Why Immune Cells ‘Remember’ Obesity and Make Weight Loss Harder to Maintain

Losing weight is difficult enough, but keeping it off can bring another challenge. Scientists are finding that the body may hold onto traces of obesity even after the number on the scale falls.

New research suggests that immune cells inside fat tissue can retain a biological memory of obesity. These changes may persist after weight loss and could help explain why maintaining a lower weight can be difficult for some people. This memory has nothing to do with remembering old eating habits. Researchers are studying molecular changes inside immune cells, including changes that affect how genes work and how cells respond to metabolic stress.

Immune Cells May Keep a Molecular Record of Obesity

One major study published in Science Translational Medicine focused on macrophages, immune cells that live inside fat tissue. These cells normally help clear damaged and dead cells while regulating inflammation.

Researchers found that obesity altered how macrophages processed genetic instructions in mice. Surprisingly, many of those changes remained after the animals lost weight and returned to a healthier body weight.

The scientists specifically examined RNA splicing, a process cells use when turning genetic instructions into proteins. About 52% of the obesity-associated RNA splicing changes remained after weight loss.

Those lingering changes appeared to affect how macrophages performed their normal jobs. The altered cells became less effective at clearing dead cells from fat tissue, which could affect signals involved in breaking down stored fat.

Other studies have identified another form of possible obesity memory in CD4 T cells. These immune cells coordinate important parts of the body’s immune response and can undergo lasting molecular changes after obesity. Researchers have found changes in DNA methylation, a process that helps control how genes are switched on or off. Some obesity-related methylation patterns may remain after substantial weight loss.

Weight Loss May Not Immediately Reset Inflammation

Pavel / Pexels / Scientists have found evidence that certain inflammatory immune cells remain in fat tissue after weight loss.

This persistence could help explain why metabolic health does not always reset as quickly as body weight changes.

Research involving people with obesity treated with semaglutide found that inflammatory CD4+ effector memory T cells remained largely unchanged after six months, despite significant weight loss. Exercise-related improvements in metabolic health have also failed to completely restore T cell balance in some research.

These findings suggest that losing excess fat and reversing obesity-related immune changes may happen on different timelines. Someone can lose substantial weight while parts of the immune system still carry signs of earlier metabolic stress.

Researchers are particularly interested in tissue-resident memory T cells. These cells can remain inside fat tissue and respond rapidly when they encounter signals connected with renewed metabolic stress. One pathway attracting attention is the CD70-CD27 axis. This signaling system helps regulate the formation and survival of memory T cells after an immune response.

Persistent inflammatory signals in fat tissue may keep these memory cells alive instead of allowing their numbers to fall normally. The cells can then remain ready to react if metabolic stress returns.

When weight returns, those cells may respond faster because they have encountered similar conditions before. They can produce inflammatory molecules such as IFN-gamma and TNF-alpha, potentially contributing to renewed inflammation in fat tissue.

Can the Immune Memory Be Erased?

Magnific / The CD70-CD27 pathway is one possible target. Experimental research has found that blocking this pathway can reduce memory T cells in fat tissue following weight loss and weight cycling.

In mouse experiments, animals lacking CD70 were protected against some of the worsening glucose tolerance associated with weight cycling. Their response to stable obesity was similar to that seen in ordinary mice.

Scientists are also interested in how long obesity-related epigenetic changes last. Researchers at the University of Birmingham have suggested that sustained weight control could gradually allow some immune changes to fade.