In a recent study published in the journal Allergy, a team of researchers in Austria conducted a longitudinal analysis among individuals who had not been vaccinated against coronavirus disease 2019 (COVID-19) to understand the changes in humoral and cellular immunity over ten months after the first severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.

Study: Differential decline of SARS-CoV-2-specific antibody levels, innate and adaptive immune cells, and shift of Th1/inflammatory to Th2 serum cytokine levels long after first COVID-19. Image Credit: ker_vii / Shutterstock

Background

The COVID-19 pandemic was one of the most severe public health calamities in the last decade, causing global morbidity and mortality in the millions. Furthermore, emerging research indicates that the disease causes long-term sequelae lasting months after recovering from the initial SARS-CoV-2 infection. These post-COVID-19 symptoms, also called post-acute sequelae of  COVID-19 (PASC) or long coronavirus disease (long COVID), impact various organ systems other than the pulmonary organs, even resulting in neurocognitive impairments and cardiovascular and musculoskeletal symptoms.

Recent studies have focused on understanding long COVID’s risk factors and mechanisms. Furthermore, while long-term sequelae have been observed in other viral infections as well, such as mumps, measles, influenza, and Epstein-Barr virus infections, PASC is unique in its ability to affect multiple organ systems, with wide-ranging symptoms such as fatigue, dyspnea, insomnia, ‘brain fog’, depression, heart palpitations, and kidney failure.

About the study

In the present study, the researchers conducted a longitudinal analysis among individuals who had not been vaccinated against SARS-CoV-2 to determine the changes in humoral and cellular immune parameters over ten months after the first infection. This study was a follow-up of their previous study, which examined immune parameter changes in unvaccinated individuals ten weeks after recovering from their first SARS-CoV-2 infection.

The study included 106 participants with SARS-CoV-2 infection confirmed through a real-time polymerase chain reaction (rtPCR) or antibody-based test and a matched control cohort of uninfected individuals.

Here, they re-evaluated the immune status of the participants from the first study and determined the levels of antibodies generated against the SARS-CoV-2 spike, receptor binding domain, and nucleocapsid proteins. They also conducted neutralization assays to determine the ability of these antibodies to inhibit the binding of the receptor binding domain with the angiotensin-converting enzyme-2 (ACE-2) receptor.

Based on the waning of antibodies against SARS-CoV-2, the participants were also categorized into groups, and the leukocyte subsets were analyzed using multiparametric flow cytometry of whole blood samples. The researchers focused on understanding the subpopulations of B and T lymphocytes present in the blood.

Additionally, multiplex technology using coated beads was used for serum analysis to determine the levels of cytokines such as interleukins (IL), as well as natural killer (NK) and T cell interferon-γ (IFN-γ) levels, and immunoglobulin (Ig) E levels.

The expression of ACE-2 receptors on recent thymic emigrants, such as T cells expressing cluster of differentiation (CD)3, CD62L, CD45RA, and CD31, and memory B cells expressing CD19, CD27, and IgD was also measured. All the immune parameter measurements taken ten weeks and ten months after the first infection for the COVID-19-positive cohort were compared with measurements taken at the same time points in the control cohort.

Results

The study found that the individuals who had recovered from COVID-19 had lower absolute counts of monocytes, granulocytes, and lymphocytes as compared to the healthy control cohort. The circulating neutrophil levels in the infected individuals were also significantly lower than those in uninfected individuals.

The expression levels of CD38 and human leukocyte antigen-DR isotype (HLA-DR) were higher in the infected individuals as compared to the healthy controls, indicating that the levels of CD8⁺ cytotoxic T cells were elevated after a SARS-CoV-2 infection. The cytotoxic T-cell levels remained elevated even after ten months in patients who had severe COVID-19, while for mild to moderate cases of COVID-19, the levels returned to baseline in ten months.

The researchers believe their results support theories proposed in other studies of extensive tissue damage due to severe COVID-19 or long-term persistence of antigens in the body and continued viral shedding. Alternate explanations also include persistent activation of T cells due to increased IL-4 and IL-17A levels in the serum, both of which were elevated in the serum samples of the SARS-CoV-2 infected cohort in the study.

The individuals in the SARS-CoV-2 infected cohort also showed elevated levels of effector memory cells expressing CD3, CD4, and CD8, as well as plasmablast and translational B cells, but lower levels of regulatory T cells. Furthermore, the levels of antibodies against the SARS-CoV-2 nucleocapsid and spike proteins were found to decline with time, especially in younger patients. However, the changes in regulatory and effector T cells and recent thymic emigrants were independent of changes in antibody levels.

Conclusions

Overall, the findings showed that SARS-CoV-2 infections resulted in long-term changes in adaptive and innate immunity parameters and immune cells, and the changes were associated with cytokine profiles dominated by T helper cells 2. These findings further our understanding of the various mechanisms that contribute to long COVID.