In our study for the first time we showed significant changes in the composition of fatty acids and endocannabinoids in the course of neuroinfections of various etiologies. Until now, the assessment of FA composition in CSF has been evaluated in few studies, but the evaluation has never been carried out in the course of neuroinfection. It was not known whether FA as well as ECB in the CNS during infection play a similar role as has been shown during infection in other parts of the body. Our study again demonstrates the important role of FA and their derivatives in the course of central nervous system pathology. There are hypothetically two mechanisms which could explain the changes in CSF composition observed in our study.
Fatty acids contribute to the systemic and local immune response
Polyunsaturated fatty acids are essential for brain homeostasis and functions and are the main components of neuronal membranes (Kong et al. 2011; Calder 2016). FA metabolism may be an important mechanism underlying rapid neuroinflammation in pathologic processes such as neuroinfections.
The first mechanism would result from the fact that FA contribute to the systemic and local immune response triggered by pathogens that reached the CNS, as such changes in FA as well as in ECB in CSF would be a reflection of the ongoing inflammatory process in the CNS. In our study, we showed a significant increase in n-6 FA in the course of neuroinfection compared to the control group, this effect occurred regardless of etiology. The strongest increase of n-6 FA, mainly C18:3 (gamma-linolenic acid, GLA), C20:2 (dihomo-gamma linolenic acid, DGLA), C20:3, C20:4 (arachidonic acid, AA) and C22:2 (adrenic acid) was observed in CSF from VM patients. DGLA, which is product of GLA conversion by elongase 5 (ELOVL5), can be enzymatically transformed to several metabolites with anti-inflammatory properties (Sergeant et al. 2016). DGLA may be also a precursor of arachidonic acid (AA). As a result of AA enzymatic oxidation, prostaglandins, leukotrienes, and lipoxins are formed and may generally promote inflammation (Serhan and Petasis 2011; Norris and Dennis 2014). Results obtained in our study suggest that n-6 FA significantly attenuate in an neuro-inflammatory responses. We also observed cluster characteristic grouping of n-6 fatty acids in patients with BM and VM. Similar to the changes in the n-6 FA in both neuroinfection groups, there was a decrease in n-3 FA in CSF in relation to healthy people. The strongest decrease was observed in the precursor for n-3 FA: linolenic acid (ALA, C18:3 n- 3) and in eicosapentaenoic acid (EPA, C20:5 n-3). The deficit of n-3 FA in cerebrospinal fluid from patiens with neuroinfections when compared to the control was seen also after cluster analysis. ALA is a precursor for n-3 series, including EPA or DHA, they show strong anti-inflammatory effects (Gdula-Argasińska et al. 2016; Wysoczański et al. 2016). Inflammation is an important element of the body’s fight with infection, however, the body also has effective mechanisms to quench the inflammatory reaction. The lack of such a mechanism and an excessive pro-inflammatory reaction would pose a significant threat to the homeostasis of the body. The process of resolution of inflammation itself does not have an immunosuppressive character, as several pro-resolving mediators increase survival from different infections. n-3 FA are one of the elements of such extinguishing, hence the observation in our study. A reflection of the changes in the n-6 and n-3 FA is more than three-fold reduction of n-3/n-6 ratio, this effect was observed for both types of neuroinfections studied.
The changed FA profile in CSF appears to be caused by the synthesis of lipid mediators, including eicosanoids, generated during inflammation by microglia and astrocyte cells. These changes may also be related to the biosynthesis of endocannabinoids, as well as being the result of a diversified expression of genes involved in the synthesis, elongation and desaturation of FA during the inflammatory process (Serhan and Petasis 2011; Gdula-Argasińska et al. 2016). Endocannabinoids are lipid mediators which play an important physiological role via cannabinoid receptor (CB1 and CB2) activation and signalling. Both DHEA and EPEA have anti-inflammatory properties and have been detected in both the brain and retina (Gdula-Argasińska and Bystrowska 2016; Wysoczański et al. 2016; McDougle et al. 2017). In our study we observed the highest content of DHAE, EPEA and NEA in the CSF of patients with VM. In samples from BM patients the ECB level was lower. ECB synthesis during neuroinfection is probably one of the mechanisms by which inflammation is resolved.
The dense clustering of the different cell types during inflammation presents a unique situation for lipid handling. In contrast to the synthesis of protein mediators (cytokines), lipid mediators can be produced along enzyme pathways that involve multiple cells, including microglia, in a process known as trans-cellular biosynthesis (Serhan and Petasis 2011; Norris and Dennis 2014; Johnson 2015). The inflamed tissue becomes a specialized organ for lipid metabolism, producing the types and amounts of lipid mediators needed to promote or resolve inflammation. However, the higher content of long-chain MUFA and PUFA in CSF obtained from patients with BM and VM may be correlated with higher expression of stearoyl-coenzyme A desaturase (SCD1) as well as elongases (Gdula-Argasińska and Bystrowska 2016; Sergeant et al. 2016).
Changes in fatty acids in CSF as a consequents of BBB permeability
The second mechanism which may explain the results observed in our study is passive FA penetration from the plasma through the damaged BBB as a result of the neuroinfection. Under physiological conditions, the BBB provides effective protection of the nervous system from external factors. The transcellular transport across the BBB involves diffusion and active transport with the use of carrier membrane proteins. The diffusion process transports mainly small-molecule nutrients and fat-soluble substances. Peptides and regulatory proteins as well as protein hormones penetrate the BBB via active transport with protein carriers. The transported substances are not transmitted directly to neurons, but to glial pellets. The astrocyte protrusions cover about 90% of the surface of the capillary walls. These cells constitute the second component, also crucial for the effectiveness of the physical barrier. They are links between the capillary wall and neurons (Ballabh et al. 2004; Nakagawa et al. 2009). The exact characteristics of the BBB are presented in review by Sorge et al. (van Sorge and Doran 2012).
CSF is a secretion of the choroid plexuses located in the wall of the cerebral ventricles and periventricular organs. The transport of substances from the blood to CSF is very selective, because glial cells (tanycetes) adhere to the endothelium and functions to assist in taking certain substances (eg. hormones) from the blood and transferring them to the CSF, as well as in the opposite direction (Lossinsky and Shivers 2004). BBB permeability disorders have been described in various neurological diseases, including inflammatory, infectious, neurodegenerative and neoplastic diseases. A special group of diseases that change the physiological properties of BBB include neuroinfections. Under physiological conditions, there is little infiltration of T-lymphocytes and monocytes through the BBB, the situation changes during infectious processes involving the nervous system, during which an increased transmission of cells of the immune system across the BBB is observed (Weiss et al. 2009). Impaired integrity of the BBB is one of the key factors in BM pathogenesis and is also extremely important for the development of VM. This may be due to the toxic effect of pathogens, their products or through the ability to interact with BBB structures, mainly AJs and TJs. For example, group B Streptococcus (GBS) and Streptococcus pneumoniae directly affect BBB by producing spore-forming toxins (Nizet et al. 1997; Zysk et al. 2001; Lembo et al. 2010). As GBS produce more toxins, they are more apt to cause BM (Doran et al. 2003). Many viruses damage the cytoskeletal actin during their life cycle (Taylor et al. 2011). In addition, in the course of induced neuroinfections with various etiologies of the immune reaction, cytokines/chemokines molecules are produced that may interfere with the functioning of the BBB, which in turn facilitates the development of neuroinfections and is associated with worsening the prognosis of such patients (van Sorge and Doran 2012). An example is the increase in systemic expression of TNF-α, which translates into an increase in BBB permeability (Sharief et al. 1992). Thus, with the BBB’s excess permeability increasing in the course of neuroinfection, FA could passively pass into CSF.
In our study, there was no significant difference in majority FA between BM and VM groups. But the general trend of changes in both groups with neuroinfection was both the same and with similar intensity. Significant differences were demonstrated for LA, GLA and DHA content.
In conclusion, FA are an important element of the inflammatory reaction in the course of neuroinfection, as demonstrated by the significant increase in n-6 FA and decrease in n-3 FA as well as significant increase of EPEA, DHEA in CSF during neuroinfections of various etiology in relation to healthy people. A limit of our study was that patients were not supplemented with PUFA, therefore it seems necessary to consider this area in the future.
The process of resolution of inflammation, so important to the health of the patient, is still not fully understood. Therefore, the description of fatty acid impaired metabolism in the course of neuroinfection appears to be an important issue. It may allow, inter alia, to develop new pharmacological strategies, therapeutic aims and modern drugs, essential in the fight against neuroinfections. It appears that fatty acids and their metabolites, which are ligands for peroxisome proliferator receptors (PPARs) and nuclear factor kappa B (NF-ĸB) transcription factors (Gdula-Argasińska and Bystrowska 2016; Qasem et al. 2018) may be used as nutraceuticals in the regulation of brain immune response.