**3. Pathogenic mechanisms of NTB**

TB is a respiratory infection with a generally latent course. The immunodeficiency status favors the extrapulmonary dissemination of mycobacteria leading to inflammatory granulomas with diverse localisations. Some granulomas arise adjacent to the meninges or to the brain paren‐ chyma and become the last station before the CNS invasion. Disruption of these granulomas into the subarachnoid space is followed by the cerebrospinal fluid (CSF) invasion with mycobacteria and meningeal infection.Release of mycobacteria from these granulomas is mainly associated with the severe depletion of macrophages and lymphocytes along with the imbalance of local cytokines. The CSF inflammatory reaction induced by mycobacteria antigens leads to a lymphocyte and fibrin-rich subarachnoid exudate which progressively envelops the blood vessels and cranial nerves. The expansion and intensity of this inflamma‐ tory exudate induces multiple complications including: the obliterative vasculitis followed by cerebral infarctions, the CSF obstruction and emerging hydrocephalus and the spinal extension of TB and chronic arachnoiditis. Some of the CNS granulomas could evolve as cerebral or spinal masses further developing into tuberculomas or tuberculous abscesses [ 10,11,12]. In addition HIV patients characteristically present several TB cerebral lesions evolving simultaneously.

Below we enlisted the factors involved in the clinical progression and persistent CNS invasion with mycobacteria in HIV patients.

**1.** *The cellular immunosuppression in TB and HIV infection.*

**2. Epidemiological data on the HIV/TB co-infection**

294 Tuberculosis - Current Issues in Diagnosis and Management

infection.

first 21 days [9].

**3. Pathogenic mechanisms of NTB**

TB is preventable and curable and its eradication was considered possible before the spread of the HIV pandemic. Since then the pathogenic mechanisms of HIV and TB have been closely entwined. Such is the complementary evolution of HIV and TB that the HIV/TB co-infection has been referred to as a ''syndemic'' by some authors [1]. The term ''syndemic'' reflects the similar social, epidemiological and pathological settings of both diseases. The close interrela‐ tion between HIV and tuberculosis overcomes by far the interactions between other commun‐ ity acquired infections. Thus epidemiological studies suggest that as many as 50% of the HIV patients develop mycobacterial infections. The rate of extrapulmonary TB could account for more than 50% of cases presenting with HIV and TB coinfection. In the pre-AIDS era the immunodeficiency status incriminated in the pathogenesis of extrapulmonary TB was induced by autoimmune diseases, aging, diabetes, alcoholism, malnutrition, malignancies or immu‐ nosuppressive chemotherapy. However the total amount of extrapulmonary TB in non-HIV immunosupressed patients did not exceed 15% of all TB cases. In addition meningitis and other forms of NTB represented less than 1% of all TB cases in non-HIV patients [2,3] but presently account for 10% of all TB cases in HIV patients [4]. Tuberculous meningitis (TBM) occurs in 5%-8% of the HIV patients [5,6] but tuberculomas and abscesses are also a common finding in late stages of AIDS [7]. Regarding the CNS infection with non-tuberculous mycobacteria one of the most important risk factors is the progressive immunodeficiency induced by HIV

Co-infection with HIV not only increases the risk for central nervous system (CNS) TB [17] but also alters the clinical signs, delays the diagnosis and worsens the prognosis [8]. Thus the mortality of HIV patients with TBM is as high as 63% and nearly half of deaths occur in the

TB is a respiratory infection with a generally latent course. The immunodeficiency status favors the extrapulmonary dissemination of mycobacteria leading to inflammatory granulomas with diverse localisations. Some granulomas arise adjacent to the meninges or to the brain paren‐ chyma and become the last station before the CNS invasion. Disruption of these granulomas into the subarachnoid space is followed by the cerebrospinal fluid (CSF) invasion with mycobacteria and meningeal infection.Release of mycobacteria from these granulomas is mainly associated with the severe depletion of macrophages and lymphocytes along with the imbalance of local cytokines. The CSF inflammatory reaction induced by mycobacteria antigens leads to a lymphocyte and fibrin-rich subarachnoid exudate which progressively envelops the blood vessels and cranial nerves. The expansion and intensity of this inflamma‐ tory exudate induces multiple complications including: the obliterative vasculitis followed by cerebral infarctions, the CSF obstruction and emerging hydrocephalus and the spinal extension of TB and chronic arachnoiditis. Some of the CNS granulomas could evolve as cerebral or spinal

The site of extrapulmonary mycobacterial infections and especially the CNS invasion depend on the efficacy of cell-mediated immunity. Both the HIV infection and TB trigger complex mechanisms which increase the cellular immunosuppresion.

On the other hand humoral immnity is increased but inefficient. The high titres of antimyco‐ bacterial antibodies are not protective and could instead result in numerous complications. The most important mechanism behind the cellular immunosuppression in the HIV-TB coinfection is the severe depletion of macrophage and lymphocyte cells.

*Macrophage and lymphocyte cells.* Macrophages play a crucial role in both HIV and mycobac‐ terial infections. As phagocytes of the innate immunity they are considered the main cells involved in the immune response against mycobacteria.Infected macrophages recruit addi‐ tional immune cells such as dendritic cells and T cell lymphocytes and release numerous chemokines and cytokines to form granulomas. The latter are specific stable inflammatory structures limiting the growth of mycobacteria. At the same time mycobacteria could devel‐ op inside macrophages from granulomas thus ensuring their persistence. In addition macro‐ phages infected with Mycobacterium tuberculosis (M. tbc) augment the expression of the C-C chemokine receptor type 5, also known as CCR5, the most important HIV coreceptor [13]. Therefore infected macrophages perform a significant role in the protection and transport of mycobacteria and HIV to other tissues including the brain.

With the passing of time some of the macrophages infected with mycobacteria suffer apoptosis leading to a numeric decrease of the most important cells involved in the defence against mycobacteria invasion. Moreover HIV is directly responsible for the depletion of CD4+ T lymphocytes through its cytopathic effect and anti-gp120 antibodies. The depletion of CD4+ T lymphocytes raises the susceptibility to TB and most notably towards neurologic forms of TB [14]. In this respect the decreasing CD4+ T cell count was proven to vary inversely with the incidence of NTB. Most patients with HIV and NTB display a CD4+ T cell count below 200 cells/ mm3 unlike patients with pulmonary TB who commonly present with a CD4+T cell count, between 250 and 550 cells/ mm3 . In conclusion in the late stages of infection the main pathogenic mechanisms of invasion with mycobacteria and HIV are closely interwined.

*The Cytokine dysregulation*. Both HIV and mycobacteria are intracellular pathogens. Their presencestimulates thereleaseofcytokinesbymacrophagesandTh1cellswhichinturnregulate the cells involved in the immune response. The stability of the granuloma is usually ensured by a high number of CD4+ T and CD8+ lymphocytes along with a Th1 cytokine profile represent‐ ed by IFN -γ and TNF-α. [15].TNF-α is a pro-inflammatory cytokine released at high levels by CD4+T cells and macrophages coinfected with mycobacteria and HIV. The role of TNF-α in the clinical outcome of the 2 diseases is contradictory. Regarding its role in the control of tuberculo‐

sis a high level of TNF-α stimulates the apoptosis of infected macrophages and the cellular activation [16,17]. On the other hand the use of TNF-*α* neutralizing antibodies in inflammato‐ ry diseases has been associated with an increased risk of extrapulmonary TB including TBM [18].CD4-T-cell deficient mice [19] as well as mice able to neutralize endogenous TNf- *α* [20] or the gene for IFN-γ [21] are subjected to fatal TB. Nevertheless an in vitro experiment on human monocytes noted that higher levels of TNF-α could be associated with more virulent or faster growing mycobacterial strains [22].The contradictory effect of TNF-α was also observed in the HIV infection. Studies conducted by Lane and Osborn proved that TNF-α is a potent inhibitor over the primary HIV infection of the macrophages but enhances the HIV replication in latent HIV infections [23,24 ]. This finding could explain why mycobacteria infections which pro‐ mote the synthesis of TNF-α could also augment HIV replication in chronic infected individu‐ als. The level of TNF-α in the blood of patients infected with mycobacteria and HIV was documented to be 3 to 10 times higher than in non-HIV patients [25] showing a major imbal‐ ance in the release of this proinflammatory cytokine. TNF- α also plays a central role in the CNS localizations of mycobacteria. The excessive amount of TNF-α could accelerate the disruption of rich tuberculous foci adjacent to the CNS. Increased levels of TNF-α as well as IFN-γ were found in the CSF of patients with TBM at the disease onset [26] as well as several months after the acute episode [27] Experimental studies on rabbits proved that the excess of TNF- α acts as a persistent trigger of the inflammatory response and as a procoagulant factor associated with both the mycobacteria CNS invasion as well as cerebral vascular complications. [28]. The therapeutic use of TNF-α inhibitors in severe forms of TBM, tuberculoma and cerebral tubercu‐ lousabscesseswas linkedtoadecreasedinflammatoryresponseandnoticeableclinicalrecovery [29-31]. The major role of TNF-α in the progression of TBM was also proved in murine models by Tsenova as well [28,33]. Studies on HIV patients with TBM also emphasized the signifi‐ cance of increased levels of CSF TNF-α and of IFN-γ in advanced TBM stages [34].

formation of granulomas. However since microglial cells are the main source of cerebral TNF-α these could also induce an aggresive inflammatory response with severe menin‐ geal inflammation, brain edema, protein accumulation, endarteritis and intracranial hypertension accounting for most of the complications described in NTB [28,38]. There‐ fore a balanced activation of microglial cells is critical against the CNS mycobacterial invasion.OntheotherhandtheintracellularHIVreplicationinmicroglial cells leads totheir activation,neuroinflammationandreleaseofneurotoxinsthatcauseAIDSassociatedneural dysfunctions. The complex role of the microglia in cerebral HIV/TB co-infection is ex‐ plained by the rich number of HIV receptors and co-receptors expressed by these cells such as CD4, CCR5, CXCR4 as well as other receptors involved in the inflammatory response including IFN-γ, TNF-α,CD14 and MHC class I and II receptors [39].The CD14 receptor promotes the uptake of both HIV and nonopsonized M.tbc strains in microglial cells [40] while CD4 and CCR5/CXCR4 co-receptors interfere with HIV cell attachment. As a result microgial cells are the main target of HIV and mycobacteria once these enter the CNS. Therapies directed towards reducing the inflammatory response in the HIV/TB coinfection include the blockage of certain receptors (such as CD14), the use of CCR5 antagonists and TNF-α blockers (as thalidomide). Another alternative is dexametasone recommendedinmostformsofCNSTB.Theclinicalbenefitsofdexametazonewereinspired by in vitro studies proving a potent inhibitory effect on the release of cytokines from

Neurotuberculosis and HIV Infection http://dx.doi.org/10.5772/54631 297

*In conclusion* simultaneous infection of the microglia with HIV and mycobacteria increases the meningeal inflammatory response, the fundamental pathogenic step in all forms of CNS TB. The synthesis of excessive inflammatory infiltrate is responsible for the clinical findings and possibly irreversible complications in NTB, such as hydrocephalus and vasculitis [41]. Moreover the excessive inflammatory response triggered in the HIV/TB co-infection could induce the immune reconstitution inflammatory syndrome – a complication that is specific for

**4. Pathogenesis of the immune reconstitution inflammatory syndrome**

The Immune Reconstitution Inflammatory Syndrome (IRIS) is an uncommon inflammatory response encountered in those cases of severe immunosuppression in which the rapid adminis‐ tration of specific treatment abruptly restores the immune response. The HIV infection is the most frequent cause of immunodeficiency predisposing to IRIS. In addition TB is the most commonopportunisticinfectionrelatedtoHIV-associatedIRIS.Theantiretroviralandantituber‐ culous treatments rapidly restore the immune response. Such a rapid treatment response may sometimes leadtoanaggressivelymphoproliferativereactionandmassivereleaseofproinflam‐ matory cytokines. There are 2 clinical presentations of IRIS known as the paradoxical IRIS and unmasking IRIS. IRIS manifestations in HIV patients with NTB follow two possible scenarios:

microglia [39].

this patient category.

*In conclusion* all these studies proved that important variations of the Th1 cytokine profile and especially of those involving the release of TNF-α represent one of the pathogenic mechanisms that aggravate the outcome of NTB in the HIV infection. Understanding these changes could be the first step towards the development of efficient complementary therapies in NTB to reduce the excessive inflammatory response. Thus TNF-α inhibition could be used as an antiinflammatory therapy in NTB with severe complications but should not be recommended in other forms of TB.

#### **2.** *The persistent activation of microglial cells.*

A significant role in the pathogenic mechanisms of CNS infections was assigned to the activation of microglial cells, the resident macrophages of the CNS. Microglial cells are involved in the local phagocytosis and play a central role in the pathogenesis of infections and inflammatory diseases [35]. These cells also represent the main target of both HIV and mycobacteria infection [36,37]. Thus the activation of microglial cells by mycobacteria induces the release of proinflammatory cytokines, some of which are able to add to the stability of cerebral granulomas. A moderate level of CXCL9 and CXCL10 chemokines released by microglial cells regulates the influx of inflammatory cells to the brain and interferes with the chemotaxis of monocytes/macrophages and T cells thus assisting the formation of granulomas. However since microglial cells are the main source of cerebral TNF-α these could also induce an aggresive inflammatory response with severe menin‐ geal inflammation, brain edema, protein accumulation, endarteritis and intracranial hypertension accounting for most of the complications described in NTB [28,38]. There‐ fore a balanced activation of microglial cells is critical against the CNS mycobacterial invasion.OntheotherhandtheintracellularHIVreplicationinmicroglial cells leads totheir activation,neuroinflammationandreleaseofneurotoxinsthatcauseAIDSassociatedneural dysfunctions. The complex role of the microglia in cerebral HIV/TB co-infection is ex‐ plained by the rich number of HIV receptors and co-receptors expressed by these cells such as CD4, CCR5, CXCR4 as well as other receptors involved in the inflammatory response including IFN-γ, TNF-α,CD14 and MHC class I and II receptors [39].The CD14 receptor promotes the uptake of both HIV and nonopsonized M.tbc strains in microglial cells [40] while CD4 and CCR5/CXCR4 co-receptors interfere with HIV cell attachment. As a result microgial cells are the main target of HIV and mycobacteria once these enter the CNS. Therapies directed towards reducing the inflammatory response in the HIV/TB coinfection include the blockage of certain receptors (such as CD14), the use of CCR5 antagonists and TNF-α blockers (as thalidomide). Another alternative is dexametasone recommendedinmostformsofCNSTB.Theclinicalbenefitsofdexametazonewereinspired by in vitro studies proving a potent inhibitory effect on the release of cytokines from microglia [39].

sis a high level of TNF-α stimulates the apoptosis of infected macrophages and the cellular activation [16,17]. On the other hand the use of TNF-*α* neutralizing antibodies in inflammato‐ ry diseases has been associated with an increased risk of extrapulmonary TB including TBM [18].CD4-T-cell deficient mice [19] as well as mice able to neutralize endogenous TNf- *α* [20] or the gene for IFN-γ [21] are subjected to fatal TB. Nevertheless an in vitro experiment on human monocytes noted that higher levels of TNF-α could be associated with more virulent or faster growing mycobacterial strains [22].The contradictory effect of TNF-α was also observed in the HIV infection. Studies conducted by Lane and Osborn proved that TNF-α is a potent inhibitor over the primary HIV infection of the macrophages but enhances the HIV replication in latent HIV infections [23,24 ]. This finding could explain why mycobacteria infections which pro‐ mote the synthesis of TNF-α could also augment HIV replication in chronic infected individu‐ als. The level of TNF-α in the blood of patients infected with mycobacteria and HIV was documented to be 3 to 10 times higher than in non-HIV patients [25] showing a major imbal‐ ance in the release of this proinflammatory cytokine. TNF- α also plays a central role in the CNS localizations of mycobacteria. The excessive amount of TNF-α could accelerate the disruption of rich tuberculous foci adjacent to the CNS. Increased levels of TNF-α as well as IFN-γ were found in the CSF of patients with TBM at the disease onset [26] as well as several months after the acute episode [27] Experimental studies on rabbits proved that the excess of TNF- α acts as a persistent trigger of the inflammatory response and as a procoagulant factor associated with both the mycobacteria CNS invasion as well as cerebral vascular complications. [28]. The therapeutic use of TNF-α inhibitors in severe forms of TBM, tuberculoma and cerebral tubercu‐ lousabscesseswas linkedtoadecreasedinflammatoryresponseandnoticeableclinicalrecovery [29-31]. The major role of TNF-α in the progression of TBM was also proved in murine models by Tsenova as well [28,33]. Studies on HIV patients with TBM also emphasized the signifi‐

cance of increased levels of CSF TNF-α and of IFN-γ in advanced TBM stages [34].

in other forms of TB.

**2.** *The persistent activation of microglial cells.*

296 Tuberculosis - Current Issues in Diagnosis and Management

*In conclusion* all these studies proved that important variations of the Th1 cytokine profile and especially of those involving the release of TNF-α represent one of the pathogenic mechanisms that aggravate the outcome of NTB in the HIV infection. Understanding these changes could be the first step towards the development of efficient complementary therapies in NTB to reduce the excessive inflammatory response. Thus TNF-α inhibition could be used as an antiinflammatory therapy in NTB with severe complications but should not be recommended

A significant role in the pathogenic mechanisms of CNS infections was assigned to the activation of microglial cells, the resident macrophages of the CNS. Microglial cells are involved in the local phagocytosis and play a central role in the pathogenesis of infections and inflammatory diseases [35]. These cells also represent the main target of both HIV and mycobacteria infection [36,37]. Thus the activation of microglial cells by mycobacteria induces the release of proinflammatory cytokines, some of which are able to add to the stability of cerebral granulomas. A moderate level of CXCL9 and CXCL10 chemokines released by microglial cells regulates the influx of inflammatory cells to the brain and interferes with the chemotaxis of monocytes/macrophages and T cells thus assisting the *In conclusion* simultaneous infection of the microglia with HIV and mycobacteria increases the meningeal inflammatory response, the fundamental pathogenic step in all forms of CNS TB. The synthesis of excessive inflammatory infiltrate is responsible for the clinical findings and possibly irreversible complications in NTB, such as hydrocephalus and vasculitis [41]. Moreover the excessive inflammatory response triggered in the HIV/TB co-infection could induce the immune reconstitution inflammatory syndrome – a complication that is specific for this patient category.
