Cancer-Derived Immunoglobulin G and Pancreatic Cancer.

Immunoglobulin (Ig) is traditionally believed to be produced solely by B cells. Nonetheless, mounting evidence has demonstrated that various types of Igs are extensively expressed in many cell types. Among them, IgG is found to be highly expressed in cancer cells and is thus labeled as cancer-derived IgG. Cancer-derived IgG shares identical fundamental structures with B cell-derived IgG, but displays several unique characteristics, including restricted variable region sequences and unique glycosylation modifications for those expressed by epithelial cancers. Cancer-derived IgG plays multiple crucial roles in carcinogenesis, including facilitating cancer invasion and metastasis, enhancing cancer stemness, contributing to chemoresistance, and remodeling the tumour microenvironment. Recent studies have discovered that cancer-derived sialylated IgG (SIA-IgG) is extensively expressed in pancreatic cancer cells and is predominantly located in the cytoplasm and on the cell membrane. Cancer-derived IgG expressed by pancreatic cancer presents a restrictive variable region sequence and contains a unique sialylation site of the Fab region. Functionally, cancer-derived IgG participates in pancreatic cancer progression via different mechanisms, such as promoting proliferation, facilitating migration and invasion, resisting apoptosis, inducing inflammation, and modulating the tumour microenvironment. SIA-IgG has shown potential as a clinical biomarker. The expression of SIA-IgG is associated with poor tumour differentiation, metastasis, and chemoresistance in pancreatic cancer. High expression of SIA-IgG can serve as an independent prognostic factor for pancreatic cancer. Additionally, SIA-IgG expression elevated with malignant progression for the precursor lesions of pancreatic cancer. These findings present a prospect of applying cancer-derived IgG as a novel diagnostic and therapeutic target in the management of pancreatic cancer, and aiding in overcoming the challenge in the treatment of this stubborn malignancy.

Non B Cell-Derived Immunoglobulins in Lung Epithelial Cells and Lung Cancer.

As the locus for air exchange, lung tissue is perpetually exposed to a significant quantity of foreign pathogens. Consequently, lung has developed a refined and intricate immune system. Beyond their physical and chemical barrier roles, lung epithelial cells can contribute to immune defence through the expression of Toll-like receptors (TLRs) and other pattern recognition receptors, along with the secretion of cytokines. Emerging evidence demonstrates that lung epithelial cells can generate and secrete immunoglobulins (Igs), including IgM, IgA, or IgG, thus performing antibody function. Moreover, malignantly transformed lung epithelial cells have been discovered to produce high levels of Ig, predominantly IgG, which do not fulfill the role of antibodies, but instead carries out tumour-promoting activity. Structural analysis has indicated that the biological activity of IgG produced by lung cancer cells differs from that of Igs produced by normal lung epithelial cells due to the unique glycosylation modification. Specifically, the sialylated IgG (SIA-IgG), characterised by a non-traditional N-glycosylation modification at the Asn162 site of Igγ CH1, is highly expressed in tumour stem cells. It has been demonstrated that SIA-IgG relies on this unique sialylation modification to promote tumorigenesis, metastasis, and immune evasion. Current results have proven that the Ig produced by lung epithelial cells has multifaceted biological activities, including immune defence functions under physiological conditions, while acquiring tumour-promoting activity during malignant transformation. These insights possess potential for the diagnosis and treatment of lung cancer as novel biomarkers and targets.

Expression and Function of Mammary Epithelial Cell-Derived Immunoglobulins.

Over the past 20 years, increasing evidence has demonstrated that immunoglobulins (Igs) can be widely generated from non B cells, including normal and malignant mammary epithelial cells. In normal breast tissue, the expression of IgG and IgA has been identified in epithelial cells of mammary glands during pregnancy and lactation, which can be secreted into milk, and might participate in neonatal immunity. On the other hand, non B-IgG is highly expressed in breast cancer cells, correlating with the poor prognosis of patients with breast cancer. Importantly, a specific group of IgG, bearing a unique N-linked glycan on the Asn162 site and aberrant sialylation modification at the end of the novel glycan (referred to as sialylated IgG (SIA-IgG)), has been found in breast cancer stem/progenitor-like cells. SIA-IgG can significantly promote the capacity of migration, invasiveness, and metastasis, as well as enhance self-renewal and tumorigenicity in vitro and in vivo. These findings suggest that breast epithelial cells can produce Igs with different biological activities under physiological and pathological conditions. During lactation, these Igs could be the main source of milk Igs to protect newborns from pathogenic infections, while under pathological conditions, they display oncogenic activity and promote the occurrence and progression of breast cancer.

Sialylated IgG in epithelial cancers inhibits antitumor function of T cells via Siglec-7.

Although effective, immune checkpoint blockade induces response in only a subset of cancer patients. There is an urgent need to discover new immune checkpoint targets. Recently, it was found that a class of sialic acid-binding immunoglobulin-like lectins (Siglecs) expressed on the surface of T cells in cancer patients inhibit T cell activation through their intracellular immunosuppressive motifs by recognizing sialic acid-carrying glycans, sialoglycans. However, ligands of Siglecs remain elusive. Here, we report sialylated IgG (SIA-IgG), a ligand to Siglec-7, that is highly expressed in epithelial cancer cells. SIA-IgG binds Siglec-7 directly and inhibits TCR signals. Blocking of either SIA-IgG or Siglec-7 elicited potent antitumor immunity in T cells. Our study suggests that blocking of Siglec-7/SIA-IgG offers an opportunity to enhance immune function while simultaneously sensitizing cancer cells to immune attack.