A microRNA Family Drives the T Cell Response in Cancer

When a threat to the body presents itself, previously quiet CD8⁺ T cells become cytotoxic, proliferating and producing enzymes poised to lyse their enemies. Once the immune cells vanquish their foes, whether they are infected cells or altered cancerous ones, most of the cytotoxic T cell soldiers die off, leaving a few behind to turn into memory T cells that will protect their host from future assaults.¹

Memory T cells are an important part of the adaptive immune response because they respond to threats more quickly than naïve T cells. When faced with a familiar antigen, they rapidly transform into effector cytotoxic T cells. However, scientists are still working out the details of how memory T cells form. Immunologists Leonid Pobezinsky and Elena Pobezinskaya from the University of Massachusetts Amherst recently examined this question in the context of cancer and published their work in Nature Communications.²

Pobezinsky’s group previously found that a family of noncoding microRNAs (miRNAs) called let-7 are important for the formation of cytotoxic T cells. When expressed in non-immune cells, these miRNAs are well-documented tumor suppressors that directly target the mRNA of genes involved in cell cycle regulation.³ They are also highly expressed in naïve T cells, but are downregulated after CD8⁺ T cell activation.⁴ In a petri dish, the researchers saw that the absence of let-7 opened the door for proliferation and differentiation into cytotoxic T cells that actively killed tumor cells.

In the new study, the researchers analyzed how the miRNA family affected T cell formation in vivo by transferring CD8⁺ T cells expressing various levels of let-7 into mice bearing melanoma tumors. In this case, let-7 overexpression promoted memory T cell formation and slowed tumor growth, while cells lacking the miRNAs failed to control the tumors. These findings were in direct opposition to the research team’s in vitro work. “We really were shocked to see that,” said Pobezinsky.

The researchers think that the formation of a memory cell pool was key to these differences in the mice. To avoid an overreactive response that harms healthy cells, cytotoxic T lymphocytes express inhibitory surface receptors, or immune checkpoint molecules. Tumors can avoid attack by binding to these receptors, which drives the T cells to a dysfunctional state called exhaustion. In contrast, memory T cells are invisible to tumors because they lack certain inhibitory surface receptors. “Mother Nature doesn't want you to inactivate memory cells, which are generated after an immune response, because you want to keep them for the rest of your life,” said Pobezinsky.

When the scientists transferred T cells with low let-7 levels into the melanoma mouse model, the cancer cells likely took advantage of the cytotoxic T cells’ inhibitory receptors, inducing exhaustion. The T cells expressing let-7 escaped this fate and instead formed memory cells that kept producing functional cytotoxic T cells in response to the cancer antigens. “We had 70-80 percent tumor-free mice, which is unheard of, especially expressing just one miRNA,” said Pobezinsky.

To understand the cellular mechanisms involved in memory formation, the researchers compared the transcriptomes of T cells expressing let-7 to those that did not. They found significant changes in the cells with let-7, including the inhibition of pathways important for reactive oxygen species (ROS) production, which shifts CD8⁺ T cells toward the memory phenotype. To test this finding, the researchers treated let-7 deficient T cells during early activation with a drug that inhibits a ROS production pathway. Once those cells were injected into tumor-bearing mice, they behaved like let-7-expressing T cells—they reduced tumor burdens and prolonged survival.

Pobezinsky’s research team is currently investigating how let-7 controls the pathways involved in ROS production. In the future, the researchers see potential in administering let-7 miRNA as an immunotherapy to control tumor growth because of its role as a tumor suppressor and its influence on memory T cell formation. “The authors perform a variety of elegant immunological and transcriptional experimentation that supports that forced overexpression of let-7 may be one strategy to tip the balance in favor of the immune system in the fight against cancer,” Ingunn Stromnes, a cancer immunologist at the University of Minnesota who was not involved in this study, said in an email. “Understanding how to translate this to the human setting will be of interest, and with the explosion in gene engineering technologies, overexpressing let-7 in non-activated T cells may indeed be feasible.”