Survivors of Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) infection frequently experience lingering neurological symptoms, including impairment in attention, concentration, speed of information processing and memory.
This long-COVID cognitive syndrome shares many features with the syndrome of cancer therapy-related cognitive impairment (CRCI). Neuroinflammation, particularly microglial reactivity and consequent dysregulation of hippocampal neurogenesis and oligodendrocyte lineage cells, is central to CRCI.
We hypothesized that similar cellular mechanisms may contribute to the persistent neurological symptoms associated with even mild SARS-CoV-2 respiratory infection. Here, we explored neuroinflammation caused by mild respiratory SARS-CoV-2 infection – without neuroinvasion – and effects on hippocampal neurogenesis and the oligodendroglial lineage.
Human brain tissue from 9 individuals with COVID-19 or SARS-CoV-2 infection exhibits the same pattern of prominent white matter-selective microglial reactivity.
In mice, pro-inflammatory CSF cytokines/chemokines were elevated for at least 7-weeks post-infection; among the chemokines demonstrating persistent elevation is CCL11, which is associated with impairments in neurogenesis and cognitive function. Humans experiencing long-COVID with cognitive symptoms (48 subjects) similarly demonstrate elevated CCL11 levels compared to those with long-COVID who lack cognitive symptoms (15 subjects).
Impaired hippocampal neurogenesis, decreased oligodendrocytes and myelin loss in subcortical white matter were evident at 1 week, and persisted until at least 7 weeks, following mild respiratory SARS-CoV-2 infection in mice. Taken together, the findings presented here illustrate striking similarities between neuropathophysiology after cancer therapy and after SARS-CoV-2 infection, and elucidate cellular deficits that may contribute to lasting neurological symptoms following even mild SARS-CoV-2 infection.
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The COVID-19 pandemic caused by severe acute respiratory syndrome coronavirus-2 (SARS- CoV-2) has to date resulted in over 285 million documented COVID-19 cases worldwide. Neurological symptoms are emerging as relatively common sequelae of SARS-CoV-2 infection, with persistent cognitive impairment affecting approximately one in four COVID-19 survivors (Nasserie et al., 2021).
While more common in individuals who had experienced severe COVID requiring hospitalization, even those with mild symptoms in the acute phase may experience lasting cognitive dysfunction (Becker et al., 2021; Nasserie et al., 2021). Colloquially known as “COVID-fog”, this syndrome of COVID-associated cognitive impairment is characterized by impaired attention, concentration, speed of information processing, memory, and executive function (Becker et al., 2021; Nasserie et al., 2021).
Together with increased rates of anxiety, depression, disordered sleep and fatigue, this syndrome of cognitive impairment contributes substantially to the morbidity of “long-COVID” and in many cases prevents people from returning to their previous level of occupational function (Davis et al., 2021; Tabacof et al., 2022). Given the scale of SARS-CoV-2 infection, this syndrome of persistent cognitive impairment represents a major public health crisis (Nath 2020).
The syndrome of cognitive symptoms that COVID survivors frequently experience closely resembles the syndrome of cancer therapy-related cognitive impairment (CRCI), commonly known as “chemo-brain”. Neuroinflammation is central to the pathophysiology of cancer therapy- related cognitive impairment (for review, see (Gibson and Monje, 2021)), raising the possibility of shared pathophysiological mechanisms.
Microglia, brain-resident macrophages, become persistently reactive following exposure to certain systemic chemotherapy drugs, after cranial radiation, or after systemic inflammatory challenge with low-dose lipopolysaccharide (Geraghty et al., 2019; Gibson et al., 2019; Monje et al., 2002; Monje et al., 2003; Monje et al., 2007).
A distinct subpopulation of microglia that reside in white matter (Hammond et al., 2019) are selectively activated by systemic insults such as exposure to the chemotherapy drug methotrexate (Gibson et al., 2019). Reactive microglia impair mechanisms of cellular homeostasis and plasticity such as the ongoing generation of myelin-forming oligodendrocytes (Gibson et al., 2019), myelin plasticity (Geraghty et al., 2019) and new neuron generation in the hippocampus (Monje et al., 2002; Monje et al., 2003; Monje et al., 2007).
Local microglial cytokine secretion contributes to at least part of this dysregulation (Monje et al., 2003). Elevated circulating cytokine/chemokine levels, particularly CCL11, can also limit neurogenesis and contribute to cognitive impairment (Villeda et al., 2011).
In addition to these direct effects of inflammatory mediators on cellular plasticity in the brain, microglia also induce neurotoxic astrocyte reactivity through cytokine signaling (Liddelow et al., 2017). Astrocytes can assume a range of reactive states (Hasel et al., 2021) that can induce further pathophysiology, with certain states of reactive astrocytes inducing oligodendrocyte and neuronal cell death (Liddelow et al., 2017) through secretion of saturated lipids contained in lipoprotein particles (Guttenplan et al., 2021).
This complex cellular dysregulation is thought to contribute importantly to cognitive impairment, and anti-inflammatory strategies correct such multicellular dysregulation and rescue cognition in disease states such as occurs after exposure to neurotoxic cancer therapies (Geraghty et al., 2019; Gibson et al., 2019; Monje et al., 2003) and in aging (Villeda et al., 2011).
While systemic and severe COVID can cause multi-organ disease and numerous potential mechanisms affecting the nervous system (Lee et al., 2021; Nath and Smith, 2021; Remsik et al., 2021), even mild COVID could result in an a detrimental neuroinflammatory response, including reactivity of these exquisitely sensitive white matter microglia.
Given this context, we hypothesized that the inflammatory response to even mild COVID-19 may induce elevation in neurotoxic cytokines/chemokines, a pattern of white matter microglial reactivity, and consequent dysregulation of myelin-forming oligodendrocytes and hippocampal neural precursor cells.
Mild respiratory SARS-CoV-2 infection impairs hippocampal neurogenesis
Reactive microglia and other aspects of inflammatory responses to systemic illness (Monje et al., 2003) or aging (Villeda et al., 2011) can impair the generation of new neurons in the hippocampus, an ongoing mechanism of neural plasticity thought to support healthy memory function (Clelland et al., 2009; Zhang et al., 2008).
Examining the mouse hippocampus following mild respiratory SARS-CoV-2 infection, we found robustly increased microglial reactivity in hippocampal white matter at 7-days post-infection (Figure 4A-B) that persists until at least 7-weeks post-infection (Figure 4C-D).
Consistent with previous observations that reactive microglia can inhibit hippocampal neurogenesis (Monje et al., 2003), a stark decrease in new neuron generation was evident, as assessed by Doublecortin-positive cell quantification, at 7-days post-infection (Figure 4E-F) and persisted until at least 7-weeks post-infection (Figure 4G-H). Considering the number of newly generated hippocampal neurons as a function of the reactive microglial load, we found an inverse correlation between neurogenesis and reactive microglia in the hippocampus (Figure 4I).
Inflammatory cytokines can directly inhibit hippocampal neurogenesis (Monje et al., 2003), including interleukin-6 (IL6) derived from reactive microglia (Monje et al., 2003) and the circulating cytokine CCL11 (also called eotaxin-1) (Villeda et al., 2011). CSF cytokine profiles at 7 days and 7 weeks following mild respiratory SARS-CoV-2 infection highlights elevated IL6 in CSF at 7 days, and persistently elevated CCL11 in CSF at 7 weeks. In the same mice, CCL11 was elevated at 7-days post-infection and – in contrast to persistent elevation in CSF – normalized in serum by 7-weeks post-infection (Figure 1D-G, Figure 4J and Supplementary Figure 4).
To further explore the relevance of elevated CCL11 levels to cognitive sequalae of COVID infection in humans, we next examined circulating CCL11 cytokine levels in the plasma of people suffering from long-COVID with and without cognitive symptoms.
These individuals chiefly experienced a relatively mild course of SARS-CoV-2 infection, without hospitalization in more than 90% of subjects. We found elevated CCL11 levels in the plasma of people with long-COVID exhibiting cognitive deficits, or “brain fog” (n = 48 subjects, 16 male/32 female, mean age 46.1±14.6 years) compared to those with long-COVID lacking cognitive symptoms (n = 15 subjects, 4 male/11 female, mean age 46.7±14.1 years; Figure 4K).
We next explored the effects of SARS-CoV-2 mild respiratory infection on oligodendroglial lineage cells. Reactive microglia can alternatively promote (Miron et al., 2013) or impair (Gibson et al., 2019) oligodendrogenesis, depending on the precise microglial cell state. In disease states like CRCI, reactive microglia cause a dysregulation of the oligodendroglial lineage (Gibson et al., 2019) and loss of myelin plasticity (Geraghty et al., 2019).
Examining oligodendroglial lineage cells in subcortical white matter (cingulum of the corpus callosum), we found maintenance of the oligodendrocyte precursor cell population marked by PDGFR-alpha at 7 days after mild respiratory SARS-CoV-2 infection (Figure 5A.) However, by 7 weeks following infection, a mild decrease (~10%) in the number of oligodendrocyte precursor cells was evident (Figure 5B-C).
Mature oligodendrocytes, assessed by ASPA and CC1 immunoreactivity, exhibited a greater magnitude of depletion (Figure 5D-F and Supplementary Figure 5), with loss of approximately one third of oligodendrocytes occurring by 7-days post-infection (Figure 5D). This depletion in mature oligodendrocytes persisted until at least 7-weeks post-infection (Figure 5E).
Consistent with the loss of oligodendrocytes described above, electron microscopy ultrastructural analyses revealed a frank loss of myelin, the insulating ensheathment of axons by oligodendrocyte processes that decreases transverse capacitance and increases the speed of action potential conductance, together with providing metabolic support to axons.
This loss of myelin persisted until at least 7 weeks following mild respiratory SARS-CoV-2 infection. Changes in myelin sheath thickness relative to axon diameter (g-ratio) were not conclusively identified in the remaining myelin sheaths (Supplementary Figure 6A-D).
Such persistent loss of myelin in subcortical projections would be predicted to impair neural circuit function and axon health, adding to the numerous deleterious neurobiological consequences of SARS-CoV-2 infection.
The study findings were published on a preprint server and are currently being peer reviewed. https://www.biorxiv.org/content/10.1101/2022.01.07.475453v1