Studies in mice and in human cells have demonstrated how melanoma immunotherapy for skin cancer can be boosted by using a drug to halt immune cell escape from melanoma tumors, and so boost the numbers of these cells that are available to fight the cancer.

Led by researchers at NYU Langone Health and its Perlmutter Cancer Center, the study found that combining a chemical blocker of immune cell exit with immunotherapy held back melanoma tumor enlargement in more than half of mice tested. Immunotherapy alone had previously failed to prevent cancer growth.

“Our study confirms for the first time how CD8 T cells are escaping melanoma tumors through chemokine signaling to their nearby lymphatic vessels, leaving tumors less susceptible to anticancer immunotherapy,” said study lead investigator Maria Steele, PhD, a postdoctoral research fellow in the Ronald O. Perelman Department of Dermatology at NYU Grossman School of Medicine and Perlmutter Cancer Center. “These findings reveal that T cells circulate out of tumors, reshaping scientific views of tumor immunology where T cells randomly find and target tumor cells.”

Steele and colleagues reported on their findings in Nature immunology, in a paper titled “T cell egress via lymphatic vessels is tuned by antigen encounter and limits tumor control.” In their paper the team concluded, “These data indicate that strategies to limit T cell egress might be an approach to boost the quantity and quality of intratumoral T cells and thereby response to immunotherapy.”

Recent advances in immunotherapies that are designed to help the body’s immune defense system detect and kill cancer cells have greatly improved cancer care, researchers say. The treatments work by boosting the action of immune cells that both directly attack the cancer and prevent cancer cells from evading recognition by the immune system.

The latest generation of immune checkpoint inhibitor immunotherapies protect antitumor T cells from inactivation and have become a mainstay in the treatment of melanoma. While these drug treatments do not work for all patients, previous research has shown that having a greater overall number of T cells, particularly when positioned in the center of tumors, makes the drugs more effective. “Antigen-specific CD8+ T cell accumulation in tumors is a prerequisite for effective immunotherapy … ” the authors noted. “CD8+ T cell infiltration into tumors is associated with improved disease outcomes and response to immune checkpoint blockade (ICB).” And while “Intratumoral accumulation of CD8+ T cells is controlled by mechanisms of recruitment, retention and exit via lymphatic vessels,” the authors continued, the mechanisms of lymphocyte transit aren’t well understood.

The authors’ studies were designed to investigate the hypothesis that lymphatic vessel-mediated effector CD8+ T cell egress effectively limits the accumulation of functional CD8+ T cells. Their results confirmed that the CD8+ T cells escape melanoma tumors when they gather near the tumor periphery as well as nearby lymphatic vessels, which carry immune cells throughout the body. “Tumor-associated lymphatic vessels sequestered CD8+ T cells at the tumor periphery, thereby increasing the probability of exit in a CXCL12–CXCR4-dependent manner,” they wrote. The researchers also found that greater numbers of T cells accumulate inside tumors in mice that have been bred to lack lymphatic vessels in their skin.

Further experiments showed that chemokine signaling molecule CXCL12 and its related receptor protein CXCR4, attract and move T cells toward lymphatic vessels. The data, they said, “surprising” indicated that “lymphatic vessel-derived CXCL12 was sufficient to affect T cell accumulation and position in tumors.”

Given that the results had linked CXCR4/CXCL12 signaling to the exit of antigen-specific CD8+ T cells from the tumor microenvironment, the team reasoned that targeting this axis, “at least in the short term,” would boost intratumoral T cell accumulation and prime the TME for immunotherapy. They found that when they then blocked either CXCL12 or CXCR4, the T cells could not emigrate away from the tumor and instead stayed at its center.

Taken together, the results demonstrate how T cells are likely drawn to the tumor’s outer rim by CXCL12 and closer to the lymphatic vessels, where CXCR4 “encourages” the T cells to exit the tumor,” the researchers suggested. “Our data indicate that lymphatic vessel-derived CXCL12 sequesters CD8+ T cells at the tumor periphery and facilitates their CXCR4-dependent egress from melanomas.”

When the team then combined immunotherapy with a chemical blocker of CXCR4, the number of T cells in mice tumors doubled and half of tumors stopped growing. “Our study shows that blocking this escape route lets immunotherapy work better in fighting the growth of skin cancer cells,” said study senior investigator Amanda Lund, PhD.

The study findings also indicated that T cell leakage depended on their potency, or how strongly they could bind to target proteins on tumor cells. The longer the most potent T cells spent inside tumors, the more likely they were to encounter their target cancer cells and the more likely these T cells were to remain inside the tumor. Increasing the initial time that these T cells spend inside the tumor, the researchers say, may help improve therapy.

“These results suggest that it is not only about getting T cells into the melanoma tumor but also about getting these T cells to the right place with the right signals to drive the most specific and durable immune responses,” said Lund, an associate professor in the Ronald O. Perelman Department of Dermatology and in the Department of Pathology at NYU Grossman School of Medicine and Perlmutter Cancer Center.

The team suggested that their combined findings indicate that the lymphatic system likely recirculates T cells out of tumors. And that for some patients, blocking the exit signals— either CXCR4 and/or CXCL12—is needed to shift the balance in favor of keeping T cells inside of tumors long enough for immunotherapy to work.

They note that prior preclinical studies in different cancer types have shown that CXR4 inhibition improves intratumoral T cell accumulation and immune checkpoint blockade efficiency. “We suggest that the mechanism of action is through the retention of CD8+ T cells, a subset of which is tumor specific and exhibits increased effector function,” they stated.  “… our data continue to build a mechanistic case for the tumor-associated lymphatic vasculature and its transport function as an active player in regional disease control that may be leveraged to improve future disease outcomes.”

Future research will look at how blocking tumor T cell exit influences immunotherapy, say the authors, who also plan to look at targeting not just the chemokine signals but also other molecular pathways in the lymphatic vessels to increase the “dwell time” of T cells inside tumors.

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