Conditioning phenomena and immune function

Pavlovian Conditioning of Immunological and Neuroendocrine Functions

The phenomenon of behaviorally conditioned immunological and neuroendocrine functions has been investigated for the past 100 yr. The observation that associative learning processes can modify peripheral immune functions was first reported and investigated by Ivan Petrovic Pavlov and his co-workers. Their work later fell into oblivion, also because so little was known about the immune system’s function and even less about the underlying mechanisms of how learning, a central nervous system activity, could affect peripheral immune responses. With the employment of a taste-avoidance paradigm in rats, this phenomenon was rediscovered 45 yr ago as one of the most fascinating examples of the reciprocal functional interaction between behavior, the brain, and peripheral immune functions, and it established psychoneuroimmunology as a new research field. Relying on growing knowledge about efferent and afferent communication pathways between the brain, neuroendocrine system, primary and secondary immune organs, and immunocompetent cells, experimental animal studies demonstrate that cellular and humoral immune and neuroendocrine functions can be modulated via associative learning protocols. These (from the classical perspective) learned immune responses are clinically relevant, since they affect the development and progression of immune-related diseases and, more importantly, are also inducible in humans. The increased knowledge about the neuropsychological machinery steering learning and memory processes together with recent insight into the mechanisms mediating placebo responses provide fascinating perspectives to exploit these learned immune and neuroendocrine responses as supportive therapies, the aim being to reduce the amount of medication required, diminishing unwanted drug side effects while maximizing the therapeutic effect for the patient’s benefit.

Placebo, nocebo, and learning mechanisms

Recent substantial laboratory and theoretical research hints for different learning mechanisms regulating the formation of placebo and nocebo responses. Moreover, psychological and biological variants may play a role as modulators of learning mechanisms underlying placebo and nocebo responses. In this chapter, we present pioneering and recent human and nonhuman research that has impressively increased our knowledge of learning mechanisms in the context of placebo and nocebo effects across different physiological processes and pathological conditions.

The mechanisms of immune suppression by high-pressure stress in mice

The effects of high-pressure stress on the induction of anti-sheep red blood cells (SRBC) and of plaque-forming cells (PFC), and on thymus weight, were studied in BALB/c mice in-vivo and in-vitro. The efficacy of high-pressure stress in suppressing PFC and thymic involution was maximum when the stress was applied 1 h day(-1) for 2 days before immunization with SRBC. Both effects were blocked by administration of indomethacin, atropine, naloxone or phentolamine before the first application of stress, whereas hexamethonium and propranolol had no such effect. Hexamethonium, naloxone and propranolol administered before the second application of high-pressure stress blocked both effects. Prostaglandin and acetylcholine given 24 h before application of high-pressure stress caused a marked reduction in PFC count, but not in thymus weight. The reduced PFC count caused by acetylcholine was blocked by pretreatment with indomethacin. When adrenaline was injected 24 h after application of high-pressure stress a marked reduction in PFC was observed, but without thymic involution. When adrenaline was injected 24 h after prostaglandin injection the PFC count was also markedly reduced, but not thymus weight. The decrease in PFC caused by two exposures to stress or one exposure to stress plus injection of adrenaline was blocked by diethylcarbamazine before the second exposure to stress or the injection of adrenaline. In addition, normal spleen cells, were induced as suppressor cells when incubated with the serum of stressed mice, but not when supplemented with anti-leukotriene C4, D4 antibody. These data suggest that mice fall into a pre-stress condition via the release of prostaglandin after the first stress, and then immunosuppression is induced in these prestressed mice via the release of leukotriene C4, D4, caused by the activation of the autonomic nervous system by the second exposure to stress.

Neurobehavioral alterations in autoimmune mice

Inbred MRL, NZB and BXSB strains of mice spontaneously develop a systemic, lupus-like autoimmune disease. The progress of autoimmunity is accompanied with a cascade of behavioral changes, most consistently observed in tasks reflective of emotional reactivity and the two-way avoidance learning task. Given the possibility that behavioral alterations may reflect a detrimental consequence of autoimmune-inflammatory processes and/or an adaptive response to chronic malaise, they are tentatively labeled as autoimmunity-associated behavioral syndrome (AABS). It is hypothesized that neuroactive immune factors (pro-inflammatory cytokines, brain-reactive antibodies) together with endocrine mediators (corticotropin-releasing factor, glucocorticoids) participate in the etiology of AABS. Since AABS develops natively, and has a considerable face and predictive validity, and since the principal pathway to autoimmunity is known, AABS may be a useful model for the study of CNS involvement in human autoimmune diseases and by extension, for testing autoimmune hypotheses of several mental disorders (major depression, schizophrenia, Alzheimer's disease, autism and AIDS-related dementia).

Effect of a conditioned aversive stimulus on the immune response in three strains of rats

In the present study we investigated the effect of a brief exposure (15 s) to a conditioned aversive stimulus (CS) on the proliferative response of spleen and peripheral blood lymphocytes (PBL) in Lewis, Fischer 344 and Sprague-Dawley rats. Plasma levels of ACTH and corticosterone were also measured. For conditioning, rats were exposed to 10 presentations of a 5 s duration foot-shock (1.6 mA) preceded by a 15 s tone. Seven days later, animals were exposed to the auditory signal without electric shock. Significant differences were found in both the kinetics and the magnitude of altered mitogenic responsiveness of PBL between the different strains of rats. Enhancement of PBL responsiveness to mitogens was observed in Fischer and Sprague-Dawley rats immediately after exposure to the CS. A significant decrease in the response of PBL to mitogens was found in Lewis and Sprague-Dawley rats 10 min after exposure to the CS. The PBL response of Sprague-Dawley and Fischer rats returned to baseline at 30 min, but not in Lewis rats. Proliferative activity of spleen lymphocytes in response to the CS was suppressed from baseline in all rat strains, but the timing and degree of suppression differed. Fischer rats had the largest percentage of suppression. The earliest suppression of spleen mitogenic function after exposure to the CS was in Fischer rats, while the Lewis rats had the latest onset of suppression, with the Sprague-Dawley rats being intermediate. Plasma levels of ACTH and corticosterone peaked at 10 min in all strains of rats. The magnitude of hormonal elevation differed in the different rat strains, suggesting that corticosterone may not have a variable immunomodulatory role in each strain. These data suggest that a brief psychological stressor results in activation of the HPA axis and is associated with strain-dependent alterations of lymphocyte responsiveness to non-specific mitogens. The short-term exposure to a CS which produces different parameters of lymphocyte functional modulation, provides a useful tool to study the mechanisms of stressor-induced immune alteration.

Increased pineal melatonin content coupled to restricted water availability in a Pavlovian conditioning paradigm in rats

The objective of this study was to assess whether rat pineal melatonin content could be modified in a classical conditioning paradigm. In rats kept under light (200 lux) from 06.00 to 18.00 h daily, the time of lights off was selected as the unconditioned stimulus (US). Restricted water availability (from 10 min before to 10 min after light-dark, LD, transition) was the conditioned stimulus (CS). The conditioned and unconditioned responses were measured as the changes in pineal melatonin levels 4 h after LD transition. In animals under regular lighting conditions, lights out at 18.00 h (the US) caused a 4.4-7.8-fold increase of pineal melatonin concentration 4 h after later, when compared to animals maintained under light for the 4 h-period. After a training period of 7 days of restricted water availability (the CS), significantly augmented pineal melatonin levels were found in rats that were exposed to water but were maintained under light for the 4 h period after expected LD transition. The control animals for this experiment, i.e., rats which had undergone the training period, were kept for 4 h under light after expected LD transition, and did not receive water at LD transition, exhibited very low pineal melatonin levels. The conditioned increase of pineal melatonin content attained lower values than those in rats exposed to normal lighting conditions. It also fulfilled the contingency criterion, that is, it caused at trial a significant elevation of pineal melatonin content only when water availability was applied from 10 min previously to LD transition during training, and not 20 min after LD transition. After a training period of 7 days, restricted water availability applied 4 h before lights off (at 14.00 h), caused an enhanced production of melatonin 4 h later, regardless of the animals being exposed either to a dark or to a light environment. The results indicate that pineal melatonin production can be manipulated in a classical conditioning paradigm, when an appropriate CS stimulus is used.

Spatial learning during the course of autoimmune disease in MRL mice

The present study examines whether autoimmune MRL-lpr mice develop impairments in learning and memory that correlate with changing severity of lupus-like disease. MRL-lpr mice (n = 20) were tested in the Morris water-maze at 12, 14, 16 and 18 weeks of age. Age-matched controls were congenic MRL +/+ mice (n = 20) that develop the disease much later. Immune status was assessed by the presence of anti-nuclear antibodies (ANA), brain-reactive antibodies, proteinuria, and haematocrit. Learning rates and memory retention did not differ between the substrains, and did not correlate or deteriorate with advancing age and autoimmunity. However, the baseline performance level in autoimmune MRL-lpr mice was shifted, as evidenced by a consistently longer task-solving latencies. Thigmotaxic swimming (along the pool wall) was pronounced in the MRL-lpr group, and was associated with the observed difference in performance. The present study does not support the notion that learning/memory abilities of autoimmune MRL-lpr mice are impaired per se, but may support the hypothesis that the rapid progress of humoral autoimmunity affects the emotionality of lupus-prone mice.

Role of central nervous system and behaviour in the immune response

Recent advances in antigen preparation and delivery have led to a renewed interest in vaccination to control infectious disease. However, relatively less attention has been devoted to host factors which affect the outcome of immune responses. There is now increasing acceptance that the central nervous and immune systems interact, and that this can be a bidirectional process. A range of behavioural and psychological states, learned responses and reactions to external stimuli have all been implicated in immune modulation. These interactions may occur via the direct innervation of lymphoid compartments, by paracrine means through the release of mediators from nerves situated in close proximity to cells involved in immunity, or by neuroendocrine signals in the form of hypothalamic, pituitary and peripheral endocrine hormones. These effects may account to some extent for unexplained variability in response to vaccination and disease challenge, through inherited or acquired differences in neuroendocrine or neurotransmitter responses to stress, circadian effects or learned behavioural responses. This review addresses the effects of environmental stimuli and behavioural manipulations on immune outcome and the response to vaccination, and the potential for practical application of these techniques in human and animal disease control and health management.

An introduction to the possible role of central nervous system structures in neuroendocrine-immune systems interaction

The involvement of different regions of the brain in the immune response was investigated with the aid of small electrolytic lesions. The lesions were placed in such a way that they covered different areas of the brain stem, basal ganglia, but also some parts of the frontal cortex. The cellular immune response as well as DNA synthesis with the aid of labelled precursors was measured. The results suggest the possibility of the existence of three circuits. One circuit represents catecholaminergic cell group A1-7 in reticular formation, nucleus parabrachialis and central ncl. amygdalae. The second circuit represents serotonergic rapheal groups B6.8, hypothalamus and ncl. basomedialis of amygdala. The third circuit represents ncl. amygdalae and the medial part of frontal cortex, namely the cingulate cortex area 1-2. The close correlation between the changes of the immune response and the CNS activity was also investigated in the experiments with immunosuppressed and immunostimulated animals by measuring of the turnover of some neurotransmitters but also by recording electrocephalographic activity.

Voluntary modulation of neutrophil adhesiveness using a cyberphysiologic strategy

In a study of voluntary immunomodulation, 45 subjects were assigned either to a control group or one of two experimental groups. All groups had blood and saliva samples collected before and after either a 30 minute rest condition (Control group) or a 30 minute cyberphysiologic strategy (Experimental groups) to increase neutrophil adherence. These samples were analyzed on a range of immunologic measurements including neutrophil adherence. The second experimental group practiced a cyberphysiologic strategy two weeks prior to the experimental session. Subjects in each group returned to repeat their exercise in a second session the following week. Analysis of all immune measurements revealed statistical significance for changes in neutrophil adherence. These studies suggest that such strategies may be used to effect changes in immune cell functions. Analysis further revealed that those subjects with prior cyberphysiologic training were able, by the second session, to induce a significant increase in neutrophil adherence.

Voluntary consumption of cyclophosphamide by nondeprived Mrl-lpr/lpr and Mrl +/+ mice

Cyclophosphamide dissolved in several dilutions of chocolate milk was presented for 20 hr daily to nondeprived, symptomatic, autoimmune Mrl-lpr/lpr and asymptomatic Mrl +/+ mice. In the absence of cyclophosphamide, daily consumption was inversely related to the concentration of the chocolate milk solutions and increased from the first to the fourth day of exposure. There were no effects of strain or sex on the consumption of plain chocolate milk. Consumption of 0.1 or 0.2 mg cyclophosphamide per ml of different dilutions of chocolate milk increased over days 1-4 and decreased on day 8. Consumption of 0.4 mg/ml cyclophosphamide did not change over days. Generally, consumption was inversely related to the cyclophosphamide concentration. Females consumed more cyclophosphamide than males. Autoimmune lpr/lpr mice consumed more cyclophosphamide than +/+ mice. Dilution of chocolate milk had no effect on consumption of cyclophosphamide. Lymphoproliferation and anti-ssDNA antibody titer were reduced by the consumption of cyclophosphamide-chocolate milk solutions. It is hypothesized that autoimmune lpr/lpr mice voluntarily consume more cyclophosphamide than asymptomatic +/+ mice in an effort to "correct" their immune system dysregulation.

Voluntary consumption of cyclophosphamide by Mrl mice

Fluid-deprived, lupus-prone Mrl-lpr/lpr and congenic Mrl +/+ mice were provided with a single drinking bottle containing varying concentrations of cyclophosphamide (CY) dissolved in chocolate milk. Eighteen- and 20-week-old Mrl-lpr/lpr males with manifest symptoms of autoimmune disease voluntarily consumed more of the CY solution than Mrl +/+ mice of the same age after 1 week of 1 hr/day exposures. The volume of CY-laced chocolate milk consumed was sufficient to attenuate lymphadenopathy and the elevated anti-DNA antibody titers in these animals. When testing began before the development of manifest symptoms of autoimmune disease, there were no differences between the two substrains. These results are consistent with the hypothesis that behavioral processes can act to correct homeostatic imbalances within the immune system.

Characterization of brain-reactive autoantibodies in murine models of systemic lupus erythematosus

Using the Western blot technique we analyzed the sera of five strains of mice that develop a disease like systemic lupus erythematosus (SLE), along with two normal strains, for their binding specificities against isolated mouse integral brain membrane proteins. This report describes the distribution and frequency of the more than 200 brain-reactive autoantibodies in the 126 animals tested and verifies the hypothesis of diversity in anti-brain antibodies produced during autoimmune conditions such as SLE. These results emphasize the importance of characterizing the brain-reactive autoantibodies in the sera or cerebrospinal fluid of SLE patients with central nervous system involvement.

Conditioned immunosuppression in young versus aged mice: differences in cells and responses to environmental stimuli lead to altered conditioning in aged animals

Aged mice (greater than 20 months of age) show a decreased immune response after antigen challenge compared to their young counterparts. In this study aged mice were also found to show a diminished conditioned immunosuppression after associative learning trials with cyclophosphamide and saccharin, followed by immune stimulation in the presence of saccharin, when compared to young (10 weeks) syngeneic mice. Adoptive transfer experiments in which cells from nonconditioned or conditioned young or aged mice were injected into irradiated conditioned young or aged syngeneic mice (exposed or not exposed to conditioned stimuli) revealed the following: (1) There was an altered responsiveness of normal cells injected into conditioned aged mice (reexposed to cues) compared to the response in young recipients; (2) Cells from conditioned young mice failed to show conditioned immunosuppression on adoptive transfer to irradiated conditioned aged mice; (3) Cells from conditioned aged mice failed to show conditioned immunosuppression on adoptive transfer to irradiated conditioned young mice; (4) The changes seen in spleen cells from conditioned aged mice (relative to similar cells from young mice) were to be found in the T cell population of these animals. These data are consistent with the idea that during aging changes in both the responding cells and the conditioned environment, along with the interaction of these, produce a decreased ability to document conditioned immunosuppression of antibody responses.