Abstract Cytokine release syndrome (CRS) is a major risk during treatment with CAR-T cells, T-cell engagers, and, occasionally, checkpoint inhibitors. Because humanized immune-system mice are frequently used for the preclinical development of such therapies, we sought to provide a platform to derisk CRS before the clinical stage. We therefore characterized CRS induction, immune dynamics, and clinical manifestation in two different humanized mouse models. We humanized the immune system of initially immunodeficient mice by engrafting them either PBMCs or CD34+ hematopoietic stem cells. In the PBMC model, T cells were the only engrafted human immune population and exhibited a chronically induced, partially exhausted (TIM3+, LAG3+, PD1-) phenotype at baseline. At one week post-PBMC engraftment, circulating T-cells were at approximately at 1000 cells per mL, and OKT3 injection depleted them, thereby failing to induce CRS. At three weeks post-engraftment, however, circulating T-cell counts reached one million cells per mL and were no longer depleted by OKT3, which instead induced severe CRS and mortality within 24 hours. CRS induction in PBMC mice was accompanied by increases in circulating TNF-α, IFN-γ, and IL-2, as well as transient (≤6 hours) T-cell proliferation. In contrast, in the CD34 model, a more complete human immune system developed after engraftment, including myeloid, NK, dendritic, B, and non-exhausted T cells. This broader immune reconstitution enabled OKT3 to induce CRS with as few as 10,000 circulating T cells. Although clinical CRS symptoms were milder than those observed in PBMC-engrafted mice, the CRS cytokine signature in the CD34 model included not only the IFN-γ, TNF-α, and IL-2 surges observed in PBMC mice but also substantial increases in hallmark myeloid-derived CRS cytokines such as IL-6, CXCL10, and CCL2. Likewise, the cellular signature of CRS was more complete in CD34 mice, with induction of T-cell activation (CD69+, CD38+, HLA-DR+), exhaustion (PD1+, TIM3+), and proliferation (Ki67) markers, along with increased monocyte mobilization into the blood. In summary, our results demonstrate that the CD34 model displays mild-to-moderate clinical symptoms while more comprehensively recapitulating the hallmark cellular and molecular features of CRS. Citation Format: Audrey Wetzel, Anaïs Meynet-Cordonnier, Clothilde Philouze, Charline Boulot, Emilie Bayon, Sebastien Tabruyn, Dan Georgess. Modeling CRS in humanized immune system mice abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 7764.
Wetzel et al. (Fri,) studied this question.