Tissue Resident Memory T Cells Play Complex Role

SEATTLE — Tissue resident memory (TRM) T cells are a hot topic late in the treatment of psoriasis. These cells reside in the skin and other tissues and promote the inflammatory response, likely contributing to psoriasis symptoms. In fact, flare-ups often recur at the same site, a phenomenon that might be driven by these resident memory cells, according to Liv Eidsmo, MD, PhD.

This has led to their use as biomarkers in clinical trials for new therapies, but TRM T cells have a complex biology that is far from fully understood, Eidsmo said at the Group for Research and Assessment of Psoriasis and Psoriatic Arthritis (GRAPPA) 2024 annual meeting. “With time, we’re understanding that the regulation of the functionality is more complicated than we thought, so following these cells as a positive outcome of a clinical trial is a little bit premature,” said Eidsmo, who is a consultant dermatologist at the University of Copenhagen, Denmark.

Treatment strategies focus on inhibition of interleukin (IL)–23, which is an activator of TRM T cells and probably keeps them alive, according to Eidsmo. “The hope is that these cells can be silenced by IL-23 inhibition, which is a great idea, and it probably works. It’s just a matter of what is the readout of long-term remission, because the big challenge in the clinical world is when do we stop these expensive biological treatments? When can we feel secure that patients are in deep remission?” she asked.

TRM cells are also far from the only immune cells involved in psoriasis. Others include keratinocytes, Langerhans cells, and fibroblasts. Eidsmo referenced a recent spatial analysis that used single-cell and spatial RNA sequencing to identify the localization of specific cell populations and inflammatory pathways within psoriasis lesions and epidermal compartments as well as also suggested crosstalk links between cell types. Epigenetic changes in stem cells may also maintain a lower threshold for tissue inflammation.

Eidsmo advised caution in eliminating TRM T cells, which play a key role in protecting against melanoma and other cancers, especially later in life. “We don’t want to get rid of them. We want to have the right balance,” said Eidsmo. She noted a study in her own lab that mapped TRM T cells in healthy epidermis and found that they could be renewed from both circulating precursors and cells within the epidermis. “So getting rid of the mature TRM T cells will most likely just lead to a new generation of the same subset,” said Eidsmo.

Other data show that there are a wide range of subsets of TRM T cells, and she recommended focusing on the functionality of TRM T cells rather than sheer numbers. “This is something we’re working on now: Can we change the functionality [of TRM T cells], rather than eradicate them and hope for the best in the next generation? Can we change the functionality of the T cells we already have in the skin?”

There is also epigenetic data in TRM T cells, keratinocytes, stem cells, and other cells thus suggesting complexity and plasticity in the system that remains poorly understood. 

Taken together, the research is at too early of a stage to be clinically useful, said Eidsmo. “We need to go back to the drawing board and just realize what we need to measure, and with the new techniques coming out, maybe spatial [measurement] at a high resolution, we can find biomarkers that better dictate the future of this. Be a little bit wary when you read the outcomes from the clinical trials that are ongoing, because right now, it’s a bit of a race between different biologics. These cells are used as a readout of efficacy of the treatments, and we’re not quite there yet,” said Eidsmo.

During the Q&A session after the presentation, one audience member asked about the heterogeneity of cells found within the skin of patients with psoriasis and pointed out that many pro-inflammatory cells likely play a role in tumor control. Eidsmo responded that her group’s analysis of a large database of patients with metastatic melanoma found that a factor that is important to the development of TRM T cells was strongly correlated to survival in patients with metastatic melanoma receiving immune checkpoint blockade. “So we really don’t want to eradicate them,” said Eidsmo.

Also during the Q&A, Iain McInnes, MD, PhD, commented about the need to understand the previous events that drove the creation of memory T cells. “For me, the question is about the hierarchy, the primacy of what really drives the memory. In the infectious world, we’re trained to think [that memory responses] are T cell driven memory, but I wonder whether you have an idea of whether the T cell is responding to other memories, particularly in the stroma. Because certainly in the arthropathies, we have really good evidence now of epigenetic change in the synovial stroma and subsets,” said McInnes, who is director of the Institute of Infection, Immunity, and Inflammation at the University of Glasgow, Scotland.

Eidsmo responded that she believes responses are different among different individuals. “We know too little about how these two systems interact with one another. I think the TRM T cells are very good at amplifying the stroma to recruit cells in. I think we need to think of two-step therapies. You need to normalize this [stromal] environment. How you can do that, I don’t know,” she said. 

McInnes agreed. “As a myeloid doctor, I strongly believe that perpetuators are innate and the adaptive is following on. But how do we test that? That’s really hard,” he said. 

Eidsmo did not list any disclosures. McInnes has financial relationships with AbbVie, AstraZeneca, Bristol Myers Squibb, Boehringer, Compugen, Cabaletta, Causeway, Dextera, Eli Lilly, Celgene, MoonLake, Pfizer, Novartis, Janssen, Roche, Versus Arthritis, MRC, and UCB.