4-1BBL-Expressing aAPCs Attenuate IL-15-Induced NK Cell Expansion and Cytokine Production In Vitro but Induce NK Cell-Mediated Gvhd In Vivo

Abstract 2536

Methods to expand natural killer (NK) cells ex vivo for adoptive cell therapy are being explored to improve outcomes after allogeneic blood and marrow transplant (alloBMT). Artificial antigen presenting cells (aAPCs) can present cytokines and/or co-stimulatory molecules that can potentially improve expansion and activity. 4-1BBL (CD137L) has demonstrated mixed results on murine and human NK cells, but the impact on murine NK cell biology after alloBMT has not been explored. NK cells were harvested from either C57BL/6 (B6) or CB6F1 spleens and cultured ex vivo with a recombinant interleukin (IL)-15/IL-15 receptor alpha (Ra) complex in the presence or absence of a CD137L⁺ aAPC. Because IL-15 is typically presented in trans by IL-15Ra, the complex was utilized to potently increase agonist bioactivity. NK cells cultured with IL-15/IL-15Ra alone showed a peak of 20-fold expansion, but this expansion was decreased with the addition of CD137L⁺ aAPCs if the ratio of aAPC to NK cells was greater than 1:1. In the presence of IL-15/IL-15Ra, the impact of CD137L⁺ aAPCs on expression of the inhibitory receptors, Ly49C+I and activating receptor Ly49H was variable and strain dependent, with increased expression in B6 NK cells, but decreased expression in CB6F1 NK cells. The expression of major histocompatibility complex (MHC) class I was not affected in NK cells from either strain by the presence of CD137L⁺ aAPCs. The production of gamma interferon and tumor necrosis factor-a was robust in NK cells expanded by IL-15/IL-15Ra alone, but attenuated with the addition of CD137L⁺ aAPCs. Animal experiments showed that administration of NK cells expanded ex vivo with IL-15/IL-15Ra alone was well tolerated after T cell depleted MHC-mismatched alloBMT (CB6F1–>B6), but surprisingly the addition of CD137L⁺ aAPCs to cultures caused NK cells to induce GVHD-associated weight loss. In summary, IL-15/IL-15Ra expanded murine NK cells demonstrate increased cytokine production and do not cause toxicity when infused after alloBMT. The presence of CD137L⁺ aAPCs attenuated cytokine production and increased Ly49 receptor expression in NK cells from B6 mice. Remarkably, NK cells expanded by IL-15/IL-15Ra in the presence of CD137L⁺ aAPCs demonstrate increased propensity to cause GVHD. Ongoing studies are exploring the anti-tumor efficacy of IL-15/IL-15Ra expanded murine NK cells cultured in the presence and absence of CD137L.