studies in mice have demonstrated cerebrospinal fluid (CSF) administered tracer drainage from meningeal lymphatics across the cribriform plate, where nasal lymphatics can direct CNS antigens to both superficial and deep cervical lymph nodes.^{18,19,102,103} Although one of several cranial lymphatic networks, the olfactory route may play a physiological role in CNS antigen drainage, and indeed lymphangiogenesis is observed at the cribriform plate lymphatic network during autoimmune neuroinflammation.^102,104 In humans, the contribution of nasal CSF drainage is understudied, however magnetic resonance imaging of intrathecal contrast has shown CSF exit into the nasal cavity.^105,106 Postmortem anatomical studies also suggest connections between human subarachnoid plate and nasal lymphatics via cribriform plate, suggesting CNS antigen may access the nasal cavity in humans.¹⁰⁷

Antigen drainage via the olfactory route therefore is important for immune surveillance of not only the olfactory tissue, but these lymphatics are dually important for CNS surveillance. How the lymphatics that collect local olfactory mucosal fluid within the nasal turbinates differ from those that handle CNS derived antigens is a crucial outstanding question in defining how olfactory lymphatics are involved in delivery of mucosal and CNS antigen to secondary lymphoid organs.

Hematopoietic innate immunity

Macrophages

Tissue resident macrophages are known to mediate important neuronal maintenance functions, promote wound repair and healing, phagocytose dead cells and debris, and respond to infection.¹⁰⁸ Macrophages are the most prevalent immune cell type in the olfactory mucosa at homeostasis in mouse and humans,^12,25,44 and they are found within the neuroepithelium, lamina propria and along olfactory axon tracts.^12,109 Two distinct tissue macrophage populations exist in the olfactory mucosa. The first population differentially expresses markers including P2ry12 and Trem2, while the second expresses major histocompatibility complex class II (MHCII) genes. P2ry12 is a purinergic receptor commonly associated with microglia, however, cell surface P2ry12 expression characterizes a population of olfactory macrophages that resemble microglia in some gene signatures and functions.¹² This population is predominantly found in the neuroepithelium where they can phagocytose dying neurons and self-renew in the tissue.¹² Indeed, consistent with a critical role in neuronal function and health, macrophage depletion reduced neurogenesis after OSN death.¹¹⁰

In contrast, MHCII-expressing macrophages are replaced by peripheral monocytes and increase in proportion following intranasal infection.¹² During pathogen defense, olfactory macrophages increase expression of inflammatory mediators like interferons, IL-6, and TNF-α.¹¹¹ These cytokines are also increased in mouse models of chronic rhinosinusitis,⁶⁰ and viral inflammation models.¹¹² Chronic rhinosinusitis (CRS) patients also have increased macrophages in the olfactory mucosa compared to controls, suggesting a pathogenic role for these cells in aberrant inflammation contributing to disease.¹¹³ Dysregulated inflammation in respiratory tissues, driven by proinflammatory signals from infiltrating immune cells and macrophages, can drive fibrosis¹¹⁴ or even metaplastic disease.¹¹⁵ In the olfactory mucosa, dysregulated inflammation and improper wound healing drives sensory impairment.¹¹⁶ Future studies to further define functional heterogeneity and diversity within the macrophage compartment in the olfactory mucosa will help our understanding of how these cells balance innate immune function and tissue homeostasis. Direct comparison to other nerve-associated, CNS border macrophages and respiratory tissue macrophage subsets will help uncover specialized physiological roles for these cells.

Other innate cells

Monocytes may differentiate into macrophages in the olfactory mucosa following recruitment from blood, especially following inflammation.¹² Dendritic cells can be detected in olfactory tissue, but their role in inflammation or even in local antigen capture or presentation is understudied.⁴⁴ At steady state, nasal mucosa including olfactory tissue harbors a population of extravascular neutrophils¹¹⁷ that may provide antigen-presentation features.¹¹⁸ Neutrophilic inflammation following pathogen infiltration is seen in several contexts and can be profound. Following SARS-CoV-2 infections, neutrophils infiltrate⁷⁸ and upon neutrophil depletion, viral titers are reduced and damage is lessened, suggesting that neutrophils contribute to SARS-CoV-2 mediated damage.¹¹⁹ Pathogens such as Naegleria fowleri, colloquially known as the “brain-eating” amoeba, invade the brain via the olfactory nerve and cause fatal amoebic meningoencephalitis. While the underlying factors that allow N. fowleri to specifically exploit the olfactory tissue remain unclear, innate immune cells, especially neutrophils that enter the airway play critical roles in slowing amoeba infection.^120–122

Elevated neutrophils are observed in human CRS¹²³ but not associated with smell loss.¹²⁴ Eosinophil numbers in the olfactory mucosa are elevated in chronic rhinosinusitis and may contribute to OSN damage,¹²⁵ but evidence regarding pathology related to degree of smell loss is contradictory.^123,124 Natural killer cells are present in human olfactory tissue, and gene signatures of these cells suggest they may signal to epithelial cells and prevent proper regeneration after SARS-CoV-2 infection.⁶¹ During allergic inflammation, mast cells and eosinophils infiltrate the olfactory mucosa and neurogenesis is perhaps reduced (fewer immature OSN) but the impact on smell is unclear.¹²⁶ Future studies to directly search for other granulocytes and innate-like cells are needed to fully delineate their roles in immune surveillance and pathogen defense.

Adaptive immunity

T cells

Prior to the SARS-CoV-2 pandemic, the majority of immune related data from the human olfactory mucosa has been studied in the context of CRS and allergic rhinitis. In CRS, the olfactory mucosa exhibits immune cell infiltration and OSN loss.^114,115 CRS with nasal polyps (CRSwNP) is characterized by a type 2 immune profile, involving elevated IL-4, IL-5, IL-13, eosinophils, mast cells, Th2 lymphocytes, and IgE⁺ B cells.^113,116,117 However, SARS-CoV-2 infections have led to several studies of human postviral nasal response—albeit these typically sample respiratory tissue due to difficult in accessing the superior olfactory turbinates.¹²⁷ Nevertheless, scRNAseq data have revealed the existence of T cell populations within the human olfactory tissues, both before and after SARS-CoV-2 infection.²⁵ In vaccinated individuals, CD8⁺ and CD4⁺ tissue resident T cells in the upper airway were only found in those who experienced breakthrough infection, suggesting immunization alone does not generate T cell residence to nasal tissues.¹²⁸ In humans, stronger CD4⁺ and CD8⁺ antigen specific T cell responses correspond with lower nasopharyngeal airway viral load within 6 d of symptom onset, implicating a role in early viral clearance.¹²⁹ In patients with olfactory dysfunction after SARS-CoV-2 infection, infiltrating T cells that produce IFN-γ are recovered in olfactory biopsies and may drive pathogenesis of post-viral olfactory dysfunction.^68,130 Similarly following olfactory SARS-Cov-2 infection, increased interferon related genes, perhaps from activated T cells, were also described in hamsters^77,130 even after viral clearance,¹³⁰ implicating pathologic inflammation driven by T cells. Other studies in humans suggest that T cell infiltration may play a role in age related olfactory loss and may send disruptive signals to stem cells to limit neurogenesis.⁶¹ Nevertheless, nasal resident memory CD8+ T cells have been demonstrated to limit influenza virus transmission from nasal tissue to lung and prevent pulmonary disease.¹³¹ Vesicular stomatitis virus (VSV), aggressively infects OSNs and rapidly spreads to infect the olfactory bulb.^132,133 CNS viral control relies on interferon response¹³⁴ and robust T cell response^135,136 to prevent further viral spread in the deeper CNS structures, but T cells remain within the olfactory mucosa tissues following infection.¹¹⁶ Nasal—and presumably also olfactory—tissue resident T cells may be important for controlling respiratory infections. In models of chronic rhinosinusitis (CR), T cells, predominantly CD4⁺ T cells, infiltrate and proliferate in olfactory tissue.⁶⁰ Th2 responses are implicated in CR pathogenesis particularly in nasal polyp development, although these polyps do not necessarily form in olfactory tissue.^137,138 T cell heterogeneity and olfactory infiltration in the context of infection, viral protection, and pathologic inflammation is complex and requires further study to demonstrate the balance between pathogen defense and pathologic inflammation in olfactory tissues.

B cell lineage

OSNs have very little, if any MHC class I expression^44,116 needed to engage CD8⁺ T cells. Thus, humoral immunity, generated via vaccination or previous infection appears critical to protect these cells.⁹⁸ Studies in ferrets have suggested that influenza replication in the nasal tissue, but not lung, leads to further virus transmission.¹³⁹ Prior studies have shown variable results regarding the efficacy of passively transferred antibody in upper respiratory tract influenza infections,^140,141 but these studies did not distinguish respiratory and olfactory protection. Similarly, human population seropositivity for N. fowleri,^142,143 can be quite high, however circulating antibody is likely not sufficient for protection, as immunization in mice does not afford full protection.¹⁴⁴ Consistent with this, we’ve shown that serum antibody titer protects respiratory, but not olfactory tissues, from viral infection. And indeed BOB based serum antibody exclusion may in part contribute to “breakthrough” infections in immunized individuals,^145,146 and why antibodies against N. fowleri are not protective. Our lab has previously shown that while the BOB excludes circulating antibody from protecting the olfactory mucosa, local antibody production by extravascular mucosal plasma cells, can be potently protective against infection, preventing local infection and CNS neuroinvasion.⁹⁸ While these mucosal plasma cells required CD4⁺ T cell help, AID expression, and CXCR3⁹⁸ to provide protection, the mechanisms underlying tissue entry and olfactory residence remain unclear. A recent study identified antigen specific IgA⁺ B lineage cells within the upper airway following intranasal vaccination and while CCL28 promoted their tissue entry, it remains unclear whether these factors are involved in homing into olfactory regions.¹⁴⁷

Work in teleosts (trout) has shown that B lineage cells are present within their olfactory organ, and indeed, local Ig secretion occurs during both homeostasis and infection.¹⁴⁸ These olfactory B cells are conserved across phylogeny as they are detected in human biopsies around Bowman’s glands, where they may secrete antibody into the mucus,⁸² and single cell RNA sequencing studies demonstrate B lineage populations including plasma cells in olfactory tissue.^25,44,149 Recent studies have suggested that activated and memory B cell populations are maintained within the human upper airway,^150,151 but the contribution of olfactory tissues to these swabs is unclear. In chronic rhinosinusitis, B cells are implicated in the pathology and are especially enriched in polyps. However, these polyps are not described in olfactory mucosa and the residence specifically in olfactory tissue has yet to be investigated.^152,153 Further understanding local B cell and plasma cell populations in homeostasis, infection, and chronic inflammation is paramount for vaccination design and treatment of olfactory pathology.

Conclusion

We often overlook the importance of olfaction in coloring our everyday experiences, taking for granted the pleasure we derive from scent, be that in eating and drinking, or the memories and associations that smells evoke.¹⁵⁴ Smell is the work of an unsung mucosal surface that must rapidly relay scent information from the outside air while also functioning as a barrier tissue to protect the upper airway and brain against pathogen invasion. As a chemosensory organ, this barrier tissue is uniquely constrained in immune capacity. However, immune surveillance at this tissue is critical for airborne pathogen defense and proper protection against neuroinvasive pathogen. Currently, technical challenges hinder olfactory mucosal sampling in studies of human upper airways, leading uncertainty in determining the olfactotropism of upper respiratory tract infections in humans. However, these forthcoming analyses of immune responses to olfactory pathologies and especially infections by olfactotropic pathogens will bolster our understanding around olfactory immunity broadly and the specialization of olfactory tissue resident immune population. For instance, N. fowleri can infect a wide variety of mammals,¹⁵⁵ but the fact that it is only pathogenic when contacting the olfactory mucosa, suggests that key basic insights into olfactory immunity could be revealed by this conserved vulnerability. While olfactotropic neuroinvasive pathogens of humans are currently quite rare, the demonstrable H5N1 avian influenza neurotropism commonly observed in other mammal infections^90,92 raises concerns about potential olfactory vulnerabilities in future influenza pandemics. And although SARS-CoV-2 was not acutely neurovirulent, the capacity for coronavirus neuroinvasion exists. Taken together, these observations suggest we would be wise to understand how to protect against olfactory neuroinvasion by airborne viruses before the emergence of viral species with enhanced ability to invade the brain. As more studies analyze this tissue, we will continue to define the immune properties of the olfactory mucosa that are critical for the design of effective mucosal vaccines against airborne and neuroinvasive pathogens.

Author contributions

S.Y.T. drafted the manuscript, E.A.M. edited the manuscript.

Skye Y. Tracey (Writing—original draft [Equal], Writing—review & editing [Equal], Visualization)

Funding

This work was supported by funding from the National Institutes of Health R21DC021260, R21NS133561, and R01NS121067 to E.A.M.

Conflicts of interest

The authors have no conflicts of interest to report.