PATHCHAT Edition No. 78
Published: March 2022
Please contact your local Ampath pathologist for more information.
Authors:
- Dr. Marieke Brauer (Clinical Virologist)
- Dr. Sylvia van den Berg (Clinical Pathologist)
- Dr. Cathy van Rooyen (Immunologist)
Introduction
✅ Understanding the Immune Response to SARS-CoV-2
- Since the emergence of COVID-19, extensive research has focused on the immune response to the virus.
- The adaptive immune response, including both antibody (humoral) and T-cell (cellular) immunity, is crucial in protection against reinfection.
- Antibody studies have been widely conducted, but T-cell responses are equally important.
📌 T-cell immunity plays a critical role in viral clearance and long-term protection against SARS-CoV-2.
T-Cell Responses Following COVID-19 Infection
✅ Key Findings from Studies on T-Cell Immunity
- Antibody levels may wane as early as 6–8 months post-infection, but memory T-cell responses can persist for over 10 years.
- SARS-CoV-2-specific T-cell activation has been observed against multiple viral proteins, including the spike and nucleocapsid proteins.
- CD8+ T-cells are crucial for viral clearance and contribute significantly to immunity.
- Cross-reactive T-cells, likely from prior exposure to common cold coronaviruses, have been detected in 35–60% of SARS-CoV-2-naïve individuals.
- Higher T-cell induction has been observed in mild COVID-19 cases compared to severe infections.
📌 Memory T-cell responses provide prolonged immune protection beyond antibody-mediated immunity.
T-Cell Responses Following COVID-19 Vaccination
✅ How Vaccines Induce Immune Protection
- Most vaccines primarily induce neutralising antibodies targeting the spike protein.
- However, evidence suggests that T-cell responses play a major role in immunity.
- Vaccination generates robust T-cell responses, even in individuals with weak antibody production.
- T-cell responses have been observed in seronegative individuals following vaccination, especially in immunocompromised patients.
📌 T-cell responses may be more durable than antibody responses following infection or vaccination.
T-Cell Responses Against SARS-CoV-2 Variants
✅ Impact of Viral Mutations on Immune Protection
- Concerns exist regarding whether new SARS-CoV-2 variants can evade vaccine-induced immunity.
- Spike protein mutations in variants like Alpha, Beta, Gamma, and Omicron can reduce neutralising antibody efficacy.
- However, T-cell responses remain largely intact against emerging variants.
✅ Findings from Studies on Variant-Resistant T-Cell Immunity
- T-cell recognition of multiple viral epitopes remains consistent across SARS-CoV-2 variants.
- CD4+ and CD8+ T-cell responses remain robust despite mutations in the spike protein.
- Individuals vaccinated with mRNA and viral vector vaccines have demonstrated preserved T-cell immunity against Omicron.
📌 Unlike antibodies, T-cell responses are less affected by SARS-CoV-2 mutations, maintaining broad protection.
Laboratory Testing of T-Cell Responses to SARS-CoV-2
✅ COVID-19 T-Cell Proliferation Tests at Ampath
- Ampath offers T-cell proliferation tests targeting:
- SARS-CoV-2 Spike (S) protein.
- SARS-CoV-2 Nucleocapsid (N) and Spike (S) proteins.
✅ Available T-Cell Assays:
✔ SARS-CoV-2 T-Cell Assay (N & S Proteins) (COVIDTPR)
- Detects T-cell responses against both nucleocapsid and spike proteins.
- More comprehensive for individuals with past natural infection or vaccination.
✔ SARS-CoV-2 Spike T-Cell Assay (COVIDTSPR)
- Detects T-cell responses specifically against the spike protein.
- Useful for assessing vaccine-induced immunity.
📌 T-cell testing should be interpreted alongside antibody results for a complete immune assessment.
Interpreting T-Cell Proliferation Results
✅ How Results Are Expressed
- Results are reported as a Stimulation Index (SI), comparing T-cell activity with and without SARS-CoV-2 proteins.
- Higher SI values indicate stronger T-cell activation.
✅ Key Considerations for Interpretation
- T-cell responses may persist even when antibodies decline.
- Cross-reactive T-cells can arise from prior exposure to other coronaviruses.
- Results should be correlated with vaccination status and past infection history.
📌 T-cell testing provides a more durable measure of immunity than antibody testing alone.
Precautions for T-Cell Testing
✅ Medications That Affect T-Cell Responses
- Oral corticosteroids and immunosuppressive drugs can reduce T-cell proliferation.
- Where safe, these medications should be discontinued 14 days before testing.
📌 Patients on immunosuppressive therapy should consult their clinician before T-cell testing.
Key Takeaways for Clinicians & Researchers
✅ T-cell responses are a crucial component of long-term immunity to SARS-CoV-2.
✅ Unlike antibodies, T-cell immunity remains stable over time and offers broad cross-protection against variants.
✅ T-cell testing is particularly useful in immunocompromised individuals and those with waning antibody levels.
✅ Ampath offers validated T-cell assays to assess COVID-19 immunity.
✅ Results should be interpreted alongside antibody testing and clinical history.
📌 Understanding T-cell immunity is essential for evaluating long-term protection against COVID-19.
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