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This study has two unexpected new findings: first, tumor cell PD-L1 does not directly inhibit the toxicity of cytotoxic T cells; second, tumor cell PD-L1 has no protective effect on the primary tumor, but inhibits T cell infiltration into metastatic lesions through binding to PD-1 in metastatic myeloid cells.
After PD-1/PD-L1 inhibitors have been used in cancer immunotherapy for nearly 10 years, scientists are still discovering new mechanisms to replenish our knowledge around PD-L1.
Recently, a research team led by Professor Liu Kebin from the Medical College of Georgia published their research results in the journal “Cancer Cells” .
This paper has two unexpected new findings: first, tumor cell PD-L1 does not directly inhibit the toxicity of cytotoxic T cells; second, tumor cell PD-L1 has no protective effect on the primary tumor, but through binding to PD-1 of metastatic myeloid cells, inhibits the infiltration of cytotoxic T cells into metastatic foci, and promotes the growth of metastatic tumors.
Today, we all know that immune checkpoint inhibitors mainly play an anti-cancer role by blocking the PD-1/PD-L1 signaling pathway. Among them, PD-1 mainly refers to PD-1 on the surface of cytotoxic T cells, and PD-L1 mainly refers to PD-L1 on the surface of cancer cells.
At present, there is little controversy about the function of PD-1 and its downstream signaling pathways. However, whose PD-L1 affects the activity of T cells through PD-1 is still controversial.
A large number of studies based on mouse models have found that PD-L1 of immune cells such as dendritic cells and macrophages plays a leading role in inhibiting the anti-tumor response of T cells; only one study found that PD-L1 of tumor cells -L1 (tPD-L1) is sufficient to suppress the toxicity of cytotoxic T cells to promote tumor immune escape in a mouse model of colon cancer . However, human clinical studies have found that tumor cell PD-L1 or tumor cell combined with immune cell PD-L1 is the strongest indicator of prognosis [5,6].
According to Professor Liu, the discrepancy between the results of the mouse study and the clinical study may be due to the different anatomical locations of the tumors. In preclinical studies, most of the deaths of mice were caused by excessive tumor burden similar to the primary tumor; while the deaths of cancer patients were mainly caused by cancer metastasis. Therefore, tumor cell PD-L1 may have an unknown important role in cancer metastasis.
In order to understand the function of tumor cell PD-L1, Professor Liu’s team selected four cancer cell lines expressing PD-L1: triple-negative breast cancer cell line 4T1, microsatellite-stable intestinal cancer cell line CT26 , metastatic microsatellite-invariant Stable intestinal cancer cell line MC38-met, melanoma cell line B16-F10. They also constructed a one-to-one corresponding PD-L1 knockout cancer cell line based on the above four cell lines.
They mixed the above 8 cell lines with a cytotoxic T cell line, and found that T cells showed dose-dependent cytotoxicity to all cancer cell lines, and the co-culture of T cells and normal cancer cell lines would also lead to both The expression levels of PD-1 and PD-L1 were increased. They also found that the expression of PD-L1 had no protective effect on cancer cells, and even the addition of PD-1 inhibitors had no protective effect.
Professor Liu believes that this shows that in the absence of a tumor microenvironment, when cancer cells and T cells are 1V1, whether or not expressing PD-L1 is the same for cancer cells. In fact, this also implies that the interaction between tumor cell PD-L1 and T cell PD-1 may not affect the toxicity of T cells.
The presence or absence of PD-L1 did not affect tumor size regardless of which cell line pair was injected into the mice. This again shows that tumor cell PD-L1 does not help the growth of the primary tumor. This really refreshed everyone’s cognition!
Role of PD-L1 in promoting cancer metastasis (DOI: https://doi.org/10.1016/j.ccell.2023.02.005)
Next, Liu’s team took triple-negative breast cancer 4T1 (which can spontaneously develop lung metastasis) as an example to explore the effect of tumor cell PD-L1 on cancer metastasis. They found that after the mouse mammary glands were inoculated with two kinds of cancer cells, tumor cells expressing PD-L1 had more lung metastases and a heavier tumor burden, while the ability of lung metastasis of 4T1 cells with PD-L1 knockout was significantly reduced. weaken. Similar phenomena were also found in several other cell lines.
In subsequent studies, Liu’s team confirmed that although tumor cell PD-L1 did not protect the primary tumor, PD-L1 on the surface of myeloid cells did protect the primary tumor. This finding well explains the previous human-mouse differences.
After determining that tumor cell PD-L1 promotes metastasis and does not protect the primary tumor, the question Liu’s team has to solve is how exactly does tumor cell PD-L1 promote cancer cell metastasis.
To answer this question, they first compared the effect of PD-L1 expression status on the primary and metastatic tumor microenvironment. It was found that after 1 day after inoculation, there was no difference in the initial cancer cells in the lungs of mice regardless of whether PD-L1 was knocked out. They also noticed that compared with normal 4T1, the infiltration of CD3-positive cells in the lung metastases of PD-L1 knockout 4T1 was increased, but the primary tumor was not; Whether or not, the same pulmonary metastases can be formed.
These data indicate that tumor cell PD-L1 does not affect the dissemination of cancer cells, but helps cancer cells in metastases evade the killing of cytotoxic T cells.
How is this possible?
After a series of complex experiments, Liu’s team found that the combination of metastatic tumor cell PD-L1 and myeloid cell PD-1 (mPD-1) would activate SHP2 and downregulate IFN-I→STAT1→CXCL9, which would recruit cytotoxicity The signaling pathway of T cells, thereby inhibiting the infiltration of T cells to metastases, rather than inhibiting the lethality of T cells through the PD-1/PD-L1 pathway.
In general, the research results of Professor Liu’s team have given us a new understanding of cancer immunity. The PD-1/PD-L1 signaling pathway is far more complicated than we thought, and its function is not only related to the cell type but also related to the anatomical site of the tumor.
More importantly, this study has led us to a deeper understanding of immune neoadjuvant and adjuvant therapy and also consolidated the theoretical basis of these two therapies.
 Klement JD, Redd PS, Lu C, et al. Tumor PD-L1 engages myeloid PD-1 to suppress type I interferon to impair cytotoxic T lymphocyte recruitment. Cancer Cell. 2023;41(3):620-636 .e9.doi:10.1016/j.ccell.2023.02.005
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