Researchers identified elevated femtogram-level inflammatory cytokines in patients with post–acute sequelae of COVID-19 cardiovascular syndrome approximately 18 months following an initial infection, with these markers demonstrating direct effects on cardiac function in laboratory studies.

In the study, published in Nature Microbiology, the researchers examined blood samples from 23 patients with post–acute sequelae of COVID-19 cardiovascular syndrome (PASC-CVS) and matched recovered controls across three Australian centers. PASC-CVS plasma samples showed significantly increased levels of interleukin (IL)-12 (41 fg/mL vs 0.007 fg/mL), IL-1β (21 fg/mL vs 0.01 fg/mL), MCP-1 (14 fg/mL vs 0.01 fg/mL), and IL-6 (21 fg/mL vs 0.004 fg/mL) compared with recovered controls.

Blood samples were collected at approximately 497 days postinfection for patients with PASC-CVS and 476 days for recovered controls. The PASC-CVS cohort demonstrated a lower incidence of mild acute disease but higher incidence of moderate acute disease compared with recovered patients, though there was no statistically significant difference in severe acute disease incidence between groups.

RNA sequencing revealed patients with PASC-CVS exhibited upregulation of multiple pathways:

• Neutrophil degranulation
• Antimicrobial peptide production
• Interferon signaling
• Monocyte and dendritic cell activation
• T cell– and natural killer cell–related blood transcription modules
• Platelet activation
• Blood coagulation.

Proteomic analysis identified elevated levels of complement cascade proteins in PASC-CVS plasma:

• Complement C1r subcomponent (P = .0390)
• Complement factor B (P = .0197)
• Complement component C9 (P = .0035)
• Complement factor H (P = .0042)
• Complement factor I (P = .0273).

Additional elevated proteins included coagulation-related factors:

• Prothrombin (P = .0130)
• Serum amyloid A-4 protein (P = .0171)
• CXCL7 (P = .0366).

The researchers used human-induced pluripotent stem cell–derived cardiomyocytes to assess functional impacts. At 24 hours posttreatment with PASC-CVS plasma, cells showed significantly reduced relaxation velocity. By 48 hours, the researchers observed:

• Reduced amplitude
• Reduced upstroke velocity
• Enhanced relaxation velocity compared with controls.

Treatment with dexamethasone (100 ng/mL) prevented the PASC-CVS plasma-induced reduction in cardiomyocyte upstroke velocity at 24 hours.

The study employed multiple advanced techniques:

• "Immunostorm chip" nanotechnology for femtogram-level cytokine detection
• RNA sequencing on whole blood samples
• Mass spectrometry–based proteomics on plasma samples
• CardioExcyte 96 system for cardiomyocyte functional analysis with 1 ms time resolution and 1kHz sampling rate.

Traditional cardiac damage markers (CK-MB, cardiac troponin I) showed no statistically significant differences between patients with PASC-CVS and recovered controls when measured using nanotechnology.

The researchers noted study limitations including the small sample size and the need for validation in cohorts infected with newer SARS-CoV-2 variants. They suggested further investigation of trace-level cytokine profiles as potential diagnostic markers for PASC-CVS.

The research was conducted by scientists from multiple Australian institutions, including the University of Queensland, South Australian Health and Medical Research Institute, and QIMR Berghofer Medical Research Institute.

One study author has historically been a consultant for Sanofi, Pfizer, Roche and NovoNordisk. The opinions and data presented in this manuscript are those of the authors' and are independent of these relationships. The remaining authors declared no competing interests.