Immune cells clear aged cells but also cause harm
Our immune system has its own category of cells that can directly clear out senescent cells. But the same cells also play a role in harmful processes during ageing. A new review maps this double-edged behaviour.
Immune cells labelled CD4+ have traditionally been seen as helpers that direct other immune cells via signalling proteins. The actual attack on damaged cells or pathogens seemed to be the job of so-called CD8+ T cells. But a growing number of studies shows that a subgroup of CD4+ T cells can also kill directly. These are called cytotoxic CD4+ T cells (CD4 CTLs).
The researchers argue in their review that CD4 CTLs carry a recognisable set of killing tools, similar to CD8+ T cells. They contain perforins and granzymes (proteins that punch holes in cell membranes and trigger breakdown processes). They arise under conditions of prolonged exposure to foreign substances, such as chronic viral infections or tumours.
Senescent cells as targets
Recent research shows that senescent cells (cells that have stopped dividing but do not die, instead secreting harmful substances) are also targets for CD4 CTLs. That sounds beneficial: if the immune system can recognise and remove senescent cells, it could slow the accumulation of those cells with advancing age. But the same mechanism can also be harmful. CD4 CTLs have been linked to autoimmune diseases, neurodegenerative conditions, and severe COVID-19. In those cases they overshoot, attacking healthy tissue as well.
No simple conclusion
This review does not offer a simple conclusion. It describes a category of cells that can be both protective and harmful, depending on context. For researchers working on ways to guide the ageing immune system, understanding when CD4 CTLs are helpful and when they contribute to disease is important. The authors stress that the boundary between the two situations is not yet clear and that more research is needed to understand their precise role. This is a review article, not a primary study with new clinical data.
Terms to search for yourself: cytotoxic CD4 T cells aging, senescent cell immune surveillance, perforin granzyme T cell cytotoxicity