Three-Dimensional Imaging of Tumor-Bearing Tissue Using the Iterative Bleaching Extends Multiplexity Approach

The spatial heterogeneity of the tumor microenvironment (TME) is a critical determinant of therapeutic response, particularly for immune-oncology agents, where success depends on the distribution of specific immune cell subpopulations. Over the past decade, multiple sophisticated technologies have been introduced to achieve detailed resolution of the TME using two-dimensional sections from either formalin-fixed, paraffin-embedded (FFPE) or fixed-frozen tissues. While these thin sections are easier to procure and analyze, they lack the three-dimensional architecture needed to reliably and comprehensively characterize a tumor. To address this limitation, a tissue mounting and imaging technique was developed to enable the three-dimensional analysis of tumor lesions in their native in vivo state. This protocol outlines the procurement of human tumor tissue, the mounting of samples on custom-printed platforms, and staining procedures for post-fixation samples. The multiplexed immunofluorescence technique, IBEX (Iterative Bleaching Extends Multiplexity), was adapted to characterize the three-dimensional TME with up to 15 markers for tumor, immune, and stromal cells using commercially available antibodies. Imaging depths of up to 100 µm were achieved using an inverted white-light laser confocal microscope with a custom-printed imaging adaptor and commercial glass-bottom dishes to ensure optimal tissue orientation. This protocol highlights the potential of the IBEX method to expand multiplexed immunofluorescence studies, providing a more comprehensive understanding of TME composition.