Integrin α6-Targeted Molecular Imaging of Central Nervous System Leukemia in Mice

Abstract

Central nervous system leukemia (CNS-L) is caused by leukemic cells infiltrating into the meninges or brain parenchyma and remains the main reason for disease relapse. Currently, it is hard to detect CNS-L accurately by clinically available imaging models due to the relatively low amount of tumor cells, confined blood supply, and the inferior glucose metabolism intensity. Recently, integrin α6-laminin interactions have been identified to mediate CNS-L, which suggests that integrin α6 may be a promising molecular imaging target for the detection of CNS-L. The acute lymphoblastic leukemia (ALL) cell line NALM6 stabled and transfected with luciferase was used to establish the CNS-L mouse model. CNS-L-bearing mice were monitored and confirmed by bioluminescence imaging. Three of our previously developed integrin α6-targeted peptide-based molecular imaging agents, Cy5-S5 for near-infrared fluorescence (NIRF), Gd-S5 for magnetic resonance (MR), and ¹⁸F-S5 for positron emission tomography (PET) imaging, were employed for the molecular imaging of these CNS-L-bearing mice. Bioluminescence imaging showed a local intensive signal in the heads among CNS-L-bearing mice; meanwhile, Cy5-S5/NIRF imaging produced intensive fluorescence intensity in the same head regions. Moreover, Gd-S5/MR imaging generated superior MR signal enhancement at the site of meninges, which were located between the skull bone and brain parenchyma. Comparatively, MR imaging with the clinically available MR enhancer Gd-DTPA did not produce the distinguishable MR signal in the same head regions. Additionally, ¹⁸F-S5/PET imaging also generated focal radio-concentration at the same head regions, which generated nearly 5-times tumor-to-background ratio compared to the clinically available PET radiotracer ¹⁸F-FDG. Finally, pathological examination identified layer-displayed leukemic cells in the superficial part of the brain parenchyma tissue, and immunohistochemical staining confirmed the overexpression of the integrin α6 within the lesion. These findings suggest the potential application of these integrin α6-targeted molecular imaging agents for the accurate detection of CNS-L.

Keywords: central nervous system leukemia; integrin α6; leukemia; molecular imaging; positron emission tomography.

GRAPHICAL ABSTRACT

FIGURE 1

Surface expression of integrin α6 in human ALL cell lines and binding of the S5 peptide to NALM6 cells. (A) Flow cytometry identified the surface expression of integrin α6 in human ALL cell lines including NALM6, Ref, Jurkat, and CCRF-CEM, and (B) the highest expression of integrin α6 was observed in NALM6 cells. (C) Confocal microscopy confirmed the cellular localization of integrin α6 and the S5 peptide in NALM6 cells (Scale bar, 4 μm).

FIGURE 2

NIFR imaging with Cy5-S5 in ALL mouse models verified by bioluminescence imaging. (A) Cy5-S5 exhibited intensive fluorescence intensity in the similar tumor location of bioluminescence imaging, while (B) Cy5-CG7C did not show significant fluorescence intensity.

FIGURE 3

MR imaging with Gd-S5 in ALL mouse models. (A) Bioluminescence imaging exhibited focal luminescence intensity around the head 19 days after injection of NALM6-luciferased cells. (B) T1-weighted MR images at the baseline and 5, 10, and 15 min post-injection of Gd-S5 and the control probe Gd-DTPA (Scale bar, 1 mm). (C) Gd-S5 generated a significantly higher signal enhancement at the site of meninges which were located between the skull bone and brain parenchyma at 5, 10, and 15 min post-injection, with the greatest gap at about 10 min post-injection (n = 3).

FIGURE 4

¹⁸F-S5 PET/MR imaging in ALL mouse models. (A) ¹⁸F-S5 generated focal radioactivity at the same location of the enhanced signals in MR imaging. (B) ¹⁸F-S5 probe generated a significantly higher tumor-to-background ratio than the ¹⁸F-FDG probe (2.71 vs. 0.58, p = 0.006).

FIGURE 5

Pathological examination of tumor tissue. HE staining (A) identified layer-displayed leukemic cells in the superficial part of the brain parenchyma tissue, and immunohistochemical staining (B) confirmed the integrin α6 overexpression of the lesion (Scale bar, 40 μm).