Learning Objectives
By the end of this section, you should be able to
- 17.3.1 Discuss how the immune system can be conditioned via learned associations.
- 17.3.2 Describe the mechanisms by which the hypothalamic-pituitary-adrenal axis and immune system communicate with each other.
- 17.3.3 Describe the mechanisms by which the autonomic nervous system communicates with immune systems.
The interactions between the immune system and brain are not a one-way street but move in both directions. Thus, our behaviors impact how our immune systems function. One particularly notable piece of evidence for brain regulation of immune function is that it turns out that learning a new association can impact how the immune system works in the body, for instance the levels of antibody in the bloodstream. Here, we’ll discuss some of the ways behavior—and for this we will define it as a perceptual process that begins in the brain and manifests as an action or output—can profoundly impact how our immune system works. In the process, we will further appreciate the two-way nature of these interactions, where the brain and immune system are constantly updating and influencing one another.
Behavioral conditioning of the immune system
How do we know that the brain talks to the immune system? In classical (or “Pavlovian”) conditioning in behavioral neuroscience, a previously neutral stimulus like a light or a bell (the conditioned stimulus) becomes associated with a particular outcome like fear-induced freezing (the conditioned response) because it is paired, over a few associations, with something like a foot shock (the unconditioned stimulus) (see Chapter 18 Learning and Memory). It turns out that our immune systems “learn” associations too. More accurately, our brains learn associations that are relevant for the immune response, and this results in a shift in its function in response to previously innocuous or irrelevant cues. An example—just as we can condition rats to “freeze” when they hear a bell because it signals a potential foot shock, it is possible to pair a particular tasting beverage with a change in antibody production. What?! Here’s how it works.
If you give a rat (or a person) a drug called cyclophosphamide and then inject the rat with a novel antigen that they would normally make antibodies against, the rat will show decreased production of the antibodies. This is because cyclophosphamide is an immunosuppressant drug used in some cancer therapies. If you give a rat some chocolate milk to drink alongside cyclophosphamide and then inject it with an antigen, it will again produce fewer antibodies. This is perhaps expected; chocolate milk itself should not change the immunosuppressant properties of cyclophosphamide. But here’s where it gets interesting—Later, if you give the rat chocolate milk alone, the rat will exhibit immunosuppression and produce fewer antibodies to the same antigen (Figure 17.15). After chocolate milk!
Experiments where stimuli get coupled to immunosuppressants work in humans too. These types of experiments, when they were first done in the early 1990s, were groundbreaking because they showed without question that the brain and the immune system must meaningfully talk to each other (Cohen et al., 1994). At that point, the race was on to determine the mechanisms underlying this crosstalk, which we cover below. But what are the larger implications of this type of conditioning? Is this just a weird party trick? It turns out the implications could be profound. For instance, one big problem in the treatment of many cancers is that patients experience nausea in response to the chemotherapeutic treatments. Worse yet is that the patients begin, because of classical conditioning, to associate the very environment in which they receive the treatments with nausea, even if they are no longer receiving any drug. This type of nausea can be completely debilitating, and for this reason some medical centers have begun to randomize the rooms in which patients receive their treatments in order to limit these associations. More troubling is the possibility that a given environment could on its own induce immunosuppression, much like that chocolate milk. This would be devastating for already immunocompromised patients during their recovery.
The good news is that just as the immune system can be suppressed via association, it can also be enhanced. There is evidence in rodents that transient activation of the so-called reward circuit of the brain, the ventral tegmental area, can augment immune responses like antibody production, bacterial killing, and even anti-tumor activity in the periphery (Ben-Shaanan et al., 2016, 2018). These results raise the possibility that such enhancement could be amenable to classical conditioning as well. Indeed, repeated pairings of a small amount of a novel antigen with chocolate milk can boost antibody production to the later presentation of the chocolate milk alone (Ader et al., 1993). This, in a way, is the opposite of the experiment where chocolate milk was coupled with an immunosuppressant drug. In this case, we pair the presentation of the antigen with the chocolatey treat, making the chocolatey treat itself stimulate antibody production (Figure 17.16). Once again, the implications for these types of associations are profound, and they likely contribute to the “placebo effect” (Belcher et al., 2018), in which patient beliefs regarding a potential treatment are often just as powerful in causing a biological effect -in this case a change in the immune response- as the drug itself (see Chapter 9 Touch and Pain). This often doesn’t mean the drug doesn’t work, but rather that our neuroimmune link works well!
Stress and the immune system
What if I told you that stress is good for you? Specifically, stress is good for your immune system. First, let’s define some terms. A stressor is the thing that induces a biological response (like running from a raging bull or having to give a presentation in class). We call this biological response in its entirety a stress response. A major portion of the stress response is coordinated by the hypothalamic-pituitary-adrenal (HPA) axis. Figure 17.17 gives an overview of the HPA axis and its connections to the immune system and cytokines. Chapter 12 Stress covers the HPA axis in more detail. As a brief overview, during a stress response, hypothalamic cells release corticotropin releasing hormone (CRH) onto the pituitary gland. The pituitary gland, in turn, secretes and releases hormones such as adrenocorticotropin releasing hormone (ACTH) into the bloodstream. ACTH induces the release of glucocorticoids from the adrenal cortex (Turnbull and Rivier, 1999).
Blood concentrations of circulating glucocorticoids increase in response to virtually any type of stimulus that poses a threat to bodily homeostasis, including immune system activation. During infection, cytokines profoundly activate the HPA axis. Cytokines produced by immune activation, such as IL-1β, IL-6, and TNFα, can act directly within the brain to induce the release of CRH via direct activation of neurons within the hypothalamus to start off the HPA cascade (Berkenbosch et al., 1987; Sapolsky et al., 1987). One of the first observations of brain-immune communication came from an experiment demonstrating that systemic administration of LPS causes an increase in the stress hormone, known as corticosterone, in rodents (Wexler et al., 1957). Since then, it is well understood that the release of stress hormones is an important component of immune activation.
Stressors mobilize the immune system, sort of like rallying the troops. If we consider the body as a battlefield (which from a pathogen’s perspective, it is!), we can think of immune organs like the spleen and lymph nodes as the barracks, where immune cell soldiers hang out playing cards and wait for a call to battle. When a stressor is perceived (which can be more “physical” like preparing to run from a bear, or “psychological” like sitting in traffic, late for class), the glucocorticoids that get released (like cortisol in humans) also bind to white blood cell soldiers in the barracks (mostly monocytes and neutrophils) which immediately jump into action, pour out of the spleen and lymph and into the bloodstream, and then crawl their way into the “battle stations” of the body—the skin, lining of the gut, and the lungs (Figure 17.18) (Dhabhar et al., 1996).
Why do immune cells pre-populate certain body areas just in response to stress hormones? Because these are the areas most likely to be injured, and subsequently infected, in response to a stressor (at least the types of stressors that we evolved to be scared of, like bears). The immune cell soldiers are poised in just the right location to respond. Indeed, if you stress a rodent prior to measuring an immune response in the skin, there is a larger inflammatory response (swelling) in response to a minor skin irritant compared to a rodent that was not stressed. This exaggerated response occurs whether you expose a rodent to cat urine odor alone (technically harmless) or an actual cat (potentially very harmful!). The important part is the stress perception in the brain is transmitted to the peripheral immune system in the form of inflammation, which is beneficial in the face of injury or infection.
The immune system and the “stress” system have co-evolved, and one doesn’t work very well without the other. In fact, I would argue that it is impossible to have an immune response without also generating a stress response, and vice versa. But this response is again a Goldilocks phenomenon—the immune system works well when the amount of stress is just right, and not too low or too high. Chronic, uncontrollable, or unpredictable stressors can be extremely harmful for the immune system, and lead to immunosuppression, disease or pathology (Dhabhar and McEwen, 1999). Some of the earliest examples of this impact came from studies of human caregivers that experience a high burden of stress due to caregiving duties, e.g. of family members suffering from dementia. When given a small, experimentally-induced wound in the skin (using a biopsy needle), caregivers heal much slower than non-caregiver controls (Kiecolt-Glaser, et al., 1995). Similarly, dental school students healed punch biopsy wounds more slowly when exams were imminent, compared to more relaxed times of the semester (Marucha et al., 1998). In observational studies in humans, a longer recovery time after surgery is consistently observed for patients reporting higher levels of anxiety or perceived stress pre-surgery (Rosenberger et al., 2006). This pattern holds true for undergraduates as well—students who received a small wound to the hard palate of the mouth healed more slowly if they reported high levels of depression (Bosch et al., 2007).
Chronic stress can also lead to changes in anxiety via changes in immune function. In one study in mice, chronic stress led to a pathological “programming” of T cells which instructed them to enter the CNS and attack myelin, similar to what you see in some autoimmune disorders, and led to chronic anxiety (Fan et al., 2019; Bordt and Bilbo, 2020). In a series of other studies, again in mice, chronic defeat stress (which essentially entails frequent, forced interactions with a big bully mouse) led to monocytes leaving the spleen, which travel not to the battle stations but to the brain, where they interact with neurons and microglia and stir up trouble, resulting in chronic anxiety (Weber et al., 2017; McKim et al., 2018). Blocking the entry of monocytes into the brain completely prevents the increase in anxiety. The reasons why monocytes and T cells get recruited to the brain (and not more helpful regions like the skin) in response to chronic stress in these studies is still not clear but is an active and important area of research in the field. In sum, there is a delicate balance when considering the impact of stressors on the immune system and the overall wellbeing of the host. But defining chronic stress can be surprisingly tricky, as different individuals respond quite distinctly to different stressors.
Exercise as a “good stressor”
One useful approach is to think about what stressors we evolved to confront, which are presumably bears and charging bulls, and not sitting in traffic. We increasingly understand that it is the resolution of a stress response that is key to its impact on our health, rather than its magnitude. That is, when the stressor is gone, our stress hormone levels need to drop back to normal and all those immune cell soldiers in the battle stations need to call it a day and go back home to their barracks. When we confront a physical stressor, we typically experience a physical resolution to that stressor (for example, the bear is gone and we stop running). This physical resolution helps signal for production of stress hormones to stop and calls our soldiers home. In contrast, psychological stressors may not involve any defining physical moment of resolution at all—they are profoundly “stressful” but the resolution is less clear. In this case, with resolution lacking, our soldiers never go back home, but stay on the front lines, getting bored and causing problems. Indeed, our modern stressful lifestyles and relative lack of activity are repeatedly associated with inflammation in our arteries and organs, which is very harmful for our health.
Fortunately, it turns out that bouts of physical activity (i.e. exercise) can help create that moment of resolution to more diffuse psychological stressors, just as if we had been running from a bear and then stopped when we successfully escaped. One reason that exercise, particularly “cardio” exercise, is so good for us is because it is, essentially, an acute stressor. When we exercise, we see a ramping up of our immune system very similar to any other stressor, including immune cell deployment to battle stations. Indeed, in one very exciting study in humans, light to moderate exercise (a brisk walk or jog) immediately after receiving a vaccine for either influenza or for SARS-CoV-2 increased serum antibody levels to each vaccine 4 weeks later (Hallam et al., 2022). Importantly, there were no impacts on vaccine side effects. Another study in mice demonstrates that a mild stressor prior to injection with a novel antigen (equivalent to a vaccination) resulted in greater antibodies to that antigen even 9 months later (Dhabhar and Viswanathan, 2005)! Perhaps the next time you go to get your flu shot you should run around the block a few times first! Importantly, at the resolution of the exercise/stressor, the soldiers go home. In fact, exercise may be critical in conditioning our immune system to resolve and return to homeostasis following activation. Over time, this has tremendous benefits for our health.
Autonomic nervous system control of immune function
Above, we discussed how the hormonal response to stress coordinates immune-brain crosstalk via the blood. The hormones and cytokines involved comprise a “humoral” route of communication. There is also a major “neural” route, in which the nervous system directly connects to the visceral organs of the body via the autonomic nervous system.
The autonomic nervous system (ANS) organizes interactions between the CNS and endocrine and immune organs (Chapter 1 Structure and Function of the Nervous System: Cells and Anatomy) (Figure 17.19).
As a review, recall that the ANS controls visceral body functions and innervates peripheral glands, and can be further divided into the sympathetic nervous system, which controls the fight or flight responses, and the parasympathetic nervous system, which controls functions under basic arousal such as digestion. The sympathetic nervous system is important in “ramping up” peripheral responses such as heart rate, and it does something similar to the immune system (Figure 17.20). For instance, during an infection, cytokines can bind to sympathetic fibers and this signals to the brain to mobilize the immune system in the periphery to fight the infection. The cells of the immune system (lymphocytes, monocytes, etc) possess receptors for many neurotransmitters and neuroendocrine mediators—many of these neurohormones are produced by the nervous system and thus allow the immune system to react to neural regulation. For instance, sympathetic activation of leukocytes via norepinephrine (NE) along with cortisol is the mechanism underlying leukocyte trafficking from the barracks to the battle stations that we discussed earlier.
Whereas the sympathetic system is especially important for ramping up, the parasympathetic system is important for restoring homeostasis after an inflammatory challenge or stressor is initiated, primarily via the actions of the vagus nerve. Vagal efferents from the brainstem innervate the heart, gastrointestinal tract, liver, biliary system, and pancreas, among numerous other tissues (see Figure 17.19) (Berthoud and Neuhuber, 2000). The vagus nerve is also the primary nerve to innervate the major immune organs in the periphery, including the thymus, spleen, lymph nodes and bone marrow (see Figure 17.21). Thus, it can directly and discretely affect immune function at the level of an individual organ or localized tissue.
The vagus nerve also transmits information regarding the state of the periphery to the brain. In fact, more than 80% of vagal fibers are afferent, conveying sensory information from the body to the brain. Some of that input is used to help initiate response to an infection–parasympathetic fibers throughout the body respond to signs of infection and further stimulate sympathetic activity, working in concert with the direct stimulation of sympathetic fibers by cytokines. For this reason, it’s been suggested that the vagus nerve is the basis of our “sixth sense” and the reason that we have a “gut feeling” we may be sick, before we actually have any symptoms (Blaylock 1995). The importance of these innervations from the periphery to the brain has been demonstrated in many experiments which use surgical division of the vagus nerve, known as a vagotomy. Vagotomy blocks the induction of fever caused by peripherally administering the cytokine IL-1 or low doses of LPS. Vagotomy can also attenuate the sickness behaviors, e.g. decreases in behavioral exploration, caused by peripheral IL-1 administration (Luheshi et al., 2000; Hansen et al., 2001). Thus, the brain has the capacity to directly influence the peripheral immune system through specific neural pathways during peripheral immune activation and in response to all types of physiologically stressful situations.
The vagus nerve and the inflammatory reflex
Given its widespread distribution and localized innervation of specific tissues, the vagal afferent system is in an excellent position to convey immune-related stimuli to central areas via a so-called inflammatory reflex pathway (Berthoud and Neuhuber, 2000; Tracey, 2002). Just as a motor reflex within the body involves a sensory perception of pain or trauma and a rapid motor response, e.g. limb withdrawal to avoid that stimulus, the inflammatory reflex essentially describes the phenomenon that an inflammatory response mediated by parasympathetic afferent signaling and subsequent sympathetic activation is quickly followed by an anti-inflammatory response that returns the system to homeostasis. For instance, let’s say that vagal afferents in the gut are activated by cytokines due to an infection, as shown in step 1 of Figure 17.22. This activation quickly sends a signal up to the brain which subsequently sends a sympathetic signal back to organs like the spleen, which activates cells like T cells and macrophages for deployment as described earlier (step 2 in Figure 17.22). This response is rapid, like a reflex. This immune activation/inflammation is short lived however, because within minutes to hours, the vagus nerve once again jumps into action, which culminates in the release of acetylcholine (ACh) that suppresses inflammatory cytokine production by macrophages (step 3 in Figure 17.22).
This “cholinergic anti-inflammatory response” is critical to prevent excessive immune activation, which could lead to organ failure and even death if not carefully held in check. For many years, however, the mechanisms by which this anti-inflammatory signal is received by the immune system was a mystery. The reason comes down to anatomy. It turns out that the splenic nerve which directly innervates the spleen produces norepinephrine (NE), not ACh. How, then, do macrophages in the spleen receive the ACh signal to stop producing cytokines? The answer is…wait for it…T cells! Truly groundbreaking experiments by Dr. Kevin Tracey completed in the early 2010s demonstrated this ingenious system (Rosas-Ballina et al., 2011)– during an immune response, sympathetic activation produces NE to rapidly activate macrophages. Shortly thereafter, the vagus nerve signals the splenic nerve to also produce NE which binds to a specialized subset of T cells that slowly ramp up production of ACh. This ACh binds to acetylcholine receptors on nearby macrophages to quickly shut off inflammatory cytokine production before it becomes dangerous (Figure 17.23). This system is a beautiful example of neuroimmune crosstalk that has stimulated a flurry of promising research designed to harness the vagus nerve “anti-inflammatory reflex” for the treatment of multiple disorders, including pathological pain and autoimmune conditions.