Scientists discover key reason for continued smell loss in long-term COVID

A recent study published in the journal Science Translational MedicineResearchers found that long-term anosmia following coronavirus disease 2019 (COVID-19) was caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from olfactory tissue.

study: Permanent olfactory loss after COVID-19 is associated with immune cell infiltration and altered gene expression in the olfactory epitheliumImage Credit: Nicole Rerk / Shutterstock

Background

One of the commonly reported symptoms of COVID-19 and acute sequelae of COVID-19 (PASC) is anosmia or loss of smell. Olfactory disturbances are thought to be caused by the effects of SARS-CoV-2 on the olfactory epithelium. The olfactory epithelium is composed of primary olfactory neurons that detect odors, a layer of supporting cells that form a barrier, and basal stem or progenitor cells that restore olfaction. olfactory epithelium. Autopsy studies in humans and animal models suggest different mechanisms to explain anosmia in COVID-19 patients. This includes changes in gene expression in olfactory sensory neurons, changes in the mucus layer around ciliated neuron cells, changes in gene expression in supporting cells, and inflammation.

However, the mechanisms behind persistent anemia in PASC patients are still unknown. Possible explanations include severe epithelial damage, neuroinflammation, and damage to the olfactory bulb or olfactory cortex of the brain. Autopsy studies of PASC patients found persistent infection of supporting cells and molecular alterations in sensory neurons, but epithelial anatomy was intact and no infection of olfactory sensory neurons was seen.

About research

In the current study, the researchers used biopsy samples of the olfactory epithelium obtained from nine PASC patients experiencing persistent anosmia and validated using an odor discrimination test prior to biopsy. Did. Olfactory biopsies and mucus samples from normoesthetic individuals with and without a history of COVID-19 were used as controls.

Biopsy samples were processed for single-cell ribonucleic acid sequencing (sc-RNA seq) to identify cellular or transcriptional alterations associated with persistent olfactory dysfunction observed in PASC patients. In addition, tubulin, cluster of differentiation (CD) 45, CD3, CD68, CD207, T cell receptor gamma/delta (TCR γ/δ), supporting cell-specific marker ermin (ERMN), olfactory marker protein (OMP) , SARS-CoV-2 nucleocapsid protein, and sex-determining region Y-box 2 (SOX2).

Additionally, mucosal samples were subjected to fluorescent bead-based multiplex assays and flow cytometry to test 13 chemokines and cytokines.

result

As a result, invasion into the olfactory epithelium was reported. T cells Changes in myeloid cell populations that secrete interferon-gamma (IFN-γ) and lead to increased CD207⁺ Decrease in dendritic cells and anti-inflammatory macrophages. Furthermore, gene expression analysis revealed a continuous response to inflammatory signaling in supporting cells despite the absence of SARS-CoV-2 protein or ribonucleic acid (RNA). We observed a lower number of olfactory sensory neurons, especially mature neurons presenting her OMP, compared to the number of supporting cells in the olfactory epithelium.

T cells infiltrate nasal olfactory epithelial biopsies of PASC hypotensive patients. (a) Representative immunohistochemistry images of nasal biopsy tissues from normotonic non-COVID-19, normotonic post-COVID-19, or PASC hypotonic individuals. Tissue sections were immunostained for TUJ1 neuronal marker, CD45 pan-immune cell marker, CD3 T-cell marker, and CD68 myeloid cell marker. PASC hypotensive tissue showed dense CD45⁺ Immune cell infiltration, including prominent CD3⁺ Lymphocytic infiltration. This was not seen in the normal osmotic group. Scattered CD68⁺ Cells were present in all conditions. (B.) Enlarged area (yellow box) in (A) shows CD3⁺ Lymphocytes with marked infiltration into the olfactory epithelium (white arrows). The white dashed line indicates the basement membrane. Scale bar, 50 μm. (C.) Additional nasal biopsies were processed for scRNA-seq to allow quantitative analysis. Uniform manifold approximate projection (UMAP) visualization of combined PASC hypotonic and control-normotonic scRNA-seq datasets, integrating 16 human nasal biopsies, enables robust cell cluster analysis and annotation became. RBC, red blood cell. pDC, plasmacytoid DC;

In contrast to acute cases of COVID-19, no transcriptomic alterations were found in olfactory sensory neurons and no severe inflammation was observed in patients with PASC-related olfactory impairment. Instead, an interferon response signature and lymphocyte populations expressing T-cell γ/δ and IFN-γ were observed in olfactory epithelium samples from PASC patients with anosmia.

A comparison of the immune cell phenotypes of olfactory-dystrophic humans during PASC and hamsters with acute SARS-CoV-2 infection showed that the infiltrates of the olfactory epithelium of infected hamsters contained the first 1 Weeks revealed that monocytes, neutrophils, and macrophages were included. by the second week. In contrast, the olfactory epithelium of anosmia PASC patients was infiltrated with T cells in the months following COVID-19. These differences indicate that immune responses during PASC-associated hyposmia or anosmia are significantly different from those during acute SARS-CoV-2 infection.

Although the mechanisms behind sensory dysfunction affecting olfactory sensory neurons, supporting cells, horizontal basal cells, and immune cells remain unclear, macrophage analysis in patients with severe COVID-19 suggests acute SARS-CoV-2 infection. Other long-term changes in immune cell function during proinflammatory reprogramming.

This result offers the potential to develop therapeutic options that can be locally delivered to the olfactory epithelium or olfactory cleft. This also helps avoid other systemic reactions to treatment.

Conclusion

In summary, this study examined olfactory epithelium biopsy samples from PASC patients with anosmia using immunohistochemical analysis and sc-RNA sequencing. Results revealed significant differences between immunological responses between acute SARS-CoV-2 infection and PASC-associated olfactory impairment. Furthermore, the olfactory epithelium of PASC patients with olfactory dysfunction is infiltrated with IFN-γ-secreting T cells, and gene expression is associated with inflammatory signals despite the absence of SARS-CoV-2 protein or RNA in the olfactory tissue. indicated that the transmission was continuing.