CD33 (Siglec-3) Signaling Pathways and Disease Mechanisms: Roles in Acute Myeloid Leukemia, Alzheimer’s Disease and Immune Regulation

1. Research Background of CD33 in Immune Regulation: From Siglec Family to Disease Target

CD33 (Sialic acid-binding immunoglobulin-like lectin-3, Siglec-3) belongs to the Siglec family, which recognizes sialic acid-modified glycan structures and plays an important negative regulatory role in the immune system ^[1],^[2]. CD33 is mainly expressed in myeloid cells, including monocytes, macrophages, dendritic cells, and myeloid progenitor cells, and is of great significance in regulating innate immune responses and maintaining immune homeostasis.

The intracellular domain of CD33 contains a typical immunoreceptor tyrosine-based inhibitory motif (ITIM). When CD33 binds to its ligand, this domain is phosphorylated and recruits SH2 domain-containing phosphatases such as SHP-1 and SHP-2, thereby inhibiting the activation of various cellular signaling pathways ^[3]. This inhibitory signaling mechanism makes CD33 an important regulatory factor in myeloid cell immune responses.

In recent years, with advancements in immunology, neurobiology, and tumor research, the role of CD33 in various diseases has gradually gained attention. Studies have found that approximately 80%--90% of leukemia cells in acute myeloid leukemia (AML) express CD33, making this molecule an important therapeutic target ^[4]. Simultaneously, genome-wide association studies have shown that CD33 gene polymorphisms are significantly associated with Alzheimer's disease risk, with its mechanism closely related to microglial function regulation ^[5]. Additionally, CD33 participates in regulating the immunosuppressive function of myeloid cells in chronic viral infections and tumor immune microenvironments. Therefore, systematic study of CD33 signaling mechanisms and their disease correlations is of great significance for understanding immune regulatory networks and developing new therapeutic strategies.

2. Molecular Biological Characteristics of CD33: Structure, Function, and Splice Variants

CD33 is a type I transmembrane glycoprotein whose structure consists mainly of an extracellular domain, transmembrane domain, and intracellular domain. The extracellular domain contains two immunoglobulin-like domains: the distal IgV domain and the proximal membrane IgC2 domain. The IgV domain recognizes sialylated glycan ligands and is the key structural element for CD33 ligand recognition. The transmembrane domain anchors CD33 to the cell membrane and maintains its spatial conformational stability.

The intracellular domain of CD33 contains a conserved ITIM sequence. When CD33 binds to its ligand and becomes activated, tyrosine residues in this sequence are phosphorylated, subsequently recruiting SHP-1, SHP-2, and other phosphatases, thereby inhibiting the activation of multiple cell signaling pathways ^[3]. This inhibitory signaling mechanism is of great importance in the immune system, preventing excessive inflammatory responses and maintaining immune balance.

At the post-transcriptional regulatory level, the CD33 gene also exhibits multiple splice variants. The most common are full-length CD33-FL and CD33-Δ2, which lacks exon 2. Since exon 2 encodes the IgV domain, its deletion prevents CD33 from effectively binding sialic acid ligands, thereby altering receptor function. Studies have shown that genetic variants associated with Alzheimer's disease protection promote the expression of CD33-Δ2, thereby reducing functional CD33 levels and enhancing microglial phagocytic capacity ^[5].

CD33 is primarily expressed in myeloid cells, with expression levels dynamically changing during hematopoietic differentiation. Early myeloid progenitor cells show higher expression levels, while mature granulocytes exhibit relatively lower expression. This expression pattern reflects the important role of CD33 in myeloid cell differentiation and immune regulation.

3. CD33-Related Signaling Pathways: ITIM-Mediated Immune Regulatory Mechanisms

3.1 CD33's ITIM-Mediated Inhibitory Signaling Pathway

The most classic signal transduction mechanism of CD33 is through its intracellular ITIM domain-mediated inhibitory signaling. When CD33 recognizes sialylated ligands on the cell surface, its intracellular ITIM sequence is phosphorylated by Src family kinases. Subsequently, SH2 domain-containing protein phosphatases SHP-1 or SHP-2 are recruited to this domain and inhibit the activation of various downstream signaling molecules through dephosphorylation ^[3]. This mechanism reduces the activation level of immune cells, thereby avoiding excessive inflammatory responses.

In myeloid cells, this signaling pathway can inhibit multiple inflammation-related signaling molecules, such as Src kinases, MAPK pathways, and NF-κB signaling pathways. Through this mechanism, CD33 can regulate the inflammatory response levels of macrophages and dendritic cells, thereby maintaining immune system homeostasis.

3.2 CD33 and Microglial Signal Regulation: Key Mechanisms in Alzheimer's Disease

In the central nervous system, CD33 is mainly expressed in microglia. Microglia are the primary immune cells in the brain, responsible for clearing pathogens and abnormal proteins. Studies have shown that CD33 primarily plays an inhibitory regulatory role in microglia. When CD33 expression increases, the phagocytic activity of microglia significantly decreases, thereby reducing their ability to clear β-amyloid protein.

In this process, there is a functional antagonistic relationship between CD33 and the TREM2 signaling pathway. TREM2 promotes microglial activation and enhances phagocytic function, while CD33 reduces cellular activity by inhibiting signal transduction. When CD33 expression increases or signaling is enhanced, microglial phagocytic function is inhibited, thereby promoting β-amyloid deposition and accelerating the progression of neurodegenerative lesions.

3.3 CD33's Signaling Role in the Tumor Immune Microenvironment: Functional Regulation of Myeloid-Derived Suppressor Cells

Recent studies have shown that CD33 also plays an important role in the tumor immune microenvironment. Tumor tissues are often enriched with large numbers of myeloid-derived suppressor cells (MDSCs), which promote tumor immune escape by inhibiting T cell activity. MDSCs typically express high levels of CD33, and their signaling pathways can enhance immunosuppressive function and inhibit anti-tumor immune responses.

Activation of CD33 signaling can regulate the expression of various immune regulatory factors, such as inhibitory cytokines and immune checkpoint molecules, thereby further strengthening the tumor immunosuppressive environment. Therefore, CD33 is considered an important molecule in regulating the tumor immune microenvironment.

4. CD33-Related Diseases: From Hematological Tumors to Neurodegenerative Diseases

4.1 Acute Myeloid Leukemia (AML): CD33 as an Important Therapeutic Target

Acute myeloid leukemia is one of the most extensively studied diseases involving CD33. AML is a malignant hematological tumor originating from myeloid progenitor cells, characterized by the proliferation of abnormal leukemia cells in the bone marrow and peripheral blood. Studies have shown that the vast majority of AML patients express CD33 molecules on the surface of their leukemia cells ^[4]. This feature makes CD33 an important molecular marker for AML diagnosis, classification, and targeted therapy.

In the pathogenesis of AML, CD33 not only exists as a surface antigen but may also participate in regulating the proliferation and differentiation of leukemia cells. Through CD33-targeted antibodies or immunotherapeutic approaches, cytotoxic drugs can be selectively delivered to leukemia cells, thereby improving treatment efficacy. However, since normal myeloid cells also express CD33, this may lead to side effects such as bone marrow suppression.

4.2 Alzheimer's Disease (AD): CD33 Gene Polymorphisms and Disease Risk

Alzheimer's disease is a common neurodegenerative disease whose main pathological features include β-amyloid deposition and neuronal degeneration. Recent genomic studies have shown that CD33 is an important risk gene associated with late-onset Alzheimer's disease ^[5].

In the nervous system, CD33 is mainly expressed in microglia. Research has found that CD33 can inhibit the phagocytic capacity of microglia for β-amyloid protein, thereby affecting its clearance efficiency. When CD33 expression levels increase, microglial phagocytic function decreases, β-amyloid deposition increases, thereby promoting the development of neurodegenerative lesions.

Furthermore, CD33 gene splicing variants are also closely related to disease risk. Some genetic variants can promote the expression of the CD33-Δ2 form, thereby reducing functional CD33 production and enhancing microglial clearance capacity, which is considered an important mechanism for its protective effect against Alzheimer's disease.

4.3 Chronic Viral Infections: CD33's Role in Immune Regulation

In chronic viral infectious diseases such as hepatitis B, CD33 also participates in immune regulatory processes. Research has shown that CD33 can inhibit the antigen presentation function of dendritic cells and reduce T cell activation levels, thereby promoting viral persistence. Blocking CD33 signaling can, to some extent, restore immune cell activity and enhance antiviral immune responses.

4.4 Tumor Immune Regulation: Synergistic Action of CD33 and Myeloid-Derived Suppressor Cells

In various solid tumors, CD33 also participates in the formation of the tumor immune microenvironment. Tumor tissues are often enriched with myeloid-derived suppressor cells, which promote tumor growth by secreting immunosuppressive factors and inhibiting T cell function. Since these cells typically express high levels of CD33, this molecule is considered an important target for regulating tumor immunosuppression.