Correlation of acute humoral response with brain virus burden and survival time in pig-tailed macaques infected with the neurovirulent simian immunodeficiency virus SIVsmmFGb

Infection of pig-tailed macaques with the simian immunodeficiency virus (SIV) isolate SIVsmmFGb frequently results in SIV encephalitis (SIVE) in addition to immunodeficiency and acquired immune deficiency syndrome. We used in situ hybridization to quantitate the number of SIV-infected cells in brain parenchyma, choroid plexus, and meninges from 17 macaques that developed acquired immune deficiency syndrome after infection with SIVsmmFGb. SIV-infected cells and histopathological lesions of SIVE were identified in 15 of 17 animals (88.2%), including 12 of 12 rapid progressors (RP) and 3 of 5 slow progressors (SP). The parenchymal virus burden was much greater in RP macaques than in the three SP macaques with SIVE (median values of 24.3 versus 0.3 infected cells/mm(2), respectively; P < 0.05). Viral load differences between RP and SP with SIVE were less marked in choroid plexus (29.6 versus 12.8 infected cells/mm(2), respectively) and meninges (133.0 versus 34.2 infected cells/mm(2), respectively). A significant negative correlation was observed between the magnitude of the anti-SIV antibody titer at 1 month after inoculation and brain virus burden at necropsy (r = -0.614; P < 0.01). The close association between immune response and SIVE in this model should prove useful for identifying correlates of immune protection against primate lentiviral encephalitis.     

Simian immunodeficiency virus encephalitis: analysis of envelope sequences from individual brain multinucleated giant cells and tissue samples

Simian immunodeficiency virus (SIV)-infected macaques develop an encephalitis (SIVE) that is pathologically virtually indistinguishable from that associated with HIV infection, with multinucleated giant cells (MNGCs) being the principal histopathological manifestation. To dissect SIV variants responsible for MNGC development, we examined the relationships between env sequences transcribed in individual MNGCs and those from genomic DNA of brain and spleen tissues. The brain-specific variant found in all brain clones was dominant among the clones from MNGCs, suggesting a role in the formation of giant cells. Furthermore, two additional minor groups of sequences were present in MNGCs. One group consisted of sequences closely related to those from spleen, indicating recent and probably multiple episodes of neuroinvasion. The second group represented clones similar or identical to the initial inoculum. The survival of archival sequences and their activation presumably by the fusion of productively and quiescently infected macrophages/microglia identify the central nervous system as a possible anatomical reservoir for latent infection.     

Longitudinal analysis of monocyte/macrophage infection in simian immunodeficiency virus-infected, CD8+ T-cell-depleted macaques that develop lentiviral encephalitis

The histopathological hallmark of lentiviral-associated encephalitis is an abundance of infected and activated macrophages. Why a subset of infected hosts develops lentiviral encephalitis and others do not is unknown. Using a CD8(+) T-cell depletion model of simian immunodeficiency virus (SIV)-infected rhesus macaques, we examined the relationship between peripheral SIV infection of monocytes/macrophages and the development of encephalitis. At the same time that cerebral spinal fluid viral load increased in macaques that developed encephalitis, we observed that monocyte-derived macrophages from these macaques produced more virus than those from macaques that did not develop encephalitis. However, during the course of infection, the number of blood monocyte-associated SIV DNA copies did not distinguish macaques that developed simian immunodeficiency virus encephalitis from macaques that did not develop encephalitis. Paradoxically, in this model, macaques that developed encephalitis had fewer SIV-infected macrophages in lungs and thymus at postmortem than macaques that did not develop encephalitis. These findings suggest that inherent differences in host monocyte viral production are related to development of encephalitis.     

Localization of SIV in the genital tract of chronically infected female rhesus macaques.

Despite the fact that human immunodeficiency virus (HIV) is transmitted primarily by sexual contact, the biology of the sexual transmission of HIV is poorly understood. Simian immunodeficiency virus (SIV) can be transmitted to female rhesus macaques by placing cell-free virus into the vaginal canal, and SIV can be isolated from the vaginal secretions of infected rhesus macaques. The authors examined the genital tracts from 16 chronically infected female rhesus macaques and localized SIV-infected cells using in situ hybridization and immunohistochemistry. SIV-infected cells were found in the genital tract of 13 of the 16 animals examined, and in most cases the SIV-infected cells were located in the submucosa of the cervix and vagina. However, SIV-infected cells were also found in the vaginal epithelium. SIV-infected cells were more common in sites of inflammation than in normal areas. These findings suggest that SIV gains access to genital tract secretions from the cervix and vaginal epithelium.     

CD163, a marker of perivascular macrophages, is up-regulated by microglia in simian immunodeficiency virus encephalitis after haptoglobin-hemoglobin complex stimulation and is suggestive of breakdown of the blood-brain barrier

Macrophages and microglia are the major cell types infected by human immunodeficiency virus and simian immunodeficiency virus (SIV) in the central nervous system. Microglia are likely infected in vivo, but evidence of widespread productive infection (ie, presence of viral RNA and protein) is lacking. This conclusion is controversial because, unlike lymphocytes, macrophages and microglia cannot be discreetly immunophenotyped. Of particular interest in the search for additional monocyte/macrophage-lineage cell markers is CD163; this receptor for haptoglobin-hemoglobin (Hp-Hb) complex, which forms in plasma following erythrolysis, is expressed exclusively on cells of monocyte/macrophage lineage. We examined CD163 expression in vitro and in vivo by multiple techniques and at varying times after SIV infection in macaques with or without encephalitis. In normal and acutely SIV-infected animals, and in SIV-infected animals without encephalitis, CD163 expression was detected in cells of monocyte/macrophage lineage, including perivascular macrophages, but not in parenchymal microglia. However, in chronically infected animals with encephalitis, CD163 expression was detected in activated microglia surrounding SIV encephalitis lesions in the presence of Hp-Hb complex, suggesting leakage of the blood-brain barrier. CD163 expression was also induced on microglia in vitro after stimulation with Hp-Hb complex. We conclude that CD163 is a selective marker of perivascular macrophages in normal macaques and during the early phases of SIV infection; however, later in infection in animals with encephalitis, CD163 is also expressed by microglia, which are probably activated as a result of vascular compromise.     

Neuropathogenesis of Simian Immunodeficiency Virus in Neonatal Rhesus Macaques

Neonatal human immunodeficiency virus (HIV) infection usually occurs intrapartum or postpartum and results in a higher incidence of neurological dysfunction than is seen in adults. To explore the neuropathogenesis of neonatal HIV infection, we infected neonatal macaques with simian immunodeficiency virus (SIV) and followed the course of infection focusing on early time points. Infected neonates had decreased brain growth and mild histological changes in brain that resembled those seen in pediatric AIDS, including perivascular infiltrates of mononuclear cells, mineralization of vessels in the basal ganglia, and gliosis. The perivascular lesions and gliosis were associated with the presence of occasional infected cells that required in situ hybridization with radiolabeled riboprobes for detection. Using this technique, SIV-infected cells were detected in the brain parenchyma within 7 days of infection. These findings were confirmed by nested PCR for SIVgag DNA in brain and RT-PCR for viral RNA in cerebrospinal fluid. Together, these techniques revealed SIV infection of the CNS in 12 of 13 neonates infected with SIVmac239, 3 of 3 infected with SIVmac251, and 2 of 2 infected with SIVmac239/316. The prevalence of CNS infection was indistinguishable from that of older animals infected with the same dose and stock of virus, but neonates appeared to have fewer infected cells in the CNS and detecting them required more sensitive techniques. This observation was true regardless of inoculum and despite the fact that neonates had equal or greater viral loads in the periphery compared with older animals. These data suggest that maturation-dependent host factors have a major impact on the neuropathogenesis of pediatric AIDS.     

Novel simian immunodeficiency virus CTL epitopes restricted by MHC class I molecule Mamu-B*01 are highly conserved for long term in DNA/MVA-vaccinated, SHIV-challenged rhesus macaques

Simian immunodeficiency virus (SIV) infection of rhesus macaques provides an excellent model for investigating the basis of protective immunity against human immunodeficiency virus (HIV). One limitation of this model, however, has been the availability of a small number of known MHC class I-restricted CTL epitopes for investigating virus-specific immune responses. We assessed CTL responses against SIV Gag in a cohort of DNA/modified vaccinia virus Ankara (MVA)-vaccinated/simian-human immunodeficiency virus (SHIV)-challenged rhesus macaques. Here, we report the identification of five novel SIV CTL epitopes in Gag for the first time (Gag(39-46) NELDRFGL, Gag(169-177) EVVPGFQAL, Gag(198-206) AAMQIIRDI, Gag(257-265) IPVGNIYRR and Gag(296-305) SYVDRFYKSL) that are restricted by the common MHC class I molecule Mamu-B*01. CTL responses to these epitopes were readily detected in cryopreserved PBMC in multiple animals up to 62 weeks post-infection, both by IFN-gamma enzyme-linked immunospot assay and intracellular IFN-gamma staining. Importantly, viral sequencing results revealed that these epitopes are highly conserved in the SIV-challenged macaques over a long period of time, indicating functional constraints in these regions. Moreover, the presence of CTL responses targeting these epitopes has been confirmed in two independent cohorts of rhesus macaques that have been challenged by SHIV or SIV. Our findings provide valuable candidates for poly-epitope vaccines and for long-term quantitative monitoring of epitope-specific CD8(+) responses in the context of this common Mamu class I allele. It may thus help increase the supply of rhesus macaques in which epitope-specific immunity can be studied in the context of SIV vaccine design.     

Localization of simian immunodeficiency virus nucleic acid and antigen in brains of fetal macaques inoculated in utero.

Neurological dysfunction has been shown to be associated with human immunodeficiency virus (HIV) infection. The incidence of these abnormalities is greater in HIV-infected children when compared with adults, and the patterns of neurological disease are also known to differ from those observed in the adult population. The reasons for these differences are unclear but are most likely related to the immaturity of the host's immune and central nervous systems at the time of infection. This is thought to be particularly true for infants infected with HIV prenatally. To examine these questions, the brains of fetal rhesus macaques that were infected with SIVmac251 at various time points in utero were examined. Direct fetal inoculations were performed on gestational day (GD) 65 (n = 8; early second trimester), GD 110 (n = 4; early third trimester) and GD 130 (n = 2; mid third trimester), with harvest of fetal tissues on GD 80, 100, 130, or 145. Eleven sham controls were included with harvest at correlative time points. Specimens were examined by routine histology, immunohistochemistry, and in situ hybridization to localize viral antigens and SIV nucleic acid. Histologically, scattered glial nodules, spongiosis, and mineralization were found in the basal ganglia and deep white matter in 4 of the 14 fetuses (3 inoculated on GD 65 and one on GD 110). These fetuses and those without histological lesions had viral nucleic acid and SIV antigen in the stroma of the choroid plexus, meninges, and external granular layer of the cerebellum and in columns of cells in the cortical plate. In contrast to juvenile and adult macaques, very few SIV-positive perivascular mononuclear cells were present. These findings suggest that SIV has a different distribution in the brain of fetal macaques after direct infection when compared with adult or juvenile animals. Furthermore, the results of these studies suggest that differences in neurological disease between pediatric and adult patients with acquired immune deficiency syndrome are most likely related to the time of infection.     

Kinetics of Lymphocyte Apoptosis in Macaques Infected with Different Simian Immunodeficiency Viruses or Simian/Human Immunodeficiency Hybrid Viruses

Increased rates of T-cell apoptosis have been detected in human immunodeficiency virus (HIV)-infected individuals and in the simian immunodeficiency model (SIV) for AIDS research. We have infected macaques with virulent SIV or SIV/HIV hybrid viruses (SHIV) of different pathogenic potentials to study the early kinetics of apoptosis in this model. Animals infected with SIV showed an increased degree of apoptosis in their peripheral blood mononuclear cells as early as 8 weeks after virus inoculation. Apoptotic cells were detected in the CD4 and CD8 cell populations of infected animals. In contrast, apathogenic SHIV did not lead to increased lymphocyte apoptosis and moderately pathogenic SHIV induced only transient apoptosis. T-cell death was temporally linked to viral replicationin vivo.Furthermore, lymphocyte apoptosis in infected macaques was associated with impaired proliferative responses of helper T-cells and with CD4 cell depletion. The monkey model described here provides the opportunity for testing early therapeutic interventions to prevent virus-induced programmed cell death and the subsequent onset of AIDS.     

Active simian immunodeficiency virus (strain smmPGm) infection in macaque central nervous system correlates with neurologic disease

Simian immunodeficiency virus strain smmPGm can induce neuropathology in macaques and is a model for the development of human HIV-related brain injury. For quantitative studies of proviral presence and expression in the central nervous system (CNS), we inoculated 8 macaques intravenously with the virus. Three animals were necropsied 2 to 4 weeks after development of infection, and we obtained lymphoid tissue biopsies from 5 animals before 5 weeks after infection. Peak plasma viral loads averaged 10 viral RNA Eq/mL at week 2, whereas cerebrospinal fluid viral loads peaked at 10 viral RNA Eq/mL. The proviral DNA loads and viral gag mRNA expression in tissues were quantified by real-time polymerase chain reaction. Two animals developed neurologic disease characterized by meningoencephalitis and meningitis. Proviral DNA levels in CNS tissues of these animals at necropsy revealed 10 and 10 copies/microg of DNA, respectively, whereas viral RNA expression in the CNS reached 100 to 1000 times higher levels than those seen in early necropsies. In sharp contrast, in 2 animals necropsied at later times without CNS disease, virus mRNA expression was not detected in any CNS tissue. Our results are consistent with the hypothesis that active virus expression in the CNS is strongly correlated with neurologic disease and that the event occurs at variable periods after infection.     

SIV Infection of Macaques as a Model for AIDS Pathogenesis

Simian immunodeficiency virus (SIV) infection of macaques is the best available animal model for studying the pathogenesis of AIDS. Experimental inoculation of macaques with SIV results in a persistent infection that leads to immunodeficiency, opportunistic infections, and death. Most aspects of the illness, including immunologic and virologic parameters, are easily quantified. Furthermore, pathologic processes can be evaluated throughout the course of experimental infection. Recently, molecular clones of SIV proviral DNA have been used to study genetic variation and specific viral determinants of pathogenesis. Considered together, these observations support the continued detailed study of SIV infection of macaques as a model for human AIDS.     

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.     

SIV infection of macaques as a model for AIDS pathogenesis.

Simian immunodeficiency virus (SIV) infection of macaques is the best available animal model for studying the pathogenesis of AIDS. Experimental inoculation of macaques with SIV results in a persistent infection that leads to immunodeficiency, opportunistic infections, and death. Most aspects of the illness, including immunologic and virologic parameters, are easily quantified. Furthermore, pathologic processes can be evaluated throughout the course of experimental infection. Recently, molecular clones of SIV proviral DNA have been used to study genetic variation and specific viral determinants of pathogenesis. Considered together, these observations support the continued detailed study of SIV infection of macaques as a model for human AIDS.     

Simian immunodeficiency virus infection of macaque bone marrow macrophages correlates with disease progression in vivo.

The pathogenesis of hematopoietic abnormalities associated with infection of susceptible hosts with either simian immunodeficiency virus (SIV) or human immunodeficiency virus (HIV) is not fully understood. To determine if bone marrow cells are infected with SIV and if the pattern of viral infection is correlated with the severity of disease and abnormalities in hematopoiesis, 23 SIV-infected rhesus monkeys were examined by immunohistochemistry and in situ hybridization. By immunohistochemistry, only four monkeys were positive for SIV core protein p27, while in situ hybridization revealed viral RNA in the bone marrow of 15 monkeys. Simian immunodeficiency virus RNA was consistently expressed in the bone marrow from monkeys with severe lymphoid depletion (11 of 11), but less so in monkeys with follicular hyperplasia (0 of 2) or mild lymphoid depletion (4 of 10). In animals with mild lymphoid depletion, bone marrow cells infected with SIV were mainly mononuclear cells that appeared to be of myelomonocytic lineage. In contrast, monkeys with severe lymphoid depletion had SIV RNA localized to larger mononuclear cells with abundant cytoplasm often located in small lucent areas of the stroma. These SIV RNA-positive mononuclear cells were positive for the macrophage determinant CD68 as demonstrated by immunohistochemistry. Furthermore the stage of simian acquired immune deficiency syndrome, as indicated by lymphoid morphology, and SIV localization in the bone marrow were correlated with the incidence of anemia, bone marrow hyperplasia, and abnormal distribution of macrophages in the bone marrow. These results indicate that, in common with other animal lentiviral infections, the macrophage is a major target of SIV infections in the bone marrow.     

Early appearance of simian immunodeficiency virus (SIV) antigen and antibodies as variables in evaluating antiviral drugs in macaques.

SIV infection in macaques has become an important animal model for HIV-1 infection in humans. An antibody assay was therefore developed and compared to a commercially available antigen assay with respect to their usefulness to monitor the course of simian immunodeficiency virus (SIV) infection in cynomolgus monkeys. A peptide, JB6T, consisting of 21 amino acids with the sequence NSWGCAFRQVCHTTVPWVNDS corresponding to a segment in the env protein of human immunodeficiency virus (HIV) type 2 was used as antigen in an enzyme-linked immunosorbent assay (ELISA). JB6T was found to detect IgG and IgM antibodies to viral antigens with high specificity. The earliest anti-SIV IgM antibodies were detected at days 13-16, with a maximum at day 20 and subsequently the levels fell. Specific IgG antibody levels increased at day 16-20 after SIV infection and reached a plateau at day 60. The commercially available HIV-1 p24/26 antigen test could, due to cross-reactivity, be employed to detect SIV antigen delay, peak and duration.     

Alterations in RANTES gene expression and T-cell prevalence in intestinal mucosa during pathogenic or nonpathogenic simian immunodeficiency virus infection.

RANTES, a beta-chemokine, can suppress human immunodeficiency virus (HIV) as well as simian immunodeficiency virus (SIV) infections in T-lymphocyte cultures in vitro. However, the association of RANTES levels in peripheral blood with viral loads and disease outcome in HIV infection has been inconclusive. SIV-infected rhesus macaques were evaluated to determine whether RANTES gene expression correlated with suppression of viral infection in intestinal lymphoid tissues. Intestinal tissues were obtained from rhesus macaques infected with either pathogenic or nonpathogenic SIVmac variants at various stages of infection (primary acute, asymptomatic, and terminal). We examined the level of SIV infection (in situ hybridization), RANTES expression (quantitative competitive RT-PCR), and T-cell counts (immunohistochemistry). The most pronounced increase in RANTES gene expression in intestinal tissues was observed in primary SIV infection, which correlated with the pathogenicity of the infecting virus and not the tissue viral loads. Our results demonstrated that in contrast to the occurrence of viral suppression by RANTES in vitro, there was no direct correlation between high RANTES gene expression and suppression of viral loads in intestinal lymphoid tissues. Thus RANTES expression in the gut lymphoid tissue may not be a correlate for viral suppression. However, RANTES gene expression in primary SIV infection may be part of early host immune response to viral infection.     

Cellular Localization of Simian Immunodeficiency Virus in Lymphoid Tissues: I. Immunohistochemistry and Electron Microscopy

Simian immunodeficiency virus (SIV) is a lentivirus with genetic relatedness to the human immunodeficiency viruses (HIV-1 and HIV-2). It induces a fatal syndrome in rhesus monkeys that closely parallels the clinical course of AIDS in humans. The authors used double-labeling immunohistochemical procedures on rhesus lymph node and spleen taken during different time periods after SIV infection to localize the p27 gag protein to specific cellular immunophenotypes. In animals with follicular hyperplasia, viral protein was found associated predominantly with follicular dendritic cells. Many of these cells showed ultrastructural alterations consisting of swollen dendritic processes containing electron-dense material. Lentiviral particles were found associated with this cell type only rarely. In lymphoid tissues with other histopathologic changes, macrophages and multinucleate giant cells were the predominant cell types containing detectable quantities of viral protein; smaller numbers of p27+ lymphocytes were present. Ultrastructurally, viral particles were found within the extracellular space adjacent to tissue macrophages and within membrane-bound vacuoles of giant cells and tissue macrophages. These results show that certain histologic patterns seen during the course of infection correlate with the localization of viral antigen to specific cellular immunophenotypes and that during the disease course, viral protein is preferentially localized in sections of lymph node and spleen to cells of the macrophage and dendritic cell lineages.     

Proliferating cellular nuclear antigen expression as a marker of perivascular macrophages in simian immunodeficiency virus encephalitis

Brain perivascular macrophages are a major target of simian immunodeficiency virus (SIV) infection in rhesus macaques and HIV infection in humans. Perivascular macrophages are distinct from parenchymal microglia in their location, morphology, expression of myeloid markers, and turnover in the CNS. In contrast to parenchymal microglia, perivascular macrophages are continuously repopulated by blood monocytes, which undergo maturation to macrophages on entering the central nervous system (CNS). We studied differences in monocyte/macrophages in vivo that might account for preferential infection of perivascular macrophages by SIV. In situ hybridization for SIV and proliferating cellular nuclear antigen (PCNA) immunohistochemistry demonstrated that SIV-infected and PCNA-positive cells were predominantly found in perivascular cuffs of viremic animals and in histopathological lesions that characterize SIV encephalitis (SIVE) in animals with AIDS. Multilabel techniques including double-label immunohistochemistry and combined in situ hybridization and immunofluorescence confocal microscopy revealed numerous infected perivascular macrophages that were PCNA-positive. Outside the CNS, SIV-infected, PCNA-expressing macrophage subpopulations were found in the small intestine and lung of animals with AIDS. While PCNA is used as a marker of cell proliferation it is also strongly expressed in non-dividing cells undergoing DNA synthesis and repair. Therefore, more specific markers for cell proliferation including Ki-67, topoisomerase IIalpha, and bromodeoxyuridine (BrdU) incorporation were used which indicated that PCNA-positive cells within SIVE lesions were not proliferating. These observations are consistent with perivascular macrophages as terminally differentiated, non-dividing cells and underscores biological differences that could potentially define mechanisms of preferential, productive infection of perivascular macrophages in the rhesus macaque model of neuroAIDS. These studies suggest that within CNS and non-CNS tissues there exist subpopulations of macrophages that are SIV-infected and express PCNA.     

CD163 identifies perivascular macrophages in normal and viral encephalitic brains and potential precursors to perivascular macrophages in blood

Perivascular macrophages are uniquely situated at the intersection between the nervous and immune systems. Although combined myeloid marker detection differentiates perivascular from resident brain macrophages (parenchymal microglia), no single marker distinguishes perivascular macrophages in humans and mice. Here, we present the macrophage scavenger receptor CD163 as a marker for perivascular macrophages in humans, monkeys, and mice. CD163 was primarily confined to perivascular macrophages and populations of meningeal and choroid plexus macrophages in normal brains and in brains of humans and monkeys with human immunodeficiency virus or simian immunodeficiency virus (SIV) encephalitis. Scattered microglia in SIV encephalitis lesions and multinucleated giant cells were also CD163 positive. Consistent with prior findings that perivascular macrophages are primary targets of human immunodeficiency virus and SIV, all SIV-infected cells in the brain were CD163 positive. Using fluorescent dyes that definitively and selectively label perivascular macrophages in vivo, we confirmed that dye-labeled simian perivascular macrophages were CD163 positive and able to repopulate the central nervous system within 24 hours. Flow cytometric studies demonstrated a subset of monocytes (CD163(+)CD14(+)CD16(+)) that were immunophenotypically similar to brain perivascular macrophages. These findings recognize CD163(+) blood monocytes/macrophages as a source of brain perivascular macrophages and underscore the utility of this molecule in studying the biology of perivascular macrophages and their precursors in humans, monkeys, and mice.