Type 1 diabetes (T1D) is an organ-specific autoimmune disease originating from the destruction of pancreatic β cells by autoreactive T cells (TCs) (1). In particular, islet-associated CD8+ TCs initiate β cell insult and trigger shedding of β cell autoantigens. This subset of CD8+ TCs is primarily restricted to the class I major histocompatibility complex (MHC) molecule H-2Kd (2), in which Vα17-Jα42 TC receptor α (TCR-α) chains are expressed for recognition of a unique β cell autoantigen (1), islet-specific glycose-6-phosphatase catalytic subunit-related protein (IGRP). After initiation, a series of immune responses occur, including loading autoantigens into antigen-presenting cells (the dendritic cells), activating autoreactive CD4+ TCs, and infiltrating TCs in pancreatic islets. T1D development consists of an insulitis phase and a diabetes phase, which are readily reproduced in a murine model called non-obese diabetic (NOD) mouse for human T1D (3, 4). In general, the progression from insulitis to diabetes in NOD mice is accompanied by alternatively repeated circulation of IGRP206-214–reactive CD8+ TC pool and avidity maturation of its islet-associated counterpart (5). Thus, IGRP has been used as a target for human diabetogenic response. NOD peptide (NRP-V7) (Lys-Tyr-Asp-Lys-Ala-Asp-Val-Phe-Leu (KYNKANVEL)) is a mimotope of IGRP that shares a 67% sequence homology with the endogenous IGRP206-214 epitope and has high avidity for H-2Kd molecule (5). A transgenic mouse (8.3-NOD) that specifically expresses H-2Kd–restricted TCR (8.3-TCR) has been used for a variety of therapeutic studies (6), in which 8.3-TCR constitutes as much as 30% to 40% of islet-associated CD8+ TCs (2).
Cross-linked iron oxide-(H-2Kd)-(CLIO-NPR-V7) is a NRP-V7 labeled paramagnetic iron oxide nanoparticle that is used to image 8.3-TCR by magnetic resonance imaging (MRI) (1, 5). This agent consists of three components: an iron oxide nanoparticle core for MRI contrast enhancement, two avidin molecules are covalently linked to the core surface, and eight biotinylated peptide/MHC complex (NRP-V7/H-2Kd) monomers bind on the avidins for recognition of 8.3-TCR. The nanoparticle contains an icosahedral core of superparamagnetic crystalline Fe3O4 (magnetite) that is caged by epichlorohydrin cross-linked dextran and functionalized with amine groups (CLIO-NH2) (7). They have a high magnetic susceptibility to induce a significant magnetization inside a magnetic field. This creates microscopic field gradients that diphase nearby protons and causes a shortening of T2 relaxation times (8). Avidin is a tetrameric protein with a molecular mass of 68 KDa that is capable of strongly binding four biotins (association constant (Ka) = 1.7 × 1015 M-1) (9). Because CLIO-NRP-V7 binds to the peripheral 8.3-TCR in the blood directly, it can be used to label islet-associated TCs for in vivo tracking through systemic administration (1, 5).