T Cells: Ready and waiting to go
Some T cells that have been activated by a herpesvirus can also respond to SARS-CoV-2, even if the original herpesvirus infection happened before the COVID-19 pandemic.
- School of Cellular and Molecular Medicine, Faculty of Life Sciences, University of Bristol, United Kingdom
- Bristol Veterinary School, Faculty of Health Sciences, University of Bristol, United Kingdom
Cells called T cells play an important role in protecting the body against infection by removing pathogens that may cause harm. Two major types of T cell are involved in the response to a viral infection. Both become activated when their receptors recognize short peptides from viral proteins called ‘epitopes’: CD8 T cells directly attack infected cells, whereas CD4 T cells help other immune cells (called B cells) to produce antibodies. Once the infection has been eliminated, some of these CD8 and CD4 T cells survive in the body as long-lived memory T cells which can immediately respond if the virus invades again.
Previous studies found that some blood samples taken before the COVID-19 pandemic already contained T cells that could recognize the SARS-CoV-2 virus (Grifoni et al., 2020; Le Bert et al., 2020). However, researchers still do not fully understand how these T cells arose, or how they impact immunity and disease outcomes for COVID-19 patients.
One possibility is that these pre-existing T cells arose due to a phenomenon called heterologous immunity (Welsh et al., 2010). This is when CD4 and CD8 T cells activated by a specific pathogen ‘cross-react’ and respond to epitopes from a different virus (Mason, 1998). It was previously thought that coronaviruses already circulating in the population before the pandemic were responsible for the existence of some T cells that could recognise SARS-CoV-2 (Grifoni et al., 2020; Swadling et al., 2022). Now, in eLife, Cilia Pothast (Leiden University Medical Center), Mirjam Heemskerk (also at Leiden) and colleagues report that another group of viruses may have also been involved (Pothast et al., 2022).
The team hypothesised that some of the T cells specific to SARS-CoV-2 had been activated by a herpesvirus called human cytomegalovirus (HCMV). This pathogen is highly prevalent in the population and has also been linked to changes in the severity of COVID-19 symptoms (Alanio et al., 2022). To investigate, they stimulated pre-pandemic blood samples with different segments of SARS-CoV-2 proteins. This led them to discover a population of ‘cross-reactive’ CD4 and CD8 T cells that can recognize epitopes from both SARS-CoV-2 and HCMV (Figure 1).
Figure 1
Infection with human cytomegalovirus (HCMV) can stimulate T cells that can recognise SARS-CoV-2.
When individuals are infected with HCMV (virus shown in pink), the population of T cells that can detect this virus expands (T cells shown here in purple). Cross-reactivity is a well-known feature of the immune response. Through this process, HCMV infection can activate T cells (shown here in green) that can recognise both HCMV and another pathogen – including the SARS-CoV-2 virus, even if the HCMV infection happened before the COVID-19 pandemic. These cross-reactive T cells may be able to contribute to the immunity of an individual to SARS-CoV-2, as well as to how COVID-19 affects their body.
Image credit: Created with BioRender.com.
Pothast et al. found that this cross-reactivity was due to a T cell receptor that is expressed in multiple individuals. However, there are very few similarities between the amino acid sequences of the SARS-CoV-2 and the HCMV epitopes, bringing into question how this T cell receptor can detect both viruses. It may be possible to explain the molecular basis for this observation by solving crystal structures of this T cell receptor in complex with either the presented HCMV or SARS-CoV-2 epitopes.
Further experiments then revealed that the cross-reactive T cells limit the replication of SARS-CoV-2 in vitro when the virus is present at low levels. However, the cross-reactive T cells did not appear to have an activated phenotype in patients with severe COVID-19. This might be because individuals included in this study were over 60 years of age, and HCMV-specific T cells do not work as well as people get older (Ouyang et al., 2004).
It has been suggested that heterologous immunity may play a beneficial role in protective immunity (Welsh et al., 2010). This is consistent with a recent study showing that T cells which cross-react with SARS-CoV-2 are associated with abortive infections (when the virus fails to spread to other cells) and asymptomatic cases of COVID-19 (Swadling et al., 2022). These pre-existing T cells may also enhance a person’s response to vaccines (Loyal et al., 2021). However, heterologous immunity is a double-edged sword, as it can also increase the severity of some viral infections. For example, in dengue infections, cross-reactive antibodies and T cells can result in an immune response that is harmful to the body (Welsh et al., 2010; Screaton et al., 2015).
Further studies are needed to establish whether other pathogens (including bacteria) can stimulate T cells capable of recognising epitopes from SARS-CoV-2. In addition, studies with larger cohorts of vaccinated individuals and patients with mild or severe COVID-19 are required to define the role that these cross-reactive T cells play in protective immunity, in response to vaccination, and in disease pathology.
References
-
Cytomegalovirus latent infection is associated with an increased risk of COVID-19-related hospitalizationThe Journal of Infectious Diseases 226:463–473.https://doi.org/10.1093/infdis/jiac020
-
Dysfunctional CMV-specific CD8(+) T cells accumulate in the elderlyExperimental Gerontology 39:607–613.https://doi.org/10.1016/j.exger.2003.11.016
-
New insights into the immunopathology and control of dengue virus infectionNature Reviews Immunology 15:745–759.https://doi.org/10.1038/nri3916
-
Heterologous immunity between virusesImmunological Reviews 235:244–266.https://doi.org/10.1111/j.0105-2896.2010.00897.x
Article and author information
Author details
Publication history
Copyright
© 2023, Rivino and Wooldridge
This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.
Metrics
-
- 884
- views
-
- 74
- downloads
-
- 1
- citations
Views, downloads and citations are aggregated across all versions of this paper published by eLife.
Download links
A two-part list of links to download the article, or parts of the article, in various formats.
Downloads (link to download the article as PDF)
Open citations (links to open the citations from this article in various online reference manager services)
Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)
T Cells: Ready and waiting to go
eLife 12:e85080.
https://doi.org/10.7554/eLife.85080
Further reading
-
- Immunology and Inflammation
In autoimmune type 1 diabetes (T1D), immune cells infiltrate and destroy the islets of Langerhans — islands of endocrine tissue dispersed throughout the pancreas. However, the contribution of cellular programs outside islets to insulitis is unclear. Here, using CO-Detection by indEXing (CODEX) tissue imaging and cadaveric pancreas samples, we simultaneously examine islet and extra-islet inflammation in human T1D. We identify four sub-states of inflamed islets characterized by the activation profiles of CD8+T cells enriched in islets relative to the surrounding tissue. We further find that the extra-islet space of lobules with extensive islet-infiltration differs from the extra-islet space of less infiltrated areas within the same tissue section. Finally, we identify lymphoid structures away from islets enriched in CD45RA+ T cells — a population also enriched in one of the inflamed islet sub-states. Together, these data help define the coordination between islets and the extra-islet pancreas in the pathogenesis of human T1D.
-
- Cell Biology
- Immunology and Inflammation
The endothelial blood-brain barrier (BBB) strictly controls immune cell trafficking into the central nervous system (CNS). In neuroinflammatory diseases such as multiple sclerosis, this tight control is, however, disturbed, leading to immune cell infiltration into the CNS. The development of in vitro models of the BBB combined with microfluidic devices has advanced our understanding of the cellular and molecular mechanisms mediating the multistep T-cell extravasation across the BBB. A major bottleneck of these in vitro studies is the absence of a robust and automated pipeline suitable for analyzing and quantifying the sequential interaction steps of different immune cell subsets with the BBB under physiological flow in vitro. Here, we present the under-flow migration tracker (UFMTrack) framework for studying immune cell interactions with endothelial monolayers under physiological flow. We then showcase a pipeline built based on it to study the entire multistep extravasation cascade of immune cells across brain microvascular endothelial cells under physiological flow in vitro. UFMTrack achieves 90% track reconstruction efficiency and allows for scaling due to the reduction of the analysis cost and by eliminating experimenter bias. This allowed for an in-depth analysis of all behavioral regimes involved in the multistep immune cell extravasation cascade. The study summarizes how UFMTrack can be employed to delineate the interactions of CD4+ and CD8+ T cells with the BBB under physiological flow. We also demonstrate its applicability to the other BBB models, showcasing broader applicability of the developed framework to a range of immune cell-endothelial monolayer interaction studies. The UFMTrack framework along with the generated datasets is publicly available in the corresponding repositories.
