The consortium of researchers, led by authors from Switzerland, evaluated anti-chemokine autoantibodies after SARS-CoV-2 infection. The findings revealed different patterns of anti-chemokine autoantibodies in recovered convalescents after COVID-19 and patients with long COVID.
COVID-19 patients often experience prolonged symptoms that can last for months, a condition known as long COVID or PASC (post-acute sequelae of COVID). The mechanisms underlying long COVID and the differences in its manifestation are poorly understood.
Chemokines are chemotactic cytokines that mediate leukocyte trafficking and activity. Acute COVID-19 is characterized by high expression of certain chemokines, which recruit neutrophils and monocytes to sites of infection. These chemokines play a key role in the pathophysiology of COVID-19 by sustaining inflammation and causing collateral tissue damage. The variety and concentrations of anti-chemokine antibodies, which are induced upon infection, may modulate the inflammatory response, the manifestation of disease and the persistence of COVID symptoms.
About the study
In the present study, researchers applied a peptide-based strategy to quantify antibodies binding to a functional region of 43 human chemokines following SARS-CoV-2 infection. The study included plasma samples from COVID-19 convalescents from three independent COVID-19 cohorts (Lugano, Milano, Zurich) at 6 and 12 months following the onset of the disease. Healthy controls were SARS-CoV-2 uninfected individuals, confirmed by negative serologic test (n = 23). The Lugano cohort comprised 71 convalescent individuals, and there was a higher proportion of males in previously hospitalized (n = 50) than in outpatient (n = 21) convalescents (60% and 38.1%, respectively).
At month 12 after the infection, the Lugano cohort was asked to answer to a questionnaire about self-reported symptoms of long-COVID. 65% of participants reported at least one persistent symptom. The average number of long-term symptoms was 3.3, and they were more frequent in formerly hospitalized (72.7%) than in outpatient (47.4%) convalescents. The age and gender of convalescents with long COVID symptoms and those without long COVID symptoms did not differ at month 12.
The authors employed an enzyme-linked immunosorbent assay and a pseudovirus-based neutralization assay to assess the binding and neutralizing capacity of antibodies against SARS-CoV-2 in plasma samples of convalescents from the Lugano cohort. The IgG levels against receptor binding domain (RBD) of SARS-CoV-2 spike (S) protein and plasma half-maximal SARS-CoV-2 neutralizing titers (NT50) were both variable, but positively correlated. The anti-SARS-CoV-2-S-RBD IgG antibodies and NT50 values were significantly higher in previously hospitalized convalescents than in outpatient convalescents, as well as in males compared to females. Furthermore, the anti-SARS-CoV-2-S-RBD IgG antibodies and NT50 values observed in convalescents with long COVID were comparable to those observed in convalescents without long COVID.
There was no correlation between antibodies against SARS-CoV-2 and autoantibodies against specific chemokines. Additionally, the levels of anti-SARS-CoV-2-S-RBD IgG antibodies and NT50 values did not correlate with concentrations of antibodies against COVID-19 signature chemokines (CCL19, CCL22 and CXCL17), or with the sum of all chemokine IgG reactivities.
Anti-chemokine autoantibodies in COVID-19 convalescents
In order to examine the kinetics of autoantibodies against COVID-19 signature chemokines (CCL19, CCL22 and CXCL17), the authors assessed convalescent individuals who had plasma samples from the acute phase of infection. The Milan cohort had higher concentrations of autoantibodies against CCL19, CCL22 and CXCL17 during the acute phase compared to healthy controls. The Lugano cohort had higher concentrations of autoantibodies to CCL19 (but not to CCL22 or CXCL17) during the acute phase compared to healthy controls, and they continued to rise until month 12.
Researchers then examined temporal dynamics of anti-chemokine autoantibodies at 6 and 12 months after the onset of COVID-19. Regardless of vaccination status, autoantibodies to CCL19, CCL8, CCL13, CCL16, CXCL7 and CX3CL1 significantly increased. Autoantibodies to CXCL17 remained generally stable, while autoantibodies to CCL22 displayed variable kinetics at 12 months, as compared to those observed at 6 months after the infection.
Researchers also assessed the correlation between autoantibodies and their corresponding antigens in convalescents from the Milan (acute and at month 7) and Lugano (acute, at months 6 and 12) cohorts. In Milan cohort, the concentrations of several chemokines were significantly elevated both during acute disease (CCL2, CCL3, CCL4, CCL19, CCL21, CCL22, CCL25, CXCL2, CXCL8, CXCL9, CXCL10, CXCL13 and CXCL16) and at month 7 (CCL19, CCL21, CCL22, CXCL2, CXCL8, CXCL10, CXCL13 and CXCL16) in convalescents compared to healthy controls. However, the concentrations of chemokines and their corresponding autoantibodies did not correlate in the acute phase and at 7 months after the infection.
In 12 individuals from the Lugano cohort, the concentrations of CCL3 and CCL4 chemokines were elevated at month 6 after the infection, but, there was no increase in autoantibodies against CCL3 and CCL4. Apparently, even though some chemokines rapidly increased and persisted in plasma for at least 6 months after the infection, their levels did not correlate with levels of corresponding autoantibodies.
Anti-chemokine autoantibodies and severity of infection
After that, researchers examined the connection between the severity of infection and anti-chemokine autoantibodies. In the Lugano cohort, outpatient convalescents exhibited a broader pattern and a higher overall level of anti-chemokine autoantibodies in comparison to hospitalized convalescents at 6 months following the onset of disease. Furthermore, outpatient convalescents had higher levels of antibodies against the chemokines associated with COVID-19 severity signature (CXCL5, CXCL8 and CCL25) than hospitalized convalescents at 6 months after the infection, and this was not related to therapy given during hospitalization.
Additionally, outpatient convalescents had higher concentrations of autoantibodies against 8 chemokines (CXCL8, CCL22, CXCL16, CCL27, CXCL7, CCL20, CX3CL1 and CCL19) compared to healthy controls. They also had significantly higher cumulative IgG reactivity against the chemokines than healthy controls.
In contrast, the convalescents who were admitted to the hospital during the acute phase of the disease had only higher concentrations of antibodies against CCL19 compared to healthy controls at 6 months following the onset of infection. In addition, in hospitalized convalescents, a negative correlation between antibodies to CXCL5 and CXCL8 and anti-SARS-CoV-2-S-RBD IgG was observed.
Similar results were found in the Milan and Zurich cohorts. These findings suggest that higher levels of autoantibodies directed against specific chemokines are associated with favorable disease outcome.
The authors noted that antibodies against COVID-19 signature chemokines (CCL19, CCL22 and CXCL17), which distinguished COVID-19 convalescents from healthy controls, were different from those associated with COVID-19 severity signature (CXCL5, CXCL8 and CCL25).
Anti-chemokine autoantibodies and long-COVID
Researchers then determined whether a specific pattern of anti-chemokine antibodies at 6 months after the infection could be predictive of long COVID syndrome. The results showed significantly lower cumulative levels and specific pattern of anti-chemokine antibodies in convalescents with long COVID, particularly outpatients and females, compared to those with no long COVID symptoms. The cumulative amount of chemokine antibodies did not correlate with the number of symptoms.
Convalescents with no long COVID had increased concentrations of autoantibodies to CCL21, CXCL13 and CXCL16 compared to individuals with long COVID. It appears that autoantibodies against CCL21, CXCL13 and CXCL16 distinguished long COVID from no long COVID group with high significance. They were defined as “long COVID signature”.
As these chemokines are important for tissue trafficking and activation of T and B lymphocytes, the authors speculated that autoantibodies to these chemokines might have positive influence to the long-term outcome by antagonizing or modulating the activation, recruitment and retention of T and B lymphocytes. In Zurich cohort, only antibodies against CCL21 were significantly different between long COVID and no long COVID convalescents at month 13.
Anti-chemokine autoantibodies in other infections or autoimmune diseases
Finally, the authors measured anti-chemokine autoantibodies in plasma from patients with other infections or autoimmune disorders, such as chronic HIV-1 infection (n = 24) or infection with Borrelia burgdorferi- Lyme disease (n = 27), ankylosing spondylitis (AS, n = 13), rheumatoid arthritis (RA, n = 13) or Sjögren syndrome (SjS, n = 13).
Autoantibodies against 14 chemokines (CCL2, CCL3, CCL4, CCL5, CCL20, CCL21, CCL22, CCL23, CCL27, CCL28, CXCL7, CXCL8, CXCL9 and CXCL12), but not against CCL19, were significantly increased in individuals with chronic HIV-1-infection compared to healthy controls. Plasma from Borrelia-infected individuals was indistinguishable from the healthy controls, except for elevated CXCL14 antibodies in the acute phase. Autoantibodies against four chemokines (CCL4, CCL19, CCL25 and CXCL9) were increased in AS, RA and SjS compared with healthy controls. These findings indicate that different patterns of anti-chemokine autoantibodies distinguished not just different COVID-19 trajectories, but also other infections and autoimmune disorders.
The authors concluded that their results discovered that antibodies against certain chemokines were omnipresent in COVID-19 convalescents. Higher overall level of anti-chemokine autoantibodies in outpatient convalescents than in convalescents who were hospitalized during the acute phase of the disease indicates their association with favorable disease outcome. Additionally, convalescents with long COVID displayed lower cumulative levels and specific pattern of anti-chemokine autoantibodies compared to convalescents with no long-COVID. Anti-chemokine antibodies were also present in chronic HIV-1 infection and autoimmune disorders, but they targeted different chemokines compared with COVID-19.
This article was published in Nature Immunology.
Muri J et al. Autoantibodies against chemokines post-SARS-CoV-2 infection correlate with disease course. Nat Immunol 24, 604–611 (2023). (Open Access) https://doi.org/10.1038/s41590-023-01445-w