Immune responses in the injured olfactory and gustatory systems: a role in olfactory receptor neuron and taste bud regeneration?

Abstract

Sensory cells that specialize in transducing olfactory and gustatory stimuli are renewed throughout life and can regenerate after injury unlike their counterparts in the mammalian retina and auditory epithelium. This uncommon capacity for regeneration offers an opportunity to understand mechanisms that promote the recovery of sensory function after taste and smell loss. Immune responses appear to influence degeneration and later regeneration of olfactory sensory neurons and taste receptor cells. Here we review surgical, chemical, and inflammatory injury models and evidence that immune responses promote or deter chemosensory cell regeneration. Macrophage and neutrophil responses to chemosensory receptor injury have been the most widely studied without consensus on their net effects on regeneration. We discuss possible technical and biological reasons for the discrepancy, such as the difference between peripheral and central structures, and suggest directions for progress in understanding immune regulation of chemosensory regeneration. Our mechanistic understanding of immune-chemosensory cell interactions must be expanded before therapies can be developed for recovering the sensation of taste and smell after head injury from traumatic nerve damage and infection. Chemosensory loss leads to decreased quality of life, depression, nutritional challenges, and exposure to environmental dangers highlighting the need for further studies in this area.

Introduction

Chemosensory cells including taste receptor cells, olfactory sensory neurons (OSN), vomeronasal neurons, and solitary chemosensory cells guide food choice, pathogen detection, allergic responses, mating, and other behavioral interactions with conspecifics (Ishii and Touhara 2019; Mattes 2003; Mohrhardt et al. 2018; Risso et al. 2020; Tepper and Barbarossa 2020; Zimmerman and Munger 2021). Upper respiratory infections, chemotherapy, radiation, and traumatic nerve injury are among the causes of chemosensory dysfunction that affect nutrition and decrease quality of life (Barlow and Klein 2015; Doty 2019; Frank and Hettinger 2018; Goodspeed et al. 1987; Graham et al. 1995; Reiter et al. 2004; Seo et al. 2013; Snyder and Bartoshuk 2016). The perception of flavor, commonly referred to as “taste,” depends on complex cortical interactions between the olfactory, gustatory, and somatosensory systems though it is especially dependent on odor maps generated by retronasal stimulation of OSNs during ingestion (Shepherd 2006; Small 2012). As a result, olfactory deficits are often experienced as taste loss (Doty 2019). More recently, millions of people infected with the SARS-CoV-2 virus responsible for the COVID-19 global pandemic have experienced distinct losses of smell and/or taste, known as anosmia and hypoguesia, respectively (Cooper et al. 2020; Croy et al. 2014; Parma et al. 2020; Risso et al. 2020; Vaira et al. 2020b; von Bartheld et al. 2020). Although chemosensory function usually returns within weeks, a subset of patients experiences long-term deficits that leave them with appetite loss and inability to smell spoiled food or leaking gas (Klein et al. 2021; Petersen et al. 2021; Vaira et al. 2020a). A better understanding of mechanisms that mediate chemosensory receptor cell degeneration, regeneration, and reinnervation is needed to develop therapies for taste and smell loss (Barlow and Klein 2015; Mainland et al. 2020; Schwob 2002; Yu and Wu 2017).

Immune cells known as leukocytes infiltrate injured chemosensory epithelia and may affect functional and behavioral recovery (Bakos and Costanzo 2011; Cavallin and McCluskey 2007a; Doursout et al. 2013; Feng et al. 2010; McCluskey 2004; Ogawa et al. 2021; Shi et al. 2012; Steen et al. 2010). Leukocyte responses at the site of axotomy in models of experimental nerve injury are well reviewed, but whether and how immune cells contribute to chemosensory sensory cell regeneration has received less attention (DeFrancesco-Lisowitz et al. 2015; Dubovy 2011; Rotshenker 2011). This review aims to summarize the roles of immune responses in the degeneration and regeneration of sensory target organs of the olfactory and gustatory systems, which have mainly been studied in rodent models. We identify gaps in our understanding of leukocyte responses to injury in chemosensory epithelia which if addressed could suggest new treatment strategies for taste and smell deficits. Insights to chemosensory–immune interactions are critical to develop strategies for improving chemosensory recovery after damage.

Immune cells in the olfactory system

Cellular structure of the peripheral olfactory system and associated immune cells

The olfactory epithelium, located bilaterally in the upper nasal cavity (Fig. 1A), contains a heterogeneous population of cells (Fig. 1B) (Byrd and Brunjes 1995; Menco and Morrison 2003; Smith and Bhatnagar 2019). Mature OSNs are bipolar neurons that detect odorants at cilia-tipped dendrites facing the nasal cavity. Olfactory-driven activity is then transmitted by axons which form the small fascicles of the olfactory nerve before crossing through foramina in the cribiform plate of the ethmoid bone (Pifferi et al. 2010; Su et al. 2009). Immature OSN which have not yet extended dendritic projections are located deeper in the epithelium (Fig. 1B). Two populations of basal progenitors, horizontal basal cells and globose basal cells, give rise to new OSNs and supporting cells in the olfactory epithelium throughout life. Bowman’s glands secrete mucous that traps inhaled odorant particles and protects OSN dendrites. Sustentacular cells face the nasal vestibule and provide metabolic and structural support to nascent and mature OSNs and basal cells (DeMaria and Ngai 2010; Menco and Morrison 2003; Moran et al. 1992a). Microvillar cells, a heterogeneous population with a modulatory role in the olfactory epithelium (Bryche et al. 2021; Byrd and Brunjes 1995; Moran et al. 1982), and sustentacular cells have received recent attention as angiotensin converting enzyme (ACE)-2 expressing targets of SARS-CoV-2 (Bilinska et al. 2020; Brann et al. 2020; Fodoulian et al. 2020; Moran et al. 1982; Seo et al. 2021).

Fig. 1.

Organization of the olfactory system and injury models. (A) Mid-sagittal view of rodent head showing the location of the olfactory epithelium (OE) in the nasal cavity and axonal projections of olfactory sensory neurons (OSNs, yellow) through the cribriform plate (CP) to the olfactory bulb (OB). The peripheral taste structures and cranial nerve (CN) input to the nucleus of the solitary tract (NTS) brainstem are shown for context. (B) Cellular structure of the olfactory epithelium and olfactory bulb. OSN dendrites terminate in cilia where olfactory transduction occurs. Neural activity is transmitted via axons of the olfactory nerve to synapse with projection neurons in the olfactory bulb in ball-like structures known as glomeruli. Blood vessels (pink) and lymphatic vessels (green) in the lamina propria (LP) carry immune cells to and from the OE. Experimental injury models that cause OSN degeneration include (C) severing the olfactory nerve, (D) surgical removal of the olfactory bulb which removes the axonal target, and (E) application of chemicals or inflammatory stimuli that lesion the olfactory epithelium.

Macrophages

Immune cells also occupy the olfactory epithelium and underlying lamina propria during homeostatic conditions (Borders et al. 2007a; Chen et al. 2019; Durante et al. 2020; Graziadei and Graziadei 1979; Jafek 1983; Kanaya et al. 2014). We briefly review major immune cell types found in the olfactory epithelium and their functions, beginning with macrophages. The resident macrophage population is relatively sparse in rodents as assessed by markers including CD68, F4/80, Iba1, and the chemokine receptor, CX3CR1 (Bryche et al. 2020; Chen et al. 2017; Getchell et al. 2002b; Hasegawa-Ishii et al. 2017; Nan et al. 2001; Pozharskaya et al. 2013; Ruitenberg et al. 2008; Suzuki et al. 1995; Vukovic et al. 2010). Macrophages inhabit the olfactory organ of non-mammalian species such as larval zebrafish (Palominos and Whitlock 2021) and are mentioned in histological studies of the olfactory epithelium of bullfrog (Takagi 1971; Takagi and Yajima 1965) and goby fish (Sarkar et al. 2020). A recent scRNA-seq analysis from nasal biopsies revealed a large pool of macrophages in the human olfactory mucosa and nearby respiratory epithelium (Durante et al. 2020). Together these studies point to a resident population of macrophages in vertebrates available to respond to OSN turnover or toxins breaching the olfactory mucosa as in other barrier epithelia (Han et al. 2021).

Macrophages are renowned professional phagocytes adept at destroying pathogens, but these innate immune cells also remodel tissues by releasing chemical mediators of inflammation and wound repair (Gordon and Plüddemann 2019; Watanabe et al. 2019; Wynn and Vannella 2016). Major phenotypes are pro-inflammatory or classically activated (M1) macrophages and anti-inflammatory or alternatively activated macrophages (M2), a scheme that we present for simplicity while emphasizing the continuum of markers expressed in vivo (Martinez and Gordon 2014; Murray 2017). M1 macrophages can be activated by interferon IFN-γ, TNF-α, and bacterial lipopolysaccharide (LPS) and produce reactive oxygen species (ROS) and inflammatory cytokines such as TNF-α, IL-12, IL-23. These inflammatory mediators have anti-microbial and anti-tumoricidal properties though they can also damage tissue and impair wound healing if not tightly regulated. M2 macrophages, in contrast, are activated by the cytokines IL-4 and IL-13, which inhibit M1 macrophage activation. M2 can dampen inflammation, mediate parasite clearance, and promote tissue remodeling. As part of their role in resolving inflammation, M2 macrophages phagocytose debris, damaged and dead cells and apoptotic neutrophils. IL-10 and TGF-β are among the anti-inflammatory cytokines released by M2 macrophages to end inflammation (Biswas et al. 2012; Locati et al. 2020; Mantovani et al. 2013; Murray 2017; Ransohoff 2016; Shapouri-Moghaddam et al. 2018). A subset of macrophages from human nasal biopsies of respiratory and olfactory epithelium express the M2 markers CD163 and IL-10 indicating their potential role in homeostasis during ongoing neurogenesis (Durante et al. 2020). Resolving the tissue-specific cytokine and signaling pathways that control macrophage phenotype could point to new strategies for generating and protecting chemosensory cells (Naik et al. 2017; Vannella and Wynn 2017).

Neutrophils

These innate immune cells share a common myeloid lineage with macrophages and are generally sparse in the uninjured olfactory epithelium of rodents (Alt et al. 2015; Hasegawa-Ishii et al. 2017; Kanaya et al. 2014; Miyake et al. 2016) and rhesus monkeys (Carey et al. 2012). However, peripheral olfactory organs in adult zebrafish are populated by a substantial population of neutrophils during homeostasis in contrast to the rest of the brain (Palominos et al. 2022). Neutrophils rapidly respond to pathogens and sterile inflammation resulting from experimental injuries as a first line of defense. Once considered a short-lived, homogenous population, neutrophils are now viewed as dynamic and phenotypically plastic depending on the type and severity of injury and tissue microenvironment (Liew and Kubes 2019). Effector functions of neutrophils include phagocytosis, pathogen killing through the release of ROS and other molecules, and the release of neutrophil extracellular traps (NETs) composed of chromatin embedded with anti-microbial proteins to trap pathogens (Tan et al. 2021). Neutrophils contribute to efficient wound repair by removing tissue debris and promoting angiogenesis (Liew and Kubes 2019; Mantovani et al. 2011; Neirinckx et al. 2014). However, the dynamics and extent of neutrophil responses must be well controlled to avoid tissue destruction through rampant inflammation in a balance typical of innate immune responses.

Lymphocytes and dendritic cells

T cells and B cells recognize antigenic determinants to achieve long-lasting and specific adaptive immunity versus rapid but less specific macrophage and neutrophil responses (Abbas et al. 2019). Most of the resident inflammatory cells in normal human olfactory biopsies were identified by immunostaining as CD3+ T cells. Single-cell RNA-sequencing further revealed diverse populations of lymphocytes in the human olfactory epithelium, including CD4+ and CD8+ T cells, dendritic cells, and B cells (Durante et al. 2020). These studies extend earlier histological studies of lymphocytes in human nasal biopsies (Durante et al. 2020; Jafek 1983; Jafek et al. 2002; Mellert et al. 1992). B cells, dendritic cells, and a small population of CD3⁺ T cells also occupy the olfactory epithelium or lamina propria of untreated mice (Chen et al. 2017, 2019; Getchell and Getchell 1991; Hasegawa-Ishii et al. 2017; Pozharskaya and Lane 2013; Ruitenberg et al. 2008). Upon injury or exposure to pathogens, lymphocytes can be recruited to the olfactory epithelium from nearby nasal-associated lymphoid tissue (NALT), which is anatomically distinct from the peripheral olfactory system (Casadei and Salinas 2019; Casteleyn et al. 2010; Das and Salinas 2020; Debertin et al. 2003; Sepahi and Salinas 2016; Tacchi et al. 2014; Yu et al. 2018). NALT has been well characterized in fish and many mammalian species but, as highlighted in a recent review, immune responses to viruses and other pathogens are poorly understood in these tissues compared to other mucosal lymphoid tissues (Casadei and Salinas 2019).

Dendritic cells and T cells work together to recognize and eliminate pathogens. Dendritic cells are professional antigen-presenting cells which traffic between barrier epithelia to lymphoid tissues during ongoing immune surveillance (Cabeza-Cabrerizo et al. 2021; Eisenbarth 2019). The major types of T lymphocytes are CD4⁺ helper and CD8+ cytotoxic T cells. CD4+ T cells recognize antigen bound to MHC class II and activate macrophages or other cells to kill microbes (Hilligan and Ronchese 2020). Helper T cells are further grouped into subsets with different effector functions. Naive CD4+ cells respond to cytokines in the extracellular environment by differentiating into Th1 cells that target intracellular microbes or Th2 cells that defend against parasites. Regulatory T cells (Treg) are a distinct population of helper T cells that exert control over excessive immune responses, maintain tolerance to self-antigens, and restore homeostasis (Shevyrev and Tereshchenko 2020). CD8+ cytotoxic T cells recognize antigen bound to MHC class I and kill pathogen-infected target cells (Abbas et al. 2019; Wong and Pamer 2003). Naive B cells also encounter peptide antigens before T cell interactions drive their differentiation and proliferation into long-lived memory B cells and plasma cells. These 2 B cell populations then secrete antibodies that prevent reinfection by pathogens (Akkaya et al. 2020; Cyster and Allen 2019). The reader is referred to 2 iconic textbooks for more on the complexity of lymphocyte responses beyond this snapshot (Abbas et al. 2019; Janeway et al. 2001).

Cellular structure of the central olfactory system and associated immune cells

Unmyelinated OSN axons travel through the cribriform plate to synapse with mitral cells in structures known as glomeruli in the first layer of the olfactory bulb (Fig. 1B) (Byrd and Brunjes 1995; Nagayama et al. 2014; Smith and Bhatnagar 2019). Regenerated OSNs must reestablish these synaptic connections within glomeruli to restore the circuitry needed for olfactory function (Imamura et al. 2020). The bundles of OSN axons comprise the bilateral olfactory nerves that are wrapped by olfactory ensheathing cells (OECs) (Doucette 1990). OECs, which provide a supportive scaffold for regenerating OSNs, offer a promising autologous strategy for regenerating spinal cord and other CNS neurons without immune rejection and even dampen harmful inflammatory responses that cause secondary injury (Jiang et al. 2022). Results from clinical trials have been mixed though variations in OEC purity and dose may be partially at fault (Bartlett et al. 2020; Hu et al. 2021; Miah et al. 2021; Ruitenberg and Vukovic 2008; Sasaki et al. 2011).

Microglia originating from the yolk sac during development are the major type of resident immune cell in the olfactory bulb (Ginhoux et al. 2010). Microglia and macrophages share functions such as cytokine production, phagocytosis, and host defense (Prinz et al. 2019) though microglia also play a critical developmental role in refining neural circuits in the olfactory bulb (Denizet et al. 2017; Wallace et al. 2020). Microglia are phenotypically plastic and become polarized on an M1/M2 continuum in response to brain pathogens and injury though as with macrophages this concept oversimplifies their heterogeneity in vivo (Jurga et al. 2020; Ransohoff 2016). Several subcategories have been added to the M1/M2 scheme in an attempt to capture their phenotypic diversity (Stratoulias et al. 2019). Microglia also exhibit morphological plasticity, from highly ramified with many fine processes during the resting state to more rounded when activated (Prinz et al. 2019). CD4+ T cells and other immune cell types infiltrate the olfactory bulb following injury, but microglia appear to be the main resident population during homeostasis (Dileepan et al. 2016; Doursout et al. 2013).

Injury models in the olfactory system

OSN regeneration following injury has been examined in a variety of animals such as zebrafish, trout, salamander, frog, pigeon, rodent, rabbit, dog, and nonhuman primate (Calvo-Ochoa and Byrd-Jacobs 2019; Calvo-Ochoa et al. 2021; Graziadei and Monti Graziadei 1983; Graziadei and Okano 1979; Moran et al. 1992b; Takagi 1971; Takagi and Yajima 1965; Zielinski and Hara 1992). The remarkable regenerative potential of the olfactory epithelium is now widely accepted, but for many years, a central question was whether OSNs degenerate and are repopulated (Takagi 1971). Studies focused on immune cell responses to OSN injury are fewer, more recent and mainly performed in mice and rats. Below we discuss 3 major types of injury models in the olfactory system in which immune responses have been examined (Table 1).

Table 1.

Immune responses to injury and effects on olfactory sensory neuron (OSN) regeneration

Location	Injury model	Species	Immune response	Timing	Actions	Reference
OE	Surgical	Rat Mouse	Macrophage	Acute	Phagocytose OSN	Graziadei and Graziadei (1979); Suzuki et al. (1995, 1996a)
OE	Surgical	Mouse	Macrophage	Acute	Protects OSNs Stimulates progenitor proliferation	Borders et al. (2007a, 2007b)
OE	Surgical	Mouse	MIP-1α	Acute	Stimulates OSN progenitor proliferation by recruiting macrophages	Kwong et al. (2004)
OE	Surgical	Mouse	Macrophage scavenger receptor A	Acute	Stimulates OSN progenitor proliferation and recruits macrophages	Getchell et al. (2006)
OE	Surgical	Mouse	LIF and IL-6	Acute	ND OSN loss coincident with inflammatory responses	Getchell et al. (2002); Nan et al. (2001)
OE	Surgical	Mouse	CX3CR1	Acute	Protects OSN Stimulates progenitor proliferation	Blomster et al. (2011)
OB	Surgical	Mouse	Macrophage/microgliaa	Acute-Chronicb	Anti-regenerative Immunosuppression improved OSN regeneration and function	Kobayashi and Costanzo (2009); Kobayashi et al. (2018b)
OB	Surgical	Mouse	TNF-α	Acute-Chronic	Anti-regenerative Antagonist improved OSN reinnervation of bulb and functional recovery	Al Salihi et al. (2017)
OB	Surgical	Mouse	IL-6 signaling HMGB	Acute-Chronic	Anti-regenerative Antagonist improved OSN progenitor proliferation and functional recovery	Kobayashi et al. (2013, 2018a)
OB	Surgical	Mouse	MMP+ Neutrophils	Acute-Chronic	NDb	Bakos and Costanzo (2011)
OE	Chemical	Zebrafish Mouse	Neutrophils	Acute Acute-Chronic	ND Increased in peripheral and central olfactory system Transitioned from inflammatory to pro-regenerative phenotype	Palominos et al. (2022), Ogawa et al. (2021)
OE	Chemical	Mouse	Macrophages	Acute-Chronic	ND Phagocytosed immature and some mature OSN	Suzuki (1998); Suzuki et al. (1998)
OE	Chemical	Mouse	TNF-α	Acute	Pro-regenerative Promoted progenitor cell proliferation and OSN regeneration	Chen et al. (2017)
OB	Chemical	Mouse Rat	Macrophage/microgliac	Acute-Chronic	ND Phagocytosed OSN axons	Burd (1993); Chang and Glezer (2018); Lazarini et al. (2012)
OE	Chronic rhinosinusitis	Human	T cells (many CD4) Ccl2, CCL20 JNK activation	Chronic	Coincident OSN loss Increased progenitor cells	Chen et al. (2019); Victores et al. (2018)
OE	Inflammatory LPS	Mouse	IL-1β+ macrophages Neutrophils T cells	Chronic	ND Coincident OSN loss	Hasegawa-Ishii et al. (2017); Yagi et al. (2007)
OE	Inflammatory LPS	Mouse	Atypical neutrophils	Acute-Chronic	ND Coincident OSN loss	Ogawa et al. (2021)
OB	Inflammatory LPS	Mouse	Gliosis	Chronic	Neurodegenerative Secondary to loss of odor activity	Hasegawa-Ishii et al. (2017, 2019; 2020)
OE	Inflammatory Poly(I:C)	Mouse	Neutrophils Macrophages T cells	Acute-Chronic	Neurodegenerative Neutrophil NET inhibitor protective	Kanaya et al. (2014)
OE	Inflammatory IOI	Mouse	TNF-α	Acute-Chronic	Acutely pro-regenerative Chronically anti-regenerative	Chen et al. (2017, 2019; Lane et al. (2010); Turner et al. (2010); Victores et al. (2018)
OE	Inflammatory IOI	Mouse	IFN-γ	Chronic	No OSN degeneration Decreased olfactory function	Pozharskaya and Lane (2013)

OB, olfactory bulb; OE, olfactory epithelium; IOI, inducible olfactory inflammation.

Cell markers used in these studies cannot discriminate macrophages and microglia.

We refer to immune responses lasting ≤7 days as acute and ≥8 days as chronic.

Effect on OSN degeneration and/or regeneration not determined (ND).

Immune responses to surgical, chemical, and inflammatory models of OSN injury in mammals share some characteristics though outcomes differ (Table 1) . In the surgical axotomy model, olfactory nerves are sectioned between the olfactory bulb and cribriform plate to mimic axonal shearing from head injury (Fig. 1C) (Coelho and Costanzo 2016; Howell et al. 2018; Schwob 2002). Following nerve transection, the olfactory epithelium recovers though the dynamics of OSN turnover are altered (Schwob 2002). In the bulbectomy model, one or both olfactory bulbs are surgically removed damaging olfactory axons and destroying their glomerular targets (Fig. 1D). After bulbectomy, the number of mature OSNs never recovers completely due to the absence of trophic support from the olfactory bulb (Schwob 2002, 2005).

Chemical injury models involve systemic or nasal administration to parallel anosmia from exposure to environmental toxins (Imamura and Hasegawa-Ishii 2016; Schwob 2002; Upadhyay and Holbrook 2004) (Fig. 1E). The range of olfactory toxicants is diverse and includes the coagulant zinc sulfate, the detergent Triton X-100, the anti-mitotic colchicine, methyl bromide gas, methimazole, and 3-methylindole (Håglin et al. 2021; Schwob 2002; Turk et al. 1987; White et al. 2015; Youngentob and Schwob 2006). Chemical lesioning typically kills OSNs and other cells though the toxin can spread unevenly through the mucosa (McBride et al. 2003) and in cases of severe damage, the olfactory epithelium may transition to respiratory epithelium (Schwob 2002). Inhalation of methyl bromide gas, however, reproducibly and reversibly injures OSNs (Hastings et al. 1991; Hurtt et al. 1988; Schwob 2002; Schwob et al. 1999), making it a useful injury model to study the effects of specific immune responses on regeneration. Remarkably, despite the destruction of over 95% of the olfactory epithelium after a single exposure to methyl bromide, OSNs regenerate and the tissue is restored within 6–8 weeks (Schwob 2005).

A variation on chemical injury is to deliver inflammatory stimuli nasally or systemically to study the effects of pathogens on olfaction. The location of OSNs in the nasal cavity leaves them vulnerable to chronic rhinosinusitus, allergic rhinitis and exposure to bacteria, viruses and environmental toxins (Fig. 1A) (Cho et al. 2020; Genter and Doty 2019; Hasegawa-Ishii et al. 2019; Hoyte and Nelson 2018; Sedaghat 2017; Welge-Luessen 2009; Yan et al. 2020; Yang and Chiu 2017). The most common noninfectious immunostimulant is lipopolysaccharide (LPS), a toll-like receptor (TLR)-4 ligand purified from gram-negative bacteria. Repeated intranasal LPS to model chronic bacterial infection kills OSNs in rodents (Hasegawa-Ishii et al. 2017, 2019, 2020; Yagi et al. 2007). OSNs also degenerate in response to polyinosinic: polycytidylic acid (poly I:C), a synthetic TLR-3 ligand used to study post-viral olfactory anosmia (Kanaya et al. 2014). In contrast to these noninfectious agonists, inoculating the olfactory epithelium with live bacteria or virus requires an ABSL-2 (or higher) facility (Bomer et al. 1998; Naclerio et al. 2006). More recently, inducible, conditional overexpression of cytokines in the OE of transgenic mice has provided mechanistic insights into inflammatory OSN loss (Chen et al. 2019; Lane et al. 2005, 2010; Pozharskaya and Lane 2013; Turner et al. 2010).

Proregenerative effects of immune cells in the injured olfactory system.

Macrophage responses to injury were the focus of a number of studies to deermine which cells phagocytose degenerating OSNs. Activated macrophages engulf OSNs during the first week after axotomy, unilateral bulbectomy, or systemic colchicine in rodents (Graziadei and Graziadei 1979; Suzuki 1998; Suzuki et al. 1995, 1996a; 1998). However, olfactory ensheathing cells engulf OSN axon debris (Su et al. 2013) and sustentacular cells take over as primary phagocytes during the chronic phase (Suzuki et al. 1995, 1996a). This conclusion is based on the reduced number of macrophages by 1 week post-bulbectomy and the presence of phagosomes in sustentacular cells. In the olfactory bulb, microglia/macrophages (indistinguishable by many commonly used markers) phagocytose OSN axons after zinc sulfate irrigation of the nasal cavity (Burd 1993; Chang et al. 2003). Overall, at least in the acute postinjury period macrophages perform their traditional role in clearing debris before responses return to baseline. Though immune responses to OSN injury have largely been in rodents, the injured zebrafish olfactory system is well suited to determine the net influence of macrophage and microglial responses on OSN regeneration (Byrd 2000; Calvo-Ochoa and Byrd-Jacobs 2019; Var and Byrd-Jacobs 2019, 2020; White et al. 2015).

Whether macrophages affect the regeneration of OSNs damaged in surgical models has been addressed mainly by loss-of-function studies using pharmacological, immunological or genetic methods. Clodronate liposomes, which kill phagocytes by releasing a non-hydrolyzable analogue of ATP that prevents mitochondrial respiration (Van Rooijen and Sanders 1994), deplete resident and recruited macrophages in the olfactory epithelium (Borders et al. 2007a). Intraperitoneal administration of clodronate ablates peripheral macrophages but spares microglia in the brain (Han et al. 2019). This approach revealed a neuroprotective effect of macrophages in mice that had undergone bulbectomies (Borders et al. 2007a; Getchell et al. 2006). Clodronate-treated mice exhibited a thinner olfactory epithelium and fewer mature OSNs due to increased apoptosis and decreased proliferation of basal progenitor cells (Borders et al. 2007a). However, macrophages must be appropriately regulated to phagocytose dying sensory neurons and promote their regeneration, as demonstrated in mice lacking the chemokine receptor, CX3CR1 (Blomster et al. 2011), or the pattern recognition receptor, macrophage scavenger receptor A (Getchell et al. 2006). Though these studies support a pro-regenerative role for correctly-activated macrophages in the olfactory epithelium, a caveat is that electrophysiological or behavioral experiments are needed to assess the functional integration of new sensory neurons into olfactory circuits.

The activation phenotype of macrophages responding to OSN injury is generally unknown. High levels of Ym1/2 protein were expressed in the olfactory epithelium following zinc sulfate irrigation of the nasal cavity, axotomy, bulbectomy or in aged mice (Giannetti et al. 2004). Ym1/2 (also known as chitenase-like-3 and -4) is a marker of alternatively activated M2 macrophages associated with allergic responses in the airway and skin (Sutherland 2018) and also with oligodendrocyte proliferation in the subventricular zone of the CNS (Starossom et al. 2019). However, Ym1/2 was expressed mainly by sustentacular cells rather than macrophages (Giannetti et al. 2004). Another study demonstrated transient upregulation of IL-6, an M1 marker, by infiltrating macrophages and OECs in the olfactory nerve layer of the olfactory epithelium after bulbectomy (Nan et al. 2001). Overall, however, further investigation of macrophage phenotype, origin, and effects on regeneration is needed.

A recent study demonstrates a proregenerative role for a subset of neutrophils that express both Ly6G and Siglec-F (Ogawa et al. 2021). Ly6G is a classic neutrophil marker, while Siglec-F is a sialic acid-binding, immunoglobulin-like lectin ordinarily expressed by eosinophils (Macauley et al. 2014). Following a single application of intranasal LPS these “double-positive” neutrophils infiltrated the olfactory epithelium and altered their morphology, surface markers and gene expression from mainly proinflammatory to “neurosupportive” (Ogawa et al. 2021). For example, Sox11, which mediates neural development and regeneration (Kavyanifar et al. 2018), was upregulated in double-positive neutrophils (Ogawa et al. 2021). In a chemical OSN injury model, double-positive neutrophil numbers remained elevated as the olfactory epithelium regained thickness, suggesting a proregenerative role (Ogawa et al. 2021). Surprisingly a spatially segregated subset of OSNs failed to regenerate even 10 weeks after chronic intranasal LPS (Hasegawa-Ishii et al. 2019). This model offers an opportunity to test region-specific recovery mechanisms.

The role of lymphocytes in OSN degeneration or regeneration after injury has not been explored in depth. Ragγc^−/− alymphoid mice were used to demonstrate the independence of microglial proliferation from T, B, or NK cell responses after deafferentation by systemic dichlobenil, but OSN survival was not tested in those animals (Lazarini et al. 2012). Intriguingly, immunodeficient RAG-1^−/− mice exhibit adult-onset olfactory deficits, a thin olfactory epithelium and disorganized glomeruli in the absence of injury, indicating that lymphocytes benefit normal olfactory function through unknown mechanisms (Rattazzi et al. 2015).

Proinflammatory cytokines are often considered detrimental to neuronal regeneration, but Lane et al. demonstrate a more complex role of TNF-α after chemical OSN injury. Macrophages, TNF-α, IL-1β, and IL-6 expression increase in the days after methimazole or methyl bromide application (Chen et al. 2017). Suppression of these transient inflammatory responses impaired OSN regeneration. The authors demonstrated that TNF-α signaling through TNF receptor I and the RelA (p65) subunit of NFκB induces progenitor cell proliferation to generate new OSNs (Chen et al. 2017). These studies support the requirement for TNF-α in OSN repair, but as discussed below, strict temporal control of its expression is critical.

Detrimental effects of immune cells in the injured olfactory system

Inducible olfactory inflammation (IOI) transgenic mice inducibly overexpress cytokines in the olfactory epithelium to mimic abnormally high levels during chronic rhinosinusitus (Chen et al. 2017, 2019). Overexpression of TNF-α for 8 weeks stimulated IL-6 expression and the entry and local proliferation of M1 macrophage and CD3⁺ T cells in the OE (Chen et al. 2019). TNF-α-induced inflammatory responses blocked progenitor cell proliferation, depleting OSNs. Studies on tissue biopsies suggest similar mechanisms might cause olfactory deficits in chronic rhinosinusitis (Chen et al. 2019). In contrast to TNF-α, chronic overexpression of the proinflammatory anti-viral cytokine, IFN-γ, did not cause OSN degeneration though functional responses to odorants decreased, perhaps through direct effects on neuronal activity (Pozharskaya and Lane 2013). These studies highlight distinct phenotypes of overexpressing proinflammatory cytokines in rhinosinusitis models.

Current evidence suggests a detrimental role of macrophages in the olfactory bulb unlike the olfactory epithelium. CD68⁺, expressed by both macrophages and microglia, was elevated in the ipsilateral olfactory bulb of mice at days 5–42 after axotomy with significantly higher expression after severe versus mild injury. Daily treatment with the anti-inflammatory steroid, dexamethasone, decreased macrophage/microglial responses, astrocyte activation, and improved regeneration of OSN fibers to the olfactory bulb after olfactory nerve transection (Kobayashi and Costanzo 2009). This treatment also improved electrophysiological and behavioral outcomes if initiated at day 7 postinjury but not later (Kobayashi et al. 2018b). Together these results suggest that macrophage/microglial responses to axotomy hinder OSN axon regeneration. However, the widespread physiological effects of the drug limit the mechanistic interpretation of these studies, and as the authors note, steroids are ineffective or have undesirable side effects in head injury treatment (Beez et al. 2017; Braakman et al. 1983; Cooper et al. 1979; Dearden et al. 1986).

Cytokines can also have harmful effects in the olfactory bulb compared to the olfactory epithelium after nerve transection. Pharmacological inhibition of TNF-α increased OSN projections to the olfactory bulb, decreased lesion size, and reduced macrophages/microglia in the olfactory bulb after nerve sectioning. Mice treated with the drug also had higher evoked field potentials and better performance on an olfactory-mediated behavioral task 2–4 months later supporting long-term recovery (Al Salihi et al. 2017). Evidence points to a similar dichotomy for IL-6 depending on location (Getchell et al. 2002a; Nan et al. 2001), since blockade improved the reinnervation of glomeruli by OSN axons and the recovery of olfactory function and behavior (Kobayashi et al. 2013). Likewise, inhibiting the high mobility group box protein (HMGB)1, a proinflammatory molecule released by damaged neurons and glia, increased the number of regenerated OSN axons reaching the olfactory bulb and improved olfactory function and behavior after bulbectomy (Kobayashi et al. 2018a). Together these studies indicate that suppressing immune responses in the olfactory bulb improved recovery. We turn next to the gustatory system which is also capable of widespread regeneration after injury.

Immune cells in the taste epithelium

The innervation of oral taste buds varies depending on the location in the oral cavity (Fig. 2A and B). Anterior taste buds within papillae are ipsilaterally innervated by one of 2 chorda tympani (CT) nerves. The CT nerves also supply taste buds in the anterior foliate papillae, while the glossopharyngeal nerve (GL) innervates taste buds in posterior foliate papillae and the circumvallate papilla. Additional taste buds located on the palate and esophagus are innervated by the greater superficial petrosal nerve and vagus nerve, respectively (Witt 2019). Because of redundant taste input to the brain injuring a single taste nerve reduces but does not typically abolish whole-mouth taste sensation (Zuniga et al. 1994). However, some patients develop taste phantoms or pain after injury to a single taste nerve perhaps from the removal of inhibitory control in central pathways (McManus et al. 2012b; Snyder and Bartoshuk 2016). The functional redundancy of taste inputs to the brain is reflected in gustatory behavioral performance in rodents. Taste behavior is unaffected by unilateral CT sectioning (St John et al. 1995), while transient behavioral deficits following bilateral CT or GL injury are reversed upon regeneration (King et al. 2000; Kopka and Spector 2001; Kopka et al. 2000).

Fig. 2.

Organization of the taste system and experimental nerve injury. (A) Sketch of rodent head illustrating the anterior and posterior tongue with papillae. (B) Taste buds located in fungiform papillae (FG) and anterior foliate papillae (Fol) on the anterior tongue are ipsilaterally innervated by the chorda tympani nerve (CT) a branch of cranial nerve (CN) VII. The glossopharyngeal nerve (GL), a branch of CN X) innervates taste buds housed in the single circumvallate papilla and posterior foliate papillae. Severing one CT nerve (shown by red “X”) causes degeneration of the nerve and taste buds (gray) on that side of the tongue followed by regeneration. (C) Taste buds on the anterior tongue are composed of type I glial-like taste receptor cells, type II taste cells which sense sweet, bitter, and umami stimuli, type III taste cells which respond to sour stimuli and type IV basal progenitor cells. Taste stimuli enter the taste pore and contact microvilli on taste receptor cells where transduction takes place. Taste-elicited activity is transmitted by the CT nerve to the nucleus of the solitary tract.

Taste buds are a morphologically and functionally heterogenous group of 50–100 neuroepithelial taste cells on the dorsal surface of the tongue facing the oral cavity (Fig. 2C) (Chaudhari and Roper 2010). Type I taste cells are glial-like, type II cells transduce sweet, bitter, and umami stimuli, and type III cells form synapses with afferent nerve fibers (Fig. 2C) (Roper and Chaudhari 2017). Type III cells also express Otop1, a recently identified channel that transduces sour taste (Teng et al. 2019). Type II cells lack conventional synapses but release ATP to communicate with nearby nerve fibers (Roper 2021). Taste signals are transmitted by afferent taste nerves to the nucleus of the solitary tract (NTS) in the brainstem (Fig. 1A) and thalamus before reaching the gustatory cortex (Vincis and Fontanini 2019). Keratin (K) 5/K14-positive basal keratinocytes proliferate to generate post-mitotic Type IV taste cells at the base of the taste bud (Okubo et al. 2009). Type IV cells then undergo morphological changes to renew Type I, II, or III taste cells (Miura et al. 2014; Yang et al. 2020). A recent review provides a thorough discussion of the molecular and functional diversity of taste cells (Finger and Barlow 2021). Taste cells express multiple cytokines and TLRs recognizing pathogens indicating their potential role in immune defense and inflammation (Cohn et al. 2010; Feng et al. 2012, 2014; Hevezi et al. 2009; Shi et al. 2012; Wang et al. 2009).

Immune cells reside in the epithelium surrounding taste buds and in the lamina propria in the absence of injury. Healthy human fungiform papillae are populated by CD4+ and CD8+ T cells, mature and immature dendritic cells and macrophages (Feng et al. 2009, 2010). Low levels of macrophages, dendritic cells, and T cells were observed in the fungiform papillae and lamina propria in rat housed in a specified pathogen-free (SPF) facility (McCluskey 2004). Dendritic cells and epithelial γδT cells were more prevalent in the lamina propria of uninjured rat circumvallate and foliate papillae suggesting a more robust immune presence in the rear of the tongue during homeostasis, or perhaps due to conventional housing (Suzuki et al. 1997). SPF housing likely reduced the complexity and magnitude of leukocyte responses in the study on anterior taste buds (McCluskey 2004), since immune responses are more robust in animals exposed to a greater variety of environmental pathogens (Abolins et al. 2017; Beura et al. 2016). Thus, differences in species, pathogen exposure and region of the tongue could explain variations in baseline immune responses.

Injury models in the taste system

Animal injury models include axotomizing the gustatory nerves and systemic treatment with chemicals or inflammatory stimuli, similar to olfactory models, though the anatomy and embryonic origin of receptor cells differ in the 2 systems (Fig. 1). The most common experimental injury model in the taste system is to surgically section one or both GL or CT nerves (Fig. 2B) (Guth 1971; Olmsted 1921; Vintschgau and Hönigschmied 1877). The CT nerve is often severed ventrally after it exits the tongue, alone or together with the lingual nerve which transmits somatosensory information from lingual papillae and the non-taste epithelium (Farbman and Hellekant 1978; Witt 2019). The CT can also be sectioned behind the tympanic membrane (Barry 1999; Spector et al. 2010). Unilateral CT sectioning is also useful to understand injury-induced effects on neighboring intact taste buds on the other side of the tongue since there is little or no cross innervation of anterior taste buds (Kinnman and Aldskogius 1988). Because of its path from the tongue through the middle ear to the NTS, the CT is vulnerable to injury from middle ear infection, middle ear surgery, and dental procedures (Berling et al. 2015; McManus et al. 2012a, 2012b; Snyder and Bartoshuk 2016).

Experimental axotomy has been used to define the morphological and physiological sequelae of taste nerve injury predominantly in rodents (Barry and Frank 1992; Ganchrow and Ganchrow 1989; Hill 2005). Postinjury events are generally similar though the timing differs depending on the species, nerve sectioned, surgical approach, and type of axotomy (e.g., crush, sectioning, or avulsion) (Cheal and Oakley 1977; Cheal et al. 1977; Ganchrow and Ganchrow 1989; Guagliardo and Hill 2007; Nakashima et al. 1990; Oakley et al. 1993; Whitehead et al. 1987). The injured nerve degenerates distally from the site of axotomy and at 18 h fibers are absent from denervated anterior taste buds (Farbman 1969). Without trophic support taste buds within the first week after CT-lingual nerve or GL sectioning (Guagliardo and Hill 2007; Guth 1957; State 1977). Taste buds also degenerate and neural taste responses are lost if axonal transport is blocked with colchicine delivered to the CT-lingual nerve in gerbils (Sloan et al. 1983) Recent work indicates that R-spondin-2, a ligand of taste stem cell markers, Lgr5 and Lgr4/6, is a neuronal factor that maintains taste bud integrity in mice (Lin et al. 2021). Lgr5 progenitor cells give rise to regenerated taste cells after GL nerve sectioning (Takeda et al. 2013) or ex vivo (Ren et al. 2014). BDNF, in contrast, is needed for the regeneration of CT fibers after axotomy (Meng et al. 2017).

Once taste axons regenerate to the base of remnant taste buds, new Type I, II, and III taste receptor cells re-form taste buds (Cho et al. 1998; Saito et al. 2011). Initially the reconnected taste nerve is silent then spontaneous activity appears before weak responses to taste stimuli can be recorded (Cheal et al. 1977). Within weeks in adult rodents the peripheral taste system becomes functional and responds normally to sweet, sour, bitter and salty stimuli (Cain et al. 1996; Cheal et al. 1977; Hill and Phillips 1994; McCluskey and Hill 2002). Normal multifiber CT responses were restored within a mean of 15 days after nerve crush and 17 days after nerve sectioning in gerbils (Cheal et al. 1977). In contrast, taste buds fail to regenerate and taste function is absent when the CT or GL is sectioned in neonatal or aged rats (He et al. 2012; Hosley et al. 1987a; Hosley et al. 1987b; Martin and Sollars 2015; Sollars and Bernstein 2000; Sollars et al. 2002). This parallels clinical reports that young adults recover taste function more readily than older individuals after CT injury (Krishna et al. 2017; Saito et al. 2012; Skoloudik et al. 2022; Sone et al. 2001).

Systemic chemical treatment has been used to model taste dysfunction experienced by cancer patients undergoing chemotherapy (Gaillard and Barlow 2021). Taste buds degenerated and neurophysiological responses were lost in mice following genetic or pharmacologic inhibition of hedgehog pathway signaling with the chemotherapeutic drug, sonedigib (Mistretta and Kumari 2017). The endotoxin, LPS, and poly (I:C) are noninfectious molecules that mimic bacterial and viral infections as described above. Staphylococcal Enterotoxin A (SEA) is another inflammatory stimulant, but in this case a superantigen that hyper-stimulates T cells by cross-linking T cell receptors to MHC II in antigen-presenting cells (Deacy et al. 2021). Chemical injury will likely become a more widely used model to understand mechanisms underlying taste loss due to inflammation and cancer treatment.

Proregenerative effects of immune responses in the injured taste system

Histological reports of inflammatory cells in denervated taste buds are long-standing (Table 2). Olmstead mentioned leukocytes in degenerating anterior lingual taste buds 8 days after combined CT and lingual nerve sectioning in dog (Olmsted 1921). A “mild leukocytic infiltration” of degenerating posterior taste buds in rats after GL axotomy was also described in a classic study in the field (Guth 1957). More recently, macrophage, dendritic cells, and γδT cells responded to GL axotomy in rat (Suzuki et al. 1996b; Takeda et al. 1996). After CT axotomy, macrophages and neutrophils rather than T cells dramatically increased in the anterior tongue at days 2–7 after injury in rat (Cavallin and McCluskey 2005; Guagliardo et al. 2009; He et al. 2012; McCluskey 2004; Steen et al. 2010). Immune cell responses may differ because of the severity of injury since both GL nerves were sectioned, while CT axotomy was unilateral. Acute macrophage responses are needed to maintain normal neural responses to tastants in the intact CT after contralateral injury (Cavallin and McCluskey 2005; McCluskey 2004; Wall and McCluskey 2008). Whether macrophages also promote taste bud regeneration and recovery is currently unknown.

Table 2.

Immune responses to injury and effects on taste receptor cell regeneration

Location	Injury model	Species	Immune response	Timinga	Actions	Reference
LE	Surgical	Dog Rat Mouse	Leukocytes Lymphocytes	Acute-Chronic	NDb	Guth (1971); Olmsted (1921); Takeda et al. (1996)
LE	Surgical	Mouse Rat	Macrophages Neutrophils T cells	Acute	ND	Guagliardo et al. (2009); McCluskey (2004); Shi et al. (2012); Steen et al. (2010); Suzuki et al. (1996b)
LE	Surgical	Aged Rat	Neutrophils	Acute	Excessive inflammation before later deficits in taste bud regeneration	He et al. (2012)
LE	Surgical	Rat	IL-1β MIP-1α MCP-1 VCAM-1 ICAM-1	Acute	ND	Cavallin and McCluskey (2007a, 2007b); Shi et al. (2012)
NTS	Surgical	Mouse Rat	Microglia	Acute-Chronic	ND	Bartel (2012); Bartel and Finger (2013); Riquier and Sollars (2017)
LE	Chemical	Mouse	TNF-α	Acute	ND	Sarkar et al. (2021)
LE	Inflammatory LPS poly(I:C)	Mouse	IFN-γ TNF-α IL-6 IL-12 MCP-1	Acute	Reduces taste cell proliferation and lifespan Increases taste cell apoptosis	Cohn et al. (2010); Wang et al. (2007)
LE	Inflammatory LPS	Rat	Macrophages	Acute	ND	Cavallin and McCluskey (2005)
LE	Inflammatory LPS Poly(I:C) SEA	Mouse	IL-10	Acute	Protective	Feng et al. (2014)

LE, lingual epithelium; NTS, nucleus of the solitary tract; LPS, lipopolysaccharide; SEA, staphylococcal enterotoxin A.

We refer to immune responses lasting ≤7 days as acute and ≥8 days as chronic.

Effect on taste cell degeneration and/or regeneration not determined (ND).

Several studies have focused on whether immune cells phagocytose apoptotic taste cells after axotomy. Clearance of degenerating taste cells is proposed to occur through desquamation through the taste pore (Guth 1957; Olmsted 1921), phagocytosis by fibroblasts (Suzuki et al. 1996b; Takeda et al. 1996) or phagocytosis by type I taste cells (Farbman 1969). Macrophages appear near degenerating GL fibers (Suzuki et al. 1996b) and are reported to engulf taste receptor cells during renewal in the absence of nerve injury (Yoshie et al. 1990). However, the fate of degenerating taste cells remains unsettled. Controlled phagocytosis by immune cells is generally considered a positive, necessary part of wound healing before new cells can regenerate (Wynn and Vannella 2016).

Cytokines and chemokines may also affect regenerating taste cells though less is known about these mechanisms than in the injured olfactory system. Several macrophage recruitment factors are acutely upregulated in the anterior tongue after unilateral CT sectioning including intracellular adhesion molecule (ICAM)-1, vascular cell adhesion molecule (VCAM)-1, and monocyte chemoattractant protein (MCP)-1 (Cavallin and McCluskey 2007b). These molecules are increased in both the denervated and intact anterior taste fields prior to peak macrophage responses at 48 hr. Vascular expression of VCAM-1 is reduced by dietary sodium restriction, a treatment that also prevents macrophage entry (Cavallin and McCluskey 2005, 2007a, 2007b; McCluskey 2004). Individual chemoattractant factors may contribute to successful regeneration by controlling the influx of immune cells.

Uninjured taste buds express a number of cytokines including IL-1, TNF-α, IFN-γ and IL-10 (Feng et al. 2014; Feng et al. 2012; Shi et al. 2012; Wang et al. 2007). Taste buds and infiltrating macrophages and neutrophils upregulate IL-1β at 48 hr post-CT sectioning. IL-1 mediates early post-injury changes in the neighboring, intact CT nerve responses to sodium as shown by treatment with a recombinant antagonist to IL-1R (Shi et al. 2012). IL-1β increased sodium flux in polarized taste buds supporting direct effects of cytokine on taste function (Kumarhia et al. 2016). A preliminary report indicates that IL-1 signaling is also critical for the recovery of taste function after CT regeneration (Dong et al. 2021) suggesting a pro-regenerative role similar to axotomized sciatic nerve and in contrast to the cytokine’s generally detrimental effect on injured CNS neurons (Brough et al. 2015; Nadeau et al. 2011). IL-10, which dampens inflammation, is also beneficial and needed to maintain taste bud structure (Feng et al. 2014). IL-10 knock-out mice exhibit reduced taste bud number, fewer type II / III taste cells and excessive T cell and TNF-α responses to challenge with LPS (Feng et al. 2014). Together, these studies indicate a complex interplay of regulatory cytokines in taste buds during homeostasis and recovery from injury.

Taste nerve axotomy stimulates central as well as peripheral inflammatory responses. Unilateral CT sectioning stimulated microglial proliferation and activation increasing their density in the NTS from 2 to 20 days postinjury in mouse and rat (Bartel 2012; Bartel and Finger 2013; Riquier and Sollars 2017). Whether microglial responses to CT sectioning relate to functional and anatomical changes in the NTS and their role in taste bud regeneration is currently unknown (Barry 1999; Reddaway et al. 2012).

Detrimental effects of immune responses in the injured taste system

Relatively little is known about immune cells that perturb taste bud regeneration and functional recovery. Neutrophils quickly infiltrate both sides of the tongue after axotomy with only transient negative effects on the function of the intact, contralateral CT nerve (He et al. 2012; Steen et al. 2010; Wall and McCluskey 2008). However, elevated neutrophil responses are associated with negative effects on taste regeneration. Few taste buds regenerate in aged rats and the regenerated CT is unresponsive to tastants in parallel with a 4-fold increase in neutrophil responses (He et al. 2012). As in other systems, this dysregulated immune response may deter regeneration (Rea et al. 2018).

Elevated cytokine levels can also harm taste receptor cells. Wang and colleagues have identified mechanistic links between systemic inflammation, increased cytokine levels, and taste cell renewal (Cohn et al. 2010; Wang et al. 2007, 2009). Inflammatory stimuli such as LPS activated TNF-α, IFN, and other cytokine signaling pathways reduced taste receptor progenitor cell proliferation and increased taste cell apoptosis as a basis for taste dysfunction during sickness (Cohn et al. 2010; Feng et al. 2012; Wang et al. 2007, 2009). The upregulation of inflammatory genes in taste buds is recapitulated in organoids grown from mouse stem cells derived from circumvallate papillae (Feng et al. 2020). Our group found that exogenous TNF-α also causes a dramatic drop in sodium transport in polarized taste buds (Kumarhia et al. 2016). These in vitro systems are useful to dissect molecular mechanisms of taste cell inflammation and cell death in combination with in vivo studies.

Chemotherapy may cause taste loss in part through harmful inflammatory responses. Cyclophosphamide, used to treat a number of cancers, upregulates TNF-α in a subset of type II taste cells (Sarkar et al. 2021). Amifostine, a drug used to protect other tissues from chemotherapy side effects (Kouvaris et al. 2007) inhibited the increase in TNF-α by taste cells (Sarkar et al. 2021). Whether TNF-α directly impacts taste cell survival in this model is currently unknown. Inflammatory side effects may also be drug dependent. For example, few macrophages invade fungiform papillae housing degenerated taste buds after pharmacologic treatment with the cancer drug sonidegib (Ermilov et al. 2016). Further studies are needed to determine individual chemotherapy drugs cause taste loss through inflammatory mechanisms.

Harnessing the immune system to promote recovery

Immune responses can have detrimental and beneficial effects on the regeneration of injured chemosensory receptors, as discussed, though most of the literature offers only correlative links. Both the cellular composition and timing of immune responses to injury must be well orchestrated to achieve optimal regeneration and recovery. For example, elevated and extended neutrophil responses to CT nerve injury or local LPS caused functional deficits in CT responses, while macrophages paralleled the recovery of normal neural function (McCluskey 2004; Steen et al. 2010; Wall and McCluskey 2008). Acute TNF-α stimulated the proliferation of OSN progenitor cells in contrast to chronic elevation of the cytokine which reduced OSN axonal regeneration and functional recovery (Al Salihi et al. 2017; Chen et al. 2019; Lane et al. 2010; Turner et al. 2010). The type of injury also determines the potential for chemosensory cell regeneration. OSNs and taste buds regenerate more readily after axotomy compared with methods that injure progenitor cells, such as some forms of chemical lesioning or removal of the entire taste papilla (Brann and Firestein 2014; Ferrell and Tsuetaki 1984; Schwob 2002; Schwob et al. 2017; Zalewski 1970).

Better understanding of complex immune responses will likely lead to strategies to promote chemosensory cell regeneration, allowing therapies to move beyond broad-spectrum immunosuppression of the past using steroids like dexamethasone (Bromley 2000; Crisafulli et al. 2018; Doty 2019; Jiang et al. 2010). Treatments to stimulate the regeneration of OSNs, which are true neurons, may diverge from those supporting the reformation of taste buds composed of neuroepithelial cells. More studies that test the functional integration of regenerated chemosensory cells are needed to determine immune mechanisms impacting sustained recovery. Transgenic mouse models are expected to be particularly useful in defining proregenerative immune effects on injured chemosensory cells with some caution. Immune cells share a common lineage and overlapping markers (Kondo 2010), so careful experimental design is needed to interpret loss-of-function approaches. Conversely, gain-of-function methods like adoptive transfer may fail if labeled donor immune cells home to undesirable sites rather than chemosensory epithelia. Thoughtful attention to physiological relevance is also needed to interpret overexpression genetic models. Overall, however, these strategies have led to gains in understanding immunologic disease and will shed light on chemosensory–immune interactions during regeneration and recovery

Funding

Supported by the Medical College of Georgia’s Medical Scholars Program (H.G.L.) and National Institutes of Health National Institutes of Deafness & Communication Disorders R01DC016668 (L.P.M.).

Conflict of interest

None declared.