White blood cells in PET studies
White blood cells (WBCs, leukocytes) are central in infections and inflammatory diseases and also in vascular diseases, including atherosclerosis. Circulating leukocytes are recruited to migrate into the interstitial space and lymph in the affected tissue region by resident immune cells, endothelial cells and platelets, which release inflammatory mediators and cytokines such as histamine, heparin, and serotonin (Petri et al., 2008).
WBCs in the blood consist of neutrophils (∼59%), lymphocytes (∼34%), monocytes (∼4%), eosinophils (∼3%), and basophils (∼0.5%). Neutrophils, eosinophils, and basophils contain granules, and are therefore called granulocytes. Based on the irregularly shaped nucleus, granulocytes are also referred to as polymorphonuclear leukocytes (PMNL), but sometimes the term PMNL is used to refer only to neutrophils. Activated granulocytes can migrate through vascular endothelium into tissues. Neutrophils can phagocytose foreign bodies, forming phagocytic vacuole, and in degranulation granules with their enzyme loads (myeloperoxidase, collagenase, etc) merge with the vacuole, producing reactive oxygen species. Neutrophils can also release the granule contents, including myeloperoxidase (MPO) and lactoferrin, to their surroundings to destroy larger targets and to attract more neutrophils to the site. Extracellular MPO induces endothelial dysfunction (Vita et al., 2004), and is involved already in the first steps of atherosclerosis (Nicholls & Hazen, 2005).
Lymphocytes consist of T lymphocytes (70-80% of lymphocytes in the blood) and B lymphocytes (10-15%), NK cells, and others. Lymphocyte precursors are formed in the bone marrow; T lymphocytes maturate mainly in the thymus, and B lymphocytes maturate in the marrow and lymphatic system. When antigens are presented for B cells, B cells transform into plasma cells which produce antibodies.
Eosinophils target parasites and cells infected with viruses, and together with basophils are involved in allergic reactions. The granules of eosinophils contain proteoglycan 2 (PRG2, major basic protein, MBP), which binds to heparan sulphate proteoglycans on cell membranes, killing the cells, and activating other WBCs. Basophils produce interleukin 4, which stimulates B lymphocytes to produce IgE. Platelets can form circulating complexes with eosinophils, priming eosinophils for adhesion to inflamed tissue via integrin interactions.
The number of monocytes in the blood is relatively low, since monocytes migrate into tissues where they transform into macrophages. Macrophages phagocytose cellular debris and pathogens, and present the antigens to lymphocytes. Tumour-associated macrophages (TAMs) may form half of the mass of tumours (Vinogradov et al., 2014). Activated macrophages are involved in organ transplant rejection and in inflammatory diseases such as arthritis, atherosclerosis, inflammatory bowel disease, and multiple sclerosis.
Numerous techniques have been developed for labelling white blood cells for inflammation imaging with isotopes for tracing the sites where WBCs accumulate (Ottobrini et al., 2011). For SPECT imaging, lipophilic compounds [99mTc]HMPAO and [111In]oxyquinoline are used to label autologous leukocytes (Love & Pellegrino, 2004). For PET imaging, in vitro [18F]FDG-labelled white blood cells are the most studied (Pellegrino et al., 2005). Labelled nanoparticles have been used to label genetically engineered T lymphocytes in vitro (Bhatnagar et al., 2014), but more often engineered T cells are transduced with retroviral vectors containg a reporter gene (Moroz et al., 2015). [89Zr]oxinate4 could be used for long-term tracking of leukocytes (Charoenphun et al., 2015). Usually, though, white blood cells are labelled in vivo, by administering radiopharmaceutical that binds to targets that are abundant on white blood cells, such as TSPO, chemokine receptors, and cellular adhesion molecules such as vascular adhesion protein 1, or using labelled monoclonal antibodies or affibodies.
Monocytes/macrophages and platelets have significant TSPO density, which is utilized in inflammation imaging. TSPO density can be altered in diseases and personality disorders (Turkheimer et al., 2015). [11C]PBR28 binding in blood cells correlates with VT in the brain (Kanegawa et al., 2016). TSPO tracers can be used to study treatment effect, for instance, a MPO inhibitor has been shown to reduce [11C]PBR28 uptake in the brains of subjects with Parkinson's disease.
Inflammation imaging using [18F]FDG PET is mainly based on the activated inflammatory cells that have an increased demand for glucose. Increased expression of glucose transporters and hexokinase results in increased uptake of [18F]FDG in the tissue crowded by activated macrophages and neutrophils.
Most cells utilize de novo pathway for DNA synthesis, but lymphoid organs and rapidly proliferating cells rely on the deoxyribonucleotide salvage pathway, where deoxyribonucleotides are converted to nucleotides by phosphorylation catalysed by nucleoside kinases. Inhibitors of this pathway can be used in treatment of cancer, and to selectively kill activated T cells and T lymphoblasts in immunosuppressive therapy.
Nucleoside analogues, such as [18F]FAC and [18F]F-AraG can be used to measure the activity of deoxyribonucleotide salvage pathway, and have shown promise in imaging lymphoid organs and immune activation, including lymphocyte infiltration in tissue (Radu et al., 2008; Brewer et al., 2010; Ronald et al., 2017; Salas et al., 2018; Levi et al., 2021).
Activated macrophages and neutrophils tend to eagerly phagocytose nanoparticles, which is used in imaging inflammation and atherosclerosis. Adenine nucleotides bind to metabotropic P2Y receptors and transmitter-gated ion-channel P2X receptors, which are abundant in monocytes and macrophages.
Many intracellular pathogens exploit the hosts response to apoptosis by exposing phosphatidylserine (PS) on their surface or by cloaking themselves in host cell derived PS-containing vesicles to facilitate binding, entry, and immune response evasion (apoptotic mimicry). PET radiopharmaceuticals developed to target PS may be useful on detecting these pathogens, although the response will not be specific, as PS is exposed also in normal apoptosis, even during non-microbial inflammation.
PET radiopharmaceuticals targeted for somatostatin receptors (SSTRs) can be used for inflammation imaging, because SSTRs are overexpressed on activated macrophages and fibroblasts. SSTR2 is exclusively expressed in proinflammatory macrophages, but not in other macrophage phenotypes, and not in monocytes, T or B lymphocytes, NK cells, platelets, neutrophils, or endothelial cells (Tarkin et al., 2017).
Folate receptor β is expressed on activated macrophages, but not in quiescent macrophages, providing a target for inflammation imaging. FRβ is localized in membrane caveolae, and it interacts with CD11b/CD18 to regulate cellular adhesion to collagen (Machacek et al., 2016).
Neutrophils express formyl peptide receptor (FPR), labelled peptides that bind to FPR have been used to visualize neutrophils in inflammation models.
Activated neutrophils can release of neutrophil elastase (NE) from azurophilic granules in high local concentrations. NE degrades elastin and other components of the extracellular matrix, allowing WBCs to traverse tissue, and cleaves proteins into signalling peptides promoting chemotaxis (Estrada et al., 2022). [11C]GW457427 ([11C]NES) can be used to assess neutrophil elastase in humans (Estrada et al., 2022).
Activated WBCs upregulate the expression of components of the cholinergic signalling pathway. Increased uptake of AChE marker [11C]donepezil and VAChT marker [18F]FEOBV have been seen in animal models and in humans with infections, inflammation, and cancer (Jørgensen et al., 2017).
P2X7 receptor is abundantly expressed on inflammatory cells, and may be largely responsible for the release of inflammatory cytokines of the interleukin family (Di Virgilio et al., 2017). Various inflammatory mediators, including IFN-γ and TNF-α, can upregulate P2X7R expression on macrophages and other cell types, while anti-inflammatory mediators, such as TGF-β1 and interleukins, can downregulate the expression (Bartlett et al., 2014). P2X7R radiotracers for neuroinflammation research are being developed.
Activated T cells express high affinity interleukin-2 receptors, which can be targeted using PET radioligands.
Plasma cells express cell surface glycoprotein CS1, which belongs to the signalling lymphocyte activating-molecule-related receptor family 7 (SLAMF7), and is important in immunomodulation. CS1 is also overexpressed in multiple myeloma cells, and can be targeted with for example elotuzumab, which has been labelled with 89Zr (Ghai et al., 2021).
- Inflammation and infection
- PET imaging of P2 purinoceptors
- Converting blood TAC to plasma TAC
Bain BJ. Blood Cells - A Practical Guide, 5th ed. Wiley Blackwell, 2015. ISBN: 978-1-118-81733-9.
Krams R, Bäck M (eds.): The ESC Textbook of Vascular Biology. Oxford University Press, 2017. ISBN-13: 9780198755777. doi: 10.1093/med/9780198755777.001.0001.
Updated at: 2022-03-12
Created at: 2019-01-31
Written by: Vesa Oikonen