To Detect and Reject, Parallel Roles for Taste and Immunity

PURPOSE OF REVIEW

From single cells to entire organisms, biological entities are in constant communication with their surroundings, deciding what to 'allow' in, and what to reject. In very different ways, the immune and taste systems both fulfill this function, with growing evidence suggesting a relationship between the two, through shared signaling pathways, receptors, and feedback loops. The purpose of this review was to explore recent reports on taste and immunity in model animals and in humans to explore our understanding of the interplay between these systems.

RECENT FINDINGS

Acute infections in the upper airway, as with SARS-CoV-2, are associated with a proinflammatory state, and blunted taste perception. Further, recent findings highlight taste receptors working as immune sentinels throughout the body. Work in humans and mice also points to inflammation from obesity impacting taste, altering taste bud abundance and composition. There is accumulating evidence that taste cells, and particularly their receptors, play a role in airway and gut immunity, responsive to invading organisms. Inflammation itself may further act on taste buds and other taste receptor expressing cells throughout the body as a form of homeostatic control.

Brain-immune interactions and the neural basis of disease-avoidant ingestive behaviour

Neuro-immune interactions are widely manifested in animal physiology. Since immunity competes for energy with other physiological functions, it is subject to a circadian trade-off between other energy-demanding processes, such as neural activity, locomotion and thermoregulation. When immunity is challenged, this trade-off is tilted to an adaptive energy protecting and reallocation strategy that is identified as 'sickness behaviour'. We review diverse disease-avoidant behaviours in the context of ingestion, indicating that several adaptive advantages have been acquired by animals (including humans) during phylogenetic evolution and by ontogenetic experiences: (i) preventing waste of energy by reducing appetite and consequently foraging/hunting (illness anorexia), (ii) avoiding unnecessary danger by promoting safe environments (preventing disease encounter by olfactory cues and illness potentiation neophobia), (iii) help fighting against pathogenic threats (hyperthermia/somnolence), and (iv) by associative learning evading specific foods or environments signalling danger (conditioned taste avoidance/aversion) and/or at the same time preparing the body to counteract by anticipatory immune responses (conditioning immunomodulation). The neurobiology behind disease-avoidant ingestive behaviours is reviewed with special emphasis on the body energy balance (intake versus expenditure) and an evolutionary psychology perspective.

Mice do not develop conditioned taste aversion because of immunity loss

OBJECTIVES

This study intends to test the generation of conditioned taste aversion and conditioned immunodepression by daily paired administration of saccharin solution with cyclophosphamide, 15 mg/kg, for 4 days.

METHODS

One group of male mice of the outbred CD1 strain drank 0.15% saccharin and received 1 injection of cyclophosphamide, 15 mg/kg, for 4 days (paired group), another group (unpaired group) received the same doses of saccharin and cyclophosphamide noncontingently, the third group (cy60) received saccharin paired with cyclophosphamide, 60 mg/kg, and the fourth group (placebo) received saccharin in the absence of cyclophosphamide. All mice were immunized with keyhole limpet hemocyanin (KLH), 0.2 mg, 1 day before the treatments.

RESULTS

Mice of the paired, unpaired and cy60 groups displayed a similarly decreased antibody response to KLH, but mice of the paired group did not develop an aversion to saccharin while mice of the cy60 group did. Besides, repeat presentation of saccharin to mice of the paired group did not alter their antibody response to ovalbumin compared with mice of the unpaired or placebo group.

CONCLUSIONS

Taste aversion was not elicited in response to impaired immunity and the conditioned stimulus (saccharin) did not impair the antibody response.

The conditioned stimulus elicits taste aversion but not sickness behavior in conditioned mice

OBJECTIVES

This article extends previous reports on (i) elicitation of taste aversion after pairing a flavored beverage (saccharin solution) with a disease-provoking microbial product (lipopolysaccharide, LPS, or polyinosinic:polycytidylic acid, poly I:C); (ii) elicitation of sickness behavior (assessed as diminished ingestion of water and food) by the conditioned stimulus, and (iii) development of tolerance to those microbial products.

METHODS

Mice of the CD1 strain were conditioned by pairing ingestion of 0.15% saccharin solution with injection of LPS (100 mug/mouse) or poly I:C (6 mg/kg). A few days later, some mice were offered saccharin solution and were injected with saline, whereas other mice were offered saccharin solution and were injected with the microbial product.

RESULTS

Regardless of the nature of the unconditioned stimulus (LPS or poly I:C), (i) taste aversion to saccharin ensued, (ii) tolerance ensued to sickness elicitation by a second administration of the microbial component, and (iii) saccharin taste did not evoke sickness.

CONCLUSIONS

Symptoms of infectious sickness in the absence of infection are hardly explained by exposure to the conditioned stimulus.

Immune cells of the human peripheral taste system: dominant dendritic cells and CD4 T cells

Taste loss or alterations can seriously impact health and quality of life due to the resulting negative influence on eating habits and nutrition. Infection and inflammation are thought to be some of the most common causes of taste perception disorders. Supporting this view, neuro-immune interactions in the peripheral gustatory system have been identified, underlying the importance of this tissue in mucosal immunity, but we have little understanding of how these interactions influence taste perception directly or indirectly. This limited understanding is evident by the lack of even a basic knowledge of the resident immune cell populations in or near taste tissues. The present study characterized the distribution and population of the major immune cells and their subsets in healthy human anterior, lingual, fungiform papillae (FP) using immunohistochemistry. Dendritic cells (DCs) were the predominant innate immune cells in this tissue, including four subtypes: CD11c(+) DCs, DC-SIGN+ immature DCs, CD83(+) mature DCs, and CD1a(+) DCs (Langerhans cells). While most DCs were localized beneath the lamina propria and only moderately in the epithelium, CD1a(+) Langerhans cells were exclusively present within the epithelium and not in sub-strata. A small number of macrophages were observed. T lymphocytes were present throughout the FP with CD4(+) T cells more prevalent than CD8(+) T cells. Very few CD19(+) B lymphocytes were detected. The results show that DCs, macrophages, and T lymphocytes are the constitutive guardians of human FP taste tissue, with DCs and CD4 T cells being dominant, while B lymphocytes are rare under normal, healthy conditions. These observations provide a basic anatomical foundation for the immune response in the healthy human tongue as a basis for subsequent disease-related studies, but none of the present data indicate that the immune cell populations identified are, in fact, altered in individuals with abnormal taste perception.

Central blockade of IL-1 does not impair taste-LPS associative learning

After saccharin intake is associated with the consequences of peripheral lipopolysaccharide (LPS) administration, rats develop a strong conditioned avoidance behavior against this gustatory stimulus. To investigate the role of central interleukin-1 (IL-1) as a key signal during taste-LPS engram formation, rats were chronically infused with IL-1 receptor antagonist into the lateral ventricle of the brain before, during and after a single association trial. The results indicate that a stable taste-LPS engram can be formed even under the chronic blockade of central IL-1 signaling during engram formation and consolidation. More importantly, our data show that animals which did not experience a fever response during association phase (due to the LPS encounter) were unable to elicit hyperthermia as part of the conditioned response. These data indicate that pairing a relevant taste stimulus with an immune challenge, such as LPS, might result in the formation of multiple engrams, specifically codifying independent information.

Murine taste-immune associative learning

Taste-immune associative learning can result from contingent pairings of an immune-competent unconditioned stimulus (US) with a gustative conditioned stimulus (CS). Recalling such an association may induce a set of physiological responses affecting behavior, endocrine, and immune functions. We have established a model of behaviorally conditioned immunosuppression employing the immunosuppressant drug cyclosporine A (CsA) as the US and saccharin as the CS in rats and humans. In order to investigate the inter-species generalization of this neuro-immune interaction, we tested the feasibility of this paradigm in mice. In a single-bottle scheme, male BALB/c mice (n=5) were conditioned by conducting three association trials and a single recall trial. Control groups (n=5/group) were designed to assure associative learning, pharmacological effects of the US, and placebo effect. Results show that CsA-conditioned animals displayed significant immunosuppression in the spleen after recall, measured by in vitro T-lymphocyte proliferation, and IL-2 production. However, the same animals did not show evidence of avoidance behavior to the CS. In contrast, evoking the association of saccharin-lithium chloride (inducing gastric malaise) in another set of animals (n=4/group) resulted in significant and pronounced avoidance of the taste (CS). These animals also displayed significant suppression of splenic T-lymphocyte responsiveness after the recall phase. The present results indicate that mice seem to be capable of associating a gustative stimulus with CsA, resulting in behaviorally conditioned immunosuppression without affecting appetitive behavior.

The effects of lipopolysaccharide and lithium chloride on the ingestion of a bitter-sweet taste: comparing intake and palatability

Activation of the immune system with lipopolysaccharide (LPS) has been shown to result in decreased consumption of normally preferred substances while at the same time not affecting palatability. The present study examined the effects LPS administration on both intake and palatability of a relatively unpalatable bitter-sweet taste. Bitter is thought to signal a danger cue to an animal representing a potential toxin-containing food. Using a one-bottle consumption test, voluntary intake of a sucrose-quinine (0.15 M sucrose + 0.00015 M quinine; S-Q) solution was assessed in rats on two conditioning days (days 1 and 4) after a systemic injection with LPS, LiCl, or NaCl. On the test day (day 7), rats were given 1h access to the same solution in the absence of any injection. In a separate experiment, rats fitted with intraoral cannulae received similar testing schedules, however, the solution was delivered intraorally, activating only the consummatory responses of the animal. During conditioning, rats received 5 brief (1 min) intraoral infusions of the taste across a 1h period following injections of LPS, LiCl or NaCl. Individual taste reactivity responses were recorded and analyzed. Both LPS and LiCl resulted in decreased consumption of the unpalatable taste relative to controls on the test day, suggesting typical conditioned taste avoidance. When the consummatory responses were examined, LPS-treatment produced an increase in active oral rejection relative to NaCl- and LiCl-treated groups on both conditioning days. The present study demonstrates that although both LPS- and LiCl-treatment result in similar conditioned avoidance using an intake measure, they do not elicit similar patterns of taste reactivity responding to intraoral infusions of the bitter-sweet taste. Furthermore, the present results suggest that immune activation with LPS-treatment results in increased rejection of a mildly aversive stimulus and supports the hypothesis that reorganization of behavioral priorities occurs during bacteria-induced sickness.

Lipopolysaccharide-induced up-regulation of activated macrophages in the degenerating taste system

Unilateral chorda tympani (CT) nerve section and maintenance on a sodium-restricted diet leads to a rapid decrease in neurophysiological taste responses to sodium in the contralateral, intact CT nerve. Up-regulation of immune function with lipopolysaccharide (LPS; 100 microg i.p.) induces a recovery of normal sodium taste responses, suggesting that the sodium-deficient diet is immunosuppressive. In fact, there is a bilateral increase in the number of lingual, activated macrophages in control-fed rats receiving CT nerve section that does not occur in sodium-deficient rats after sectioning. In the current study, we hypothesized that the LPS-induced recovery of normal taste function in sodium-deficient rats is based on an increase in the activated macrophage response to denervation. Rats receiving a unilateral CT nerve section, a sodium-restricted diet, and/or an injection of LPS (100 microg; i.p.) were overdosed with pentobarbital at day 2 postsectioning, and tongues were rapidly dissected and frozen. Cryosections were then immunohistochemically stained to determine the percentage of ED1 staining for activated macrophages or the number of alphabeta or gammadelta T cells. Activated macrophage levels were significantly increased in sodium-restricted rats that received LPS following unilateral CT nerve section, supporting our hypothesis. These novel findings suggest that LPS overcomes the immunosuppression induced by the sodium-restricted diet and also indicate that the immune system plays a role in regulating taste function after neural injury.

Immune activation paired with intraoral sucrose conditions oral rejection

The effects of lipopolysaccharide (LPS) and LiCl on conditioned taste aversion acquisition using intraoral infusions as the method of taste delivery was examined. Rats received two pairings of an intraorally delivered sucrose (5 ml) taste with the effects of a systemic injection of LPS, LiCl or NaCl. The magnitude of conditioning was quantified by scoring taste reactivity responses to a brief intraoral infusion of sucrose in the absence of any drug injection. Rats previously conditioned with LiCl or LPS displayed clear evidence of conditioned aversion with increased oral rejection responses relative to saline controls. Our results suggest activation of the immune system with LPS can condition consummatory aspects of ingestion when this conditioning involves intraoral fluid presentation.

Up-regulation of activated macrophages in response to degeneration in the taste system: Effects of dietary sodium restriction

Dietary sodium restriction combined with unilateral chorda tympani nerve section leads to a rapid and specific decrease in neurophysiological taste responses to sodium in the contralateral, intact chorda tympani (Hill and Phillips [1994] J. Neurosci. 14:2904-2910). Previous work demonstrated that dietary sodium restriction may induce these early functional deficits by inhibiting immune activity after denervation (Phillips and Hill [1996] Am. J. Physiol. 271:R857-R862). However, little is known about the leukocyte response to denervation of taste buds in fungiform papillae. In the current study, it was hypothesized that T cells and macrophages are increased in the tongue after unilateral denervation in control-fed but not sodium-restricted animals. Adult, specified pathogen-free rats received unilateral chorda tympani nerve section or sham section followed by dietary sodium restriction or maintenance on control diet. At day 1, 2, 5, 7, or 50 postsectioning, immunostaining was used to detect the percentage of staining for activated macrophages, the number of alpha beta T cells, and the number of delta gamma epithelial T cells in the tongue. The number of lingual T cells did not significantly differ between treatment groups following denervation. However, there was a dramatic bilateral increase in ED1(+) staining for activated macrophages in control-fed rats that peaked at day 2 postsectioning. In contrast, sodium-restricted rats did not show an increase in activated macrophages above baseline at any time postsectioning. Further analysis of extralingual macrophages indicated that the deficit in immune activity in sodium-restricted rats is localized to the tongue and is not widespread. A model for immune modulation of taste receptor cell function is proposed based on these novel findings.

Immune-induced flavor aversion in mice: modification by neonatal capsaicin treatment

OBJECTIVE

This study was designed to evaluate the role of c-sensitive fibers in the establishment of immune-induced flavor aversion in mice.

METHODS

Mice were treated neonatally with capsaicin in order to destroy c-sensitive fibers; after such treatment, adult animals, immunized or not with ovalbumin, were submitted to a two-bottle preference test, with a choice between water and a sweetened egg white solution.

RESULTS

Neonatal capsaicin treatment was unsuccessful in preventing the development of immune-induced aversion to the sweetened solution containing the antigen. Nonetheless, amongst immunized mice, those which had been previously treated with capsaicin showed a significant increment in the preference for the sweetened egg white solution. Furthermore, our data showed that neonatal capsaicin treatment did not interfere with either IgG1 or IgE production.

CONCLUSION

The present results suggest that c-sensitive fibers have a role in the transmission of the signals generated by this immune response to the central nervous system, thus contributing to the development of a flavor aversion in mice.

Conditioned taste aversion produced by cyclosporine A: concomitant reduction in lymphoid organ weight and splenocyte proliferation

The classical conditioning of immune parameters is commonly conducted within a conditioned taste aversion (CTA) paradigm. In this study, the immunosuppressive drug cyclosporine A (CsA) was investigated for its ability to produce both taste aversion to a novel stimulus and conditioned alterations in immune functioning. The paradigm comprised the pairing of a 0.2% saccharin solution (the conditioned stimulus; CS) with an intraperitoneal injection of 20 mg/kg CsA (the unconditioned stimulus; UCS). Upon saccharin re-presentation, a marked reduction in fluid consumption was observed, indicating aversion to the novel substance (=CTA). By using a single CsA/saccharin pairing the CTA lasted for one CS representation. However, by implementing three pairings, this effect could be extended for up to seven representations. No noticeable difference was recorded by adjusting the saccharin representation from every consecutive day to every second day. The most effective paradigm in creating CTA was subsequently investigated for its effectiveness in producing conditioned immune alterations. Animals were killed on the day of the third CS re-presentation, and immune functions assessed. Conditioned animals displayed a significant reduction in thymus and spleen weights. Effects on the spleen were further investigated, revealing a significantly reduced proliferative ability of isolated splenocytes to concanavalin A. These results demonstrate that the physiological effects produced by CsA are sufficiently salient to elicit CTA. Furthermore, the reduction in lymphoid organ weight and splenocyte proliferation induced by CsA are also conditionable using this paradigm.

Immunological induction of flavour aversion in mice. II. Passive/adoptive transfer and pharmacological inhibition

Mice immunized with ovalbumin develop a strong aversion to ingesting sweetened egg white dilutions or ovalbumin solutions. In immunized animals, gavage or voluntary ingestion of ovalbumin triggers an increase of vascular permeability in the intestine; pretreatment with a mixture of histamine and serotonin antagonists blocked this reaction, but not the aversion; dexamethasone inhibited both the aversion and the increase in permeability. The aversion was transferred to normal recipient mice with high-titre anti-Ova sera obtained with complete Freund's adjuvant, but not with lower-titre serum pools of mice immunized with the help of Al(OH)3 adjuvant. However, the aversion was also (adoptively) transferred with whole spleen cells from immune donors. This later condition is inefficient to transfer the formation of high titres of specific antibodies.

Topographic organization of Fos-like immunoreactivity in the rostral nucleus of the solitary tract evoked by gustatory stimulation with sucrose and quinine

Fos immunohistochemistry was used to elucidate the pattern of activation elicited by two qualitatively and hedonically distinct taste stimuli, sucrose and quinine, within the first-order gustatory relay, the rostral division of the nucleus of the solitary tract. Compared to unstimulated controls, both sucrose and quinine elicited significant increases in Fos-like immunoreactivity in the rostral central subnucleus, the region of the rostral solitary nucleus that receives the densest primary afferent input. Within the rostral central subnucleus, neurons that exhibited Fos-like immunoreactivity following quinine stimulation were concentrated medially, but neurons that exhibited Fos-like immunoreactivity following sucrose stimulation were distributed more evenly along the mediolateral axis. Despite their differential distribution, sucrose- and quinine-activated neurons also demonstrated notable intermingling. Further, the chemotopic arrangement was only partially consistent with what would be predicted if chemotopy was merely an outcome of orotopy. Our results suggest that a rough chemotopy characterizes the organization of taste responses in the nucleus of the solitary tract, and that the topographic pattern of taste afferent terminations in this nucleus is related to their chemosensitivity as well as to their peripheral spatial distribution.

Immunological induction of flavor aversion in mice

1. Young adult BALB/c and B6D2F1 mice of both sexes (20 +/- 2 g) immunized ip with 2 doses of 10 micrograms ovalbumin (Ova), but not with 2 doses of 10 micrograms bovine gammaglobulins (BGG), show aversion to the ingestion of sweetened egg white or crystallized Ova solutions which are avidly ingested by normal mice. In 24 h, normal mice or mice immunized with BGG ingested, respectively, 340 +/- 80 and 265 +/- 56 mg of sweetened egg white per gram of body weight (mg/gbw); in the same period, Ova-immunized mice ingested less than one tenth these amounts (18 +/- 5 mg/gbw). ELISA-titers of anti-Ova and anti-BGG antibodies in immune mice were of similar magnitude. 2. Aversion arises coincidentally with the emergence of anti-ovalbumin antibodies in serum in the primary response, 14 days after primary immunization. 3. Previous induction of oral tolerance to ovalbumin by a single gavage with 20 mg Ova 7 days before primary ip immunization, which blocks the increase of specific antibodies in serum, also blocks the development of the aversive phenomenon. 4. Aversion was induced to 1 mg/ml but not 0.1 mg/ml sweetened crystallized ovalbumin solutions and was already noticeable 2 h after exposure of immunized mice to sweetened egg white solutions. 5. We conclude that, at least in experimental situations, immunological factors may be of decisive importance in diet selection.