Neuropathogenesis of simian immunodeficiency virus in neonatal rhesus macaques

Non-human Primate Models to Investigate Mechanisms of Infection-Associated Fetal and Pediatric Injury, Teratogenesis and Stillbirth

A wide array of pathogens has the potential to injure the fetus and induce teratogenesis, the process by which mutations in fetal somatic cells lead to congenital malformations. Rubella virus was the first infectious disease to be linked to congenital malformations due to an infection in pregnancy, which can include congenital cataracts, microcephaly, hearing impairment and congenital heart disease. Currently, human cytomegalovirus (HCMV) is the leading infectious cause of congenital malformations globally, affecting 1 in every 200 infants. However, our knowledge of teratogenic viruses and pathogens is far from complete. New emerging infectious diseases may induce teratogenesis, similar to Zika virus (ZIKV) that caused a global pandemic in 2016-2017; thousands of neonates were born with congenital microcephaly due to ZIKV exposure in utero, which also included a spectrum of injuries to the brain, eyes and spinal cord. In addition to congenital anomalies, permanent injury to fetal and neonatal organs, preterm birth, stillbirth and spontaneous abortion are known consequences of a broader group of infectious diseases including group B streptococcus (GBS), Listeria monocytogenes, Influenza A virus (IAV), and Human Immunodeficiency Virus (HIV). Animal models are crucial for determining the mechanism of how these various infectious diseases induce teratogenesis or organ injury, as well as testing novel therapeutics for fetal or neonatal protection. Other mammalian models differ in many respects from human pregnancy including placentation, labor physiology, reproductive tract anatomy, timeline of fetal development and reproductive toxicology. In contrast, non-human primates (NHP) most closely resemble human pregnancy and exhibit key similarities that make them ideal for research to discover the mechanisms of injury and for testing vaccines and therapeutics to prevent teratogenesis, fetal and neonatal injury and adverse pregnancy outcomes (e.g., stillbirth or spontaneous abortion). In this review, we emphasize key contributions of the NHP model pre-clinical research for ZIKV, HCMV, HIV, IAV, L. monocytogenes, Ureaplasma species, and GBS. This work represents the foundation for development and testing of preventative and therapeutic strategies to inhibit infectious injury of human fetuses and neonates.

The Brain Retains: Nonhuman Primate Models for Pediatric HIV-1 in the CNS

PURPOSE OF REVIEW

Perinatal HIV-1 infection is associated with an increased risk for neurologic impairments. With limited access to clinical specimens, animal models could advance our understanding of pediatric central nervous system (CNS) disease and viral persistence. Here, we summarize current findings on HIV-1 CNS infection from nonhuman primate (NHP) models and discuss their implications for improving pediatric clinical outcomes.

RECENT FINDINGS

SIV/SHIV can be found in the CNS of infant macaques within 48 h of challenge. Recent studies show an impermeable BBB during SIV infection, suggesting neuroinvasion in post-partum infection is likely not wholly attributed to barrier dysfunction. Histopathological findings reveal dramatic reductions in hippocampal neuronal populations and myelination in infected infant macaques, providing a link for cognitive impairments seen in pediatric cases. Evidence from humans and NHPs support the CNS as a functional latent reservoir, harbored in myeloid cells that may require unique eradication strategies. Studies in NHP models are uncovering early events, causes, and therapeutic targets of CNS disease as well as highlighting the importance of age-specific studies that capture the distinct features of pediatric HIV-1 infection.

Perivascular macrophages in the neonatal macaque brain undergo massive necroptosis after simian immunodeficiency virus infection

We previously showed that rhesus macaques neonatally infected with simian immunodeficiency virus (SIV) do not develop SIV encephalitis (SIVE) and maintain low brain viral loads despite having similar plasma viral loads compared to SIV-infected adults. We hypothesize that differences in myeloid cell populations that are the known target of SIV and HIV in the brain contribute to the lack of neonatal susceptibility to lentivirus-induced encephalitis. Using immunohistochemistry and immunofluorescence microscopy, we examined the frontal cortices from uninfected and SIV-infected infant and adult macaques (n = 8/ea) as well as adults with SIVE (n = 4) to determine differences in myeloid cell populations. The number of CD206+ brain perivascular macrophages (PVMs) was significantly greater in uninfected infants than in uninfected adults and was markedly lower in SIV-infected infants while microglia numbers were unchanged across groups. CD206+ PVMs, which proliferate after infection in SIV-infected adults, did not undergo proliferation in infants. While virtually all CD206+ cells in adults are also CD163+, infants have a distinct CD206 single-positive population in addition to the double-positive population commonly seen in adults. Notably, we found that more than 60% of these unique CD206+CD163- PVMs in SIV-infected infants were positive for cleaved caspase-3, an indicator of apoptosis, and that nearly 100% of this subset were concomitantly positive for the necroptosis marker receptor-interacting protein kinase-3 (RIP3). These findings show that distinct subpopulations of PVMs found in infants undergo programmed cell death instead of proliferation following SIV infection, which may lead to the absence of PVM-dependent SIVE and the limited size of the virus reservoir in the infant brain.

Lack of susceptibility in neonatally infected rhesus macaques to simian immunodeficiency virus-induced encephalitis

Despite combination antiretroviral therapies making HIV a chronic rather than terminal condition for many people, the prevalence of HIV-associated neurocognitive disorders (HAND) is increasing. This is especially problematic for children living with HIV. Children diagnosed HAND rarely display the hallmark pathology of HIV encephalitis in adults, namely infected macrophages and multinucleated giant cells in the brain. This finding has also been documented in rhesus macaques infected perinatally with simian immunodeficiency virus (SIV). However, the extent and mechanisms of lack of susceptibility to encephalitis in perinatally HIV-infected children remain unclear. In the current study, we compared brains of macaques infected with pathogenic strains of SIV at different ages to determine neuropathology, correlates of neuroinflammation, and potential underlying mechanisms. Encephalitis was not found in the macaques infected within 24 h of birth despite similar high plasma viral load and high monocyte turnover. Macaques developed encephalitis only when they were infected after 4 months of age. Lower numbers of CCR5-positive cells in the brain, combined with a less leaky blood-brain barrier, may be responsible for the decreased virus infection in the brain and consequently the absence of encephalitis in newborn macaques infected with SIV.

Chronic Viral Neuroinflammation: Speculation on Underlying Mechanisms

Viral infection in the brain can be acute or chronic, with the responses often producing foci of increasingly cytotoxic inflammation. This can lead to effects beyond the central nervous system (CNS). To stimulate discussion, this commentary addresses four questions: What drives the development of human immunodeficiency virus (HIV)-associated neurocognitive disorders, does the phenotype of macrophages in the CNS spur development of HIV encephalitis (HIVE), does continual activation of astrocytes drive the development of HIV-associated neurocognitive disorders/subclinical disease, and neuroinflammation: friend or foe? A unifying theory that connects each question is the issue of continued activation of glial cells, even in the apparent absence of simian immunodeficiency virus/HIV in the CNS. As the CNS innate immune system is distinct from the rest of the body, it is likely there could be a number of activation profiles not observed elsewhere.

Of mice and monkeys: can animal models be utilized to study neurological consequences of pediatric HIV-1 infection?

Pediatric human immunodeficiency virus (HIV-1) infection remains a global health crisis. Children are much more susceptible to HIV-1 neurological impairments than adults, which can be exacerbated by coinfections. Neurological characteristics of pediatric HIV-1 infection suggest dysfunction in the frontal cortex as well as the hippocampus; limited MRI data indicate global cerebral atrophy, and pathological data suggest accelerated neuronal apoptosis in the cortex. An obstacle to pediatric HIV-1 research is a human representative model system. Host-species specificity of HIV-1 limits the ability to model neurological consequences of pediatric HIV-1 infection in animals. Several models have been proposed including neonatal intracranial injections of HIV-1 viral proteins in rats and perinatal simian immunodeficiency virus (SIV) infection of infant macaques. Nonhuman primate models recapitulate the complexity of pediatric HIV-1 neuropathogenesis while rodent models are able to elucidate the role specific viral proteins exert on neurodevelopment. Nonhuman primate models show similar behavioral and neuropathological characteristics to pediatric HIV-1 infection and offer a stage to investigate early viral mechanisms, latency reservoirs, and therapeutic interventions. Here we review the relative strengths and limitations of pediatric HIV-1 model systems.

Atypical nodular astrocytosis in simian immunodeficiency virus-infected rhesus macaques (Macaca mulatta)

BACKGROUND

Simian immunodeficiency virus (SIV), a model for HIV pathogenesis, is associated with neuropathology.

METHODS

Five SIV-infected animals were selected following a database search of 1206 SIV-infected animals for nodular or astrocytic lesions. Two of five had neurologic dysfunction, and 3 of 5 were incidental findings.

RESULTS

Histologic examination revealed multifocal nodular foci in the gray and white matter formed by interlacing astrocytes with abundant cytoplasm and large, reactive nuclei. Nodules were often enmeshed with small capillaries. Immunohistochemistry revealed variable immunoreactivity for a panel of markers: GFAP (4/5), vimentin (5/5), Glut-1 (1/5), CNPase (0/5), S100 (5/5), Iba1 (0/5), Ki67 (0/5), and p53 (4/4). In situ hybridization failed to detect any SIV RNA (0/5). Immunohistochemistry for simian virus 40, rhesus cytomegalovirus, and rhesus lymphocryptovirus failed to detect any antigen within the lesions.

CONCLUSION

The immunoreactivity of p53 in the lesions compared with adjacent tissue suggests a local derangement in astrocyte proliferation and function.

MCP-3/CCL7 production by astrocytes: implications for SIV neuroinvasion and AIDS encephalitis

Monocyte/macrophages and activated lymphocytes traffic through normal brain, and this trafficking is increased in inflammatory conditions such as HIV encephalitis (HIVE). HIVE is characterized in part by perivascular accumulations of macrophages. The earliest events in this process are poorly understood and difficult or impossible to address in humans. The SIV-infected macaque model of neuroAIDS has demonstrated migration of monocytes into the brain early in disease, coincident with peak SIV viremia. The chemotactic signals that initiate the increased emigration of mononuclear cells into the CNS have not been described. Here, we describe astrocytes as a primary source of chemokines to facilitate basal levels of monocyte trafficking to CNS and that increased chemokine (C-C motif) ligand 7 (CCL7) production may be responsible for initiating the increased trafficking in neuroAIDS. We have previously published complementary in vivo work demonstrating the presence of monocyte chemoattractant protein 3 (MCP-3)/CCL7 within the brain of SIV-infected macaques. Here, we demonstrate that MCP-3/CCL7 is a significant chemokine produced by astrocytes, that basal monocyte migration may be facilitated by astrocyte-derived CCL7, that production of CCL7 is rapidly increased by TNF-α and thus likely plays a critical role in initiating neuroinvasion by SIV/HIV.

Enteric ganglionitis in rhesus macaques infected with simian immunodeficiency virus

Gastrointestinal (GI) disease is a debilitating feature of human immunodeficiency virus (HIV) infection that can occur in the absence of histopathological abnormalities or identifiable enteropathogens. However, the mechanisms of GI dysfunction are poorly understood. The present study was undertaken to characterize changes in resident and inflammatory cells in the enteric nervous system (ENS) of macaques during the acute stage of simian immunodeficiency virus (SIV) infection to gain insight into potential pathogenic mechanisms of GI disease. Ganglia from duodenum, ileum, and colon were examined in healthy and acutely infected macaques by using a combination of routine histology, double-label immunofluorescence and in situ hybridization. Evaluation of tissues from infected macaques showed progressive infiltration of myenteric ganglia by CD3+ T cells and IBA1+ macrophages beginning as early as 8 days postinfection. Quantitative image analysis revealed that the severity of myenteric ganglionitis increased with time after SIV infection and, in general, was more severe in ganglia from the small intestine than in ganglia from the colon. Despite an abundance of inflammatory cells in myenteric ganglia during acute infection, the ENS was not a target for virus infection. This study provides evidence that the ENS may be playing a role in the pathogenesis of GI disease and enteropathy in HIV-infected people.

In vitro characterization of primary SIVsmm isolates belonging to different lineages. In vitro growth on rhesus macaque cells is not predictive for in vivo replication in rhesus macaques

We report in vitro characterization of 11 SIVsmm strains of six lineages co-circulating in naturally infected sooty mangabeys (SMs) from US Primate Centers and showed no major differences in the in vitro replication pattern between different SIVsmm lineages. Primary SIVsmm isolates utilized CCR5 and Bonzo co-receptors in vitro. SIVsmm growth in human T cell lines was isolate-, not lineage-specific, with poor replication on Molt4-Clone8, CEMss and PM1 cells and better replication on MT2, SupT1 and CEMx174 cells. All primary SIVsmm isolates replicated on SM and human PBMCs. In vitro replication in macaques varied widely, with moderate to high replication in pig-tailed macaque PBMCs, enhanced by CD8+ T cell depletion, and highly variable replication on rhesus macaque (Rh) PBMCs. Primary SIVsmm isolates replicated in Rh monocyte-derived dendritic cells (MDDCs) and monocyte-derived macrophages (MDMs). In vivo, SIVsmm isolates replicated at high levels in all SIVsmm-infected Rh. The poor in vitro replication of primary SIVsmm isolates in Rh cells did not correlate with in vivo replication, emphasizing the value of in vivo studies.

Cognitive and motor deficits associated with HIV-2(287) infection in infant pigtailed macaques: a nonhuman primate model of pediatric neuro-AIDS

Lentivirus-infected nonhuman primates exhibit behavioral and neurological pathology similar to human immunodeficiency virus (HIV)-infected humans and offer a means to examine the effects of lentivirus infection while controlling for confounding factors inherent in human populations. The purpose of this study was to examine cognitive and motor development in infant macaques vertically infected with HIV-2287. Subjects were 20 infant pigtail macaques (Macaca nemestrina); 8 controls born to uninfected dams, and 12 infants whose dams had been inoculated and infected with HIV-2287 in the third trimester of pregnancy. Eight of these pregnancies had undergone surgical procedures in the form of maternal amniotic catheters or maternal amniotic and fetal carotid artery and jugular vein catheters. Data indicated that catheterization had little or no impact on behavioral development. Seven infants were vertically infected (as measured by polymerase chain reaction (PCR) at birth) and five were not infected (as measured by PCR and coculture on repeated testing). Infected infants attained cognitive and motor milestones at significantly later ages than controls. Uninfected infants, born to infected dams, attained developmental milestones at later ages than controls on all tasks, but this reached statistical significance only for the Fine Motor Task. Attainment of milestones was not correlated with viral dose, maternal CD4+ levels at parturition or infant viral RNA levels at birth. Attainment of milestones was negatively correlated with infants' proportions of CD4+ lymphocytes at birth and significantly correlated with proportions of CD4+ lymphocytes 2 weeks after birth, indicating poorer performance in those infants with a more rapid CD4+ depletion. These cognitive and motor deficits closely resemble those observed in human infants and children infected with HIV and indicate that HIV-2287-infected infant macaques represent an excellent model of pediatric neuro-acquired immunodeficiency syndrome (neuroAIDS).

Cell tropism of simian immunodeficiency virus in culture is not predictive of in vivo tropism or pathogenesis

SIVmac239/316 is a molecular clone derived from SIVmac239 that differs from the parental virus by nine amino acids in env. This virus, unlike the parental SIVmac239, is able to replicate well in alveolar macrophages in culture. We have not however, observed macrophage-associated inflammatory disease in any animal infected with SIVmac239/316. Therefore, we sought to examine the cell tropism of this virus in vivo in multiple tissues using in situ hybridization combined with immunohistochemistry and multilabel confocal microscopy for viral nucleic acid and multiple cell-type-specific markers for macrophages and T lymphocytes. Tissues examined included brain, heart, lung, lymph nodes, spleen, thymus, and small and large intestine. Matched tissues from macaques infected with the parental SIVmac239 and uninfected macaques were also examined. Many infected cells were detected in the tissues of animals infected with SIVmac239 and SIVmac239/316 although there appeared to be fewer positive cells in animals infected with SIVmac239/316. Surprisingly, in light of the cell culture observations, nearly every simian immunodeficiency virus-infected cell in animals inoculated with SIVmac239/316 was a T lymphocyte rather than a macrophage. This was true both during early infection (first 2 months) and in terminal disease. In contrast, as previously described, SIVmac239 was found in both T cells and macrophages in tissues as early as 21 days after infection. These studies indicate that during both acute and chronic SIVmac239/316 infection T lymphocytes rather than macrophages are the principal targets in vivo. These data combined with the absence of macrophage-associated lesions in SIVmac239/316-infected animals indicate that in vitro cell tropism is not predictive of in vivo tropism or disease pathogenesis.

Simian immunodeficiency virus infection in neonatal macaques

Children with human immunodeficiency virus infection often have higher viral loads and progress to AIDS more rapidly than adults. Since the intestinal tract is a major site of early viral replication and CD4(+) T-cell depletion in adults, we examined the effects of simian immunodeficiency virus (SIV) on both peripheral and intestinal lymphocytes from 13 neonatal macaques infected with SIVmac239. Normal neonates had more CD4(+) T cells and fewer CD8(+) T cells in all tissues than adults. Surprisingly, neonates had substantial percentages of CD4(+) T cells with an activated, memory phenotype (effector CD4(+) T cells) in the lamina propria of the intestine compared to peripheral lymphoid tissues, even when examined on the day of birth. Moreover, profound and selective depletion of jejunum lamina propria CD4(+) T cells occurred in neonatal macaques within 21 days of infection, which was preceded by large numbers of SIV-infected cells in this compartment. Furthermore, neonates with less CD4(+) T-cell depletion in tissues tended to have higher viral loads. The persistence of intestinal lamina propria CD4(+) T cells in some neonates with high viral loads suggests that increased turnover and/or resistance to CD4(+) T-cell loss may contribute to the higher viral loads and increased severity of disease in neonatal hosts.

Neurovirulence depends on virus input titer in brain in feline immunodeficiency virus infection: evidence for activation of innate immunity and neuronal injury

Lentiruses cause neurological disease depending on the virus strain and its neurotropism, yet it remains uncertain to what the impact of infectious virus quantity in the brain early in infection is on the subsequent development of neurological disease or neurovirulence. We investigated the relationship between infectious virus input titer and the resulting neurovirulence, using ex vivo and in vivo assays of feline immunodeficiency virus (FIV)-induced neurovirulence. FIV infection of cell cultures and neonatal cats was performed using 10(2.5) (low-titer) or 10(4.5) (high-titer) 50% tissue culture infectious doses (TCID(50))/ml of the neurovirulent FIV strain, V1CSF. Ex vivo neurotoxicity assays revealed that conditioned medium (CM) from feline macrophages infected with high-titer (P <.001) or low-titer (P <.01) V1CSF induced greater neuronal death than CM from mock-infected cells. In vivo, animals infected intracranially with high-titer V1CSF showed neurodevelopmental delays compared to mock-infected animals (P <.001) and animals infected with low-titer V1CSF (P <.02), concurrent with reduced weight gains and greater depletion of CD4+ cells over a 12-week period. Neuropathological changes, including astrogliosis, macrophage activation, and neuronal damage, were evident in V1CSF-infected animals and were viral titer dependent. In vivo magnetic resonance (MR) spectroscopy and proton nuclear magnetic resonance ((1)H-NMR) spectroscopy of tissue extracts revealed evidence of neuronal injury, including reduced N-acetyl aspartate/creatine (P <.05) and increased trimethylamine/creatine (P <.05) ratios, in the frontal cortex of high-titer V1CSF-infected animals compared to the other groups. T2-weighted MR imaging detected increased signal intensities in the frontal cortex and white matter of V1CSF-infected animals relative to controls, which was more evident as viral titer increased (P <.01). The present findings indicate that lentivirus infectious titers in the brain during the early stages of infection determine the severity of neurovirulence, reflected by neurobehavioral deficits, together with neuroradiological and neuropathological findings of activation of innate immunity and neuronal injury.

Envelope gene-mediated neurovirulence in feline immunodeficiency virus infection: induction of matrix metalloproteinases and neuronal injury

The release of neurotoxins by activated brain macrophages or microglia is one mechanism proposed to contribute to the development of neurological disease following infection by lentiviruses, including feline immunodeficiency virus (FIV). Since molecular diversity in the lentiviral envelope gene influences the expression of host molecules implicated in neuronal injury, the role of the envelope sequence in FIV neuropathogenesis was investigated by using the neurovirulent FIV strain V1CSF, the nonneurovirulent strain Petaluma, and a chimera (FIVCh) containing the V1CSF envelope gene in a Petaluma background. All three viruses replicated in primary feline macrophages with equal efficiency, but conditioned medium from V1CSF- or FIVCh-infected cells was significantly more neurotoxic than medium from Petaluma-infected cultures (P < 0.001) and could be attenuated in a dose-dependent manner by treatment with either the matrix metalloproteinase (MMP) inhibitor prinomastat (PMT) or function-blocking antibodies to MMP-2. Although FIV sequences were detectable by PCR in brain tissue from neonatal cats infected with each of the viral strains, immunohistochemistry revealed increased astrogliosis and macrophage activation in the brains of V1CSF- and FIVCh-infected cats relative to the other groups, together with elevated markers of neuronal stress that included morphological changes and increased c-fos immunoreactivity. Similarly, MMP-2, but not MMP-9, mRNA and protein expression was increased in brain tissues of V1CSF- and FIVCh-infected cats relative to Petaluma-infected animals (P < 0.01). Infection with V1CSF or FIVCh was also associated with greater CD4(+) cell depletion (P < 0.001) and neurodevelopmental delays (P < 0.005), than in Petaluma-infected animals; these deficits improved following PMT therapy. These findings indicated that diversity in the envelope gene sequence influenced the neurovirulence exhibited by FIV both in vitro and in vivo, possibly through a mechanism involving the differential induction of MMP-2.

Macaques with rapid disease progression and simian immunodeficiency virus encephalitis have a unique cytokine profile in peripheral lymphoid tissues

The influence of host cytokine response on viral load, disease progression, and neurologic lesions was investigated in the simian immunodeficiency virus (SIV)-infected macaque model of AIDS. Cytokine gene expression (interleukin-1beta [IL-1beta], IL-2, IL-6, IL-10, gamma interferon [IFN-gamma], and tumor necrosis factor alpha [TNF-alpha]) and viral loads were evaluated by semiquantitative reverse transcription-PCR in lymph nodes of 5 control animals and 28 animals infected with SIVmac251 at the terminal stages of AIDS. Infected animals showed higher expression of IFN-gamma, IL-6, and IL-10 mRNAs compared with controls. Levels of all cytokines were comparable between animals with rapid (survival, <200 days) or slow/normal (survival, >200 days) disease progression. However, among rapid progressors, the eight animals with SIV encephalitis had a unique cytokine profile (increased IL-2, IL-6, and IFN-gamma) that was associated with higher viral loads. These observations provide evidence that host cytokine responses may influence SIV neuropathogenesis independent of disease progression.

Suboptimal nucleotides in the infectious, pathogenic simian immunodeficiency virus clone SIVmac239

We analyzed virus sequences in two monkeys infected with SIVmac239 and two monkeys infected with SHIVnef that maintained high, persisting viral loads. Sequence changes were observed consistently at four loci in all four animals: a single nucleotide change in the Lys-tRNA primer binding site in the 5' long terminal repeat; two nucleotide changes that resulted in two amino acid changes in the pol gene product; and a single nucleotide change in the region of the simian immunodeficiency virus genome where the rev and env genes overlap, resulting in changes in the predicted amino acid sequences of both gene products. None of these mutations were seen in short-term cultures of CEMx174 cells infected with SIVmac239 or SHIVnef. At all four positions in all four animals, the new sequences represented consensus sequences for primate lentiviruses, whereas the inoculum sequences at these four loci have either never been or rarely been reported outside of SIVmac239. Thus, although cloned SIVmac239 is consistently pathogenic and consistently induces high viral load set points, it is clearly less than optimal at these four nucleotide positions.