A glial endogenous cannabinoid system is upregulated in the brains of macaques with simian immunodeficiency virus-induced encephalitis

Abstract

Recent evidence supports the notion that the endocannabinoid system may play a crucial role in neuroinflammation. We explored the changes that some elements of this system exhibit in a macaque model of encephalitis induced by simian immunodeficiency virus. Our results show that profound alterations in the distribution of specific components of the endocannabinoid system occur as a consequence of the viral infection of the brain. Specifically, expression of cannabinoid receptors of the CB2 subtype was induced in the brains of infected animals, mainly in perivascular macrophages, microglial nodules, and T-lymphocytes, most likely of the CD8 subtype. In addition, the endogenous cannabinoid-degrading enzyme fatty acid amide hydrolase was overexpressed in perivascular astrocytes as well as in astrocytic processes reaching cellular infiltrates. Finally, the pattern of CB1 receptor expression was not modified in the brains of infected animals compared with that in control animals. These results resemble previous data obtained in Alzheimer's disease human tissue samples and suggest that the endocannabinoid system may participate in the development of human immunodeficiency virus-induced encephalitis, because activation of CB2 receptors expressed by immune cells is likely to reduce their antiviral response and thus could favor the CNS entry of infected monocytes.

Figure 1.

CB₁ immunoreactivity in brain tissue samples of frontal cortex from control (A) and SIVE (B) animals. In both groups, a moderate signal could be observed in pyramidal neurons (A, inset) together with a diffuse neuropilic labeling. No significant modifications were observed in SIVE animals even within cellular infiltrates in white matter areas (B, inset). Scale bars: A, B, 400 μm; insets, 200 μm.

Figure 2.

Western blot of CB₁ immunoreactivity in cortical tissue homogenates from control (A, lane 1) and SIVE (A, lane 2) animals. Two bands of 85 and 52 kDa were detected. Preincubation with the immunizing peptide completely prevented the signal in control (B, lane 1) and SIVE (B, lane 2) samples.

Figure 3.

CB₂ staining in brain cortical tissue samples of the frontal cortex from control (A) and SIVE (B-F) animals. A, Note the absence of any detectable signal in tissue sections from control animals compared with SIVE animals (B). B, Selected cells on a perivascular location show strong immunoreactivity for CB₂ receptors. Longitudinal (C) and transverse (D) sections of blood vessels show intense signal in the outer surface of the vessel walls. E, CB₂-positive cells within cellular infiltrates commonly found in perivascular areas of SIVE brains. Cells located both on incipient microglial nodules (arrows) and on well developed, mature infiltrates (dashed circle) show CB₂ labeling. F, High-magnification image of an incipient infiltrate in the proximity of a blood vessel wall (asterisk) showing CB₂ immunoreactivity (brown color). Scale bars: E, 800 μm; A, B, 400 μm; C, 200 μm; D, F, 100 μm.

Figure 4.

CB₂ receptors are expressed by microglial cells associated with blood vessels (A, B) as well as in pathological cellular infiltrates (C, D) and by T-lymphocytes (E, F). A, Immunofluorescent signal for CB₂ receptors is located in the outer portion of a blood vessel. B, CD68 immunoreactivity (a common marker of macrophages/microglia) in the same vessel shown in A. Note the partial overlap of CB₂ and CD68 staining. C, Immunofluorescent signal for CB₂ receptors in groups of perivascular cells. D, The same groups of cells were positive for CD68 staining. E, F, Cellular infiltrates show strong immunoreactivity for CB₂ (E) and CD3 (F). Note the almost total match between both patterns. Scale bars, 50 μm.

Figure 5.

FAAH immunoreactivity in brain cortical tissue samples of the frontal cortex from control (A) and SIVE (B-F) animals. A, FAAH staining in control animals was limited to gray matter (Gm) portions of the cortex, specifically in pyramidal neurons (inset). Note the almost complete absence of labeling in white matter areas (Wm). B, FAAH immunoreactivity in SIVE was dramatically increased in white matter areas of the cortex. This increased signal corresponded to the astrogliotic process characteristic of SIVE. C, D, FAAH is massively expressed in white matter astrocytes and, remarkably, in those associated with blood vessels (C, asterisks) having perivascular infiltrates (arrows). Interestingly, a lesser degree of expression seemed to occur around those vessels lacking these pathological structures (C, white arrow). FAAH immunoreactivity was present both in somata and in astrocytic processes. E, F, FAAH immunoreactivity (black arrows) colocalizes with GFAP-positive cells (white arrows), confirming their astrocytic nature. Scale bars: A, B, 800 μm; C, 200 μm; insets in A, D, 100 μm; E, F, 50 μm.