CNS–immune system interactions: Conditioning phenomena

Repeated intraperitoneal injections of saline attenuate the antibody response to a subsequent intraperitoneal injection of antigen

The results of two experiments with male C3H/HeJ mice indicated that repeated intraperitoneal (ip) injections of sterile saline over a 2-week period resulted in an attenuated antibody response to the subsequent ip injection of a soluble protein antigen, keyhole limpet hemocyanin. There were no differences among experimental groups that received a different number of preimmunization injections of saline, and comparable effects were obtained simply by the daily handling of mice. Neither repeated ip injections nor handling altered preimmunization (baseline) corticosterone levels, and only previously unmanipulated mice showed an elevation in corticosterone levels 30 min after ip immunization. These latter results suggest that the depressed IgM and IgG responses were not due to the immunosuppressive effects of endogenously elevated adrenocortical steroids.

Pavlovian conditioning of rat mucosal mast cells to secrete rat mast cell protease II

Antigen (egg albumin) injections, which stimulate mucosal mast cells to secrete mediators, were paired with an audiovisual cue. After reexposure to the audiovisual cue, a mediator (rat mast cell protease II) was measured with a sensitive and specific assay. Animals reexposed to only the audiovisual cue released a quantity of protease not significantly different from animals reexposed to both the cue and the antigen; these groups released significantly more protease than animals that had received the cue and antigen in a noncontingent manner. The results support a role for the central nervous system as a functional effector of mast cell function in the allergic state.

Interaction of HIV infection with nervous tissue may play a crucial role in the development of AIDS

In this note, we first review some of the immunopathogenic mechanisms of HIV infection and discuss the possible role of the idiotypic network in producing immunodeficiency; we then propose that the interaction of HIV with the nervous system is such as to generate signals which are misinterpreted by the nervous system: the feedback reaction of the nervous system may play a crucial role in the development of AIDS. We conclude by studying a very simple model for the dynamics of AIDS.

Neuropsychological correlates of serum lymphocytotoxic antibodies in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by frequent neuropsychiatric (NP) manifestations. At least two different pathogenetic mechanisms have been proposed for NP-SLE, including vasculitis and antibodies against neuronal antigens, the latter as expressed by the presence of brain cross-reactive lymphocyte antibodies. We have previously reported a high prevalence of cognitive dysfunction in SLE which can remain subclinical and which cannot be accounted for on the basis of disease activity, general distress, or steroid medication. In the present study, we undertook the same extensive, standardized neuropsychological testing in 98 consecutive female SLE patients in order to evaluate central nervous system functioning in relation to serum lymphocyte antibodies which were measured at the time of neuropsychological testing by a microcytotoxicity test. A significant association was observed between the presence of serum lymphocytotoxic antibodies (LCA) and cognitive impairment in patients with SLE. The pattern of impairment which predominated in the LCA-positive patients involved deficits in anteriorly associated, primarily visuospatial functions. These findings support the hypothesis of localization of a particular antigen-antibody interaction in the brain in SLE, suggesting the existence of immunological control mechanisms for normal brain functioning.

Behaviorally conditioned suppression of mitogen-induced proliferation and immunoglobulin production: effect of time span between conditioning and reexposure to the conditioning stimulus

Rats were subjected to taste aversion conditioning using the immunosuppressive drug cyclophosphamide (CY) as the unconditioned stimulus (UCS) paired with saccharin, the conditioned stimulus (CS), and were reexposed to the CS at 2, 5, or 10 days after a single conditioning trial. Twenty-four hours after reexposure the rats were sacrificed and spleen cells assayed for mitogen-induced proliferation and immunoglobulin production. A robust conditioned taste aversion (CTA) was observed irrespective of the day of CS reexposure. However, only conditioned rats reexposed to the CS 2 days after training displayed a conditioned reduction in proliferative responses to PHA and PWM. These rats also exhibited a reduction in the synthesis of IgM, but not IgG or IgA, by spleen cells cultured with PWM. These effects were not observed in conditioned rats reexposed 5 or 10 days after conditioning. In another experiment, rats were subjected to a backward conditioning (UCS prior to CS) training trial, tested 2 days later for the presence of CTA, and sacrificed 24 h later for assessment of immune function as described above. The results of this experiment demonstrated that rats do not develop an aversion to saccharin when it is first presented 4 h after CY, and no alterations in spleen cell proliferation and immunoglobulin production were noted. The data show that the CTA response established by explicit association between CY and saccharin depresses in vitro spleen cell proliferation and IgM production only when elicited shortly after the conditioning trial.

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.

Behavioral trait associated with conditioned immunity

Inbred strains of mice were tested for their activity in an open field. Animals selected for high or low activity ("tails" of the normal distribution) were further inbred through nine generations (brother x sister) with further selection at each generation. Reciprocal skin grafts between the two groups were performed to ensure that little/no genetic drift occurred. Using a cyclophosphamide:saccharin conditioning paradigm (R. Ader & N. Cohen (1975) Psychosom. Med. 37, 333-342) we show that mice preselected for high activity in an open field were those in which it was most easy to demonstrate conditioned immunosuppression. There was no difference in the conditionability of the two groups as assessed by taste aversion. By use of a cross-fostering design we conclude that immunological conditioning (in adult mice) can be affected by a characteristic of the nursing mother which is associated with activity in an open-field trial.

Behaviorally conditioned enhancement of delayed-type hypersensitivity in the mouse

Cyclophosphamide (CY), previously used to condition suppression of a number of different immune responses, was used to condition an enhancement of a delayed-type hypersensitivity (DTH) response in mice. Three weeks before sensitization with sheep red blood cells (SRBC), mice were conditioned by pairing their consumption of a saccharin solution (SAC) with injection of CY. Two, three, and four days after sensitization (Day 0), animals were reexposed to SAC and, on Day +3, they were injected with a low dose of CY. This dose suppressed the DTH response to a challenge with SRBC on Day +4 in both conditioned and control animals. Following rechallenge with SRBC on Days +11 and +18, however, DTH responses were enhanced. Conditioned mice that had been reexposed to SAC showed a significantly greater enhancement than either nonconditioned animals or conditioned mice that were not reexposed to SAC. Thus, enhancement as well as suppression of immune responses can be conditioned with CY.

Analysis of lymphocytes in, and host environment of, mice showing conditioned immunosuppression to cyclophosphamide

Mice were subjected to repeated exposures to cyclophosphamide: saccharin (conditioned) or cyclophosphamide:saccharin followed by saccharin only (conditioned:extinguished). Animals in the former group but not the latter subsequently showed diminished IgG antibody-forming cells (AFC) after challenge with sheep red blood cells followed by reexposure to immunologically inert cues (saccharin). When these animals were used as irradiated recipients of syngeneic spleen lymphocytes, reconstituted irradiated conditioned mice showed augmented IgG AFC on transfer of naive spleen cells and reexposure to saccharin. The expected diminished IgG AFC response was seen when cells from conditioned mice were transferred. However, the latter cells gave augmented IgG AFC when transferred to naive irradiated mice. Both of the effects seen with cells from conditioned animals (increased IgG AFC in control recipients; decreased IgG AFC in conditioned mice reexposed to saccharin) were regulated by adoptively transferred T cells in the spleen cell population.

The impact of single versus repeated exposure of mice to Toxocara canis

Infection with T. canis can alter dramatically the brain and behavior of the host. Previous results suggest that if the mammalian host is exposed either simultaneously to lead, or has a history of prior exposure to that toxic substance, the magnitude of the behavioral reaction to T. canis may be modified or even reduced. The present data suggest that the magnitude of both the behavioral and tissue/immune reactions may be less if the organism has multiple, instead of a single exposure, to T. canis. Lead, and perhaps other environmental toxicants may alter neurotropic products of the parasite, the behavior of the parasite, and/or reactivity of the host in the presence of the parasite. Such considerations may help explain, in part, the relative rarity of reported toxocariasis in humans, despite the fact that serological indices suggest that exposure to T. canis may be as high as 7% of the world population.

Anticipation of Pharmacological and Nonpharmacological Events: Classical Conditioning and Addictive Behavior

Typically, pharmacological phenomena such as tolerance, sensitization, and dependence have been viewed as resulting from the operation of feedback mechanisms: pharmacologically disturbed homeostatic functioning is countered by compensatory responses that restore physiological equilibrium. We summarize the results of research indicating that feedforward mechanisms (i.e., regulatory responses made in anticipation of a drug) also importantly contribute to drug effects. Such feedforward mechanisms operate on the basis of Pavlovian conditioning principles. We also discuss the role of such physiological feedforward mechanisms in areas that are not primarily pharmacological: immunology, exercise physiology, and stress.

Behavioral influences on the immune system: stress and conditioning

Several investigators have demonstrated that immune responses may be subject to classical conditioning processes. Conditioning effects in both cell-mediated and humoral immune responses have been reported. Strong evidence for behavioral effects on immune responses has been provided by studies of stress-induced immunosuppression. Stressors may increase morbidity due to infectious agents, depress antibody responses, inhibit lymphocyte reactions, and attenuate several other cell-mediated immune functions. In our studies, acute inescapable footshock inhibited natural killer cell (NK) activity in CD-1 and C57BL/6J mice, but not in DBA/2J mice. Genetic factors play a role in the immunological responses to stress. In contrast to the effects of acute stress, NK activity of mice exposed to chronic inescapable footshock was not reduced from control levels. Adaptive processes may be invoked during repeated stress exposure, thereby limiting the potentially damaging effects of the stressor.