Multimodality imaging methods for assessing retinoblastoma orthotopic xenograft growth and development

Abstract

Genomic studies of the pediatric ocular tumor retinoblastoma are paving the way for development of targeted therapies. Robust model systems such as orthotopic xenografts are necessary for testing such therapeutics. One system involves bioluminescence imaging of luciferase-expressing human retinoblastoma cells injected into the vitreous of newborn rat eyes. Although used for several drug studies, the spatial and temporal development of tumors in this model has not been documented. Here, we present a new model to allow analysis of average luciferin flux ([Formula: see text]) through the tumor, a more biologically relevant parameter than peak bioluminescence as traditionally measured. Moreover, we monitored the spatial development of xenografts in the living eye. We engineered Y79 retinoblastoma cells to express a lentivirally-delivered enhanced green fluorescent protein-luciferase fusion protein. In intravitreal xenografts, we assayed bioluminescence and computed [Formula: see text], as well as documented tumor growth by intraocular optical coherence tomography (OCT), brightfield, and fluorescence imaging. In vivo bioluminescence, ex vivo tumor size, and ex vivo fluorescent signal were all highly correlated in orthotopic xenografts. By OCT, xenografts were dense and highly vascularized, with well-defined edges. Small tumors preferentially sat atop the optic nerve head; this morphology was confirmed on histological examination. In vivo, [Formula: see text] in xenografts showed a plateau effect as tumors became bounded by the dimensions of the eye. The combination of [Formula: see text] modeling and in vivo intraocular imaging allows both quantitative and high-resolution, non-invasive spatial analysis of this retinoblastoma model. This technique will be applied to other cell lines and experimental therapeutic trials in the future.

Figure 1. Illustration of the neonatal rat left eye, viewed from above, showing approximate injection angle and location at the lateral equator.

Figure 2. Characterization of a Y79 retinoblastoma cell line expressing an EGFP-luciferase fusion protein (Y79-EGFP-luc).

(A) Y79-EGFP-luc cells display identical growth kinetics to the parent cell line (F-test p = 0.49). Y79-EGFP-luc data points shifted right for clarity. (B–D) Correlations between growth parameters: (B) Luminescence versus cell count; (C) Fluorescence versus cell count; (D) Fluorescence versus luminescence. Mean ± SD shown, n = 3 (some error bars are smaller than the data point size).

Figure 3. Modeling of luciferin flux from bioluminescence imaging (BLI) data reveals slowing of tumor growth over time.

(A) Conventional peak luminescence imaging of xenografts in six individual animals (each animal is one colored line) shows exponential tumor growth in the majority of animals. (B) Calculated luciferin flux () modeled (Equations 1–3) from BLI data over the 14-day study. (C) Pseudo-color parametric images of for three representative animals at three timepoints, color-coded as in (A).

Figure 4. Xenograft fluorescence correlates with luminescence.

(A) Ex vivo quantitative fluorescence imaging of six right eyes from six P14 rats, all of which were injected with Y79-EGFP-luc at P0. (B) Correlation between fluorescence intensity measured from individual eyes shown in (A) and peak bioluminescence.

Figure 5. Characterization of pathology induced by Y79-EGFP-luc retinoblastoma cell xenografts or control injections.

Brightfield (A,D,G,J), green fluorescence (B,E,H,K), and OCT (C,F,I,L) imaging over a 4 week period shown. Tumors were highly vascularized (white arrowheads) and had well-defined edges as seen on brightfield and fluorescence imaging. OCT provided some additional depth resolution not possible with the other two modalities, although this was limited by shadowing of posterior features. OCT was also able to identify small, distinct satellite tumors growing independent of the main tumor mass (red arrows). Red lines indicate the OCT planes; L, lens; R, retina.

Figure 6. Histologic analysis of Y79 xenografts reveals intravitreal tumors with morphology closely resembling in vivo OCT imaging.

On OCT imaging in(lower right), this small tumor was seen to be closely apposed to the phakic lens and was noted to have a small finger-like extension off the main tumor mass. Histologic H&E sections confirmed this same appearance after fixation. L, lens; original magnification, top = 25×, lower left = 100×.