COVID-19-related neuropathology and microglial activation in elderly with and without dementia

Abstract

The actual role of SARS-CoV-2 in brain damage remains controversial due to lack of matched controls. We aim to highlight to what extent is neuropathology determined by SARS-CoV-2 or by pre-existing conditions. Findings of 9 Coronavirus disease 2019 (COVID-19) cases and 6 matched non-COVID controls (mean age 79 y/o) were compared. Brains were analyzed through immunohistochemistry to detect SARS-CoV-2, lymphocytes, astrocytes, endothelium, and microglia. A semi-quantitative scoring was applied to grade microglial activation. Thal-Braak stages and the presence of small vessel disease were determined in all cases. COVID-19 cases had a relatively short clinical course (0-32 days; mean: 10 days), and did not undergo mechanical ventilation. Five patients with neurocognitive disorder had delirium. All COVID-19 cases showed non-SARS-CoV-2-specific changes including hypoxic-agonal alterations, and a variable degree of neurodegeneration and/or pre-existent SVD. The neuroinflammatory picture was dominated by ameboid CD68 positive microglia, while only scant lymphocytic presence and very few traces of SARS-CoV-2 were detected. Microglial activation in the brainstem was significantly greater in COVID-19 cases (p = 0.046). Instead, microglial hyperactivation in the frontal cortex and hippocampus was clearly associated to AD pathology (p = 0.001), regardless of the SARS-CoV-2 infection. In COVID-19 cases complicated by delirium (all with neurocognitive disorders), there was a significant enhancement of microglia in the hippocampus (p = 0.048). Although higher in cases with both Alzheimer's pathology and COVID-19, cortical neuroinflammation is not related to COVID-19 per se but mostly to pre-existing neurodegeneration. COVID-19 brains seem to manifest a boosting of innate immunity with microglial reinforcement, and adaptive immunity suppression with low number of brain lymphocytes probably related to systemic lymphopenia. Thus, no neuropathological evidence of SARS-CoV-2-specific encephalitis is detectable. The microglial hyperactivation in the brainstem, and in the hippocampus of COVID-19 patients with delirium, appears as a specific topographical phenomenon, and probably represents the neuropathological basis of the "COVID-19 encephalopathic syndrome" in the elderly.

Keywords: COVID-19; dementia; elderly; microglia; neurocognitive disorders; neuropathology.

FIGURE 1

COVID‐19 neuropathology. H&E reveals diffuse cortical oedema (A), inflammatory perivascular infiltrates (B), and micro nodules in the basal ganglia (C) and subependymal zone (D), which are also identified with CD68 marker (E). LFB shows myelin loss in the subcortical WM due to SVD (F). Rare foci of perivascular T‐lymphocytes (G) and B‐lymphocytes (H) are identified using anti‐CD3 and anti‐CD20 antibodies, respectively. B‐lymphocytes are also observed within some inflammatory nodules (I). Rare SARS‐Cov‐2 positive cells are detected only in the lower brainstem of the one case (Cov3) (J, K).CD34 staining displays foci of abnormal tuft‐like capillary features, particularly frequent in the pons (L, M). GFAP reveals mild to moderate gliosis with frequent reactive astrocytes (N, N’). GFAP enhancement in astrocytic end feet around blood vessels is a peculiar picture (O). Scale bars: 512 μm (F); 288 μm (A, L); 180 μm (D, E); 106 μm (B, J, O); 76 μm (C, I); 47 μm (H, K, M, N); 31 μm (G); 17 μm (N’)

FIGURE 2

Microglial activation. The first row compares the neuropathological findings present in the frontal cortex (A–D) and the subiculum (E–H) of 2 AD cases (one AD/COVID‐19 and one AD/non‐COVID) and a non‐AD/non‐COVID case. The AD/COVID‐19 case shows amyloid plaques (A, E), as well as moderate to severe microglial activation and nodules (CD68+) in a topographical distribution similar to that of the plaques (B, F). This microglial activation does not appear different from that seen in the AD/non‐COVID case (C, G), while the same areas in the non‐AD/non‐COVID case are clearly diverse, with a normal microglial representation (D, H). The second row shows different pictures of activated microglia (CD68+) in COVID‐19 cases, including amoeboid cells, perivascular infiltrates and nodules in the olfactory bulb (I), frontal cortex (J, K), hippocampus (L, M), midbrain (N: red nucleus; O: detail of a nodule with neuronophagia in the upper right sector), pons (P: locus coeruleus Q & R: raphe nuclei), and medulla oblongata (S: DMV area). Scale bars: 315 μm (A‐H, L); 137 μm (I, M, N, P, Q); 60 μm (J, K, R); 48 μm (O, S)

FIGURE 3

The degree of microglial activation in brain areas (A) and whole brainstem (B). (A) Microglial grading in COVID‐19 and non‐COVID cases per areas. (B) Comparison of microglial grading from the whole brainstem (midbrain+pons+medulla) between COVID‐19 and non‐COVID cases. Small circles represent out values, stars show far out values. FCGM, frontal cortical gray matter; FWM, frontal white matter; HC, hippocampus; MB, midbrain; MO, medulla oblongata

FIGURE 4

The degree of microglial activation in the frontal cortex considering the COVID‐19 and dementia groups. Boxplot showing average microglial grading in the frontal cortical gray matter of COVID‐19 and non‐COVID cases, with or without Alzheimer's Dementia (only significant p‐values are shown). Number of cases: COVID‐19/no dementia, n = 5; COVID‐19/dementia, n = 4; non‐COVID/dementia, n = 3; non‐COVID/no dementia, n = 3

FIGURE 5

Microglial activation and Braak stage in COVID‐19 cases with delirium. (A) Boxplot showing microglial activation in the hippocampus of COVID‐19 cases comparing cases with and without delirium (delirium n = 5; no delirium n = 4). (B) Boxplot showing association between the degree of tau pathology and delirium onset in COVID‐19 cases. Early delirium n = 3, Late delirium n = 2, no delirium n = 4

FIGURE 6

Microglial activation and Alzheimer's disease pathology. Scatter plot showing correlation between microglial activation and amyloid‐β burden (A) and tau pathology (B). FGM, frontal cortical gray matter; HC, hippocampus