NK cells are the primary effectors of the innate immune response against infections pathogens and malignant transformation through their efficient cytolytic activity and cytokine secretion. Nevertheless, tumor cells have developed mechanisms to evade innate immune surveillance and the molecular basis for target resistance to NK cell-mediated lysis is not yet completely understood. To identify novel pathways that modulate tumor cell resistance to NK cells, we previously developed a cell-cell interaction based screening approach using a large sub-set of a lentiviral shRNA library containing multiple independent shRNAs targeting more than 1,000 human genes. Using this approach we found that silencing JAK1 and JAK2 significantly increased secretion of INF-γ from NK cells and increased tumor cell susceptibility to NK cell lysis.

To examine the role of the JAK signaling pathway in the modulation of tumor cell susceptibility to NK lysis, we analyzed down-stream signaling pathways in several cell lines (IM9, U937, K562, RPMI, MM1S KM12BM) and primary tumor cells (AML, MM, ALL). In the absence of NK cells, silencing JAK1 or JAK2 did not affect the basal activation of STAT proteins (STAT1(pY701), STAT1(pS727), STAT3(pY705), STAT3(pS727), STAT4(pY693), STAT5(pY694), STAT6(pY641)) or AKT(pS473) and ERK1/2(pT202/pY204) or expression of activating or inhibitory ligands on tumor cells. Because JAK1 and JAK2 transduce signals downstream of the IFN-γ receptor, we hypothesized that JAKs may play a role in tumor cell evasion of NK cell activities such as cytolysis and IFN-γ secretion. To test this hypothesis we pre-incubated various tumor cell lines or primary tumor cells with activated NK supernatant or recombinant human IFN-γ. Tumor cell activation in this fashion resulted in activation of STAT1 (pSTAT1(pY701)) but none of the other STATs, ERK or AKT. As expected, STAT1 activation was blocked when JAK1 or JAK2 were silenced or inhibited by a JAK inhibitor. Silencing of STAT1 with 2 independent shRNAs also resulted in increased tumor susceptibility to NK cell cytolysis in 3 different tumor cell lines tested. To confirm that IFN-γ secreted by activated NK cells induced resistance in tumor cell targets we used a blocking IFN-γ antibody (D9D10). 10μg/ml D9D10 completely blocked STAT1 phosphorylation and in different experiments using U937, IM-9, KM12BM, MM1S and RPMI we found that D9D10 significantly increased specific NK target cell lysis by 51.8%, 78.5%, 25.1%, 20.6% and 28.5% compared to IgG1 isotype controls. Similar results were obtained whit different primary tumor cells.

To determine whether IFN-γ stimulation affected expression of ligands involved in NK cell recognition of tumor cells, we analyzed the effect of activated NK supernatant or IFN-γ on the expression of MHC Class I, β2M, HLA-C, HLA-A2, NKG2D, NKP44, NKP46, NKP30 ligands using chimeric FC proteins, MICA/B, DNAM-1 ligands (CD112, CD155), 2B4 ligand (CD48), TRAIL ligands (TRAIL-R1, TRAIL-R2), Fas ligand (CD95) and PD1 ligands (PDL1, PDL2, B7H3, B7H4). The basal expression of these ligands varied among the various tumor cell lines or primary tumors tested but the only ligand that was significantly up-regulated in every tumor sample tested was PDL1. PDL1 expression by tumor cells is known to inhibit T cell immunity. To test whether increased levels of PDL1 could also inhibit NK cell killing, we co-cultured primary NK cells with U937, IM9, KM12BM, RPMI, K562, MM1S, primary MM, AML and ALL cells with or without 10μg/ml anti-PDL1 antibody (recombinant mab with Fc mutated to eliminate FcR-mediated effects). Blocking PDL1 significantly increased NK cell killing of U937, IM9, KM12BM, RPMI, MM, AML and ALL (p=0.03, p=0.02, p=0.03, p=0.005, p=0.009, p=0.03 and p=0.02 respectively). NK cell killing activity did not further increase when a JAK inhibitor was added to the co-culture.

These results show that NK cell secretion of IFN-γ results in IFN receptor signaling and activation of JAK1, JAK2 and STAT1 in the tumor cell targets, followed by rapid up-regulation of PDL1 expression and increased resistance to NK cell lysis. Blockade of JAK pathway activation prevents subsequent PDL1 up-regulation resulting in increased susceptibility of tumor cells to NK cell activity suggesting that JAK pathway inhibitors may work synergistically with other immunotherapy regimens by eliminating IFN-induced PDL1 mediated immunoinhibition.


Freeman:Bristol-Myers-Squibb/Medarex: Patents & Royalties; Roche/Genentech: Patents & Royalties; Merck: Patents & Royalties; EMD-Serrono: Patents & Royalties; Boehringer-Ingelheim: Patents & Royalties; Amplimmune: Patents & Royalties; CoStim Pharmaceuticals: Patents & Royalties; Costim Pharmaceuticals: Membership on an entity’s Board of Directors or advisory committees.

Author notes


Asterisk with author names denotes non-ASH members.