Study of the Activation of Proinflammatory Pathways of Toll-like Receptors in Schizophrenia Patients

Schizophrenia is a mental illness that affects approximately 1% of the population. It supposes an important impact on patients and the socio-health system, since most patients will have difficulty to have an autonomous and independent life and they will need health, economic and social resources for lifetime.

Until recently, the etiopathogeny of schizophrenia was explained by the combination of genetic and environmental factors. However, in recent years there are increasing evidence of the involvement of the immune system in the development and evolution of schizophrenia in at least a subset of patients. Although several meta-analyzes have been published where the increases or decreases in some interleukins and other proinflammatory molecules are described, immune pathways involved in the development and evolution of schizophrenia are still unknown.

The first observations that relate the immune system disorders with the development of non-affective psychosis were made a century ago where it is described the association between pathogen infections (viruses, protozoa or spirochetes) and the occurence of psychosis. Pathogens can directly affect neurons and brain structures, increasing the release of free radicals of oxygen and nitrogen or indirectly altering the activation of the innate or adaptive immune system. They have been published current epidemiological studies that support this theory and they argue that there is an increased risk of having a psychotic decompensation after hospitalization. Furthermore, it is known that many of the genes involved in schizophrenia match those genes involved in the defense against pathogens attack.

Several epidemiological studies have objectified the association of autoimmune diseases and/or chronic inflammatory diseases (intestinal, vasculitis, etc.) with some mental disorders such as schizophrenia, being stress one of the most important triggers factors in the development of both pathologies. Considering that in most cases the onset of autoimmune disease or chronic inflammatory precedes the first psychotic episode, it is hypothesized the autoimmune reaction as a trigger for psychotic symptoms. However, although these studies show statistically significant the involvement of some interleukins (increase in interleukin 6 (IL-6) mainly) in the development or exacerbation of schizophrenia, the levels of interleukins can be altered by other causes such as cardiovascular diseases, obesity and metabolic syndrome; conditions highly prevalent in patients with schizophrenia.

The innate immune system consists of various immune cells (macrophages, mast cells, dendritic cells, microglia, leukocyte, etc.) that recognize pathogen-associated molecular patterns (PAMPs) or damage-associated molecular pattern molecules (DAMPs) through a receptorial system called pattern-recognition receptors (PRRs), located on the cell surface or in cytoplasm. Toll-like receptors (TLRs) are one of the most important families of PRRs. They are transmembrane proteins that have an extracellular domain which binds to the ligands and other intracellular domain which binds to other protein complexes to initiate intracellular transduction pathways. Among the 11 known, the TLRs most studied in schizophrenia has been the TLR-4. Multiple studies have objectified that TLRs are expressed during neurodevelopment in early stages of life and they also have an important role in neurodevelopment and neuronal plasticity during adulthood. Microglia are phagocytic cells located in the central nervous system (CNS) and it is known that they express various types of TLRs. Its role as regulatory cells of CNS is done via intracellular signaling pathways that initiate TLRs. Depending the activated signaling routes, microglia perform different functions: involved positively in the immune response, in neurodevelopmental, in neuronal plasticity and in synaptogenesis, or otherwise through production of proinflammatory cytokines and free radicals of oxygen and nitrogen that favors both neuroinflammation and cerebral neurotoxicity.

Some epidemiological and experimental studies in both animals and humans suggest that infections by viruses, bacteria and protozoa in the prenatal phase (especially in the first quarter) represent a major risk factor for prenatal inflammation, abnormalities in neurodevelopment, obstetric complications and behavioral changes similar to those observed in schizophrenia patients. TLRs can cause neuronal damage in the fetus through two intracellular signaling pathways: increasing cytokine production and/or antimicrobial factors and producing a maintained activation of microglia. Both increased cytokines and antimicrobial factors such as maintained activation of microglia will cause the accumulation of free radicals of oxygen and nitrogen, resulting in neurodegeneration, necrosis and/or apoptosis, which will lead to changes both in the structure and brain function, and in the normal process of child neurodevelopment. It is known that excessive activation of microglia may lead to a reduction in the synapses between glutaminergic neurons in some brain areas such as cerebral cortex, hippocampus and basal ganglia, similar to glutamatergic hypofunction found in brains of schizophrenia patients. In addition, several studies have been published demonstrating the anti-inflammatory and antioxidant properties of antipsychotics, suggesting that its effectiveness is not only due to its action on dopaminergic receptors (classical theory) but they also have a modulatory effect on the immune system.