Cancerous tumors consist of transformed cells that are surrounded by stromal cells (or healthy, normal host cells) consisting of cells such as the fibroblasts, endothelial cells, various myeloid cells, and the extracellular matrix (1). The various cell types in the tumor interact with each other constantly so as to maintain homeostasis in the lesion. In a cancerous tumor, the stromal cells produce ligands that assist in the maintenance and growth of the tumor (e.g., by inducing angiogenesis), and the neoplastic cells reciprocate by generating factors that help in the survival, growth, and proliferation of the stromal cells (2). In addition, a large number of macrophages infiltrate and reside in the tumor stroma (known as tumor-associated macrophages; TAMs), and these cells are also known to promote growth and proliferation of the tumor because they produce factors that are inflammatory, pro-angiogenic, and immunosuppressive, and that assist in the metastasis of the cancer (3). On the basis of their gene and protein expression profile, two distinct types of TAMs are found in the tumor stroma. The TAMs either have a low expression of the major histocompatibility complex class II (MHC II) (MHClow TAMs) or express high levels of MHC II (MHChi TAMs). The MHClow TAMs are present mainly in the hypoxic areas of the tumor, are strong promoters of angiogenesis, and overexpress the macrophage mannose receptor (MMR, CD206), an endocyte C-type lectin receptor (3). The MHChi TAMs are found primarily in the perivascular regions of a cancerous lesion and exhibit a low angiogenic potential (3). Therefore, MMR could be a valuable target for the noninvasive imaging of MMR+ stromal cells with single-photon emission computed tomography (SPECT) to visualize tumor stroma, and the stromal cells could be targeted to treat neoplastic lesions.
There is a great deal of interest to develop imaging probes that are based on a nanobody (Nb) scaffold because Nbs are miniature antibodies (Ab) (~15 kDa compared with ~150 kDa for a normal Ab) that are rapidly eliminated through the kidneys and, if labeled with a radionuclide, generate a high signal/noise ratio at the target site (4). To learn about the unique source, structure, properties, and application of Nbs, see Vaneycken et al. (5). Movahedi et al. generated an anti-MMR Nb, labeled it with 99mTc, and showed that the radiolabeled Nb ([99mTc]MMR Nb) can be used to detect MMR+ stromal cells with SPECT in mice bearing either TS/A cell tumors (a mouse mammary adenocarcinoma cell line) or 3LL-R cell tumors (a C57BL/6 Lewis lung carcinoma cell line) (3, 4).