Complement component C3 plays a central role in activating the complement system and is the most abundant complement protein in human plasma, primarily synthesized in the liver. As part of the innate immune system, the complement system is activated during tissue damage and pathogen invasion, playing a crucial role in the inflammatory response, host homeostasis, and pathogen defense. The complement cascade is activated through the classical pathway, alternative pathway, and lectin pathway, all of which generate C3 convertase, which cleaves C3 into C3a and C3b. C3a is a potent anaphylatoxin with pro-inflammatory activity, while C3b is a regulator that induces C5 cleavage, thereby participating in the dissolution and clearance of immune complexes. Mutations in this gene are associated with atypical hemolytic uremic syndrome (aHUS) and age-related macular degeneration (AMD). Deficiencies in C3 and C3-derived peptides can lead to autoimmune diseases (such as rheumatoid arthritis, systemic lupus erythematosus, and vasculitis) and make individuals susceptible to recurrent respiratory infections and infections caused by encapsulated organisms. Conversely, excessive activation of C3 and related complement components is associated with kidney diseases (immune complex glomerulonephritis, hemolytic uremic syndrome, lupus nephritis, membranous nephropathy, and immune-mediated nephropathy) ^[1-2].

The Transferrin receptor (TFRC) gene encodes Transferrin Receptor 1 (TFR1), a protein that is expressed at low levels in most normal cells but shows increased expression in highly proliferative cells, such as basal epidermal cells, intestinal epithelium, and certain activated immune cells. Brain capillary endothelial cells, which constitute the blood-brain barrier (BBB), also express this receptor at high levels ^[3]. TFR1 plays a critical role in maintaining iron metabolism and homeostasis by facilitating receptor-mediated endocytosis of iron-bound transferrin (Tf) via Tf cycling, thereby promoting iron uptake ^[4]. Cellular iron deficiency can lead to apoptosis, while cellular transformation requires substantial iron to sustain proliferation, with iron overload contributing to tumor progression. The high expression of TFR1 in many tumors makes it a potential tumor marker, offering a target for therapies to inhibit tumor growth and metastasis ^[3]. Moreover, TFR1 is implicated in anemia and iron metabolism disorders. Studies have shown that elevated TFR1 expression in cardiomyocytes is associated with exacerbated inflammation in myocarditis patients ^[5]. Various clinical drugs targeting TFR1 are currently under development, including antisense oligonucleotides (ASOs), antibody-drug conjugates (ADCs), and antibody-oligonucleotide conjugates, applicable to diseases such as cancer, anemia, and neurodegenerative disorders. Research indicates that enhancing antibody transport across the blood-brain barrier via TFR1, by forming specific bispecific antibodies with anti-β-amyloid antibodies, can improve therapeutic outcomes in Alzheimer's patients ^[6-7]. As research progresses, TFR1 is expected to become an effective clinical target for multiple diseases and a synergistic target for drug delivery across the blood-brain barrier (BBB).

The B6-hC3/hTFRC(CDS) mouse model is a humanized model obtained by breeding B6-hC3 mice (Catalog No.: I001135) with B6-hTFRC(CDS) mice (Catalog No.: C001584). This model can be used for research on complement-mediated diseases, iron metabolism disorders, neurodegenerative diseases, and tumor development, aiding in studying C3/TFRC-targeted drugs.