In a recent study posted to the bioRxiv* preprint server, researchers examined patients with pulmonary post-acute sequelae (PASC) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) to determine the frequency and function of SARS-CoV-2-specific T cells in their blood. Additionally, they evaluated the relationship of SARS-CoV-2-specific T cells with the expression of plasma inflammatory markers and pulmonary function.
Among patients of PASC, coronavirus disease 2019 (COVID-19) symptoms, such as shortness of breath, cough, chest pain, irregular heartbeat, brain fog, fatigue, intermittent fever, lung-related conditions, etc., exceed four weeks from symptom onset. SARS-CoV-2 infection is the major factor causing PASC, although the underlying mechanisms responsible for long-term lung dysfunction are unclear.
Moreover, COVID-19 treatments are ineffective at resolving pulmonary symptoms. T cells, a key component of adaptive immunity, have been associated with generalized PASC and most likely play a role in PASC; however, the characteristics of SARS-CoV-2-specific T cells and their connections to systemic inflammation or pulmonary symptoms are currently unknown.
About the study
In the present study, researchers recruited patients with confirmed SARS-CoV-2 between July 2020 and April 2021.
The pulmonary PASC study cohort had participants who experienced prolonged tussis, dyspnea, fatigue, and their symptoms lasted for a median duration of six months from symptom onset or hospital discharge. Patients with resolved COVID-19 (RC) constituted the control cohort and reported no symptoms at the time of sample collection. The median time from symptom onset to blood collection for PASC and RC cohorts was 225 and 32 days, respectively.
Concerning clinical characteristics, the PASC and RC cohorts were similar. Those with PASC were older than RC participants, with a median age of 54 in PASC versus 33 in RC. Also, 40% of PASC participants required hospitalization during acute COVID-19 infection. The average symptom duration for PASC and RC cohorts was six months and 12 days, respectively. Likewise, PASC and RC participants reported a median of nine and six symptoms during initial infection, respectively. As expected, PASC participants had an average of five prolonged symptoms while RC participants had none.
Of all the PASC patients, compared to hospitalized participants (PASC-H), those who were not hospitalized (PASC-NH) had more symptoms during the acute and post-acute phases of the disease. During statistical analyses, age, the number of comorbidities, time to sample collection, and hospitalization status did not affect the frequency of SARS-CoV-2 T cells and plasma IL-6 levels between PASC and RC cohorts.
The frequency of SARS-CoV-2-specific T cells in blood was measured using intracellular cytokine tumor necrosis factor (TNF-α), interferon gamma (IFN-γ), and interleukin-10 (IL-10) staining after stimulation with peptide pools of the SARS-CoV-2 spike (S), nucleocapsid (N) or membrane (M) surface-expressed proteins.
First, the researchers analyzed the combined frequency of SARS-CoV-2-specific T cells for all three proteins. Next, frequencies of IFN-γ- and TNF-α-producing SARS-CoV-2-specific T cells were measured separately, as these cytokines had the most significant differences overall. Finally, they compared the expression of cytokines and phenotypic markers on SARS-CoV-2-specific T cells in pulmonary PASC and RC participants.
Compared to study participants with RC, patients with pulmonary PASC had significantly elevated frequencies of TNF-a-producing CD8+ T cells that expressed the highest levels of Ki-67, indicating recent activation and proliferation. Further, these T cells were significantly higher in female PASC participants than males.
TNF-α-producing CD8+ T cells, having a half-life between three and five months, were found in PASC participants who donated blood over six months from symptom onset, thus, suggesting a possible explanation of PASC pathophysiology.
Further, the study findings could not rationalize that PASC occurs due to damage done by disease severity during acute infection since 60% percent of the pulmonary PASC cohort initially experienced mild disease yet developed PASC. Moreover, there was no difference in the frequency of SARS-CoV-2-specific T cells between PASC-H and PASC-NH participants, thus, confirming that pulmonary PASC participants have elevated levels of T cells months after initial infection.
The simplified presentation of incredibly complex evaluations (SPICE) analysis revealed that most SARS-CoV-2-specific T cells in individuals with pulmonary PASC only produced one of the three cytokines tested with TNF-α dominating the virus-specific CD4+ T cell response and IFN-γ dominating the CD8+ T cell response. RC cohort participants had multiple cytokines, indicating the T cell cytokine response was more restricted in PASC than RC.
Although there is a strong connection between T cell cytokine production and lung function in SARS-CoV-2 infections, this association had not been examined in pulmonary PASC. The authors observed that SARS-CoV-2-specific T cell frequencies were strongly associated with increased markers of inflammation and decreased lung function in PASC.
The elevated frequencies of IFN-γ- and TNF-α-producing SARS-CoV-2-specific T cells were positively associated with decreased lung function in pulmonary PASC. They also found the duration of dyspnea correlated with increased frequencies of CD8+ IFN-γ- and TNF-α-producing SARS-CoV-2-specific T cells and decreased levels of CD4+ IL-2 producing T cells.
Similar to the effects of systemic cytokines and T cell expression of inflammatory cytokines in other pulmonary conditions, these findings suggested that the presence of persistently activated SARS-CoV-2-specific T cells in PASC likely contributes to lung dysfunction.
Taken together, the study findings showed that pulmonary PASC could be driven partly by the production of inflammatory cytokines by SARS-CoV-2-specific T cells. Further, these findings provided insight into the pathophysiology of pulmonary PASC needed for the development of new treatments for the affected population.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.