The immunosuppressive Prostate Cancer (PCa) microenvironment has a major influence on the progression of the disease and is a key challenge for immunotherapy considering that immune effector cells activated by such modalities, will be rendered anergic or regulatory once they enter this microenvironment. We have previously shown that IL-15 can activate NK and NKT cells to mediate cytotoxicity against PCa cells in lymphocyte-PCa cocultures and also that IL-15 can modulate NK receptors to shift the balance between activation and inhibition of NK cells. From initial data in PCa-lymphocyte cocultures, we also observed that IL-15 downregulated the inhibitory NK ligand HLA-BW4 on PCa cells, whereas the ligands MICA/MICB were not significantly affected. Due to the critical role of MICA and other activatory NK receptor ligands (aNKRL) such as Nectin-2 and ULBP1 on NK cell function we have now studied effects of IL-15 on the expression of aNKRLs MICA, Nectin-2 and ULBP1 both on the cell surface of PCa cells and in the supernatants of PCa-lymphocyte co-cultures.
PC-3 and LNCaP PCa cell-lines were incubated with non-adherent PBMCs at effector:target ratios of 8:1 and cytokines IL-2 or IL-15. After 1 week, tumour cells were stained with antibodies to Nectin-2, MICA, and ULBP1 ligands. Flow cytometric analysis followed staining. MICA, Nectin-2 and ULBP1 ELISAs were performed with supernatants from these co-cultures of PCa cells and lymphocytes (and from PCa cells alone, treated with or without cytokines) to determine whether there was any effect of IL-15 on shedding of these ligands from the PCa cells.
We now show that MICA, Nectin-2 and ULBP1 ligands expressed on PC-3 cells were not significantly changed with IL-15 treatment in PCa-lymphocyte cocultures, however, on LNCaP cells, IL-15, (but not IL-2) downregulated MICA, Nectin-2 and ULBP1 ligands by upto 90%* (* = p<0.01,1-way-anova, post-hoc Newman-Keuls,n = 5).
In ELISAs measuring concentrations of MICA and Nectin-2 in the PCa-lymphocyte co-culture sups, IL-15, but not IL-2, upregulated concentrations of MICA by upto 145%* (* = p<0.01, 1-way-anova, post-hoc Newman-Keuls,n = 5), but also directly increased shed MICA in LNCaP without the presence of lymphocytes. Concentrations of Nectin-2 in the sups were however, not affected by IL-15 or IL-2. Shed ULBP1 is being currently investigated.
In contrast to previous findings with IL-15 where aNKRLs (e.g. MICA, ULBP1) on certain cell types are upregulated, we see a downregulation of MICA, ULBP1 and Nectin-2 ligand expression on LNCaP cells with a concomitant increase in shed MICA. Although the current paradigm has been that shed MICA can inhibit NK cell function, other groups are now showing that the shed ligand may actually activate NKG2D on NK cells and our findings that IL-15 dramatically increases NK cell functions against PCa cells may confirm this alternative paradigm.
Citation Format: Christina Alexandra Sakellariou, Oussama Elhage, Richard A. Smith, Prokar Dasgupta, Christine Galustian. IL-15 increases NK functions in the PCa-lymphocyte microenvironment by a profound increase in shedding of MICA from PCa cells - a novel paradigm. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2340. doi:10.1158/1538-7445.AM2015-2340