Engineered drug resistant γδ T cells kill glioblastoma cell lines during a chemotherapy challenge: a strategy for combining chemo- and immunotherapy

Abstract

Classical approaches to immunotherapy that show promise in some malignancies have generally been disappointing when applied to high-grade brain tumors such as glioblastoma multiforme (GBM). We recently showed that ex vivo expanded/activated γδ T cells recognize NKG2D ligands expressed on malignant glioma and are cytotoxic to glioma cell lines and primary GBM explants. In addition, γδ T cells extend survival and slow tumor progression when administered to immunodeficient mice with intracranial human glioma xenografts. We now show that temozolomide (TMZ), a principal chemotherapeutic agent used to treat GBM, increases the expression of stress-associated NKG2D ligands on TMZ-resistant glioma cells, potentially rendering them vulnerable to γδ T cell recognition and lysis. TMZ is also highly toxic to γδ T cells, however, and to overcome this cytotoxic effect γδ T cells were genetically modified using a lentiviral vector encoding the DNA repair enzyme O(6)-alkylguanine DNA alkyltransferase (AGT) from the O(6)-methylguanine methyltransferase (MGMT) cDNA, which confers resistance to TMZ. Genetic modification of γδ T cells did not alter their phenotype or their cytotoxicity against GBM target cells. Importantly, gene modified γδ T cells showed greater cytotoxicity to two TMZ resistant GBM cell lines, U373(TMZ-R) and SNB-19(TMZ-R) cells, in the presence of TMZ than unmodified cells, suggesting that TMZ exposed more receptors for γδ T cell-targeted lysis. Therefore, TMZ resistant γδ T cells can be generated without impairing their anti-tumor functions in the presence of high concentrations of TMZ. These results provide a mechanistic basis for combining chemotherapy and γδ T cell-based drug resistant cellular immunotherapy to treat GBM.

Figure 1. Transitory increase in stress-associated antigens on TMZ-resistant cell line U87 after exposure to TMZ.

U87 cells were cultured to confluence and incubated in media containing 100 µM TMZ. Stress antigens were assessed at the time intervals noted on the x axis by flow cytometry and the increase in median fluorescence intensity over isotype control were calculated. Data are shown as percentage increase over unmanipulated U87 cells. SD of 3 experiments are shown.

Figure 2. Genetic modification of expanded/activated γδ T cells by HIV (a) and SIV-GFP (b) lentiviral vectors 6 days following transduction with an MOI of 15.

Mean fluorescence intensity (MFI) is approximately the same for both vectors, but transduction efficiency and expression from the SIV-derived vector is higher when measured on day+6. Quadrant values are noted to the right of each plot.

Figure 3. Transduction of γδ T cells with lentivirus vector was performed on day 6, 7 and 8 of expansion culture (see text) with increasing MOI.

(a) On day +14 cells were incubated in media supplemented with 400 µM TMZ and viable cell counts were obtained for each MOI. Two separate experiments are shown. (b) Quantitative PCR analysis to measure P140KMGMT copy numbers of the bioengineered γδ T cells in the presence of increasing concentrations of TMZ, which are indicated in the figure.

Figure 4. Expanded/activated γδ T cells were manufactured as described in the text.

Flow cytometry from two separate donors shown from (a) unmanipulated and (b) P140KMGMT-transduced γδ T cells. For both panels (a) and (b) quadrant 2 (Q2) represents γδ T cells. As discussed in the text, the yield of γδ T cells was slightly lower than control due to loss of cells during the transduction procedure; however, purity of the final product was not affected as both products from a single donor show >90% purity of γδ T cells. (c and d) Cytotoxicity assays from two separate expansions (panel c and d, respectively) of unmodified γδ T cells (solid line) versus TMZ P140KMGMT transduced γδ T cells (dashed line) against the TMZ-resistant glioma cell line U87 were conducted to determine if genetic modification impairs γδ T cell function. Cytolytic activity of γδ T cells against U87 cells was nearly equivalent at all E:T ratios, verifying that P140KMGMT transduced γδ T cells function is equivalent to that of unmodified γδ T cells.

Figure 5. TMZ-resistant clones of the GBM cell line (a) U373^TMZ-R and (b) SNB-19^TMZ-R were selected by incubation in increasing concentrations of TMZ over 60 days.

The cell lines were labeled with PKH-26 and incubated for 4 hours in the presence of 100 µM TMZ alone (upper panel) and with P140KMGMT transduced γδ T cells (γδ^TMZ-R) at a 10∶1 effector:target ratio. The culture was then labeled with ToPro Iodide and acquired for flow cytometric phenotyping. A minimum of 5000 PKH26+ events was acquired to insure statistical validity of the data. All plots gated on PKH-26+ target cells. Note that the cloned cell lines are resistant to killing in media supplemented with TMZ with SNB-19^TMZ-R showing less cell loss than U373^TMZ-R. Addition of γδ^TMZ-R results in much greater incorporation of ToPro Iodide after 4 h incubation suggesting that the increased cytotoxicity is overwhelmingly due to genetically modified γδ T cells. Dose-dependent cytotoxicity of γδ^TMZ-R is significantly less when assayed against SNB-19^TMZ-R (c) with no TMZ in the media vs. γδ^TMZ-R against SNB-19^TMZ-R in the presence of TMZ (p = 0.0085). Cytotoxicity was also trended greater against TMZ-resistant U373 with γδ^TMZ-R as effectors as well when the assay was conducted in the presence of TMZ (p = .0875). These assays were conducted as separate experiments from different donors.