• E-mail
• Print
• Comment
• Font Size
• Digg
• del.icio.us
• Discuss article

Do Pathogen Exposure and Innate Immunity Cause Brain Diseases?

Posted on: Tuesday, 1 November 2005, 03:01 CST

By Simard, Alain R; Rivest, Serge

It had long been thought that the central nervous system was isolated from the immune system owing to the blood-brain barrier and that this organ was unable to mount an immune reaction of its own when challenged by invading pathogens. It is now clear that the immune system has a profound impact on the central nervous system, because immune molecules found in the blood stream are able to stimulate cells within the brain. Moreover, recent studies have demonstrated that cells within the central nervous system have the capacity to produce molecules of the innate immune system and that this organ is able to generate a proper immune reaction. This topic has been extensively studied in recent years, and it is becoming clear that the innate immune system is an important modulator of the fate of neurons. Indeed, the precise role(s) of the innate immune response in neurodegenerative diseases is currently under intensive debate. In this review paper, we present evidence either supporting or opposing a role for the innate immune response in these events. The mechanisms by which pathogens interact with the brain and whether such an interaction leads to neurodegenerative disorders are also discussed. [Neurol Res 2005; 27: 71 7725]

Keywords: Cytokines; gliogenesis; inflammation; innate immunity; antigen-presenting cells; macrophages; Toll-like receptors

INTRODUCTION

In comparison with all other physiological systems, the central nervous system (CNS) is probably the one in which our knowledge is the most limited. Nonetheless, many scientists devote their careers to the study of neuroscience. The discovery of the blood-brain barrier (BBB) was a crucial step for our understanding of how the organism protects this organ. In essence, the BBB consists of tight junctions between the endothelial cells of the blood vessels located in the brain. For a long period of time, the BBB was thought to prevent the entry of all molecules and microorganisms into the CNS. However, more recent findings have changed our perspective of this dogma, because it has been discovered that certain molecules can indeed enter the brain via its blood vessels.

Another of the most popular frontiers of science is the study of our immune system, how the human body reacts to combat and eliminate pathogens. Our knowledge of immune mechanisms is quickly expanding, yet there is still much to learn. Indeed, the discovery that the immune system functions within the CNS is fairly recent, and we are only beginning to understand the complicated roles of neuroimmunity. The brain can only mount an innate immune response, therefore CNS immunity is a simpler version of the systemic immune reaction, which consists of innate and adaptive immunities. It is important to understand these mechanisms because many studies have demonstrated that there are numerous links between the innate immune response in the CNS and neurodegenerative diseases. There are many controversial issues regarding the role of the immune response in neurotoxicity; some researchers claim that the immune response protects cells of the nervous system, whereas others argue that it is the cause of neurodegeneration. In this review, recent data supporting both hypotheses will be presented. To this end, the mechanisms by which the innate immune response functions in the CNS will be discussed. Having a broader understanding of neuroimmunity may eventually help treat patients afflicted with neurological diseases such as multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD) and Parkinson's disease (PD), to name a few.

INNATE IMMUNITY

Host organisms detect the presence of infectious agents by recognizing specific molecular elements that are produced by micro- organisms such as bacteria1. These elements are called pathogen- associated molecular patterns (PAMPs) and are recognized by specific cells of the innate immune system in order to mount a rapid response to bacterial infection. The endotoxin lipopolysaccharide (LPS), which is the major component of the outer membrane of Gram-negative bacteria, is one of the most characterized molecules known to stimulate the innate immune system and to induce a strong inflammatory response2. An essential element of the innate immune response is the secretion of proinflammatory cytokines, which are produced as a result of a PAMP's recognition by their specific receptors.

The secretion of cytokines by immune cells such as monocytes, neutrophils and macrophages requires a cascade of mechanisms. In the case of LPS, the endotoxin first binds to LPS-binding protein (LBP), a 60 kDa glycoprotein'. The complex then activates its receptor, termed CD14(4), ultimately leading to an increase in nuclear factor kappa B (NF-κB) activity and expression of pro-inflammatory genes, such as tumor necrosis factor alpha (TNF-α) and interleukin-1 beta (IL-Iβ)5. Although CD14 is certainly a receptor for LPS4 and other endogenous molecules6, it is a glycosylphosphatidylinositol-anchored protein and lacks transmembrane and intracellular domains, which implies that CD14 requires a co-receptor to induce intracellular signaling. The characterization of the human homologues of the Toll receptor has helped identify another family of receptors, called Toll-like receptors (TLRs), to be the missing link between LPS and NF-κB activity (for a review, see ref. 7). A large family of TLRs, consisting of at least 12 highly homologous TLRs (10 in human and 12 in mouse), has been characterized8. The extracellular domains of all TLRs comprise of 18-31 leucine-rich repeats and are very divergent from one another, indicating that these receptors recognize different molecules. The cytoplasmic domain of these proteins is highly similar to the cytoplasmic portion of the interleukin-1 receptor (IL-1R), and is therefore named the Toll/IL-1 receptor (TIR) homologous region1,9.

It is now proposed that the various TLRs are the key to the selective recognition of the major PAMPs produced by Cram-negative or Cram-positive bacteria. Cell wall components from Cram-positive bacteria are recognized by TLR2 conjugated to TLR6 or TLR1, doublestranded RNA (dsRNA) viruses are recognized by TLR3; LPS from Cram-negative bacteria binds to TLR4, flagellin to TLR5, and CpG bacterial and viral DNA triggers signaling via TLR9(8,1011). Single- stranded RNAs (ssRNAs) from viruses (e.g. HIV-1 and influenza) are now believed to be the physiological ligands of TLR7 and TLR812,13, whereas TLR11 is activated by uropathogenic bacteria14. It is also believed that TLR1, 2, 4, 5, 6 and 11 recognize extracellular PAMPs, while TLR3, 7, 8 and 9 are sensors for intracellular PAMPs in the phagosome (e.g. CpC DNA, ssRNA and dsRNA). Human TLR10 has no mouse counterpart and mouse TLR11-13 has no known ligands yet.

NF-κB SIGNALING

To date, it is not precisely known how LBP, CD14 and TLR4 interact to become the functional LPS receptor. However, it is well established that the activation of LBP/CD14/TLR4 complex by LPS ultimately leads to the activation of NF-κB and mitogen- activated protein (MAP) kinases. It is thought that CD14 acts as the major LPS/LBP-binding protein and then activates an adjacent TLR4 receptor, which subsequently transduces the LPS signal in the cytoplasm. Supporting evidence for this notion was obtained when Muroi et al. demonstrated that TLR4 co-precipitates with CD14 and that labeling of TLR4 does not occur when using photoactivable LPS in cells devoid of CD14(15). Once a TLR is activated, the intracellular adaptor protein, termed myeloid differentiation factor 88 (MyD88), has the ability to interact with its carboxyl-terminal TIR domain16. MyD88 then binds and activates the kinase known as IL- I receptorassociated kinase (IRAK) via its death domain (DD)17, consequently leading to the recruitment of TNF receptor-associated factor 6 (TRAF6)1. Several IRAK proteins have been identified. IRAKI and IRAK4 are Ser/Thr kinases that activate TRAF6, while IRAK-M negatively regulates TLR signaling, and the function of IRAK2 remains elusive17. TRAF6 then activates NF-??inducing kinase (NIK)18, a mitogen-activated protein kinase kinase kinase (MAPKKK), which in turn phosphorylates the IvB kinase (IKK) complex. Even though IKK is comprised of 3 subunits, α. β and γ, only the β subunit is required for NF-κB activation by PAMPs19. Afterwards, the newly activated IKK phosphorylates IκB, which targets the latter for polyubiquitination and degradation by proteasomes1820. NF-κB is a p50/ p65 heterodimer that is normally complex with kB and found within the cytoplasm. The degradation of IκB frees NF-κB in the cytoplasm and allows the p50/p65 heterodimer to translocate into the nucleus where it can bind its consensus DNA sequence and induce gene transcription. Following its degradation, IκB is rapidly re-synthesized to act as an endogenous inhibitory signal for NF-κB. NF-κB proteins are sequence-specific transcription factors that control the synthesis of a wide variety of molecules involved in orchestrating the innate immune response21,22.

Even though the NF-κB signaling pathway is the prevalent signaling mechanism used by TLRs, certain isoforms of this receptor can utilize other signaling pathways17. It is known that the TIR domain-containing adaptor protein (TIRAP) functionsdownstream of TLR2 and TLR4, but not other TLRs, to induce NF-κ and MAP kinase signaling pathways''4. Similarly, the TIR domain-containing adapt or-indue ing IFN-κB (TRIF) functions downstream of TLR3 and possibly TLR425,26. This signaling pathway activates the interferon regulatory factor 3 (IRF3), a transcription factor required for induction of interferon (IFN) genes, via two IKKs (IKKε and TBK-1)27,28. Therefore, myeloid cells can selectively identity infectious agents and start producing various pro- inflammatory proteins via different signaling pathways.

INNATE IMMUNITY AND THE CENTRAL NERVOUS SYSTEM

It had long been thought that the brain was a privileged organ, because the BBB was said to be able to prevent the entry of all molecules and small organisms into the CNS. As a result of the discovery of the BBB, it was postulated that the immune system does not function in the brain, because invading pathogens and immune cells do not have access to the CNS. It has, however, been shown recently that this is far from being the case. Indeed, it is now well documented that the innate immune system functions to a great extent in the brain (for a review, see ref. 29). There is accumulating evidence that cells of the brain itself are capable of synthesizing and releasing pro-inflammatory cytokines. It has been demonstrated that some of these cytokines induce or promote neurodegeneration, while others exert neuroprotective effects29. Some cytokines, like TNF-α and IL-1β, seem to have dual roles, because they can be either neurodegenerative or neuroprotective, depending on the circumstances.

Figure 1: Circumventricular organs (CVOs). There are four regions of the brain that are devoid of the BBB (denoted in black). These areas are named the CVOs and consist of the area postrema (AP), median eminence (ME), vascular organ of the lamina terminalis (OVLT) and the subfornical organ (SFO)

Much of the evidence supporting either of these hypotheses depends on recent developments in molecular biology, using both in vitro and in vivo models. Many of the experimental models consist in the generation of genetically engineered animals, either to completely abolish the expression of certain cytokines or to constitutively over-express these molecules. One must remain a little skeptical about the validity of the results taken from these experiments because these molecules are not necessarily constitutively expressed in the cerebral tissue, and thus the use of a forced expression of these molecules to obtain experimental data remains a controversial issue. Although it is clear that the transcriptional activation of pro-inflammatory cytokines occurs in the CNS among a wide variety of models and diseases, it is generally found under basal conditions that extremely low mRNA levels are undetectable. This is actually the case not only in the CNS, but also in all other tissues. Therefore, the physiological relevance of increased constitutive cytokine levels must be questioned because these molecules are usually regulated by sophisticated transcriptional processes that are triggered only in case of emergency, such as the detection of invading pathogens or degenerating cells.

COMMUNICATION ROUTES BETWEEN THE BLOOD AND THE CNS

While immune molecules can be produced by both systemic and cerebral cells, invading pathogens and cytokines that are produced by the systemic immune system are unlikely to diffuse in concentrations high enough to the cerebral tissue owing to the BBB, therefore, they must target cells that can be reachable from the systemic circulation. Indeed, communication between the CNS and the blood supply exists within brain structures that contain a rich vascular plexus with specialized arrangements of the blood vessels. These compartments are known as circumventricular organs (CVOs) and consist of 4 organs (Figure 1), namely the vascular organ of the lamina terminalis (OVLT), the subfornical organ (SFO), the median eminence (ME) and the area postrema (AP). The tight junctions between the endothelial cells that normally make up the BBB are modified or lacking in these areas, explaining the ability of large molecules to diffuse into the perivascular region30. These structural properties make them accessible target sites not only for pro-inflammatory cytokines, but also for infectious agents. The choroid plexus (chp) and leptomeninges are also recognized as being highly vascularized regions and are very sensitive to infectious agents. These two regions of the brain exhibit a rapid transcriptional activation of different inflammatory molecules. However, the chp and leptomeninges are devoid of neurons and therefore they are not considered as being CVOs.

Another method by which circulating cytokines may communicate with cells of the CNS is through a direct interaction with the endothelial cells that generate the BBB. Cells forming the BBB are in an ideal location to transfer information from the blood circulation to the brain parenchyma (Figure 2). Receptors for many molecules of the immune system are often constitutively expressed, or their expression can be induced, in vascular-associated cells of the CNS. Therefore, systemic cytokines can trigger a series of events leading to the activation of MAP kinases and NF-κB or JAK/STAT transduction pathways, when they bind their receptors on endothelial cells of the brain capillaries. Depending on the circumstances, the transduction signals evoked in cells of the BBB may activate the transcription and production of soluble factors, such as prostaglandins (PGs)31. These molecules are generally known for their positive effects on immunity, such as the induction of pro- inflammatory molecules in macrophages. However, they have immunosuppressive properties in the CNS, because they play a critical role in the activation of the hypothalamic-pituitary- adrenal (HPA) axis and the production of the ultimate endogenous immunosuppressor, e.g. glucocorticoids (for reviews, see refs. 32 and 33).

Figure 2: Summary of the innate immune response in the central nervous system. Upon the infection by an invading pathogen (i.e. Gram-negative bacteria), substantial amounts of PAMPs (i.e. LPS) can be found in the blood. These molecules are recognized by blood immune cells via their respective Toll-like receptors (TLRs). Myeloid cells (i.e. monocytes) then start the production of TNF- α, which has paracrine effects on other cells of the same type, inducing the production of higher levels of TNF-α, as well as other cytokines such as IL-1β and IL-6. These cytokines and the PAMPs are able to enter the brain parenchyma via the CVOs (upper half of the figure), because these regions lack the BBB. Once in the brain tissue, the molecules can bind to their receptors on microglia, resulting in the activation of these cells and the production of cytokines, proteins of the complement system and polyamines. These molecules can have paracrine effects on neighboring cells, which causes a wave of pro-inflammatory activation. The circumstances that dictate whether this response has neuroprotective or neurodegenerative effects are still unclear. However, a proper innate immune reaction is unlikely to cause brain damages. Alternatively (bottom half of the figure), TNF-α, IL- 1β and IL-6 in the blood stream can bind endothelial cells and induce the production of the chemokine MCP-I. As a result, monocytes and other immune cells (i.e. T lymphocytes) are recruited to the site of inflammation, and migrate across the BBB into the brain parenchyma. The precise mechanisms allowing for this immigration are not yet fully understood. The infiltrating monocytes transdifferentiate into microglia and remain powerful antigen- presenting cells (APCs). Upon another subsequent infection, these new APCs in the brain could serve to recruit and activate lymphocytes, which would most likely be detrimental to the CNS. On the other hand, the possibility still remains that these microglia could have neuroprotective effects under certain circumstances. A third cytokine-dependent mechanism in the CNS is the activation of cyclooxygenase 2 (COX-2) in endothelial cells, which activate corticotropin-releasing factor (CRF) neurons and ultimately leads to the production of glucocorticoids and the inhibition of the innate immune response. This is a very efficient endogenous antiinflammatory mechanism. It is thought that the innate immune system has neuroprotective effects when cytokine levels are controlled. However, if the actions of glucocorticoids are altered in any fashion, cytokine levels could be too high and neurodegeneration would be favored

Table 1: The Role of TNF-α and IL-1B in Neuronal Death

ROLE OF INNATE IMMUNE PROTEINS IN THE CNS

It is generally well accepted that TNF-α has detrimental systemic effects such as tissue necrosis, vascular coagulation, high fever and respiratory problems34. When neurons are concerned, however, results showing the effects of TNF-α and IL-I on neurodegeneration in vitro are quite contradictory (Table 7). Certain studies have shown that addition of TNF-α and IL-1 β to cultured neurons does not have neurodegenerative effects35. Another report demonstrated that when added individually, these cytokines did not induce the death of neurons, but when administered together, neurodegeneration was observed36. In contradiction to these results, Strijbos and Rothwell demonstrated that low concentrations of IL-1 have neuroprotective effects, whereas high concentrations are neurotoxic over a longer term37. Westmoreland et al. observed that TNF-α is neurodegenerative while IL-1 is not38. These contradictory results may be due to differences in the cytokine concentrations that were administered to the cultured neurons. Moreover, these in vitro results may not necessarily reflect the exact nature of the cytokines in \vivo, where neurons are accompanied by many other cell types, such as microglia, astrocytes and oligodendrocytes to name a few. Nonetheless, these results are somewhat indicating that TNF-α and IL-1 may be toxic to neurons.

In this regard, there are also many signs that these cytokines may have an active role in the development of neuronal death and the etiology of neurodegenerative diseases. Indeed, positive correlations have been observed between the degree of neuronal death during bacterial meningitis and the levels of TNF-α, IL-1 and IL-1R239-42. Moreover, TNF-α concentrations are higher than normal in MS patients43 and in AD patients44, while TNF-α and IL-1 levels are increased in those afflicted by PD45 Moreover, TNF- α KO mice are protected against neuronal death in models of PD46. It has also been demonstrated that TNF-α mRNA synthesis is increased during ischemia and that microglia are the source of the cytokine production47.

It has been recently established that, when left unchecked, TNF- α induces neural damages and chronic TNF-α infusion in the brain causes neuronal death by apoptosis48. In this paper, the authors reported that IL-1 is also neurotoxic, but to a much lesser extent than TNF-α. However, it must be noted that the phenomenon only occurred when the production of the cytokines was unregulated, or when high physiological concentrations of the cytokines were chronically administered in the CNS, and these data have been supported by another study49. Indeed, other reports have shown that these cytokines can be neuroprotective during ischemia or cranial trauma when synthesized within physiological concentrations50,51, while their overexpression causes neurodegeneration52. Moreover, it has been suggested that TNF- α is neuroprotective when it interacts with TNF-R2, whereas the cytokine induces neurodegeneration when it binds TNF-R1(53). Evidence that IL-1β may also be neuroprotective has been provided by a study demonstrating that IL-1 can protect retinal ganglion cells following optic nerve transection14. Taken together, these data confirm the notion that certain cytokines have dual roles, either promoting death or survival depending on the circumstances.

In a very elegant study, Arnett and colleagues have used mice lacking TNF-α and its associated receptors to study a model of demyelination and remyelination55. Surprisingly, the lack of TNF- α led to a significant delay in remyelination, which was associated with a reduction in the pool of proliferating oligodendrocyte progenitors followed by a reduction in the number of mature oligodendrocytes. This reparative role of TNF-α in the CNS is mediated via the TNFR2, not TNFR1, indicating that the dual role of TNF-α in demyelination and remyelination may depend on the receptor type and/or the cellular source of the cytokine. One must therefore to be careful before designing new strategies to either prevent or increase the production of specific innate immune proteins for treating neurodegenerative and demyelinating disorders.

OUR VIEW AND CONCLUDING REMARKS

Although the innate immune system has long been described as a primitive defense mechanism, it is now clear that it is much more complex and efficient than we first thought. With over 12 TLRs and numerous accessory proteins, which in combination can specifically recognize different molecular patterns from Grampositive and Gram- negative bacteria, parasites and viruses, the innate immune system provides an effective first line of defense. Innate immunity is sometimes sufficient to counteract a simple infection, and in these cases the adaptive response is not required to eliminate the pathogen. In circumstances where the infectious agent is not eradicated within a few days, macrophages begin producing IL-12, which activates cells of the adaptive immune response, and the transfer between innate and adaptive immunity occurs.

In the first few days where the innate immune response is active, however, many events take place (Figure 2). Monocytes, which are the principal mediators of this immune response, produce and secrete substantial amounts of cytokines, especially TNF-α IL-1β and IL-6. These pro-inflammatory cytokines serve to recruit and enhance the activity of other immune cells, and thus improve the overall immune response. Macrophage phagocytic activity is increased, and the pathogen is thus eliminated. On the other hand, cytokines of the innate immune response also activate the HPA axis, which consequently leads to the production of glucocorticoids. Thus, cytokines mediate their own down-regulation, and the strength of the immune reaction is kept in tight control via this mechanism. Other effects of cytokine release are the induction of fever, and even septic shock in extreme cases. All in all, even though the innate immune response does not employ antibodies that are specific to individual proteins, it is nonetheless a very strong and efficient means to counteract the invasiveness of infectious agents.

Although the BBB presents an effective barrier against the entry of many blood molecules and invading pathogens in most areas of the CNS, it is clear that molecules of the systemic innate immune system are able to stimulate the immune cells of the brain. This is achieved by two viable mechanisms, which are via regions that lack the BBB or by a direct stimulation of the endothelium of brain capillaries (Figure 2). A large number of studies support both of these processes. Many researchers have observed that systemic immune challenges often initiate the immune response in the CNS first by activating immune cells that are in or near the CVOs. Once these cells are stimulated, they start producing substantial amounts of cytokines, mainly TNF-α, which thereafter bind to their receptors on neighboring immune cells in a paracrine manner. As a result, a wave of cytokine expression is instigated, and immune cells across the brain parenchyma are activated and begin synthesizing more immune molecules. The second mechanism consists of an indirect pathway by which CNS vascular cells are targeted by cytokines found in the blood stream. To this end, systemic cytokines can bind their receptors that are expressed on endothelial cells, resulting in the activation of the enzyme COX-2 and the expression of PCs56. These soluble mediators are then released into the brain parenchyma and can diffuse to stimulate nearby neurons. The principal effect of PG synthesis by brain endothelial cells is fever induction and activation of the HPA axis, which ultimately results in the production of glucocorticoids31. It is a well established fact that glucocorticoids are potent inhibitors of inflammation, and this is actually the most powerful endogenous antiinflammatory mechanism in all vertebrates33. To the contrary, COX-2 activation in peripheral systems has pro-inflammatory properties, because it results in the activation of polymorphonuclear cells and the expression of molecules that stimulate the immune system. Despite the dual nature of COX-2 in peripheral organs, this enzyme plays an overall anti-inflammatory role via the production of PCs, the activation of the HPA axis and finally the synthesis of glucocorticoids. Cytokines found in the blood circulation therefore stimulate their own inhibition via the BBB, but they also induce a strong immune response inside the brain parenchyma via the CVOs.

The discovery that the CNS was not isolated from the immune system, and that this organ is able to mount a very strong immune response against invading pathogens opened the door to many possibilities. Since then, the major topic for discussion in this scientific field was whether the innate immune system was beneficial or detrimental for the neural environment. Arguments for both cases are very convincing, because it is well known that cytokines such as TNF-α are potent inducers of apoptosis, while they also activate NF-κB-dependent signaling pathways, which often promote cellular growth and survival. Moreover, cytokines amplify the immune response, and they induce the expression of chemokines, which serve to recruit other immune cells to the site of inflammation. This phenomenon can also have both positive and negative effects, because immigrating cells can help to rapidly eliminate the invading pathogens, but they can also be harmful to neurons, oligodendrocytes and other cells of the CNS.

Indeed, studies performed over the last decade have provided evidence supporting both sides of the story. In general, it seems that TNF-α and IL-1β have neuroprotective effects at low concentrations, whereas they cause neuronal death when they are produced in large quantities. However, most of these studies were performed with cultured neurons, and may not necessarily reflect physiological conditions, where neurons are surrounded by many other types of cells, such as microglia, astrocytes and oligodendrocytes. On the other hand, in vivo studies tend to support the idea that these cytokines are neurotoxic rather than neuroprotective. However, in vivo models generally consist of transgenic animals that either over-express the cytokines or lack their receptors. These animal models are not necessarily representative of normal physiological conditions and of an appropriate immune response. Therefore, one cannot reject the possibility that TNF-α and IL-1β may protect neurons during inflammation; this is clearly the case during acute demyelination and remyelination. Nevertheless, these cytokines may certainly have detrimental effects on neurons when their secretion is left uncontrolled, which emphasizes the importance of their self-regulation via the activation of the HPA axis and the production of glucocorticoids.

It is now known that infiltration of immune cells into the CNS is one of the first \steps in many neurological disorders, such as MS. The mechanisms by which these immigrating cells contribute to neurotoxicity are not yet fully understood, however recent evidence suggests that infiltrating monocytes are potent APCs compared with resident microglia57. This could create profound complications if a second infection occurs in the CNS later in the life of the subject. In this regard, microglia that have a systemic stem cell origin would present the antigen to a much greater extent than native microglia, and this could lead to the recruitment and activation of multiple other types of immune cells58. The mere presence of these cells in the CNS, which is highly abnormal, could be very destructive to the neural environment. This phenomenon would implicate the adaptive immune response, which the CNS is usually protected from under normal circumstances. Such mechanisms would allow the immune system to function to a much greater extent.

The possible outcomes of such a response are extremely variable, and such an occurrence could explain the etiology of many neurodegenerative diseases. Indeed, it is possible that some neurological disorders are caused by invading pathogens, which infected the hosts on separate occasions. For example, if a subject is severely infected by a certain pathogen early in life or in the gestational stage, resulting in monocyte immigration inside the CNS and their differentiation into potent APCs, a subsequent infection later in life could lead to these cells presenting the antigen to other specialized cells of adaptive immunity. Large amounts of pro- inflammatory cytokines would be secreted in the neural environment, and cells normally found in the CNS may not have the capacity to control an adaptive immune response. Such an extraordinary reaction could lead to catastrophic results, even to the death of neurons and their support cells. In other words, certain individuals could be more susceptible to neurodegenerative diseases if they have been severely infected by any given bacteria or virus at an earlier stage in life.

One of the principal restraints that prevent researchers deciphering the precise roles of molecules of the immune system in neurodegeneration is the fact that there are too many molecules to study at once. Many cytokines share the same physiological effects, which can easily confuse the analysis of experimental data. To date, there has been a lack of laboratory procedures that would permit a simultaneous study of multiple molecules in animal disease models. However, recent developments in molecular biology have been provided with new technology that may prove helpful in this cause. This new procedure is termed RNA interference (RNAi), and consists of small dsRNAs that target specified mRNAs for degradation, thus abolishing the synthesis of the protein itself. The advantage of this technique over the currently popular knock-out animal models is that RNAi might permit us to prevent the expression of many proteins simultaneously. Therefore, one could inhibit the synthesis of multiple cytokines or receptors, like TNF-α and IL-1β for example. The problem of redundancy would hence be eliminated. It is also possible to target the expression of the small interfering dsRNAs in specific populations of cells and at a certain stage of differentiation. For instance, one could specifically inhibit the expression of key cytokines and their receptors in bone marrow stem cells once they differentiate into macrophages or even microglia. Therefore, by transplanting these bone marrow stem cells into irradiated animals, one could discover the exact role of these molecules in the migration of stem cells into the CNS and understand what effects this may have on the onset of neurodegenerative diseases. Similar procedures can be applied to other animal models, which would undoubtedly lead to a better understanding of the effects of all immune molecules in the CNS. It is quite possible that the key to determining whether the immune system is beneficial or detrimental for the CNS lies in the simultaneous study of multiple molecules rather than the study of one molecule at a time.

In conclusion, the short-term effects of the immune system on neural integrity have been extensively studied over the past decade. More data need to be collected regarding the precise circumstances where innate immunity is beneficial or detrimental to neurons. Moreover, the effects of the immune response on the CNS must be studied over longer periods, and the consequences of multiple infections must also be determined in order to fully understand their role in neurodegenerative diseases. Finally, studying the cooperation between TLRs and their respective ligands may provide new clues for better understanding how the different families of pathogens interact with the host, which may become more vulnerable and ultimately exhibit brain diseases. The results that will be obtained in the coming years should prove to be invaluable and very useful in our understanding of the etiology of neurological disorders and the development of therapies to either prevent or remedy these ailments.

ACKNOWLEDGMENTS

The Canadian Institutes in Health Research (CIHR) supports this research. Alain Simard is supported by a PhD studentship from the CIHR and Serge Rivest is a CIHR scientist and holds a Canadian Research Chair (Junior) in neuroimmunology.

REFERENCES

1 Andersen KV. Toll signaling pathways in the innate immune response. Curr Opin lmmunol 2000; 12: 13-19

2 Wright SD. Toll, a new piece in the puzzle of innate immunity. J ftp Med 1999; 189: 605-609

3 Schumann RR. Leong SR, Flaggs CW, el al. Structure and function of lipopolysaccharide binding protein. Science 1990; 249: 1429-1431

4 Wright SD, Ramos RA, Tobias PS, et at. CD 14. a receptor for complexes of lipopolysaccharide (LPS) and LPS binding protein. Science 1990; 249: 1431-1433

5 Nadeau S, Rivest S. Endotoxemia prevents the cerebral inflammatory wave induced by intraparenchymal lipopolysaccharide injection: Role of glucocorticoids and CDI4. J lmmunol 2002; 169: 3370-3381

6 Perera PY, Vogel SN. Delore CR. ef al. CD14-dependent and CD14- independent signaling pathways in murine macrophages from normal and CDI4 knockout mice stimulated with lipopolysaccharide or taxol. J lmmunol 1997; 158: 4422-W29

7 Triantafilou M, Brandenburg K, Cutsmann T, el al. Innate recognition of bacteria: Engagement of multiple receptors. Crif Rev lmmunol 2002; 22: 251-268

8 O'Neill LA. Immunology. After the toll rush. Science 2004; 303: 1481-1482

9 Muzio M, Polenlarutti N. Bosisio D. et al. Toll-like receptors: A growing family of immune receptors that are differentially expressed and regulated by different leukocytes. I Leukoc Biol 2000: 67: 450-456

10 Beutler B. Hoebe K, Du X. ef al. How we detect microbes and respond to them: The Toll-like receptors and their transducers. I Leukoc Biol 2003; 74: 479-485

11 Kopp E, Medzhitov R. Recognition of microbial infection by Tolllike receptors. Clin Opin Immunol 2003; 15: 3960-401

12 Diebold SS, Kaisho T. Hemmi H, ef al. Innate antiviral responses by means of TLR7-mediated recognition of single-stranded RNA. Science 2004; 303: 1529-1531

13 Heil F, Hemmi H, Hochrein H, ef al. Species-specific recognition of single-stranded RNA via loll-like receptor 7 and 8. Science 2004; 303: 1526-9

14 Zhang D, Zhang C, Hayden MS, ef al. A toll-like receptor that prevents infection by uropathogenic bacteria. Science 2004; 303: 1522-1526

15 Muroi M, Ohnishi T, Tanamoto K. Regions of the mouse CD14 molecule required for toll-like receptor 2- and 4-mediated activation of NF-kappa B. J Biol Chem 2002; 277: 42372-42379

16 Kawai T, Adachi O, Ogawa T, et al. Unresponsiveness of MyD88deficient mice Io endotoxin. Immunity 1999; 11: 115-122

17 Barton CM, Medzhitov R. Toll-like receptor signaling pathways. Science 2003; 300: 1524-1525

18 Baeuerle PA. Pro-inflammatory signaling: Last pieces in the NF- kappaB puzzle? Curr Biol 1998; 8: Rl9-22

19 Delhase M, Hayakawa M, Chen Y, ef al. Positive and negative regulation of IkappaB kinase activity through IKKbeta subunit phosphorylalion. Science 1999; 284: 309-313

20 Baeuerle PA, Baltimore D. NF-kappa B: Ten years after. Cell 1996; 87: 13-20

21 Karin M, Ben-Neriah Y. Phosphorylation meets ubiquitination: The control of NF-|kappa|B activity. Annu Rev lmmunol 2000; 18: 621- 663

22 Karin M. The beginning of the end: IkappaB kinase (IKK) and NF- kappaB activation. J Biol Chem 1999; 274: 27339-27342

23 Horng T, Barton CM, Flavell RA, ef al. The adaptor molecule TIRAP provides signalling specificity for Toll-like receptors. Nafure 2002; 420: 329-333

24 Yamamoto M, Sato S, Hemmi H, ef al. Essential role for TIRAP in activation of the signalling cascade shared by TLR2 and TLR4. Nature 2002; 420: 324-329

25 Yamamoto M, Sato S, Mori K, et al. Cutting edge: A novel Toll/ IL-1 receptor domain-containing adapter that preferentially activates the IFN-beta promoter in the Toll-like receptor signaling. J Immunol 2002; 169: 6668-6672

26 Oshiumi H, Matsumoto M, Funami K, et at. TICAM-I, an adaptor molecule that participates in Toll-like receptor 3-mediated interferon-beta induction. Nat Immunol 2003; 4: 161-167

27 Fitzgerald KA, McWhirter SM, Faia KL, et al. IKKepsilon and TBK1 are essential components of the IRF3 signaling pathway. Nat Immunol 2003; 4: 491-496

28 Sharma S, tenOever BR, Grandvaux N, et al. Triggering the interferon antiviral response through an IKK-related pathway. Science 2003; 300: 1148-11 51

29 Nguyen MD, Julien JP, Rivest S. Innate immunity: The missing link in neuroprotection and neurodegeneration? Nat Rev Neurosci 2002; 3: 216-227

30 Oldfield BJ, Mckinley MJ. Circumventricular organs. In: G P, ed, The rat nervous system, San Diego: Academic Press, 1995: pp. 391- 403

31 Zhang J, Rivest S. Is survival possible without arachidonate metabolites in the brain during systemic infection? News Physiol Sci 200\1; 18: 137-142

32 Turrin NP, Rivest S. Unraveling the molecular details involved in the intimate control of the immune and neuroendocrine systems: A revised and innovative hypothesis. Exp Bio Med 2004; 229: 996-1006

33 Glezer I, Rivest S. Glucocorticoids: Protectors of the brain during innate immune responses. Neumscientist 2004; 10: 538-552

34 Tracey KJ, Cerami A. Tumor necrosis factor: A pleiotropic cytokine and therapeutic target. Annu Rev Med 1994; 45: 491-503

35 Piani D, Spranger M, Frei K, et al. Macrophage-induced cytotoxicity of N-methyl-D-aspartate receptor positive neurons involves excitatory amino acids rather than reactive oxygen intermediates and cytokines. Eur J lmmunol 1992; 22: 2429-2436

36 Chao CC, Hu S, Ehrlich L, et al. Interleukin-1 and tumor necrosis factor-alpha synergistically mediate neurotoxicity: Involvement of nitric oxide and of N-methyl-D-aspartate receptors. Brain Behav lmmun 1995; 9: 355-365

37 Strijbos PJ, Rothwell NJ. Interleukin-1 beta attenuates excitatory amino acid-induced neurodegeneration in vitro: Involvement of nerve growth factor. J Neurosci 1995; 15: 3468-3474

38 Westmoreland SV, Kolson D, Gonzalez-Scarano F. Toxicity of TNF alpha and platelet activating factor for human NT2N neurons: A tissue culture model for human immunodeficiency virus dementia. J Neurovirol 1996; 2: 118-126

39 Mustafa MM, Lebel MH, Ramilo O, et al. Correlation of interleukin-1 beta and cachectin concentrations in cerebrospinal fluid and outcome from bacterial meningitis. J Pediatr 1989; 115: 208-213

40 Mustafa MM, Ramilo O, Olsen KD, et al. Tumor necrosis factor in mediating experimental Haemophilus influenzae type B meningitis. 7 Clin Invest 1989; 84: 1253-1259

41 Arditi M, Manogue KR, Caplan M, et al. Cerebrospinal fluid cachectin/tumor necrosis factor-alpha and platelet-activating factor concentrations and severity of bacterial meningitis in children. J Infect Dis 1990; 162: 139-147

42 van Deuren M, van der Ven-Jongekrijg J, Vannier E, et al. The pattern of interleukin-1 beta (IL-1beta) and its modulating agents IL-1 receptor antagonist and IL-1 soluble receptor type Il in acute meningococcal infections. Blood 1997; 90: 1101-1108

43 Mauser SL, Doolittle TH, Lincoln R, et al. Cytokine accumulations in CSF of multiple sclerosis patients: Frequent detection of interleukin-1 and tumor necrosis factor but not interleukin-6. Neurology 1 990; 40: 1 735-1 739

44 Sheng JG, Boop FA, Mrak RE, et al. Increased neuronal betaamyloid precursor protein expression in human temporal lobe epilepsy: Association with interleukin-1 alpha immunoreactivity. J Neumchem 1994; 63: 1872-1879

45 Mogi M, Harada M, Narabayashi H, et al. Interleukin (IL)-I beta, IL-2, IL-4, IL-6 and transforming growth factor-alpha levels are elevated in ventricular cerebrospinal fluid in juvenile parkinsonism and Parkinson's disease. Neurosci Lett 1996; 211: 13- 16

46 Sriram K, Matheson JM, Benkovic SA, et al. Mice deficient in TNF receptors are protected against dopaminergic neurotoxicity: Implications for Parkinson's disease. FasebJ2002; 16: 1474-1476

47 Gregersen R, Lambertsen K, Finsen B. Microglia and macrophages are the major source of tumor necrosis factor in permanent middle cerebral artery occlusion in mice. J Cereb Blood Flow Metab 2000; 20: 53-65

48 Nadeau S, Rivest S. Glucocorticoids play a fundamental role in protecting the brain during innate immune response. J Neurosci 2003; 23: 5536-5544

49 Stepanichev MY, Zdobnova IM, Yakovlev AA, et al. Effects of tumor necrosis factor-alpha central administration on hippocampal damage in rat induced by amyloid beta-peptide (25-35). J Neurosci Res 2003; 71: 110-120

50 Bruce AJ, Boling W, Kindy MS, et al. Altered neuronal and microglial responses to excitotoxic and ischemic brain injury in mice lacking TNF receptors. Nat Med 1996; 2: 788-794

51 Scherbel U, Raghupathi R, Nakamura M, et al. Differential acute and chronic responses of tumor necrosis factor-deficient mice to experimental brain injury. Proc Natl Acad Sci USA 1999; 96: 8721- 8726

52 Probert L, Akassoglou K, Pasparakis M, et al. Spontaneous inflammatory demyelinating disease in transgenic mice showing central nervous system-specific expression of tumor necrosis factor alpha. Proc Natl Acad Sci U S A 1995; 92: 11294-11298

53 Fontaine V, Mohand-Said S, Hanoteau N, et al. Neurodegenerative and neuroprotective effects of tumor necrosis factor (TNF) in retinal ischemia: Opposite roles of TNF receptor 1 and TNF receptor 2. J Neurosci 2002; 22: RC216

54 Diem R, Hobom M, Grotsch P, et al. Interleukin-1 beta protects neurons via the interleukin-1 (IL-1 ) receptor-mediated Akt pathway and by IL-1 receptor-independent decrease of transmembrane currents in vivo. MoI Cell Neurosci 2003; 22: 487-500

55 Arnett HA, Mason J, Marino M, ef al. TNF alpha promotes proliferation of oligodendrocyte progenitors and remyelination. Nat Neurosci 2001 ; 4: 1116-1122

56 Laflamme N, Lacroix S, Rivest S. An essential role of interleukinlbeta in mediating NF-kappaB activity and COX-2 transcription in cells of the blood-brain barrier in response to a systemic and localized inflammation but not during endotoxemia. J Neurosci 1999; 19: 10923-10930

57 Simard AR, Rivest S. Bone marrow stem cells have the ability to populate the entire central nervous system into fully differentiated parenchymal microglia. Faseb I 2004; 18: 998-1000

58 Simard AR, Rivest S. Role of inflammation in the neurobiology of stem cells. Neuroreport 2004; 15: 2305-2310

Alain R. Simard and Serge Rivest

Laboratory of Molecular Endocrinology, CHUL Research Center and Department of Anatomy and Physiology, Laval University, 2705 boul. Laurier, Qubec, Canada G1V 4G2

Correspondence and reprint requests to: Dr Serge Rivest, Department of Anatomy and Physiology, Laval University, 2705 boul. Laurier, Qubec, Canada G1V 4G2. [Serge.Rivest@crchul.ulaval.ca] Accepted for publication July 2005.

Copyright Maney Publishing Oct 2005

Source: Neurological Research

More News in this Category