Article

Chronic inflammation, persistent SARS-CoV-2 antigens, reactivation of latent herpes viruses, and reduced cortisol levels may contribute to post-COVID syndrome

Prospective studies suggest that one in eight people infected with the severe acute respiratory syndrome coronavirus type-2 (SARS-CoV-2) have persistent symptoms. This novel disease, known as long-COVID or post-acute COVID-19 syndrome (PACS), is more prevalent among hospitalization survivors, but even people who have experienced mild acute COVID-19 have a wide range of organ dysfunction and clinical symptoms. The authors from the United States conducted this study to identify biological (immunological, virological, and hormonal) features that persist in participants diagnosed with post-COVID syndrome more than one year after acute infection.

The underlying pathogenesis of post-COVID syndrome remains unclear. Current hypotheses discuss the persistence of virus or viral remnants in various tissues, the development or aggravation of autoimmunity, microclotting and platelet hyperactivation, immune dysregulation, the reactivation of non-SARS-CoV-2 latent viral infections, and widespread organ damage resulting from chronic inflammation.

 

About the study

The study included 268 participants, categorized into five different groups: healthcare workers infected with SARS-CoV-2 before vaccination, post-COVID patients, post-COVID patients from an independent study, healthy uninfected vaccinated controls, and vaccinated COVID-19 convalescent controls without post-COVID symptoms. Most (76%) post-COVID patients were infected in 2020 when the majority of cases were caused by the parental SARS-CoV-2 strain (WA-1).

The COVID-19 vaccinated convalescents and post-COVID patients had mild acute SARS-CoV-2 infections that did not require hospitalization.

On average, samples were collected more than a year after the acute COVID-19. Anti-viral reactivity was examined using three complementary methodologies, namely rapid extracellular antigen profiling (REAP), serum epitope repertoire analysis (SERA), and enzyme-linked immunosorbent assay (ELISA).

 

 

 

Results

The most common symptoms in the post-COVID group were fatigue (87%), brain fog (78%), memory impairment (62%) and confusion (55%). The postural orthostatic tachycardia syndrome (POTS) was also frequently reported.

 

The analysis of the circulating immune cells

 

Compared to convalescent controls, post-COVID patients had significantly changed populations of circulating immune cells. They had an increased number of non-conventional monocytes (CD14lowCD16high) and a decreased number of conventional type 1 dendritic (cDC1) cells, which are responsible for antigen presentation and cytotoxic T-cell priming. The numbers of neutrophils, eosinophils, conventional and intermediate monocytes, plasmacytoid dendritic, and cDC2 cells did not differ significantly between the groups.

Individuals with post-COVID had a significantly higher median relative percentage of B lymphocytes in activated populations (CD86highHLA-DRhigh) and double-negative subsets (IgDCD27CD24CD38), as well as the absolute count of double-negative B cells in comparison to convalescent controls. The circulating levels of other B cell subsets, including naïve B cells, were not significantly different among the groups.

An analysis of the circulating T lymphocyte populations showed a reduced number of CD4+ T central memory cells (CD45RACD127+CCR7) and an increased absolute number of exhausted CD4+ T-cells (PD-1+TIM3+) in post-COVID patients compared to convalescent controls. The naïve CD4+ and CD8+ T-cells did not differ significantly between the groups.

The cerebrospinal fluid of post-COVID participants had an increased number of TIGIT+CD8+ T-cells, which is consistent with possible immune exhaustion.

After being stimulated with phorbol myristate acetate and ionomycin, CD4+ cells from individuals with post-COVID participants produced significantly higher median levels of intracellular interleukin (IL)-2 and IL-4, as well as IL-2 and IL-6 among CD8+ T cells compared to vaccinated convalescents and unvaccinated healthy controls. Notably, individuals with post-COVID also had uniquely elevated median levels of IL-4/IL-6 double-positive CD4+ T cells and IL-4/IL-6 double-positive CD8+ T cells. The levels of interferon (IFN)-γ and IL-17 (in CD4+ cells) and tumor necrosis factor and granzyme B (in CD8+ cells) did not significantly differ across groups.

 

The antibody responses to SARS-CoV-2-specific antigens

An initial analysis of antibody responses to SARS-CoV-2-specific antigens was performed by ELISA only in post-COVID participants who had received two doses of COVID vaccine. They showed increased anti-S1 IgG levels compared to the vaccinated convalescent controls. The levels of total anti-S IgG and anti-RBD IgG did not significantly differ between the two groups.

Importantly, unvaccinated participants diagnosed with post-COVID syndrome had higher levels of anti-N IgG than unvaccinated convalescent controls.

 

The antibody responses to other viruses

The antibody responses to other viruses were only analyzed in participants who had received two doses of COVID vaccines. Reactivities against 38 viral epitopes were detected in 98 post-COVID patients and 38 controls.

Post-COVID patients had higher REAP scores for several herpes virus antigens, including the Epstein–Barr virus (EBV) minor viral capsid antigen gp23, the EBV fusion-receptor component gp42, and the varicella-zoster virus (VZV) glycoprotein E.

The correlation analysis demonstrated that the number of polyfunctional CD4+ T cells co-expressing IL-4/IL-6 detected in post-COVID patients was correlated with reactivity to EBV lytic antigens, but not with reactivity to SARS-CoV-2-specific antigens. The authors stated that these results may indicate that CD4+ T cell activation may shift in response to EBV, as previously speculated in individuals with myalgic encephalomyelitis/chronic fatigue syndrome.

Since anti-EBV IgM levels were not elevated in post-COVID participants and there was no evidence of EBV viremia, the authors suggested that the higher reactivity to lytic EBV antigens may be due to recent EBV reactivation, rather than acute infection.

To summarize, the results of the REAP and SERA revealed an increase in IgG reactivity to EBV and VZV surface antigens, without evidence of primary EBV infection or acute viremia among post-COVID participants.

The analysis of hormones and soluble immune mediators

A parallel analysis of circulating hormones and immune mediators showed that post-COVID participants had higher median levels of complement C4b, chemokines CCL4, CCL19, CCL20, galectin-1, a proliferation-inducing ligand (APRIL), and luteinizing hormone (LH), and lower median levels of IL-5. Following viral infection, a cytokine APRIL, a member of the tumor necrosis factor family, is expressed to play an important role in B cell response and antibody production, but its role following SARS-CoV-2 infection is still not fully investigated.

Importantly, the circulating cortisol levels decreased markedly in both the post-COVID groups, post-COVID patients, and post-COVID patients from an independent study. This result remained significant even after variations in demographics and sample-collection time adjustment.

The decrease in cortisol levels did not correlate with a compensatory increase in adrenocorticotropic hormone (ACTH) levels, suggesting an inappropriately blunted hypothalamic-pituitary axis response. The authors pointed out that the half-life of ACTH in plasma is extremely short, potentially compromising the accuracy of change detection.

Subsequent statistical modeling revealed an association between reduced cortisol levels and post-COVID syndrome, taking into account differences in age, sex, body mass index, time of sample collection, and cohort. In addition, serum cortisol levels were found to be the most significant predictor of post-COVID status in this model.

 

Conclusion

The results of this study show that several mechanisms, such as persistent SARS-CoV-2 antigens, reactivation of latent herpesviruses, chronic inflammation and decreased cortisol levels may contribute to pathophysiological mechanisms of the post-COVID syndrome. The authors emphasized that the finding of persistently lower cortisol levels in post-COVID participants more than a year after the acute infection warranted further investigation.

 

This article was published in Nature.

 

Journal Reference

Klein J, Wood J, Jaycox JR. et al. Distinguishing features of long COVID identified through immune profiling. Nature 623, 139–148 (2023). (Open Access)   https://doi.org/10.1038/s41586-023-06651-y

 

 

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