This protocol describes how the combination of EcoHIV infection with Tmem119-EGFP mice offers a valuable biological system for investigating microglial alterations and viral reservoirs in rodent models of HIV-associated neurocognitive disorders.
Combined antiretroviral therapy (cART) has dramatically improved the quality of life for people living with HIV (PLWH). However, over 4 million PLWH are over the age of fifty and experience accompanying HIV-associated neurocognitive disorders (HAND). To understand how HIV impacts the central nervous system, a reliable and feasible model of HIV is necessary. Previously, a novel biological system using chimeric HIV (EcoHIV) inoculation was developed in a rat model to investigate neurocognitive impairments and synaptic dysfunction. Nevertheless, a significant challenge remains in clarifying EcoHIV's neuroanatomical distribution, particularly its differential expression in various cell types in the brain. In the current study, EcoHIV with mScarlet fluorescence labeling was modified and retro-orbitally injected into Tmem119-EGFP knock-in mice (which express enhanced green fluorescence protein primarily in microglia) to determine if microglia are the major cell type responsible for viral expression and reservoirs of HIV in the brain. The current data show that: (1) in vitro, EcoHIV-mScarlet fluorescence signals were predominantly localized in microglia-like cells among primary rodent brain cells; (2) in vivo, injection of EcoHIV-mScarlet into Tmem119-EGFP mice induced significant HIV expression in the mouse brain. The co-localization of mScarlet and EGFP signals suggests that microglia are the main cell type harboring HIV in the brain. Overall, EcoHIV in rodents offers a valuable biological system to study microglial alterations, viral reservoirs in the brain, and the neurological mechanisms of HIV-associated neurocognitive disorders.
Despite the profound benefits derived from antiretroviral therapy, people living with HIV (PLWH) still experience neurocognitive disorders. To better understand the neuronal mechanisms of HIV-associated neurocognitive disorder (HAND), there is a critical need for HIV models to further elucidate the specific cell type involvement in NeuroHIV.
The HIV-1 transgenic rat, which has constitutive exposure to HIV-1 viral proteins, is a popular rodent model used to investigate neurocognitive disorders1,2,3,4and neuroanatomical alterations5,6,7 associated with HAND. The functional deletion of the gag and pol domains prevents viral replication, rendering the HIV-1 transgenic rat noninfectious8,9. Recently, a chimeric HIV (EcoHIV) infection model in mice was initially reported by Potash et al.10 and later extended to rats, which may be advantageous for further studies on HAND and substance use disorders11. In this novel biological system, systemic HIV-1 infection was observed along with many clinical features of HIV-1 in humans, including lymphocyte and macrophage involvement, antiviral immune responses, neuroinvasiveness, and brain inflammation.
Microglia play a critical role as specialized brain-resident macrophages in maintaining brain function and homeostasis. To distinguish microglia from closely related cell types (e.g., blood monocytes, perivascular macrophages, meningeal macrophages), the current study used the Tmem119-EGFP knock-in mouse line. Previous studies have reported that transmembrane protein 119 (Tmem119) exhibits an exclusively microglia-specific expression pattern in rodent and human brain tissue12,13,14,15. The EGFP signal in Tmem119-EGFP knock-in mice was observed throughout the brain and localized specifically to microglial cells.
In the present study, Tmem119-EGFP knock-in mice were inoculated with the EcoHIV-mScarlet virus, and cells positive for EcoHIV in the central nervous system were identified. Here, we present a protocol for EcoHIV-mScarlet inoculation in Tmem119-EGFP knock-in mice, providing a reliable model for therapeutically targeting microglial alterations in HIV.
The Animal Care and Use Committee at the University of South Carolina approved all animal protocols (federal assurance number: D16-00028). All experiments strictly followed the guidelines established by the National Institutes of Health in the Guide for the Care and Use of Laboratory Animals. Tmem119-EGFP knock-in mice (30 days old, male, 23-26 g body weight) were obtained from a commercial source and group-housed in AAALAC-accredited facilities. All animals were housed under a 12/12 h light-dark cycle with free access to food and water. The details of the animals, reagents, and equipment used in this study are listed in the Table of Materials.
1. EcoHIV-mScarlet packaging in 293FT cells
2. EcoHIV-mScarlet infection in primary rat brain cells
3. EcoHIV-mScarlet virus infection in primary glia cells from adult mice
4. EcoHIV-mScarlet virus retroorbital injection into Tmem119-EGFP mice
5. Visualization of brain tissue slices
A fragment of mScarlet (1858 bp) containing enzyme sites of "Cla1" at the 3' end and "Not1" at the 5' end was inserted into pNL4-3-EcoHIV lentiviral vector (Figure 1). To validate the expression of EcoHIV-mScarlet, primary brain cells isolated from the cortex of rat E18 embryos were treated with EcoHIV-mScarlet (60 µL, 1.26 × 106 TU/mL) for 6 days in vitro. The data in Figure 2 showed that red fluorescent signals of mScarlet were mainly localized in glial types of cells based on the different cell morphology. Furthermore, CD11b/c and Iba1 (cell markers for microglia) labeling showed that mScarlet signals were co-localized with CD11b/c + and/or Iba1 + cells. The data indicated that microglia were the major cell type of EcoHIV-mScarlet distribution in vitro. There was no significant neuronal infection in the cultured cells (Supplementary Figure 1).
Next, the infection of EcoHIV-mScarlet was tested on adult mouse primary mixed glial cells. To do so, mixed glia cells were first isolated and purified from adult mice (2 months) and infected with EcoHIV-mScarlet (8 µL, 1.26 × 106 TU/mL) for 2 days. The images in Figure 3 showed that EcoHIV-mScarlet successfully infected adult mouse glia.
To further address the distribution pattern of EcoHIV-mScarlet in the mouse brain, specifically for identifying the infected cell type, EcoHIV-mScarlet was retro-orbital injected into Tmem119-EGFP knock-in mouse line in which microglia were specifically labeled with EGFP signals without any other macrophage types of conflicts5. The results in Figure 4 (also in Supplementary Figure 2) show that mScarlet red fluorescence signals were observed in EGFP-positive cells, suggesting microglia as the major cell type of EcoHIV expression in mouse brains.
Figure 1: The vector map of EcoHIV-NL-4-3-mScarlet. Please click here to view a larger version of this figure.
Figure 2: The EcoHIV-mScarlet infection in primary rat brain cells. (A,E) Representative images of CD11b/c staining at primary brain cells. The brain cells were isolated from E18 rat embryos and infected with the EcoHIV-mScarlet virus for 6 days. (B,F) Representative images of mScarlet fluorescent signals from in vitro brain cells. (C,G) Representative images of Iba1 staining at primary brain cells. (D,H) Merged images of triple labeling of CD11b/c, mScarlet, and Iba1. Scale bars: 50 µm. Please click here to view a larger version of this figure.
Figure 3: EcoHIV-mScarlet infection in primary mouse mixed glial cells. (A,B) Representative confocal images of mScarlet distribution in vitro. The primary mixed glia cells were isolated from adult C57BL6 mice (2-month-old) and cultured for 2 weeks before viral infection. The EcoHIV-mScarlet was added into the culture medium for two days and captured the images under the 60× objective of the Confocal microscope. Scale bar: 10 µm. Please click here to view a larger version of this figure.
Figure 4: EcoHIV-mScarlet distribution in Tmem119-EGFP knock-in mouse line. (A,B) Merged images of mScarlet/EGFP/ToPro3 signals in brain sections. The yellow frame indicates the target area of (B). Scale bar: 300 µm. (C) Representative image of mScarlet distribution in the external plexiform layer of the olfactory region of Tmem119-EGFP knock-in mouse line. (D) Representative image of EGFP distribution. The fluorescent signals were localized in microglia cells in the Tmem119-EGFP knock-in mouse line. (E) Representative image of TO-PRO-3 nucleus staining. Scale bars: 300 µm. Please click here to view a larger version of this figure.
Supplementary Figure 1: MAP2 and MOG staining of primary rat brain cells infected with EcoHIV-mScarlet virus. Scale bars: 50 µm. Please click here to download this File.
Supplementary Figure 2: Confocal images of EcoHIV-mScarlet infection in Tmem119-EGFP knock-in mouse line. Scale bars: 75 µm. Please click here to download this File.
Supplementary File 1: EcoHIV-mScarlet plasmid DNA sequence. Please click here to download this File.
In the present study, it was found that (1) the novel EcoHIV-mScarlet successfully infected primary rat brain cells in vitro; (2) the triple labeling of mScarlet, CD11b/c, and Iba1 identified microglia as the predominant cell type for this EcoHIV expression in rat brain cells in vitro; (3) primary mouse brain cells from adults in vitro further evidence the EcoHIV-mScarlet infection; (4) the EcoHIV-mScarlet distribution in the Tmem119-EGFP knock-in mouse line demonstrated a microglia-specific distribution pattern of EcoHIV infection.
Emerging studies suggested that various types of brain cells identified within the central nervous system (neurons, astrocytes, microglia, oligodendrocytes, etc.) exhibit functional and transcriptomic alteration during HIV and HIV-associated neurocognitive disorders18,19. For example, astrocytes contribute 30%-70% of the brain and perform surveillance to maintain brain homeostasis20. Astrocytes also modulate immune function and regulate the secretion of multi-cytokines and chemokines, especially in the situation of brain inflammation and neurodegeneration in HIV21,22. HIV infection of microglia not only results in a continuous release of viral proteins and proinflammatory cytokines and chemokines, but also provides predominant sources for HIV viral reservoirs23,24,25,26. Furthermore, activated microglia contribute a critical immunological function in the CNS; however, prolonged activation may also exacerbate neurodegeneration in HIV progression10,27. Oligodendrocytes also play an important function, including releasing several neurotrophins (such as nerve growth factor, brain-derived neurotrophic factor, etc.)28. A previous study found that the number of oligodendrocytes significantly decreased in the brains of AIDS patients, which may indicate direct damage to oligodendrocytes from HIV viral proteins29. Therefore, a specific type of cell-manipulated infection model of HIV should provide fundamental means of identifying the differential functions of various brain cells following infection. In the current study, a biological system was developed to mimic features of HIV-1 by chimeric HIV (EcoHIV) inoculation. This HIV inoculation was also combined with the Tmem119-EGFP knock-in mouse line to generate and validate a microglia-manipulated rodent model of HIV.
However, the limitations of the present study must be recognized. There were a few Iba1/CD11b/c negative cells that presented mScarlet fluorescent signals in vitro. Other types of brain cells, such as brain macrophages or pericytes, may be involved in EcoHIV infection, or alternatively, the cell culture environment may promote aberrant infection patterns relative to in vivo inoculation. Future, whole animal studies should further clarify the function of mScarlet+ microglia in the process of EcoHIV infection, and further define the regional distribution of mScarlet+EcoHIV microglia in the brain. Additionally, neurocognitive alterations occurring as a result of microglial infections may also be addressed in this rodent model. Collectively, the EcoHIV-mScarlet inoculation of Tmem119-EGFP knock-in mice provides a novel model and research strategy for investigating the microglial-driven mechanisms of HIV-associated neurocognitive disorders.
This work was funded by NIH grants DA059310, DA058586, AG082539, and GM109091. We appreciate the generous gift of the EcoHIV-NL4-3-EGFP from Dr. Potash of the Icahn School of Medicine at Mount Sinai.
Name | Company | Catalog Number | Comments |
293FT cells | ThermoFisher Scientific | R70007 | |
33 G, Small Hub RN Needle, (Point Style: 3, Needle Length: 19.25 mm) | Hamilton | 7803-05 | |
Antibiotic-Antimycotic solution | Cellgro | 30004CI | 100x |
C57BL/6-Tmem119em2Gfng/J | The Jackson Laboratory | Strain #:031823 | |
Corning BioCoat Gelatin 75cm² Rectangular Canted Neck Cell Culture Flask with Vented Cap | Life Technologies | 354488 | |
Corning DMEM with L-Glutamine, 4.5 g/L Glucose and Sodium Pyruvate | Life Technologies | 10013CV | |
Cover glass | VWR | 637-137 | |
Dumont #5 Forceps | World Precision Instruments | 14095 | |
Dumont #7 Forceps | World Precision Instruments | 14097 | |
EndoFree Plasmid Maxi Kit (10) | Qiagen | 12362 | |
Eppendorf Snap-Cap Microcentrifuge Biopur Safe-Lock Tubes | Life Technologies | 22600028 | |
Feather Double Edge Carbon Steel Blades | Ted Pella, inc. | 121-9 | |
Fisher BioReagents Microbiology Media: LB Broth, Miller | Fisher Scientific | BP1426-500 | |
Human recombinant anti-CD11b antibody | Miltenyi Biotec | 130-120-214 | 1:50 dilution |
Intraocular Injector Syringe (6.5 µL), Removable Needle | Hamilton | 6609071-01 | |
Invitrogen Lipofectamine 3000 Transfection Reagent | Life Technologies | L3000015 | |
Invitrogen One Shot TOP10 Electrocomp E. coli | Fisher Scientific | C404052 | |
Iris Forceps | World Precision Instruments | 15914 | |
Iris Scissors | World Precision Instruments | 500216 | |
Lentivirus-Associated p24 ELISA Kit | Cell Biolabs, inc. | VPK-107-5 | |
Lenti-X Concentrator | Takara | PT4421-2 | |
Opti-MEM I Reduced Serum Medium | Life Technologies | 11058021 | |
Paraformaldehyde | Sigma-Aldrich | 158127-500G | |
Paraformaldehyde | Sigma | P6148 | |
PELCO easiSlicer Vibratory Tissue Slicer | Ted Pella, inc. | 11000 | |
PELCOÂ Pro CA44 Tissue Adhesive | Ted Pella, inc. | 10033 | |
PELCOÂ Pro Specimen Retrievers | Ted Pella, inc. | 101-33 | |
ProLong Gold | Fisher Scientific | P36930 | |
Rabbit monoclonal anti-Iba1 antibody | Abcam | ab178847 | 1:500 dilution |
RN Compression Fitting 1 mm | Hamilton | 55750-01 | |
Sevoflurane | Merritt Veterinary Supply | 347075 | |
Sprague Dawley pregnant rat | Inotiv | ||
SuperFrost Plus Slides | Fisher Scientific | 12-550-154%Â | |
To-Pro-3 | ThermoFisher Scientific | T3605 | Nucleus staining kit |
Trypsin-EDTA (0.25%), phenol red | ThermoFisher Scientific | 25200-056 | |
Vannas Scissors | World Precision Instruments | 500086 |
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