e21064 Background: Interleukin-15 (IL-15) is a pro-inflammatory cytokine that stimulates the differentiation and proliferation of T, B, and NK cells. IL-15 is normally tightly bound by its receptor, IL-15Rα. Dendritic cells and monocytes/macrophages primarily express both IL-15 and IL-15-Rα; however, expression has been detected on epithelial cells including those of the lung. In the lung, IL-15 is thought to play a role in the induction of immune responses to infection. Over expression of IL-15 has been shown to induce NK cell activation and cytotoxic T-lymphocyte (CTL) responses leading to tumor regression. Little is known about IL-15 or IL-15Rα expression in lung cancer. Methods: mRNA from 146 primary lung cancers were analyzed by multiplex qPCR for expression of IL-15, IL-15Rα and b-actin (internal control) and compared to expression in normal lung tissue from 45 patients. Of the 146 patients, 50 were stage I (IA=20, IB=30), 49 stage II (IIA=9, IIB=40), 36 stage III (IIIA=18, IIIB=18), and 11 stage IV. Results: Comparing the expression of IL-15 between normal lung and tumor, we found tumors expressed significantly less IL-15 than normal lung (P<0.001). Lung cancers at any given stage, expressed significantly less IL-15 than that seen in normal lung. When comparing stages, stage IV tumors expressed significantly less IL-15 than tumors of stages I (P=0.021), II (P=0.020), or III (P=0.024). There was no difference in expression between stages I, II or III (P>0.05). When examining the expression of IL-15Rα we found no differences between normal lung and tumor. No differences were seen between normal lung and stages I-IIIA; however, significantly less IL-15Rα expression was noted in stages IIIB and IV compared to normal lung. Between stages, stages I and II had significantly more IL-15Rα expression than stage IV tumors. There were no differences between stages I, II and III. Conclusions: Pro-inflammatory cytokines such as IL-15 may be important for the induction of lung immunity. IL-15 expression is down regulated in lung cancers and this down regulation increases with stage. We posit that the down regulation of IL-15 by lung cancers may aid in their evasion of immune responses allowing progression and dissemination of tumor.