Clusters of cells labeled with Iba-1 in parenchyma and perivascular regions were observed in frontal brain of all eight SIV-infected monkeys (Table 1, Fig. 1). As both microglia and macrophage express Iba-1, the MN might represent microgliosis (Fig. 1C, D, F and G) and the PC were probably infiltrated monocytes and derived macrophages (Fig. 1E–H). In gray matter, the MNs were scattered in frontal cortex (Ctx) (Fig. 1F and G) with more MNs located near by the cortex surface or by border of gray and white matter. In white matter, these MNs were observed in the whole frontal corpus callosum (CC), a large white matter tract linking bilateral hemispheres, and in deep white matter bridging between lobes and gyri (Fig. 1). We judged whether they are MN or PC by looking for a blood vessel cavity inside of the cell cluster and we identified the nodule as PC if a central or eccentric cavity was regarded. The number of MNs in gray and white matters of each SIV-infected monkey (Table 1) was statistically compared and the number of MNs in frontal white matter was significantly higher than that in frontal cortex (Fig. 2A).
Monkey's No. 003 004 005 009 417 418 495 553 MN in white matter 39 20 35 27 20 31 24 34 MN in gray matter 13 0.5 10 7 3 8 3 4 PC in white matter 15 11 5 3 20 12 5 1 PC in gray matter 4 1 1 2 3 1 1 1 MN: microglia nodule; PC: perivascular cuffing; SIV: simian immunodeficiency virus.
Table 1. Number of MN and PC per cm3 in frontal brain white and gray matters of 8 SIV-infected monkeys
The characteristic of the PC is that small vasculature structure was generally visualized and layers of Iba-1 labeled cells surrounded the hollow of blood vessel (Fig. 1E–H). Sometimes, MN and PC were intermingled with each other (Fig. 1F). However, some PCs around brain capillaries might be mistaken as MNs if the cavity is compressed to close by aggregated cells (Fig. 1F). In this case, overlapped circled cellular layers observed under higher magnification lens is a criterion to distinguish PC from MN. Evidently, more PCs were in white matter than in gray matter (Table 1). Statistic comparison of the values showed the number of PCs in white matter was significantly higher than that in gray matter (Fig. 2A).
Nef p27 is synthesized at an early stage of infection and can enhance infectivity of the newly assembled viral particles during viral replication[20–21]. More importantly, Nef p27 represents neurovirulence of the virus in SIV infection, and a full-size p27 is crucial for viral replication and diffusion in the monkey's brains[21–22]. Thus, we used p27 to label infected microglia. A representative counting window is shown in Fig. 1A inset. The ratio of infected to uninfected microglia in gray and white matter of each monkey is shown in Fig. 2B. Then, we statistically compared the number of SIV infected microglia in the gray and white matter MNs, and found no significant difference (Fig. 2C). As we have noticed, p27 positive cells, namely infected microglia, in many white matter MNs were few (Fig. 3A and B) or even absent (Fig. 4D). As shown in Fig. 4D that was montaged figure from a serial of 10× microimages, only 3 among 12 MNs in that CC contained p27 labeled cells (arrows). Interestingly, some infected cells preferred to surround a MN than to situate inside of the MN (Fig. 4C).
Whereas, Iba-1 positive cells in PCs of both gray and white matters were constantly co-labeled with p27. Thus, the number of p27-Iba-1 co-labeled cells, representing SIV infected macrophages, in the white matter was significantly higher than that in gray matter (Fig. 2C; Fig. 3C and D). In parallel, in the white matter, infected PC macrophages were significantly more than infected MN microglia (Fig. 2C, P<0.001). In addition, more or less p27 single labeled cells around PC (Fig. 3D and E) and/or MN (Fig. 4C) were visualized in both gray and white matters. It is not clear what cells are those Iba-1 negative infected cells, but some of them might be lymphocytes since infiltration of HIV-1 infected lymphocytes into the brain in AIDS cases had been reported previously.
In the gray matter, it seemed that more infected cells were distributed in areas close to superficial pia mater (Fig. 4A), ventricle ependyma and border between gray and white matter (Fig. 4B). In the white matter, it appeared that SIV infected cells preferred to situate nearby or surround the blood vessels (Fig. 4C), or distributed close to the ventricles (Fig. 4D). In summary, the number of SIV infected cells correlated with the number of PCs in both gray and white matters. However, the number of infected microglia variated greatly in MNs and was not correlated with the number of MNs, especially in the white matter (Fig. 4D).
MBP is a major structural protein in the myelin sheath and disruption of the MBP positive linear structure reflects the injury of myelin sheath, then myelinated nerve fibers. NF is an essential structural protein of myelinated and unmyelinated fibers; disconnection of NF positive linear structure indicates the broken-down of the nerve fibers. We indeed observed an abundance of intruded injuries by nodular lesions on myelin sheath or nerve fibers by Iba-1 and MBP or NF double immunostaining (Fig. 5).
In the frontal cortex, heavier MBP labeling was observed in the upper layer Ⅳ (internal granular layer) and layer Ⅴ (pyramidal layer). In contrast, no apparent MBP staining was seen in layers Ⅰ–Ⅲ (Fig. 5A and B). Fig. 5A shows a MN (arrowhead) located in cortical upper layer IV where the MBP stain is heavier, a MBP negative cavity is seen over there (small arrows). However, a MN found in layersⅡ and Ⅲ where there is no evident MBP labeling does not crop a cavity (Fig. 5B). This phenomenon suggests MN microglia might be electively encroaching myelin compound during formation of the MNs.
Both MNs (arrowhead) and PCs (opened arrowheads) could damage the surrounding myelin sheaths and/or nerve fibers of either myelinated or unmyelinated. Fig. 5C shows a large nodule lesion composed of both MN and PC has encroached adjacent MBP positive myelin sheathes (small arrows) in the CC. Meanwhile, both MNs (arrowhead) and PCs (opened arrowhead) cut off NF positive nerve fibers (Fig. 5D and E) in the CC. These nerve fibers could be either myelinated or unmyelinated axon bundles.
Severer nodular lesion in white matter than in gray matter in simian immunodeficiency virus-infected monkey, but not closely correlated with viral infection
- Received Date: 2018-05-11
- Accepted Date: 2019-06-04
- Rev Recd Date: 2019-04-16
- Available Online: 2019-08-28
- Publish Date: 2020-07-01
- simian immunodeficiency virus-infected monkey /
- white matter tract /
- nodular lesions /
- microglia nodules /
- perivascular cuffing /
- nerve fiber disconnection
Abstract: Immune cell accumulation and white matter anomaly are common features of HIV (human immunodeficiency virus) -infected patients in combination antiretroviral therapy (cART) era. Neuroimaging tests on cART treated patients displayed prominent diffuse white matter lesions. Notably, immune cell nodular lesion (NL) was a conspicuous type of pathological change in HIV/SIV (simian immunodeficiency virus) infected brain before cART. Therefore, we used SIV infected brain to investigate the distribution of those NLs in gray and white matters. We found a significant higher number of NLs in white matter than that in gray matter. However, virus infection correlated with macrophage NLs but not with microglia NLs, especially in white matter. In addition, NLs interrupted white matter integrity more severely, since even tiny nodules could disconnect nerve fibers in white matter tracts. In the gray matter with dense myelinated axons, NLs obviously encroached those fibers; in the area of few myelinated axons, small nodules well co-localized with extracellular matrix between neurons.
|Citation:||Jingdong Zhang, Howard Fox, Huangui Xiong. Severer nodular lesion in white matter than in gray matter in simian immunodeficiency virus-infected monkey, but not closely correlated with viral infection[J]. The Journal of Biomedical Research, 2020, 34(4): 292-300. doi: 10.7555/JBR.33.20180047|