When human immunodeficiency virus meets chemokines and microglia: neuroprotection or neurodegeneration?

Abstract

Chemokines are chemotactic cytokines that were originally discovered as promoters of leukocyte proliferation and mobility. In recent years, however, evidence has demonstrated constitutive expression of chemokines and chemokine receptors in a variety of cells in the central and peripheral nervous system and has proposed a role for chemokines in neurodegenerative diseases characterized by inflammation and microglia proliferation. In addition, chemokine receptors, and in particular CXCR4 and CCR5, mediate human immunodeficiency virus type 1 (HIV) infection of immunocompetent cells as well as microglia. Subsequently, HIV, through a variety of mechanisms, promotes synapto-dendritic alterations and neuronal loss that ultimately lead to motor and cognitive impairments. These events are accompanied by microglia activation. Nevertheless, a microglia-mediated mechanism of neuronal degeneration alone cannot fully explain some of the pathological features of HIV infected brain such as synaptic simplification. In this article, we present evidence that some of the microglia responses to HIV are beneficial and neuroprotective. These include the ability of microglia to release anti-inflammatory cytokines, to remove dying cells and to promote axonal sprouting.

Figure 1. Expression of CCL5 in adult rat brain

Sections (40 µm) were obtained from the somatosensory cortex (A) and substantia nigra (B) of 3 month old male SD rats. A. Sections were stained with CCL5 (red, 1:100 dil) and class III β–tubulin (green, 1:1000 dil) antibodies. Cy3 TSA Biotin System (Perkin-Elmer) was used to detect CCL5 according to the manufacturer’s instructions. White arrows indicate CCL5 positive neurons, black arrows indicate non-neuronal cells positive for CCL5. Mag 60X. B. Sections were stained with CCL5 (red) and tyrosine hydroxylase (TH, green, 1:1000 dil) antibodies. Mag 20X. Fluorescence was visualized with FV300 laser confocal scanning system attached to an Olympus IX-70 upright microscope. Orange denotes co-localization.

Figure 2. Activity-dependent release of CCL5

Cortical neurons were prepared from E17 rat embryos as described (Avdoshina et al., 2010). Neurons were exposed to KCl (25 M) or NMDA (15 µM) for 30 min, the medium was collected and CCL5 content determined by ELISA (R&D), according to the manufacturer’s instructions. *p<0.001 vs control (ANOVA and Scheffe’s test).

Figure 3. mRNA levels for pro-inflammatory cytokines are not elevated in the hippocampus of HIV tg rats

The hippocampus was dissected from 5 month old Fischer 344 controls (white bars) and HIV tg (black bars) male rats [Hsd:HIV-1(F344); Harlan Laboratories]. Total RNA was isolated, and cDNA subjected to quantitative real time polymerase chain reaction for IL-1β, IL-1 receptor antagonist (IL-1RA), the microglia marker Iba-1, and TNF-α. Data are represented as mean δCt +/- SD (n=6).

Figure 4. Microglia morphology in the hippocampus of HIV tg rats

Representative images of Iba-1+ cells (red) within the CA1 pyramidal cell layer and molecular layer of the hippocampus of control (A) and HIV tg (B) rats at 8 months of age. The nuclear marker 4',6-diamidino-2-phenylindole or DAPI (blue) was used as a counterstain. Note that microglia in HIV tg rats maintain a normal appearance with fine ramified processes and has no prominent evidence of activation. Scale bar = 50µm.

Figure 5. Gp120 activates microglia

Rats received X4 gp120 into the superior colliculus (gp120IIIB, Immunodiagnostic Inc., 400 ng in 0.1% bovine serum albumin). Rats were euthanized 18 days after the injection and coronal sections through the visual cortex were prepared. Example shows a section of gp120-treated rats stained for caspase-3 (green), the microglia marker CD11b (red) and counterstained with DAPI (blue). Immunoreactivity was analyzed by confocal imaging. Note that CD11b positive cells have their processes encircling the outside of caspase-3 positive cells that were previously identified as neurons. Mag 40X. (Adapted from Ahmed et al., 2009).