Natural killer (NK) cells are part of the innate immune system, our first line of defense. We already know that they can develop immune memory after infection with Streptococcus pneumoniae. More recently, scientists from the Institut Pasteur have shown that memory NK cells undergo reprogramming that alters their gene expression and makes them more effective in responding to bacteria.
In 2023, scientists from the Institut Pasteur discovered that natural killer (NK) cells can develop an immune memory against Streptococcus pneumoniae, also known as pneumococcus (Camarasa, Torne et al., 2023) – bacteria responsible for pneumonia, meningitis and septicemia. But the mechanisms used by memory NK cells to fight against bacteria remained unknown. In a new paper, scientists from the Institut Pasteur describe how memory NK cells develop a more effective response against bacteria by changing their gene expression program.
What are natural killer cells?
Natural killer (NK) cells are part of the innate immune system, our first line of defense. They are mainly known for their role in fighting against cancer cells and viruses. While T and B cells develop an immune memory – the principle behind immunization –, it was previously thought that NK cells did not have this ability. They were seen as effective soldiers but with no capacity for learning. But over the past few years, scientists have demonstrated that NK cells can learn to react to viruses. More recently, scientists from the Institut Pasteur have discovered that they can also "remember" pneumococcal infections.
How do memory NK cells develop a better defense against bacteria?
The scientists observed that there is a difference between "memory" NK cells and naive NK cells. After a first infection, the cells respond more quickly and effectively upon re-exposure – they are activated earlier, produce more defensive molecules and destroy enemy cells more effectively. The team also identified several subgroups of memory NK cells, some of which display enhanced proliferation on reinfection, while others are kept in reserve to provide lasting protection.
The key to this response is the reprogramming of gene expression in memory NK cells, giving them a new identity and new functions.
Unlocking the prospect of antibacterial immunotherapy
These new discoveries help us understand the unique role played by NK cells in antibacterial defense. The results suggest that these cells could be harnessed in novel therapeutic strategies to tackle bacterial infections – in a similar way to their already well established use in immunotherapy for cancer.
In view of the growing threat of antibiotic resistance, this is a promising approach. Rather than relying solely on antimicrobial drugs, we could strengthen and direct our own natural defenses against pathogenic bacteria like Streptococcus pneumoniae, which claims thousands of lives every year.
Source
NK cells undergo transcriptional and functional reprogramming following Streptococcus pneumoniae infection, Mucosal Immunology, November 28, 2025. Julia Torné et al.
