Monocyte/macrophage trafficking in acquired immunodeficiency syndrome encephalitis: lessons from human and nonhuman primate studies

Abstract

Here the authors discuss evidence in human and animal models supporting two opposing views regarding the pathogenesis of human immunodeficiency virus (HIV) in the central nervous system (CNS): (1) HIV infection in the CNS is a compartmentalized infection, with the virus-infected macrophages entering the CNS early, infecting resident microglia and astrocytes, and achieving a state of latency with evolution toward a fulminant CNS infection late in the course of disease; or alternatively, (2) events in the periphery lead to altered monocyte/macrophage (MPhi) homeostasis, with increased CNS invasion of infected and/or uninfected MPhis. Here the authors have reevaluated evidence presented in the favor of the latter model, with a discussion of phenotypic characteristics distinguishing normal resident microglia with those accumulating in HIV encephalitis (HIVE). CD163 is normally expressed by perivascular MPhi s but not resident microglia in normal CNS of humans and rhesus macaques. In agreement with other studies, the authors demonstrate expression of CD163 by brain MPhi s in HIVE and simian immunodeficiency virus encephalitis (SIVE). CNS tissues from HIV-sero positive individuals with HIVE or HIV-associated progressive multifocal leukoencephalopathy (PML) were also examined. In HIVE, the authors further demonstrate colocalization of CD163 and CD16 (Fcgamma III recptor) gene expression, the latter marker associated with HIV infection of monocyte in vivo and permissivity of infection. Indeed, CD163(+) MPhis and microglia are often productively infected in HIVE CNS. In SIV infected rhesus macaques, CD163(+) cells accumulate perivascularly, within nodular lesions and the parenchyma in animals with encephalitis. Likewise, parenchymal microglia and perivascular MPhi s are CD163(+) in HIVE. In contrast to HIVE, CD163(+)perivascular and parenchymal MPhi s in HIV-associated PML were only associated with areas of demyelinating lesions. Interestingly, SIV-infected rhesus macaques whose viral burden was predominantly at 1 x 10(6) copies/ml or greater developed encephalitis. To further investigate the relationship between CD163(+)/CD16(+) MPhis/microglia in the CNS and altered homeostasis in the periphery, the authors performed flow-cytometric analyses of peripheral blood mononuclear cells (PBMCs) from SIV-infected rhesus macaques. The results demonstrate an increase in the percent frequency of CD163(+)/CD16(+) monocytes in animals with detectable virus that correlated significantly with increased viral burden and CD4(+) T-cell decline. These results suggest the importance of this monocyte subset in HIV/SIV CNS disease, and also in the immune pathogenesis of lentiviral infection. The authors further discuss the potential role of CD163(+)/CD16(+) monocyte/MPhi subset expansion, altered myeloid homeostasis, and potential consequences for immune polarization and suppression. The results and discussion here suggest new avenues for the development of acquired immunodeficiency syndrome (AIDS) therapeutics and vaccine design.

Figure 1

HIVE CNS shows significant accumulation of CD163⁺ cells that colocalizes with CD16 and harbors productive HIV-1 infection. All panels are shown at 40× magnification. A significant number of CD163⁺ MPs are observed in HIVE when compared to HIV-1–infected individuals without dementia and seronegative controls (A). Double-immunofluorescence studies of HIVE CNS tissue using sequential application of antibodies against human CD163 and CD16 on the same tissue section shows that virtually all CD163⁺ cells colocalize with CD16 (B). Similar analyses on HIVE CNS tissue using antibodies against CD163 and HIV-1 p24 revealed HIV-1 p24⁺ cells in patchy areas of the brain parenchyma, around blood vessels, and within nodular lesions (C). The predominance of these cells located perivascularly and within nodular lesions were found to be CD163⁺. Additionally, patches of ramified CD163⁺ cells in the parenchyma appear to harbor productive infection. PML CNS tissue reveals a different pattern of CD163 positivity from that seen in HIVE. (D) Panels A and B illustrate brain parenchyma. Panels C and D show blood vessels. Panels A and C demonstrate a PML lesion. Panels B and D represent areas outside of the PML lesions. A significant number of CD163⁺ MPs are observed within PML lesions. These cells are seen both in the parenchyma and the perivascular space. Interestingly, CD163⁺ cells with ramified microglial morphology are not observed in PML, as is seen in HIVE, but have a round, foamy morphology. Outside of PML lesions, only limited CD163⁺ cells are observed perivascularly. Parenchymal CD163⁺ cells are not observed outside of lesions.

Figure 2

CD163⁺ cell accumulation and productive SIV infection in the CNS of rhesus macaques. In situ hybridization using α-sense RNA probe against SIVmac239 reveals productive SIV infection confined primarily to cells located perivascularly and within nodules in macaques with encephalopathy (A). Separate immunohistochemistry studies revealed that these cells are also CD163⁺ (B). Rare positive cells are also observed within the brain parenchyma (A). Productive infection is not seen in SIV-infected animals without encephalopathy. (A) Panels A and D demonstrate the specificity of the probe used where panel A shows a nodular lesion from a section of tissue treated with a sense probe and panel D shows the same nodule from tissue treated with the α-sense probe. (B) Infrequent CD163⁺ perivascular MΦs are observed in seronegative CNS tissue, with slightly more seen in SIV-infected animals without encephalopathy. SIVE, however, shows significant accumulation of CD163⁺ MΦs located perivascularly and within nodular lesions. Numerous CD163⁺ ramified microglia are also observed in the brain parenchyma.

Figure 3

The frequency of CD14⁺/CD163⁺/CD16⁺ monocytes is increased in SIV-infected animals, with detectable viral loads and correlates with viral burden and CD4⁺ T-cell decline. Flow-cytometric studies showed that the percent frequency of CD163⁺/CD16⁺ monocytes (CD14⁺) from SIV+ animals with detectable virus correlates with viral load (A). An inverse correlation was observed between the percent frequency CD14⁺/CD163⁺/CD16⁺ monocytes and absolute number of CD4⁺ T cells in animals with counts less than 1100 cells/µl (B). Together, these data suggest that expansion of this monocyte subset may contribute to and/or result from increased virus production and AIDS progression (CD4⁺ T-cell loss). One-way analysis of variance (ANOVA) with Tukey-Kramer multiple comparisons post test and correlation tests were performed on flow cytometry data using Graph Pad Prism version 3.00 software, San Diego, CA.

Graph 1

Development of CNS disease is associated with high viral loads in SIV-infected rhesus macaques. All data points show viral load (copies/ml) at designated times post infection. Animals with closed data points did not demonstrate encephalopathy at necropsy. Animals with open data points demonstrated pathology consistent with SIVE at necropsy. SIVE was seen predominantly among animals whose viral loads generally remained at 1 × 10⁶ copies/ml (purple line on graph) or greater during the course of disease.