Y RNA is a reason why monocytes and macrophages assist, rather than assault, the cancer cells that proliferate in chronic lymphocytic leukemia (CLL). Not much is known about Y RNA, but one particular Y RNA, hy4, has been found in great abundance in the plasma of CLL patients. This discovery, from the German Cancer Research Center (GCR) in Heidelberg, points to a link between Y RNA and CLL’s immunosuppressive character. Breaking this link, say the GCR researchers, could lead to new therapeutic approaches against CLL.
The key to the discovery was exosome analysis. Exosomes, little bubbles, or vesicles, that cells transmit to their surroundings, are released in elevated numbers by leukemia cells. Accordingly, they attracted the notice of the GCR team, which was led by molecular geneticist Martina Seiffert, Ph.D. Ultimately, the GCR scientists found that the exosomes contained, among other things, Y RNA.
This finding appeared July 28 in the journal Science Immunology, in an article entitled “Tumor-Derived Exosomes Modulate PD-L1 Expression in Monocytes.” The article also explains how CLL-derived exosomal RNAs promote monocytes in CLL patients to adopt an immunosuppressive phenotype, including promoting expression of programmed death-ligand 1 (PD-L1).
“We carried out RNA sequencing and proteome analyses of CLL-derived exosomes and identified noncoding Y RNA hY4 as a highly abundant RNA species that is enriched in exosomes from plasma of CLL patients compared with healthy donor samples,” wrote the article’s authors. “Transfer of CLL-derived exosomes or hY4 alone to monocytes resulted in key CLL-associated phenotypes, including the release of cytokines, such as C-C motif chemokine ligand 2 (CCL2), CCL4, and interleukin-6, and the expression of PD-L1.”
The CDR scientists also observed that monocyte responses to hy4 were abolished in Toll-like receptor 7 (TLR7)-deficient monocytes. This finding, the scientists say, indicates that exosomal hY4 may be a driver of TLR7 signaling. “Pharmacologic inhibition of endosomal TLR signaling,” the article’s authors added, “resulted in a substantially reduced activation of monocytes in vitro and attenuated CLL development in vivo.”
Tumor cells influence their environment so as to avoid an immune response and facilitate favorable conditions for growth. It has been known for a long time that solid tumors, those which grow as solid tissue inside an organ, manipulate macrophages, the “big eater” cells of the immune system, for their own purposes.
“Recently, we have seen more and more evidence that something similar must be happening in leukemia,” noted Dr. Seiffert. So leukemia cells, acquired by the patient through CLL, could only survive in a cell culture if it also contains macrophages or monocytes, the precursors of the big eaters. They serve as a form of nourishment for cancer cells.
Dr. Seiffert's team decided to study the interplay between leukemia cells and monocytes and how it could favor cancer development. “We know that the so-called PD-L1 receptor occurs more frequently on the surface of these nourishing cells, and suppresses the immune response,” continued Dr. Seiffert. “What we have here is a so-called immune checkpoint, which prevents excessive immune responses.”
In this case, however, the immune response is suppressed so much that the cancer cells can multiply unopposed. In addition, the monocytes send out semiochemicals, which belong to the inflammation response of the immune system and support the growth and multiplication of the cancer cells.
The decisive question has been how the leukemia cells can manipulate the monocytes in their environment. The scientists initially presumed that exosomes might play a role. Exosomes help cells communicate with each other and influence each other's behavior.
To evaluate the effect of the Y RNA, the scientists treated monocytes and macrophages of humans and mice with suspect exosomes, as well as purified Y RNA from those exosomes, in a culture dish. In both cases the cells changed similarly to how they would in CLL patients. “They carry more PD-L1 receptors to their surface and emit semiochemicals which accelerate the immune response and create favorable growth conditions for leukemia cells,” explained Franziska Haderk, Ph.D., first author of the new study.
The GCR scientists also discovered that the Y RNA message of the so-called TLR7/8 is found in the monocytes. These serve to register foreign RNA, such as from pathogens, and to activate the immune response. At the same time, the activation of the TLRs also strengthens the immune inhibitor PD-L1.
“This creates an environment that supports the survival of the cancer cells and recruits cells of the immune system, but at the same time stops an effective response of the immune cells via the PD-L1,” details Dr. Haderk.
With this result, the GCR researchers have identified multiple new therapy approaches. In addition to a suppression of the PD-L1 receptor, it is conceivable to inhibit the recognition of the Y RNA message.
“This could perhaps succeed by adding TLR inhibitors such as chloroquine, a medication that is used for malaria and rheumatic inflammation,” speculated Dr. Seiffert. In experiments with mice given CLL cells, the agent was able to markedly suppress the reproduction of cancer cells. “That makes chloroquine an interesting candidate for a combination therapy along with other agents,” Dr. Seiffert asserted.