Current HIV Research - Volume 12, Issue 2, 2014

Monocytes, macrophages, and microglia, play multiple roles in the pathogenesis of HIV infection. In addition to providing a tissue reservoir of infection, HIV infected and non-infected macrophages enter and accumulate in the Central Nervous System, where virus infection spreads to resident glial cells including astrocytes and microglia. This process contributes to the secretion of toxic molecules and inflammatory cytokines, which contribute to neuronal damage and promote a spectrum of HIVassociated neurological disorders ranging from HIV dementia to asymptomatic neurocognitive impairment. There appears to be an alteration in the dynamics of monocyte/macrophage subsets as a result of HIV infection, with resulting alterations in immune polarization status. It is likely that such alterations contribute to T cell dysfunction in HIV infection and may present important challenges to immune therapeutic and vaccine-based eradication strategies. As the macrophage/microglial reservoir of HIV infection is long-lived and not susceptible to the direct action of current anti-retroviral compounds (as cells already infected), this reservoir likely accumulates in infected persons over-time and may thereby provides an important target for therapeutic strategies aimed at HIV eradication and/or the treatment of HIV associated neurocognitive disorders. In the articles of this issue, experts have provided insights and perspectives regarding host-viral interaction mechanisms and specific molecular pathways involved in HIV induced inflammation, oxidative stress and neuronal injury. The articles within this Hot Topic issue, represent a series of review articles as well as original research papers. The areas covered represent diverse, yet related areas of investigation regarding HIV infection, virus induced alterations gene expression in infected cells, effects on cells of the blood brain barrier, neural injury and protection, and identify new avenues for therapeutic intervention. The monocyte/macrophage lineage, including microglia, are infected by HIV by a process that predominantly involves “R5” virus, interacting with the cellular coreceptor CCR5. The inability of X4 viruses to transmit de novo are unclear, but their appearance during the course of disease progression might suggest they are immunologically unfit and immune dysregulation would be required for successful CXCR4 tropic virus propagation in vivo. If this is the case, effects of HIV in polarizing macrophages toward M2 function may be a critical step in viral pathogenesis as well as persistence. As R5 virus appears critical for transmission and prominent in the pathogenesis of HIV induced CNS disease, the identification of determinants involved in infection through various coreceptors are of major importance, particularly in view of the association with evolution toward X4 tropic virus with AIDS progression. In the paper by Aiamkitsumrit, bioinformatics approaches to the prediction of viral tropism are discussed [6]. The processes whereby monocytes differentiate into specific subsets appear to be dysregulated in AIDS, promoting altered immune polarization and expansion of CD16+ monocytes. CD16+ monocytes are preferentially infected by HIV and have increased capacity to cross the blood barrier, responding to specific cytokines and promoting CNS dysfunction, as review by Williams et al. [2]. HIV encephalitis and consequent neuronal injury has been studied by a number of laboratories and it is clear that virus infection, macrophage/microglial activation, as well as macrophage transmigration of the blood brain barrier are important in the disease progress. It has been poorly understood, however, to what extent, alterations in macrophage/microglial activation and/or trafficking lead to minor neurocognitive impairment in HIV infection. This is currently an important issue, where minor neurocognitive impairment remains prominent in the setting of antiretroviral therapy. Results from Tavazzi et al. [3] here demonstrate that inflammation, in the absence of any obvious CNS infection is evident in both morphologic and phenotypic markers in CNS tissue from patients with minor neurocognitive disorders. The findings emphasize the likely importance of the physical interface, i.e. the blood brain barrier and perivascular region, between the CNS and the periphery in neuroAIDS in the current era of antiretroviral therapy. It will be also be of interest to determine how microglial activation reflects some of the altered dysregulation pathways described in this issue. Once inside target cells, HIV induced dysregulation of host gene expression plays an important role in the disease progress, amplified by autocrine and paracrine mechanisms. The paper by Lynn Pulliam [1] suggests the dual role of virus induced interferon alpha as an important initiator of the immune response, but also having deleterious effects in the setting of chronic infection. Thus, the activation of type I interferon, to which HIV appears to be resistant, may be an important aspect of immune dysregulation during the chronic phase of HIV infection. Interferon alpha secretion leads to a specific “alarm signature” which is likely important for further investigation regarding pathogenesis, development of diagnostics, as well as new targets for therapy. 76 Current HIV Research, 2014, Vol. 12, No. 2 Editorial One mechanism which may account for the indirect effects of HIV infection leading to neuronal injury, appears to be involved in the increased production and secretion of potentially neurotoxic cathepsin B, with altered interactions of cystatin B in patients with HIV infection and neurocognitive disorders, as reviewed by Rivera et al. [4]. Another inflammatory mediator likely involved in the pathogenesis of HIV infection and CNS disorders via microglial production and receptor interactions appears to be mediated by the neurokinin, substance P (SP). As substance P appears to have immune polarizing properties on the myeloid lineage, and promotes inflammation and virus replication, it likely plays a role in both immune dysfunction and neuroAIDS. The modeling and simulation to support clinical investigation with an SP antagonist are discussed in the paper by Barrett et al. [5]. Altered macrophage biology in HIV infection obviously presents unique opportunities for therapeutic intervention. In view of the topics and issues raised by the authors contributing here in Part I and in the next issue, Part II, of this Hot Topic issue, it is likely that strategies targeting monocytes/macrophages and microglia will have broad implications with relevance to AIDS, as well as a wide range of end-organ diseases. We hope that the research and review articles in this two-part issue will promote significant interest in further research in this area.

With successful antiretroviral therapy, HIV-1-infected subjects can achieve undetectable peripheral viral loads and immune homeostasis. However, in a subset of individuals on therapy, peripheral monocytes have a gene expression profile characteristic of a type 1 interferon α (IFN) response. This type 1 IFN response correlates with a number of pathogenic conditions including neural cell injury and in combination with HCV infection, cognitive impairment. Lessons from the non-human primate models of pathogenic and nonpathogenic SIV suggest that returning the initial IFN spike in acute SIV infection to normal allows the immune system to control infection and return to homeostasis. An IFN “alarm” signature, defined as monocyte activation with overexpression of the type1 IFN genes IFI27 and CD169, would be useful for identifying a subset of subjects with HIV-1 infection that could progress to a number of pathologies associated with immune activation including cognitive dysfunction. This strategy is being actively pursued for autoimmune diseases that are characterized by an IFN signature. Therapies to block the IFN signature are under investigation as a means to reset the immune system and in a subset of HIV-1-infected subjects may be an adjuvant to standard antiviral therapy to return cognitive function.

HIV infected people are living longer due to the success of combined antiretroviral therapy (cART). However, greater than 40-70% of HIV infected individuals develop HIV associated neurocognitive disorders (HAND) that continues to be a major public health issue. While cART reduces peripheral virus, it does not limit the low level, chronic neuroinflammation that is ongoing during the neuropathogenesis of HIV. Monocyte transmigration across the blood brain barrier (BBB), specifically that of the mature CD14+CD16+ population that is highly susceptible to HIV infection, is critical to the establishment of HAND as these cells bring virus into the brain and mediate the neuroinflammation that persists, even if at low levels, despite antiretroviral therapy. CD14+CD16+ monocytes preferentially migrate into the CNS early during peripheral HIV infection in response to chemotactic signals, including those from CCL2 and CXCL12. Once within the brain, monocytes differentiate into macrophages and elaborate inflammatory mediators. Monocytes/macrophages constitute a viral reservoir within the CNS and these latently infected cells may perpetuate the neuropathogenesis of HIV. This review will discuss mechanisms that mediate transmigration of CD14+CD16+ monocytes across the BBB in the context of HIV infection, the contribution of these cells to the neuropathogenesis of HIV, and potential monocyte/macrophage biomarkers to identify HAND and monitor its progression.

HIV-associated neurocognitive disorders (HAND) describes different levels of neurocognitive impairment, which are a common complication of HIV infection. The most severe of these, HIV-associated dementia (HIV-D), has decreased in incidence since the introduction of combination antiretroviral therapy (cART), while an increase in the less severe, minor neurocognitive disorder (MND), is now seen. The neuropathogenesis of HAND is not completely understood, however macrophages (Mφ)s/microglia are believed to play a prominent role in the development of the more severe HIV-D. Here, we report evidence of neuroinflammation in autopsy tissues from patients with HIV infection and varying degrees of neurocognitive impairment but without HIV encephalitis (HIVE). Mφ/microglial and astrocyte activation is less intense but similar to that seen in HIVE, one of the neuropathologies underlying HIV-D. Mφs and microglia appear to be activated, as determined by CD163, CD16, and HLA-DR expression, many having a rounded or ramified morphology with thickened processes, classically associated with activation. Astrocytes also show considerable morphological alterations consistent with an activated state and have increased expression of GFAP and vimentin, as compared to seronegative controls. Interestingly, in some areas, astrocyte activation appears to be limited to perivascular locations, suggesting events at the blood-brain barrier may influence astrocyte activity. In contrast to HIVE, productive HIV infection was not detectable by tyramide signal-amplified immunohistochemistry or in situ hybridization in the CNS of HIV infected persons without encephalitis. These findings suggest significant CNS inflammation, even in the absence of detectable virus production, is a common mechanism between the lesser and more severe HIV-associated neurodegenerative disease processes and supports the notion that MND and HIV-D are a continuum of the same disease.

Mononuclear phagocytes including monocytes and macrophages, are important defense components of innate immunity, but can be detrimental in HIV-1 infection by serving as the principal reservoirs of virus in brain and triggering a strong immune response. These viral reservoirs represent a challenge to HIV-1 eradication since they continue producing virus in tissue despite antiretroviral therapy. HIV-1 associated neurocognitive disorders (HAND) involve alterations to the blood-brain barrier and migration of activated HIV-1 infected monocytes to the brain with subsequent induced immune activation response. Our group recently showed that HIV replication in monocyte-derived macrophages is associated with increased cystatin B. This cysteine protease inhibitor also inhibits the interferon-induced antiviral response by decreasing levels of tyrosine phosphorylated STAT-1. These recent discoveries reveal novel mechanisms of HIV persistence that could be targeted by new therapeutic approaches to eliminate HIV in macrophage reservoirs. However, cystatin B has been also associated with neuroprotection. Cystatin B is an inhibitor of the cysteine protease cathepsin B, a potent neurotoxin. During HIV-1 infection cystatin B and cathepsin B are upregulated in macrophages. Reduction in cystatin/cathepsin interactions in infected macrophages leads to increased cathepsin B secretion and activity which contributes to neuronal apoptosis. Increased intracellular expression of both proteins was recently found in monocytes from Hispanic women with HAND. These findings provide new evidence for the role of cathepsin /cystatin system in the neuropathogenesis induced by HIV-infected macrophages. We summarize recent research on cystatin B and one of its substrates, cathepsin B, in HIV replication in macrophages and neuropathogenesis.

Psychiatric illness is common in HIV-infected patients and underlines the importance for screening not only for cognitive impairment but also for co-morbid mental disease. The rationale for combining immunomodulatory neurokinin- 1 receptor (NK1-R) antagonists with combined antiretroviral therapy (cART) is based on multimodal pharmacologic mechanisms. The NK1-R antagonist aprepitant’s potential utility as a drug for depression is complicated by >99.9% protein binding and both enzyme inhibition and induction of CYP3A4. A population-based PK model developed from a pilot Phase 1B trial in 19 HIV-infected patients (125 or 250 mg/d aprepitant for 2 weeks) was modified to account for enzyme induction and impact of an exposure enhancer on CYP3A4 metabolism. Likelihood of clinical success in depression was assessed based on achievement of target trough plasma concentration and evaluated using Monte Carlo simulation. Scenarios were generated for varying daily dose (375, 625, 750 and 875 mg), pharmacokinetic variability, exposure enhancement (EE), duration (2 and 6 months) and sample size (n=12 and 24/arm). Daily dosing of ≥625 mg with EE yielded desirable troughs (based on in vitro infectivity experiments) of > 2.65 ug/mL for the majority of virtual patients simulated. Results are dependent on the degree of exposure enhancement and extent of enzyme induction. Actual threshold exposure requirements for aprepitant in HIV-associated depression are unknown though preclinical evidence supports trough levels > 2.65 ug/mL. If 100% NK1r blockage is necessary for efficacy, doses of 875 mg (625 mg with EE) or higher may be required. The benefit of aprepitant on innate immunity(natural killer cells) and absence of negative effects onex vivo neutrophil chemotaxis alleviates concerns regarding drug dependent inhibition (DDI)-mediated infection risk.

The evolution of human immunodeficiency virus type 1 (HIV-1) with respect to co-receptor utilization has been shown to be relevant to HIV-1 pathogenesis and disease. The CCR5-utilizing (R5) virus has been shown to be important in the very early stages of transmission and highly prevalent during asymptomatic infection and chronic disease. In addition, the R5 virus has been proposed to be involved in neuroinvasion and central nervous system (CNS) disease. In contrast, the CXCR4-utilizing (X4) virus is more prevalent during the course of disease progression and concurrent with the loss of CD4+ T cells. The dual-tropic virus is able to utilize both co-receptors (CXCR4 and CCR5) and has been thought to represent an intermediate transitional virus that possesses properties of both X4 and R5 viruses that can be encountered at many stages of disease. The use of computational tools and bioinformatic approaches in the prediction of HIV-1 co-receptor usage has been growing in importance with respect to understanding HIV-1 pathogenesis and disease, developing diagnostic tools, and improving the efficacy of therapeutic strategies focused on blocking viral entry. Current strategies have enhanced the sensitivity, specificity, and reproducibility relative to the prediction of co-receptor use; however, these technologies need to be improved with respect to their efficient and accurate use across the HIV-1 subtypes. The most effective approach may center on the combined use of different algorithms involving sequences within and outside of the env-V3 loop. This review focuses on the HIV-1 entry process and on co-receptor utilization, including bioinformatic tools utilized in the prediction of co-receptor usage. It also provides novel preliminary analyses for enabling identification of linkages between amino acids in V3 with other components of the HIV-1 genome and demonstrates that these linkages are different between X4 and R5 viruses.