Low Cerebrospinal Fluid Beta Amyloid1-42 in Patients with Tuberculous Meningitis

Abstract


Background
Central nervous system (CNS) infections are uncommon diseases characterized by signi cant morbidity, disability and mortality.Tuberculous meningitits (TBM) is the most severe manifestation of extrapulmonary infection by Mycobacterium tuberculosis.It is characterized by a slowly progressing granulomatous in ammation of the basal meninges, an in ammatory reaction that can lead to complications such as hydrocephalus, cerebral vascular infarction, cranial nerve palsy and, if untreated, death.Vulnerable populations are at higher risk of infection and complications.Rapid diagnosis and initiation of treatment is therefore necessary to reduce the high mortality and severe sequelae associated with the disease.Diagnosing TBM can be di cult as the symptoms are non-speci c and they mimic other infections or vascular disorders.The identi cation of speci c plasma or cerebrospinal uid (CSF) biomarkers may be relevant for an early diagnosis and for prognosis.TBM is traditionally characterized by CSF pleyocitosis, increased proteins, decrease of glucose concentration.Although other CSF biomarkers have been investigated, none has reached clinical practice.S100b, NSE (neuron-speci c enolase) and interleukynes have been advocated to be predictive of disease's severity and outcome [1-3, 29, 30].Recently, several infectious agents have been called out to be possible triggers in causing neurodegenerative diseases, especially Alzheimer Disease (AD).The total burden of infectious agents has been linked to the development of AD in sporadic cases [16]; this appears to be substantially due to microglia activation [24], long acting in ammation neuronal alteration, oxidative stress and amyloid-beta accumulation but also to a direct effect by infectious agents.Speci cally, viruses from the Herpesviridae family have since long been called out to play a decisive role (together with APOE phenotype) [9,15,33] in affecting disease onset and clinical progression.Other bacteria have also been suggested to have a causative role including Spirochetaceae, Chlamydia and Gram-negative bacteria [26,31].Recent reports suggest also a role for parasites in stimulating different pattern of in ammation [27] and no data have been outlined for fungi.Beside this, amyloid-beta has also been identi ed as a protein acting as an antiinfective peptide playing a direct role in the clearance of different infections in various animal models [7].
HSV6 appears to be capable of directly enhance the seeding and acceleration of amyloid-beta deposition despite a debated pathogenic potential [10].Following important reviews [26,32,36], this suggestive hypothesis could link the accumulation of amyloid-beta during infection and the subsequent development of neurodegenerative disease.Aim of this analysis was to study the CSF concentrations of several biomarkers in patients with TBM

Methods
We collected cerebrospinal uid samples from patients among hospitals of Turin between 2001 and 2018, undergoing lumbar puncture (LP) for clinical reasons.All of them were morning LP.Patients signed a written informed consent for CSF withdrawal, storage and analysis.The retrospective analysis of the collected data was approved by the Ethics Committee (Città della Salute e della Scienza, Ospedale Molinette, RetroNEG Protocol, n 0094995, October 4th 2017).

Discussion
In this small case study we measured several cerebrospinal uid biomarkers in meningeal tuberculosis.
We con rmed the presence of classical TBM CSF ndings such as BBB impairment, in ammation and report here, for the rst time, very low level of Aβ1-42 [1,2,3,21,28].Neuronal damage is a classical feature of TBM due to its devastating in ammation and disruptive process.14.3.3positivity was found in 5/13 (38,5%) of TBM; this cellular-cycle protein, previously associated with prionic disease, accumulates in the CSF after neuronal damage especially during bacterial involvement of CNS and it is cleared from the CFS after successful treatment [4].BBB impairment and IgG synthesis were observed; CSAR and IgG ratios were high in TBM, con rming results in literature where a signi cant impairment in BBB due to TBM is described [3,21].A raised level of neopterin can be found in TBM, denoting intrathecal production by macrophage-derived cells and, as the BBB has a low permeability for peripheral neopterin, it represents a relevant index of local in ammation.[4,8,9] Moreover, we found out that classical markers of TBM disease activity had a good correlation with Aβ1-42: low glucose and higher cells correlates with lower amyloid, BBB damage expressed by CSAR, as well as FTau, resulted higher in lower Aβ1-42 [5,21].These ndings outline the possibility for amyloid-beta of being a good proxy of precocious disease activity and a potential marker to follow over time.Also, lower Aβ1-42 level was associated with worse outcomes, thus suggesting a possible prognostic of this marker in clinical practice.Additionally, the observation of low levels of Aβ1-42 in patients with TBM is of potential interest and should be interpreted in the context of the recent discovery of a possible antimicrobial role of amyloid-beta [7,12,25,35] and of a hypothetical infectious "trigger" for Alzheimer Disease [6].Amyloid-beta protein seems to be shed and playing an anti-infective role in response of several infections in a murine model [14].In vivo low levels of CSF amyloid-beta have been observed in patients with pneumococcal meningitis and other bacterial meningitis [28,11] That is critical because observing amyloid metabolic alterations during TBM is perhaps the key passage for understanding amyloid's antimicrobial role.This may show how amyloid metabolism is potentially altered by several infections, as seen for HSV6 and 7 that have been recently associated with development of AD, probably playing an important role in driving alterations such oxidative damage and progression to accumulation of neuro brillary tangles.
Several mechanisms regarding the nding of low Aβ1-42, besides amyloid deposition in the brain parenchyma, can be hypothesized.Amyloid-beta levels could be reduced because of the interaction of amyloid-beta fragments with albumin, usually elevated in CSF TBM, thus lowering levels of the free peptide.Additionally, Aβ1-42 can cross the BBB by leaking in CNS and then accumulating (even if it is known that in peripheral tissues Aβ1-40 is prevalent), in the context of increased permeability, thus being lower in the CSF/CNS.Data on the potential measurement of serum amyloid-beta peptides in the setting of Alzheimer's dementia may con rm this hypothesis [8, 21,22].Another mechanism could be an impaired and reduced amyloid-beta clearance from the CNS [11]: the ISF/CSF ow is believed now to be partially convective and through perivascular spaces that can be harmed during tubercular infections of the CNS and systemic in ammation [24].That could be particularly relevant following the recent discovery of the so called Glymphatic Central Nervous System [19,23].TBM it is known to affect the basal anatomic section of the brain with a reduced CSF recirculation, a brosant effect and a possible central hypertensive syndrome.In view of these observations it is possible that even the glymphatic recirculation is altered; unfortunately, data are scarce and there are no reliable markers up to date.
To conclude, the analysis regarding the time to normalization for Aβ1-42 in our population deserves an additional remark: relying on our data, only three patients normalized amyloid-beta during follow-up.Patient 4 at day 22, patient 1 at day 190, patient 3 at day 267 (Fig. 3).Acknowledging that data are limited and we were not able to measure these equally for all patients, it is still of great interest that the vast majority of patients did not normalize amyloid-beta while hospitalized nor under treatment; moreover, the time to normalization was not homogenous between patients suggesting a persistent and unpredictable ongoing accumulation and probable undergoing slight but constant and enduring in ammation, which is coherent with TBM physiopathology and such a life-threatening condition.Following a recent article and debate [29,30], could be tested as a prognostic marker in both pediatric and adult population, backing the routine use of available neuromarkers for both a better tailored approach to patients and in research.An adjunctive information may come from retesting Aβ1-42 levels at the end of therapy (one-year follow-up retesting).To our knowledge, this is the rst case showing such low levels of Aβ1-42 in TBM; its accumulation, drove by neuroin ammation related to infections, can be central in understanding neurodegenerative diseases.This study has several limitations: study design lacking an age-matched or Alzheimer's control group, sample size, impossibility to perform homogenous number of LP at follow-up for all patients and incomplete data on Neuro laments (NFL).Nevertheless, the nding of low Aβ1-42 concentrations warrant further analysis in controlled settings.

Conclusions
CSF Biomarkers from patients with TBM were compatible with in ammation, blood brain barrier damage and impairment in beta amyloid metabolism.Aβ1-42 could be tested as a prognostic marker, backing the routine use of available neuromarkers for both a better tailored approach to patients and in research.
To our knowledge, this is the rst case showing such low levels of Aβ1-42 in TBM; its accumulation, drove by neuroin ammation related to infections, can be central in understanding neurodegenerative diseases.Further studies are needed in order to understand the relevance of these observations List Of Abbreviations TBM Tuberculous meningitis; NSE neuron-speci c enolase; AD Alzheimer Disease; CNS Central nervous system; LP lumbar puncture; BBB Blood-brain barrier; CSF cerebrospinal uid; HHV6 Human Herpes Virus 6; Aβ1-42 1-42 Amyloid-βeta.