Not a Crazy Idea: TLRs Implicated in Psychiatric Disorders

Innate immunity lays the foundation of the immune response and provides the first line of defense against invading pathogens. Toll-like Receptors (TLRs) are an essential aspect of this response, think of them as security guards on the outside of a cell. TLRs belong to a class of receptors which recognize unique, yet general, patterns on bacteria, viruses, or even fungi, making them members of the pathogen-recognition receptor (PRR) group. TLRs generally reside on leukocytes, mostly monocytes, dendritic cells (DCs), and neutrophils. When a pathogen binds to the receptor an intracellular signaling cascade is initiated which leads to the activation of Nf-kB, a pro-inflammatory transcription factor. Simply put, NF-kB leads to the production of cytokines, immune messengers, which can affect other immune cells and promote inflammation. The inflammatory response is characterized by localized swelling caused by vasodilation, an increase in diameter of the blood vessels. This response allows more leukocytes to travel to the area of invasion to remove the pathogen.

When asked to imagine the immune response the first thing which comes to the mind of most is the illness, usually a cold, virus, or something more severe, such as HIV. However, recent research has linked TLR deregulation to two psychiatric illnesses, schizophrenia and bipolar disorder. These are novel, intriguing directions for immunological research. Schizophrenia affects approximately 1% of the population and is characterized by positive symptoms, such as hallucinations and delusions, and negative symptoms, namely social withdrawal and lack of an emotional response. There’s no clear cut factor which causes schizophrenia but there are several genetic factors and prenatal effects. For example, there is increasing evidence that bacterial and viral infections which may occur in utero may have lasting effects on the developing fetus. Any sort of maternal immune response may lead to the production of cytokines in the mother which may cross the placenta and even the infant’s blood-brain barrier, leading to neurodevelopmental problems which may present as Schizophrenia. On the other hand, bipolar disorder affects roughly 2% of the population and is characterized by drastic mood swings. Previous research in both disorders has revealed heightened cytokine levels in both.

Work by McKernan et al. (2011) examined the role of TLRs in these disorders and revealed interesting results. Overall, the study was simply designed but yielded practical insight into the immune response in these disorders. The authors took whole blood samples from 40 healthy controls (HCs), 40 Schizophrenia patients, and 20 Bipolar patients, making sure to exclude subjects who had any sort of immune disease or immunodeficiencies. The samples were centrifuged, spun, to separate the plasma from the whole blood layer. The blood samples (containing the white blood cells of interest) were then cultured either with or without a TLR agonist, the specific molecule which binds to and activates the TLR. There are ten different TLRs so many agonists were used. After a period of time the fluid from the culture, the supernatant, was extracted for analysis of cytokines unsing an enzyme-linked immunosorbent assay (ELISA). Simply, ELISA’s usually contain an antibody specific to a cytokine of interest tethered at the bottom of a cell plate. After a few more steps the authors could use a detected signal to indicate the quantity of cytokine which was present in that particular culture.

There are many different classes of cytokines, and the authors primarily examined interleukins (ILs) and Tumor-necrosis factors (TNFs). For instance, in blood cultures which were not treated with TLR agonists, IL-8, IL-6, and TNFalpha were elevated in the schizophrenia and bipolar disorder groups compared to HCs. However, the meat of their results concern the TLR agonists and their effect on the cytokine release in the HCs compared to both psychotic disorders. When the authors stimulated TLR2 with HKLM (heat-killed Listeria monocytogenes) there was a significant increase in IL-1B in both psychotic groups compared to the HCs. Also interestingly, the IL-1B release was significantly higher in the bipolar group over the schizophrenia group. Treating the blood cultures with lipopolysaccharide, a component of the bacterial cell wall, activated TLR4 and led to greater IL-1B release in both psychotic groups compared to HCs. Plus, when using ssRNA to stimulate TLR8 the authors found greater IL-1B release in the psychotic groups once again. Interestingly, when also using ssRNA, there was increased TNFalpha release in only the bipolar group compared to the HCs. Yet, when using ODN2006 (oligodeoxynucleotides~think DNA) to agonize TLR9 there was elevated IL-8 release in only schizophrenics compared to HCs. The agonists used for TLR1, TLR3, TLR5, TLR6, and TLR7 showed no significant effect between the groups, suggesting the effect may be specific to only a subset of TLRs.

At the end the authors let the reader know what directions they plan on pursuing in the future. One question they hope to answer is if any of the agonists they used in the experiment exist endogenously in those with schizophrenia and bipolar disorder. Personally, I believe examining receptor density on white blood cells may be an attractive question for them to pursue. An up-regulation of certain TLRs may lead to greater cytokine release. One suggestion they posit to explain their findings is that increased cytokine release can lead to dangerous inflammation which may harm early neural plasticity and development. Another idea is that the increased cytokine release is that individuals suffering from these disorders are under increased stress which may affect the immune system. Their hypothesis is supported by Bailey et al. (2007) who found that TLR2 and TLR4 increased in expression on macrophages after mice had been in involved in a social defeat-stress paradigm. This type of social defeat also has been shown to activate DCs in the spleen and increase the expression of TLRs on these cells (Powell et al. 2009). Overall, the authors believe their results are practical; if psychiatric disorders have a unique cytokine and expression profile then it may become easier to distinguish between disorders. Most importantly, before we get too excited, the authors mentioned inherent limitations in their study. For one, the lack of drug-free patients may have confounded some of their results, especially anti-psychotic medications. While these medications should not have a large effect it can’t be said for sure. Lastly, they state epigenetic effects may explain why in some cases one disorder had a larger cytokine release than another.

These results are invaluable. For one, they provide stronger support for the neurodevelopmental hypothesis of Schizophrenia. Also, since cytokines are one of the few molecules capable of getting into the brain there are many more directions the authors could take. For instance, radiolabeling experiments could be used to examine where many of these cytokines are acting in the brain and on what. Therapeutically, immunosuppressant drugs aimed at dampening the inflammatory response may be examined for their potential to mitigate symptoms of both disorders. Yet, as their name suggests, these drugs may also make the immune system prone to more opportunistic infections. The main question in this study was simple yet novel and the authors examined it in a direct way. This approach makes their work and findings clearer. By expanding the domain of immunology into psychiatric disorders the authors are helping to lead more interdisciplinary research in the future.

McKernan DP, Dennison U, Gaszner G, Cryan JF, and Dinan TG (2011). "Enhanced peripheral toll-like receptor responses in psychosis: further evidence of a pro-inflammatory phenotype." Translational Psychiatry. 1, e36, published online August 30, 2011.

Other References

Bailey MT, Engler H, Powell ND, Padgett DA, Sheridan JF. Repeated social
defeat increases the bactericidal activity of splenic macrophages through a
Toll-like receptor-dependent pathway. Am J Physiol Regul Integr Comp Physiol 2007;
293: R1180–R1190.

Powell ND, Bailey MT, Mays JW, Stiner-Jones LM, Hanke ML, Padgett DA et al. Repeated
social defeat activates dendritic cells and enhances toll-like receptor dependent cytokine
secretion. Brain Behav Immun 2009; 23: 225–231