Long covid & the immune system23 May 2021 17:24
From New Scientist article 15/7/21 -
People who survive severe covid-19 appear to end up with a prematurely-aged immune system and other persistent immunological problems, which may be the underlying cause of long covid. The immune response to acute covid-19 is now well understood, but the longer-term effects are only just coming to light. The preliminary results from three studies looking into these long-term effects were reported last month at a virtual conference hosted by the UK Coronavirus Immunology Consortium and the British Society for Immunology. Together they suggest that the immune system gets a nasty hangover from the virus, but that it may be reversible.
University of Birmingham studied the immune systems of 46 people who had been hospitalised with severe covid-19. Three months after discharge, a significant number showed signs of premature immunosenescence: an age-related decline in the ability to mount an immune response. This included a loss of naive B- and T-cells, which are immature immune cells that have yet to be called into action against a pathogen. They also saw another sign of immune ageing: accumulation of memory B- and T-cells, which remain in circulation after an infection, ready to respond to reinfection. Additionally, there was also an excess of senescent T-cells, which can secrete damaging chemicals, driving inflammation that may underlie many age-related diseases.The subjects were aged between 30 and 68. Their immune systems were compared with a similar group who hadn’t had covid-19. Immunosenescence usually starts around age 60, but the survivors under 60 showed signs of it.
It isn’t possible to rule out that the survivors became severely ill because they already had prematurely-aged immune systems. Immune age varies widely between individuals; somebody who is 40 years old can have an immune age of anything between 20 and 60. Is it cause or effect? Probably a bit of both, as an analysis of people in the UK Biobank who developed covid showed they were biologically 10 to 14 years older.
A separate study at Manchester Uni looked at neutrophils in about 50 people who experienced severe covid-19. Neutrophils are general-purpose immune cells that patrol the body, destroying invading bacteria. They also promote inflammation, and are activated by viruses, including SARS-CoV-2.
About half of the people in the study survived - around a third had elevated and dysfunctional neutrophils up to 9 months after discharge from hospital. It is possible that this neutrophil imbalance contributes to long covid symptoms. For example, there is some evidence that neutrophils promote excessive blood clotting, one symptom of long covid.