CD19: One of The Most Important Targets for Car-T Cell Therapy

What is CD19

Understanding CD19: Definition and Role in the Immune System

CD19, also known as Cluster of Differentiation 19, is a cell surface protein belonging to the immunoglobulin superfamily. It serves as a crucial marker for B cells, playing a pivotal role in the human immune system. While predominantly found on the surface of B cells, CD19 is rarely expressed on other immune cells. Its significance lies in its involvement in B cell development and activation, making it a valuable target for research and clinical applications.

The Structural Composition of CD19

The human CD19 antigen is a 95 kd transmembrane glycoprotein that belongs to the immunoglobulin (Ig) superfamily [1-2]. It is encoded by the cd19 gene, which spans 7.41 kilobites and is located on the short arm of chromosome 16, specifically at 16p11.2 [3]. The gene consists of 15 exons and codes for the CD19 molecule comprising 556 amino acids. While there are multiple mRNA transcripts, only two transcript isoforms have been isolated in vivo [2-4]. Interestingly, the gene possesses an unusually short 5'-untranslated region. The proximal cd19 promoter lacks a TATA box, and the major start sites are located within 50 bp of the initiation codon [6].

CD19 is categorized as a type I transmembrane protein, characterized by a single transmembrane domain, a cytoplasmic C-terminus, and an extracellular N-terminus. It stands apart from other known proteins due to the absence of significant homology [7]. The extracellular component consists of two C2-type Ig-like domains, separated by a smaller potential disulfide-linked non-Ig-like domain. Additionally, the extracellular portion contains N-linked carbohydrate addition sites. The cytoplasmic domain, highly conserved, comprises 242 amino acids, with nine tyrosine residues located near the C-terminus [7-9]. Research suggests that CD19's biological functions rely on three cytoplasmic tyrosine residues, namely Y391, Y482, and Y513. Experimental studies have demonstrated that substituting phenylalanine for tyrosine at positions Y482 and Y513 leads to the inhibition of phosphorylation at the other seven tyrosines [10-11].

Fig.1 CD19 molecular structure[12]

Function of CD19

CD19 plays a critical role in establishing intrinsic B cell signaling thresholds by modulating both B cell receptor (BCR)-dependent and independent signaling [13-14]. It is actively involved in antigen-independent B cell development and immunoglobulin-induced B cell activation, making it essential for mounting an optimal immune response. CD19 forms a complex with the BCR and other surface molecules, facilitating direct and indirect recruitment and binding of various downstream protein kinases [15]. These kinases include members of the Src family (Lyn, Fyn), Ras family, Abl, Btk, adapter molecules (Vav, Grb2), and PI3K [9]. Recent findings highlight CD19's role in optimal MHC class II-mediated signaling, where it modulates tyrosine phosphorylation and Akt kinase signaling [16].

As a dominant signaling component, CD19 forms a multimolecular complex on the surface of mature B cells, along with complement receptor CD21 (CD2), tetraspanin membrane protein CD81 (TAPA-1), and CD225 [17] (Figure 2). The CD19 complex functions to lower the threshold for receptor-dependent signaling by regulating both intrinsic and receptor-induced signals [16]. As a critical co-receptor for BCR signal transduction, CD19 plays a vital role in the activation of protein tyrosine kinases (PTKs) required for BCR signaling. Notably, it recruits and amplifies the activation of Src-family protein tyrosine kinases such as Lyn and Fyn upon BCR activation [19]. Additionally, CD19 enhances BCR-induced signaling, crucial for B cell expansion, by recruiting and activating PI3K and downstream Akt kinases [20].

The CD19/CD21 complex can also independently react and bind to activated complement fragment C3d, leading to its translocation into membrane "lipid rafts" domains. In this location, tyrosine phosphorylated CD19 can interact with co-localized kinases in the membrane, modulating BCR signaling [21]. It is worth noting that more CD19 is typically associated with CD21 than in complex with the BCR. The simultaneous binding of surface antigen by B-cell Ig and C3d by CD21 dramatically increases B-cell activation. Studies have shown that CD19 does not necessarily require CD21 for signal transduction, as CD19/21 complex signaling can be initiated solely by the binding of C3d to CD21.

The Role of CD19-Complex in B Cell Activation

The CD19-complex, also known as the B cell co-receptor complex, is a group of cell surface proteins that play a critical role in B cell activation and signaling. It consists of several membrane proteins, including CD19, CD21, CD81, and [CD225]( (Leu-13).

1. CD21 (Complement receptor 2, CR2): CD21 acts as a receptor for complement fragments C3d and iC3b. It forms a complex with CD19 and CD81 on the B cell surface. The binding of C3d or iC3b to CD21 enhances B cell signaling, promoting B cell activation and antibody production.
2. CD81 (TAPA-1): CD81 is a tetraspanin protein that associates with CD19 and CD21, forming a stable complex on the B cell membrane. It plays a crucial role in the assembly and stabilization of the CD19-complex, contributing to efficient B cell signaling and activation.
3. CD225 (Leu-13): CD225 is another protein associated with the CD19-complex and plays a role in the regulation of B cell activation and signaling.

The CD19-complex serves as a critical signaling hub on the B cell surface, facilitating the transmission of signals from the B cell receptor (BCR) and complement receptors into the cell. This signaling cascade is essential for B cell activation, proliferation, and differentiation, ultimately leading to the production of antibodies and a robust immune response against pathogens.

Fig.2 CD19 associated signaling complex.[12]

CD19 Signaling Pathways in B Cell Activation

CD19 plays a central role in B cell activation through various signaling pathways:

1. B Cell Receptor (BCR) Signaling Pathway: CD19 acts as a crucial co-receptor in the BCR complex. When B cells encounter foreign antigens, the BCR binds to the antigen and becomes activated. Subsequently, the activated BCR forms a complex with CD19. CD19's activation triggers interactions with several signaling proteins within its intracellular phosphorylation domain, leading to the activation of important signaling pathways such as PI3K (phosphatidylinositol 3-kinase) and PLCγ2 (phospholipase Cγ2). These pathways work together to promote the proliferation and differentiation of B cells, ultimately generating an immune response.
2. CD19-PI3K Signaling Pathway: The association between CD19 and PI3K initiates the PI3K signaling pathway. Upon activation, PI3K generates the second messenger PIP3 (phosphatidylinositol triphosphate), which, in turn, activates Akt (protein kinase B). The activation of Akt is vital for supporting B cell survival and proliferation.
3. CD19-Btk Signaling Pathway: CD19 interacts with B cell-specific protein tyrosine kinase (Btk) and stimulates the Btk signaling pathway. Btk activation leads to the release of Ca2+ and the activation of PKC (protein kinase C). This process further enhances the activation of B cells and the subsequent immune response.
4. CD19-Vav Signaling Pathway: CD19 binds to Vav protein, initiating the Vav signaling pathway. Vav proteins activate small G proteins of the Rac/Rho family, which regulate cytoskeletal remodeling and cell migration.

The combined effect of these signaling pathways ensures the full activation of B cells, enabling their active participation in the immune response. The CD19 signaling pathway is of critical importance in the biological function of B cells and the proper functioning of the immune system. Moreover, dysregulation of CD19 signaling has been implicated in the development of B cell malignancies. Understanding these pathways is crucial for advancing our knowledge of B cell biology and developing targeted therapies for B cell-related diseases.

Fig.3 CD19 signal transduction pathway.[22]

Diseases Associated with CD19

CD19 is implicated in various significant diseases, including:

1. B-Cell Lymphoma: B-cell lymphoma, a type of leukemia and lymphoma arising from lymphoid tissue, shows overexpression of CD19 on the surface of B cells in certain patients. This heightened expression of CD19 makes it a valuable target for treating these diseases. CAR-T cell therapy, an immunotherapy, has demonstrated efficacy in treating B-cell lymphoma by genetically modifying the patient's T cells to carry CD19-specific receptors. These engineered T cells selectively recognize and eliminate CD19-positive B cells.
2. Chronic Lymphocytic Leukemia (CLL): In CLL, a type of chronic lymphocytic leukemia, the majority of patients display CD19 on the surface of leukemia cells. Consequently, CAR-T cell therapy targeting CD19 has gained widespread use as a treatment for CD19-positive CLL, showing promising therapeutic outcomes.
3. Autoimmune Diseases: Autoimmune diseases stem from the immune system's attack on its own healthy tissues and organs. Conditions like rheumatoid arthritis and systemic lupus erythematosus involve abnormal activation and function of B cells, and CD19 might play a role in the pathogenesis of these diseases.
4. B Cell Immune Deficiency: CD19 deficiency can lead to impaired B cell function, thereby affecting the immune system's normal operations. Notably, CD19 deficiency has been linked to some primary immunodeficiencies.

Clinical Significance of CD19 Detection

The detection of CD19 holds crucial clinical significance in various aspects:

1. Diagnosis of B-Cell Lymphoma and Chronic Lymphocytic Leukemia: CD19 serves as a surface marker for B cells, and since B-cell lymphoma and chronic lymphocytic leukemia are diseases caused by malignant B cells, clinicians can aid in diagnosing these conditions by assessing the expression level of CD19 in patients' body fluids or tissues. This diagnostic approach plays a pivotal role in guiding treatment decisions and evaluating disease progression.
2. Predicting Treatment Response: In CD19-related diseases like B-cell lymphoma and chronic lymphocytic leukemia, the expression level of CD19 in patients can function as a biomarker to forecast their response to treatment. Studies have indicated that CD19-positive patients tend to exhibit a more favorable therapeutic response to CAR-T cell therapy, as this revolutionary therapy targets CD19-positive malignant B cells.
3. Guiding the Application of CAR-T Cell Therapy: CAR-T cell therapy is an innovative technique that selectively identifies and attacks CD19-positive B cells. By genetically modifying a patient's own T cells to carry CD19-specific receptors, this immunotherapy has shown remarkable promise. Prior to administering CAR-T cell therapy, evaluating the expression level of CD19 in patients helps determine the feasibility and effectiveness of the treatment.
4. Monitoring Therapeutic Effect and Prognosis Evaluation: For patients undergoing CAR-T cell therapy, monitoring the expression level of CD19 allows for the assessment of treatment effectiveness. A decrease in CD19-positive cells after therapy indicates positive treatment outcomes. Furthermore, the expression level of CD19 correlates with patient prognosis, with high CD19 expression potentially indicating a poorer prognosis for the disease.

CD19 Protein

Recombinant Human B-Lymphocyte Antigen Cd19 (CD19) Protein (His&Myc)

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