Crefmirlimab (IAB22M2C) is a humanized anti-CD8 monoclonal antibody fragment (minibody) designed for PET imaging. Upon radiolabeling (e.g., with 89Zr), it serves as a non-invasive imaging biomarker that binds with high affinity and specificity to the CD8 receptor on CD8+ T cells, enabling real-time, dynamic tracking of T-cell distribution, infiltration, and trafficking in vivo in both preclinical models and clinical studies. It is a critical tool for evaluating the pharmacodynamics of T-cell engaging therapies (such as the GUCY2C-CD3 bispecific antibody PF-07062119) and for investigating the tumor immune microenvironment