Membrane-Bound IL-21 Promotes Sustained Ex Vivo Proliferation of Human Natural Killer Cells

NK cells have therapeutic potential for a wide variety of human malignancies. However, because NK cells expand poorly in vitro, have limited life spans in vivo, and represent a small fraction of peripheral white blood cells, obtaining sufficient cell numbers is the major obstacle for NK-cell immunotherapy. Genetically-engineered artificial antigen-presenting cells (aAPCs) expressing membrane-bound IL-15 (mbIL15) have been used to propagate clinical-grade NK cells for human trials of adoptive immunotherapy, but ex vivo proliferation has been limited by telomere shortening. We developed K562-based aAPCs with membrane-bound IL-21 (mbIL21) and assessed their ability to support human NK-cell proliferation. In contrast to mbIL15, mbIL21-expressing aAPCs promoted log-phase NK cell expansion without evidence of senescence for up to 6 weeks of culture. By day 21, parallel expansion of NK cells from 22 donors demonstrated a mean 47,967-fold expansion (median 31,747) when co-cultured with aAPCs expressing mbIL21 compared to 825-fold expansion (median 325) with mbIL15. Despite the significant increase in proliferation, mbIL21-expanded NK cells also showed a significant increase in telomere length compared to freshly obtained NK cells, suggesting a possible mechanism for their sustained proliferation. NK cells expanded with mbIL21 were similar in phenotype and cytotoxicity to those expanded with mbIL15, with retained donor KIR repertoires and high expression of NCRs, CD16, and NKG2D, but had superior cytokine secretion. The mbIL21-expanded NK cells showed increased transcription of the activating receptor CD160, but otherwise had remarkably similar mRNA expression profiles of the 96 genes assessed. mbIL21-expanded NK cells had significant cytotoxicity against all tumor cell lines tested, retained responsiveness to inhibitory KIR ligands, and demonstrated enhanced killing via antibody-dependent cell cytotoxicity. Thus, aAPCs expressing mbIL21 promote improved proliferation of human NK cells with longer telomeres and less senescence, supporting their clinical use in propagating NK cells for adoptive immunotherapy

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Gene expression in NK cells stimulated with mbIL15 or mbIL21 as assessed using the nCounter platform.

<p>Paired aliquots of purified NK cells from 4 donors were stimulated for one week with either Clone 4 (mbIL15) or Clone 9.mbIL21. Total RNA was purified and equal quantities hybridized for detection of expression of 96 genes. Gene expression was normalized to LDH (mean 6,076 copies detected), and the remaining data for each gene plotted as mean +/− SEM for each expansion condition. Genes with detection below background (set at ≤10 detected copies) were excluded as not biologically significant (grey box). The ten highest-expressed genes in addition to LDH are labeled, as are genes that are differentially expressed by >2 fold (red) or <0.5 fold (blue) in mbIL21-expanded cells. Differentially expressed genes were replotted (inset) for comparison, and two-tailed <i>t-</i>test applied.</p

Direct cytotoxicity and cytokine secretion of fresh NK cells, mbIL15-expanded, and mbIL21-expanded NK cells.

<p>NK cells were purified from four normal donor buffy coats and assessed directly, and then retested after being expanded for 21 days with Clone 9.mbIL21 and Clone 4 (mbIL15) (with CD3 depletion on day 14). NK cells were assessed for cytotoxicity against 721.221 (A) or cytokine secretion in response to K562 (B). NK cells from another four donors were purified and cryopreserved, and expanded with Clone 9.mbIL21 and then cryopreserved. Fresh (circles) and expanded (squares) NK cells were then thawed and activated in parallel for 24 h in 50 IU/mL IL-2, and then tested for cytotoxicity against HLA^null 721.221 targets (C), C*0702-transduced (Group C1) 721.221 targets (D), or B*5801-transduced (Group Bw4) 721.221 targets (E). All data points shown are mean +/− SEM of the means of four donors tested in triplicate.</p

Surface expression of CD15 and transgene receptors on Clone 9.mbIL21 aAPC, and stability of expression of relevant receptors.

<p>After limiting-dilution cloning, cloned aAPCs were assessed for expression of transgenes, <i>A</i>. Since CD15 expression was variable among the clones and is felt to play a role in NK cell activation, its expression was also assessed. After selection of a clone with expression of all surface proteins, the clone was maintained in culture for 6 months, with periodic re-evaluation of CD137L and mbIL21 expression, <i>B</i>.</p

Schema for NK cell manufacturing with aAPCs.

<p>Artificial antigen-presenting cells (aAPCs) were produced by genetic modification of K562 to express costimulatory molecules and membrane-bound cytokines. To expand NK cells <i>ex</i> vivo, unfractionated PBMC are stimulated weekly with irradiated PBMC, inducing rapid proliferation of NK cells and in some cases non-specific expansion of T cells. Contaminating T cells may be depleted, and the remaining purified NK cells may be stimulated weekly by the aAPCs as needed to obtain sufficient numbers. Expanded NK cells may be used directly or cryopreserved for future use.</p

NK cell expansion and purity after repeated weekly stimulation with aAPCs expressing membrane-bound cytokines.

<p><i>A</i>, Mean expansion of CD3^neg/CD16-or-56^pos NK cells from 22 donors expanded for 21 days using aAPCs bearing mbIL15 or mbIL21. NK cells from five donors were also expanded with aAPCs expressing both mbIL15 and mbIL21. <i>B</i>, Mean expansion of NK cells from 4 donors expanded for 42 days using aAPCs bearing mbIL15 or mbIL21. <i>C</i>, Percent of CD3^pos T/NKT cells and CD3^neg/CD16-or-56^pos NK cells in the starting PBMC product (day 0) and at the end of 21 days of expansion on Clone 9.mbIL21 from 20 donors. <i>D</i>, Mean expansion of CD3^neg/CD16-or-56^pos NK cells on Clone 9.mbIL21 from unfractionated PBMC (n = 19), unfractionated PBMC followed by NK purification at day 14 of expansion (n = 3), or from NK cells purified from PBMC at day 0 prior to expansion (n = 13). Mean +/− SD is shown for each plot. All p values indicated are for <i>t-</i>test of fold expansion at the end of the expansion period.</p

Change in relative telomere length of purified NK cells after expansion on aAPCs with or without membrane-bound cytokines.

<p>NK cells were purified from normal donor buffy coats. An aliquot was viably frozen for later comparison, and the remaining cells were stimulated with IL-2 and irradiated aAPCs bearing no cytokine (Clone 9), mbIL15 (Clone 4), mbIL21 (Clone 9.mbIL21), or IL-15 fused with a linker to IL15Rα (Clone 27). (A) Telomere length of the NK cells was determined after 7 days by flow cytometry after hybridization with FITC-labeled PNA probe for the TTAGGG telomeric repeat sequence. Mean fluorescence of the NK cells was normalized to the reference cell line CEM-1301, and the telomere length of expanded NK cells was normalized to that of fresh NK cells for each individual donor. A 1-way ANOVA with Dunnett's multiple comparisons test was applied to compare each aAPC to Clone 4. (B) NK cells from four additional donors stimulated weekly for 21 days prior to assessing telomere length, and again normalized to that of fresh cells from each donor. P value shown for two-tailed <i>t-</i>test.</p