Perfluoropolyethylene glycol–labeled BDC2.5 T cells

Excerpt

T cells (TCs) are responsible for regulating immune responses and maintaining immune tolerance via recognition of peptide antigens that are bound to human leukocytes (1). Some TCs possess autoimmunity or self-tolerance through recognition of self-antigens. Loss of this required self-tolerance can result in an autoimmune disorder. For example, type 1 diabetes (T1D) is characterized by a spontaneous loss of immunological tolerance of pancreatic β-cells, which leads TCs to attack insulin-producing β-cells in the islets of Langerhans in the pancreas (2). The development of T1D has two stages: the insulitis phase when mixed leukocytes invade the islets and destroy β-cells, and the diabetes phase when the bulk of β-cells (up to 90%) have been destroyed and no longer produce sufficient insulin to control blood glucose levels (2). These stages can be reproduced easily in the non-obese diabetic (NOD) mouse model for human T1D (3), a spontaneous murine model in which insulitis and diabetes appear mainly in females in ~5 and ~13 weeks, respectively (4). Studies in NOD mice demonstrate that the islets in the pancreas are infiltrated with autoreactive CD4⁺ TCs as majority cells, as well as other cells such as CD8⁺ TCs, dendritic cells, B cells, and macrophages, in the insulitis phase (3). From the spleen and lymph nodes of female NOD mice, islet antigen-specific TCs, known as BDC2.5 TCs (BTCs), are derived and identified to be diabetogenic CD4⁺ TCs (5). Adoptive transfer of BTCs to NOD severe combined immunodeficiency (SCID) mice can accelerate diabetes (6). Expressing TC receptor genes (TCR) of BTCs in transgenic (Tg) mice produce a murine model (BDC2.5 TCR Tg/NOD SCID mice) with monoclonal TC repertoire in which all of the TCs are CD4⁺ type (5). Imaging these TCs in the progression of T1D is important to the understanding of the pathogenesis of autoimmunity and to the design of immunotherapeutic interventions (6).

Being the only stable isotope of fluorine with a natural abundance of ~100%, ¹⁹F has a nuclear spin 1/2 with a large gyromagnetic ratio (γ ~40.05 MHz/T) (7). The small γ difference between the ¹⁹F and ¹H (~6%) allows the use of existing proton nuclear magnetic resonance (NMR) instrumentation with minor adjustments to detect fluorinated species at high sensitivity (~83% relative to ¹H). Endogenous fluorine in vivo is found primarily in bones and teeth as solid fluorides, which have very short T₂ relaxation times and result in an undetectable signal with magnetic resonance imaging (MRI). Therefore, exogenously administered fluorinated tracers can be used to track various biological processes in vivo. For example, perfluorocarbons (PFC) such as perfluoro-15-crown-5 (15C5) have been used to measure oxygen tensions in tissues and tumors (8) and to label dendritic cells (9). The lack of background ¹⁹F signal is advantageous in in vivo applications, but additional ¹H images are required to provide anatomic interpretations. PFCs are extremely hydrophobic and do not dissolve in blood directly; they normally are formulated as biocompatible emulsions for intravenous administration (7). Inside the body, the PFC particles are cleared from circulation by phagocytes/macrophages and/or by respiration within several hours or days, depending on the administered dose, particle size, and PFC (10). Many commercial PFC emulsions have been found to be nontoxic or do not cause any health problems other than tissue swelling (7). Perfluoropolyethylene glycol (PFPE, Mw 1750) is a linear PFC that contains a large number of ¹⁹F atoms per molecule for enhancing sensitivity (6). The center CF₂ groups can generate a strong peak (>0.9 at -92 ppm) that dominates 90% of its ¹⁹F signal, whereas the end CF₂ groups yield a weak signal (<0.1 at -79 ppm) that is below the in vivo MRI detection limit. The spin relaxation time (T₁) is 2.2 times shorter than that of 15C5 at 11.7 T, which allows a shorter imaging acquisition time. PFPE can be emulsified to form particles ~100–200 nm in diameter, allowing for cellular uptake via endocytosis (9). BTCs are labeled with PFPE (PFPE-BTCs) for ¹⁹F MRI.