Cancer Immunotherapy involving Natural Killer Cells and Adenosine.

Tumor immunology is a very interesting subset of immunology as the dynamics of the immune system-tumor relationship is a long and very complicated one. Over years and years of cat-and-mouse, many immune evasion techniques have been discovered by researchers. One avenue of evasion is the release of immunosuppressive compounds by the tumor. Although tumor cells release molecules that can be recognized by the immune system and trigger a response (i.e. tumor-specific antigens (TSAs) and tumor associated antigens (TAAs)), these cells are not easy to detect. The tumor cells further ensure that these antigens are not recognized by release of immunosuppressive cytokines like IL-10 and TGF-beta (Mak and Saunders, 2011).

http://en.wikipedia.org/wiki/Metastasis

Another immunosuppressive compound is Adenosine, which is a useful molecule in the body as it is a DNA building block, a component of the main energy source in the body (ATP), and can be used to signal through one of its many receptors (A1, A2A, A2B, and A3). So it goes without saying that adenosine is present, in some form, all over the body.

Previous to the study by Beavis et al (2013), adenosine has been know to be produced from the breakdown of AMP by CD73 - a marker present on many types of cells. It has also been shown that anti-CD73 has resulted in delays in tumor expression, altogether rejection of tumor grafts in mice, inhibiting de novo carcinogensis, and preventing/reducing metastasis. In order to elucidate how exactly this immunosuppressive compound, adenosine, was suppressing tumor growth and metastasis, the authors set out to investigate adenosine in depth.



After first confirming previous research that CD73 facilitates the metastasis of tumors, the authors turned to pinpoint the receptor though which adenosine was working in preventing metastasis. In order to do this, the authors first used an adenosine non-specific receptor agonist NECA. As expected, the receptor agonist NECA significantly increased metastasis in CD73+ tumor cells. A2A receptor antagonist SCH58261 and A2B antagonist PSB-1115 were both shown to reverse this increased metastasis caused by NECA. This made sense to the authors, at it was known that these receptors have role in the immune system. To support that previous finding, the authors submitted the same paradigm in RAG-/- mice, which are mice that lack a viable immune system. In theory, if the CD73 tumor cells were being regulated by immune dependent mechanisms for metastasis, the NECA-induced increase in metastasis should be unaffected by A2A and A2B antagonists. This was found to be the case - although the authors mis-attribute the smaller decrease by A2B as an immune-independent result - something they cannot corroborate at this time. Nonetheless, the authors confirmed the role of A2A and A2B in metastasis formation and solidly showed that A2A's metastasis control was involved the immune system.

The next step in the process is to elucidate how exactly adenosine and the A2A receptor were involved in the immune response, by pinpointing which cell has A2A and is involved in tumor suppression. One candidate cell was the natural killer (NK) cell, which has A2A receptors. In order to investigate the role of adenosine in tumor suppression via NK cells, the authors co-cultured CD73+ tumor cells in the presence of NK cells to evaluate the amount of tumor cells killed. The addition of NECA to the co-culture significantly suppressed NK cell-mediated killing activity, which was only reversed by the addition of SCH58261 - the A2A receptor antagonist. Further, NK cells from A2A-/- mice were not modulated by NECA in the same tumor cell co-cultures. So, in vitro, the authors proved that NK cells were tumor killing cells, that could be inhibited by A2A receptor activation by adenosine.

Not bring enough for the authors, they pursued what mechanism the NK cell was using to kill the tumor cells. In an amazing experimental paradigm, the authors took mice with CD73+ tumors and after harvesting the tumor cells, they evaluated the NK cell infiltration via flow cytometry. Through this method, they were able to screen the amount of NK cells that had granzyme B in high concentrations, as this would indicate that the NK cells were using cytotoxic perforin cell-mediated killing of CD73+ tumor cells. In three different conditions, control, A2A, and A2B antagonist-administered mice, it was shown that the highest amount of granzyme B+ NK cells were in the A2A antagonist mice - corroborating the in vitro data described above.

In summary, the authors of this study conducted a comprehensive investigation of the role of adenosine and NK cells in tumor metastasis with great results. They elucidated that adenosine secreted by tumor cells to evade immune activity is responsible for inhibiting the killing activities of NK cells via the A2A receptor - resulting in increased ability to metastasize in the body. Further, they authors discovered that NK cells use perforin cell-mediated cytotoxicity to kill tumor cells.

I thought this study was very well done, systematically evaluating everything about adenosine and tumor cells. This research can help provide the basis for future immunotherapies for cancer - notably the CD73+ breast cancer noted in the study. I did not agree with the conclusion that the authors came to in regards to A2B, as it did seem - by their data in their figures - that it was not completely immune system independent in its anti-metastasis effects. I do agree with the authors that CD73 may have other effects that facilitate metastasis such as increased migration, and the study they proposed would be very clever and interesting by using a kinase-dead CD73 so that its only function would be that of cell-cell adhesion properties. In the end, this research is what is the basis for immunotherapies that can either supplement and hopefully potentially replace radiation or chemotherapy for certain cancers.

Reference:
     Beavis PA, Divisekera U, Paget C, Chow MT, John LB, Devaud C, Dwyer K, Stagg J, Smyth MJ, and Darcy PK. Blockade of A2A receptors potently suppresses the metastasis of CD73+ tumors. PNAS 110: 14711-14716 (2013).
     Mak TW and Saunders ME. Primer to the Immune Response. (Amsterdam: Elsevier) 2011.