HLF identified as key regulator of tissue-resident T cells — Driver of organ-specific chronic inflammation

A research group led by Assistant Professor Masahiro Kiuchi and Professor Kiyoshi Hirahara from the Graduate School of Medicine at Chiba University has revealed that the mechanisms by which tissue-resident memory CD4⁺ T cells (CD4⁺ TRM cells) remain in tissues such as the lungs and intestines for extended periods, as well as their sustained production of inflammatory cytokines, are regulated by the transcription factor HLF (hepatic leukemia factor), a protein that controls gene function. This achievement advances our understanding of organ-specific inflammatory diseases, such as atopic dermatitis, food allergies, and hay fever, and paves the way for the development of new treatments. The findings were published in Science. Hirahara stated, "We want to clarify what inflammatory signals induce HLF and connect this to clinical application and drug development."

CD4⁺ T cells receive signals from antigen-presenting cells and produce antibodies through B cell activation. They also become activated themselves, producing cytokines and transmitting signals to various cells. Once antigens have been eliminated, they continue to function as memory cells. In essence, they are lymphocytes that serve as the command center of the immune system.

There was existing knowledge of two types of memory CD4⁺ T cells: central memory T cells that circulate through lymph nodes and effector memory T cells that circulate in the blood. In recent years, however, tissue-resident CD4⁺ T cells that remain in organs and other tissues have been discovered and are attracting attention.

It has become clear that tissue-resident CD4⁺ T cells can rapidly induce secondary immune responses without passing through lymph nodes. When chronic inflammation leads to the formation of ectopic lymphoid structures (tertiary lymphoid structures, TLS) in inflamed tissues, tissue-resident CD4⁺ T cells are preserved in the TLS for extended periods, even after inflammation subsides. This enables rapid and robust immune responses without involving lymph nodes. On the other hand, their prolonged presence within tissues also makes them a cause of chronic inflammation.

The research group conducted comprehensive single-cell gene expression analysis on mouse lung tissue with fungal-induced chronic inflammation. It discovered that the transcription factor HLF is specifically expressed in tissue-resident CD4⁺ T cells. HLF is known for maintaining liver function and as a causative gene in pediatric leukemia, but there had been no reports of its role in CD4⁺ T cells.

In HLF-deficient mice, tissue-resident CD4⁺ T cells were markedly reduced, resulting in suppressed inflammation and fibrosis. Normally, when S1P (sphingosine-1-phosphate) binds to its receptor S1pr1, lymphocytes attempt to exit tissues into the bloodstream. In tissue-resident CD4⁺ T cells, CD69 is highly expressed and forms a complex with S1pr1, sequestering S1pr1 within the cell membrane (suppressing its expression). This prevents S1P from binding to S1pr1 and thereby inhibits lymphocyte migration out of tissues. In other words, the study revealed that in HLF-deficient mice, reduced expression of tissue retention factors leads to increased expression of migration factors, resulting in decreased tissue residency.

In patients with eosinophilic chronic rhinosinusitis, polyps form in the nasal cavity. Examination of these tissues revealed extensive infiltration by HLF-positive tissue-resident CD4⁺ T cells. This finding was also confirmed at the protein level in the livers of patients with primary sclerosing cholangitis. Furthermore, analysis of comprehensive single-cell RNA sequencing databases confirmed this result in data from patients with lung diseases, including COVID-19.

Selecting HLF was the key

One of the key points of this study was selecting HLF from among numerous highly expressed genes. Hirahara explained, "There was quite a bit of serendipity involved. We chose HLF because Mr. Kiuchi said, 'Let's do this one.' At first, I was skeptical and reluctant, saying, 'I've never heard of HLF, let's not bother.' But when I asked him to do preliminary experiments, the results were positive, so we began researching in earnest. We selected HLF in 2018 and proceeded with the research, with the results coming together around 2022." Kiuchi said, "It was novel and highly expressed, so I thought it might be interesting."

The late Toshinori Nakayama, former President of Chiba University, is also included as a co-author on this paper. "His sudden passing in 2023 was a very sad event, and there were several periods of interruption. However, I am glad we were able to complete this work as the last major project that Professor Nakayama was involved in," said Hirahara.

Journal Information
Publication: Science
Title: Hepatic leukemia factor directs tissue residency of proinflammatory memory CD4⁺ T cells
DOI: 10.1126/science.adp0714