CD19

Last updated
CD19
Identifiers
Aliases CD19 , B4, CVID3, CD19 molecule
External IDs OMIM: 107265 MGI: 88319 HomoloGene: 1341 GeneCards: CD19
Orthologs
SpeciesHumanMouse
Entrez
Ensembl
UniProt
RefSeq (mRNA)

NM_001178098
NM_001770
NM_001385732

NM_009844
NM_001357091

RefSeq (protein)

NP_001171569
NP_001761

NP_033974
NP_001344020

Location (UCSC) Chr 16: 28.93 – 28.94 Mb Chr 7: 126.01 – 126.01 Mb
PubMed search [3] [4]
Wikidata
View/Edit Human View/Edit Mouse

B-lymphocyte antigen CD19, also known as CD19 molecule (Cluster of Differentiation 19), B-Lymphocyte Surface Antigen B4, T-Cell Surface Antigen Leu-12 and CVID3 is a transmembrane protein that in humans is encoded by the gene CD19. [5] [6] In humans, CD19 is expressed in all B lineage cells. [7] [8] Contrary to some early doubts, human plasma cells do express CD19, [9] as confirmed by others. [10] CD19 plays two major roles in human B cells: on the one hand, it acts as an adaptor protein to recruit cytoplasmic signaling proteins to the membrane; on the other, it works within the CD19/CD21 complex to decrease the threshold for B cell receptor signaling pathways. Due to its presence on all B cells, it is a biomarker for B lymphocyte development, lymphoma diagnosis and can be utilized as a target for leukemia immunotherapies. [8]

Contents

Structure

In humans, CD19 is encoded by the 7.41 kilobase CD19 gene located on the short arm of chromosome 16. [11] [12] It contains at least fifteen exons, four that encode extracellular domain and nine that encode cytoplasmic domains, with a total of 556 amino acids. [12] Experiments show that there are multiple mRNA transcripts; however, only two have been isolated in vivo . [11]

CD19 is a 95 kd Type I transmembrane glycoprotein in the immunoglobulin superfamily (IgSF) with two extracellular C2-set Ig-like domains and a relatively large, 240 amino acid, cytoplasmic tail that is highly conserved among mammalian species. [11] [13] [14] [15] The extracellular C2-type Ig-like domains are divided by a potential disulfide linked non-Ig-like domain and N-linked carbohydrate addition sites. [14] [16] The cytoplasmic tail contains at least nine tyrosine residues near the C-terminus. [11] [14] Within these residues, Y391, Y482, and Y513 have been shown to be essential to the biological functions of CD19. [17] Phenylalanine substitution for tyrosine at Y482 and Y513 leads to the inhibition of phosphorylation at the other tyrosines. [11] [18]

Expression

CD19 is widely expressed during all phases of B cell development until terminal differentiation into plasma cells. During B cell lymphopoiesis, CD19 surface expression starts during immunoglobulin (Ig) gene rearrangement, which coincides during B lineage commitment from hematopoietic stem cell. [8] Throughout development, the surface density of CD19 is highly regulated. [11] CD19 expression in mature B cells is threefold higher than that in immature B cells. [11] CD19 is expressed on all normal, mitogen-stimulated, and malignant B cells, excluding plasma cells[ inconsistent ]. CD19 expression is even maintained in B lineage cells that undergo neoplastic transformation. [7] [18] Because of its ubiquity on all B cells, it can function as a B cell marker and a target for immunotherapies targeting neoplastic lymphocytes. [8] [11]

Function

Role in development & survival

Decisions to live, proliferate, differentiate, or die are continuously being made during B cell development. [19] These decisions are tightly regulated through B cell receptor (BCR) interactions and signaling. The presence of a functional BCR is necessary during antigen-dependent differentiation and for continued survival in the peripheral immune system. [14] Essential to the functionality of a BCR is the presence of CD19. [20] Experiments using CD19 knockout mice found that CD19 is essential for B cell differentiative events including the formation of B-1, germinal center, and marginal zone (MZ) B cells. [14] [21] [22] Analysis of mixed bone marrow chimeras suggest that prior to an initial antigen encounter, CD19 promotes the survival of naive recirculating B cells and increases the in vivo life span of B cells in the peripheral B cell compartment. [23] Ultimately, CD19 expression is integral to the propagation of BCR-induced survival signals and the maintenance of homeostasis through tonic signaling.

BCR-independent

Paired box transcription factor 5 (PAX5) plays a major role in B cell differentiation from pro B cell to mature B cell, the point at which the expression of non-B-lineage genes is permanently blocked. [23] [24] [25] Part of B cell differentiation is controlling c-MYC protein stability and steady-state levels through CD19, which acts as a PAX5 target and downstream effector of the PI3K-AKT-GSK3β axis. CD19 signaling, independent of BCR functions, increases c-MYC protein stability. Using a loss of function approach, researchers found reduced MYC levels in B cells of CD19 knockdown mice. [23] CD19 signaling involves the recruitment and activation of phosphoinositide 3-kinase (PI3K) and later downstream, the activation of protein kinase B (Akt). The Akt-GSK3β axis is necessary for MYC activation by CD19 in BCR-negative cells, with higher levels of Akt activation corresponding to higher levels of MYC. [23] [26] CD19 is a crucial BCR-independent regulator of MYC-driven neoplastic growth in B cells since the CD19-MYC axis promotes cell expansion in vitro and in vivo. [23] [26]

CD19/CD21 complex

On the cell surface, CD19 is the dominant signaling component of a multimolecular complex including CD21 (CR2, a complement receptor), TAPA-1 (a tetraspanin membrane protein), and CD225. [11] [23] The CD19/CD21 complex arises from C3d binding to CD21; however, CD19 does not require CD21 for signal transduction. CD81, attached to CD19, is a part of the tetraspanin web, acts as a chaperone protein, and provides docking sites for molecules in various different signal transduction pathways. [11]

BCR-dependent

While colligated with the BCR, the CD19/CD21 complex bound to the antigen-complement complex can decrease the threshold for B cell activation. CD21, complement receptor 2, can bind fragments of C3 that have covalently attached to glycoconjugates by complement activation. [27] Recognition of an antigen by the complement system enables the CD19/CD21 complex and associated intracellular signaling molecules to crosslink to the BCR. This results in phosphorylation of the cytoplasmic tail of CD19 by BCR-associated tyrosine kinases, ensuing is the binding of additional Src-family kinases, augmentation of signaling through the BCR, and recruitment of PI3K. The localization of PI3K initiates another signaling pathway leading to Akt activation. Varying expression of CD19 on the cell surface modulates tyrosine phosphorylation and Akt kinase signaling and by extension, MHC class II mediated signaling. [11]

Activated spleen tyrosine kinase (Syk) leads to phosphorylation of the scaffold protein, BLNK, which provides multiple sites for tyrosine phosphorylation and recruits SH2-containing enzymes and adaptor proteins that can form various multiprotein signaling complexes. In this way, CD19 can modulate the threshold for B cell activation. This is important during primary immune response, prior to affinity maturation, amplifying the response of low affinity BCRs to low concentrations of antigen. [11] [27]

Interactions

CD19 has been shown to interact with:

In disease

Autoimmunity & immunodeficiency

Mutations in CD19 are associated with severe immunodeficiency syndromes characterized by diminished antibody production. [28] [29] Additionally, mutations in CD21 and CD81 can also underlie primary immunodeficiency due to their role in the CD19/CD21 complex formation. [30] These mutations can lead to hypogammaglobulinaemia as a result of poor response to antigen and defective immunological memory. [31] Researchers found changes in the constitution of B lymphocyte population and reduced amounts of switched memory B cells with high terminal differentiation potential in patients with Down Syndrome. [32] CD19 has also been implicated in autoimmune diseases, including rheumatoid arthritis and multiple sclerosis, and may be a useful treatment target. [13] [16] [33]

Mouse model research shows that CD19 deficiency can lead to hyporesponsiveness to transmembrane signals and weak T cell dependent humoral response, that in turn leads to an overall impaired humoral immune response. [21] [22] Additionally CD19 plays a role in modulating MHC Class II expression and signaling, which can be affected by mutations. CD19 deficient B cells exhibit selective growth disadvantage; therefore, it is rare for CD19 to be absent in neoplastic B cells, as it is essential for development. [23]

Cancer

Since CD19 is a marker of B cells, the protein has been used to diagnose cancers that arise from this type of cell - notably B cell lymphomas, acute lymphoblastic leukemia (ALL), and chronic lymphocytic leukemia (CLL). [8] The majority of B cell malignancies express normal to high levels of CD19. The most current experimental anti-CD19 immunotoxins in development work by exploiting the widespread presence of CD19 on B cells, with expression highly conserved in most neoplastic B cells, to direct treatment specifically towards B-cell cancers. [13] [34] However, it is now emerging that the protein plays an active role in driving the growth of these cancers, most intriguingly by stabilizing the concentrations of the MYC oncoprotein. This suggests that CD19 and its downstream signaling may be a more attractive therapeutic target than initially suspected. [23] [26]

CD19-targeted therapies based on T cells that express CD19-specific chimeric antigen receptors (CARs) have been utilized for their antitumor abilities in patients with CD19+ lymphoma and leukemia, first against Non-Hodgkin's Lymphoma (NHL), then against CLL in 2011, and then against ALL in 2013. [8] [35] [36] [37] CAR-19 T cells are genetically modified T cells that express a targeting moiety on their surface that confers T cell receptor (TCR) specificity towards CD19+ cells. CD19 activates the TCR signaling cascade that leads to proliferation, cytokine production, and ultimately lysis of the target cells, which in this case are CD19+ B cells. CAR-19 T cells are more effective than anti-CD19 immunotoxins because they can proliferate and remain in the body for a longer period of time. This comes with a caveat since now CD19 immune escape facilitated by splice variants, point mutations, and lineage switching can form as a major form of therapeutic resistance for patients with ALL. [38]

Related Research Articles

<span class="mw-page-title-main">B cell</span> Type of white blood cell

B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or inserted into the plasma membrane where they serve as a part of B-cell receptors. When a naïve or memory B cell is activated by an antigen, it proliferates and differentiates into an antibody-secreting effector cell, known as a plasmablast or plasma cell. In addition, B cells present antigens and secrete cytokines. In mammals, B cells mature in the bone marrow, which is at the core of most bones. In birds, B cells mature in the bursa of Fabricius, a lymphoid organ where they were first discovered by Chang and Glick, which is why the B stands for bursa and not bone marrow, as commonly believed.

<span class="mw-page-title-main">T-cell receptor</span> Protein complex on the surface of T cells that recognises antigens

The T-cell receptor (TCR) is a protein complex found on the surface of T cells, or T lymphocytes, that is responsible for recognizing fragments of antigen as peptides bound to major histocompatibility complex (MHC) molecules. The binding between TCR and antigen peptides is of relatively low affinity and is degenerate: that is, many TCRs recognize the same antigen peptide and many antigen peptides are recognized by the same TCR.

<span class="mw-page-title-main">CD23</span> Low-affinity" receptor for IgE

CD23, also known as Fc epsilon RII, or FcεRII, is the "low-affinity" receptor for IgE, an antibody isotype involved in allergy and resistance to parasites, and is important in regulation of IgE levels. Unlike many of the antibody receptors, CD23 is a C-type lectin. It is found on mature B cells, activated macrophages, eosinophils, follicular dendritic cells, and platelets.

<span class="mw-page-title-main">P110δ</span> Protein-coding gene in the species Homo sapiens

Phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit delta isoform also known as phosphoinositide 3-kinase (PI3K) delta isoform or p110δ is an enzyme that in humans is encoded by the PIK3CD gene.

Co-stimulation is a secondary signal which immune cells rely on to activate an immune response in the presence of an antigen-presenting cell. In the case of T cells, two stimuli are required to fully activate their immune response. During the activation of lymphocytes, co-stimulation is often crucial to the development of an effective immune response. Co-stimulation is required in addition to the antigen-specific signal from their antigen receptors.

<span class="mw-page-title-main">B-cell receptor</span> Transmembrane protein on the surface of a B cell

The B-cell receptor (BCR) is a transmembrane protein on the surface of a B cell. A B-cell receptor is composed of a membrane-bound immunoglobulin molecule and a signal transduction moiety. The former forms a type 1 transmembrane receptor protein, and is typically located on the outer surface of these lymphocyte cells. Through biochemical signaling and by physically acquiring antigens from the immune synapses, the BCR controls the activation of the B cell. B cells are able to gather and grab antigens by engaging biochemical modules for receptor clustering, cell spreading, generation of pulling forces, and receptor transport, which eventually culminates in endocytosis and antigen presentation. B cells' mechanical activity adheres to a pattern of negative and positive feedbacks that regulate the quantity of removed antigen by manipulating the dynamic of BCR–antigen bonds directly. Particularly, grouping and spreading increase the relation of antigen with BCR, thereby proving sensitivity and amplification. On the other hand, pulling forces delinks the antigen from the BCR, thus testing the quality of antigen binding.

<span class="mw-page-title-main">Complement receptor 2</span> Protein found in humans

Complement receptor type 2 (CR2), also known as complement C3d receptor, Epstein-Barr virus receptor, and CD21, is a protein that in humans is encoded by the CR2 gene.

<span class="mw-page-title-main">CD20</span> Mammalian protein found in Homo sapiens

B-lymphocyte antigen CD20 or CD20 is expressed on the surface of all B-cells beginning at the pro-B phase and progressively increasing in concentration until maturity.

<span class="mw-page-title-main">ZAP70</span> Protein-coding gene in the species Homo sapiens

ZAP-70 is a protein normally expressed near the surface membrane of lymphocytes. It is most prominently known to be recruited upon antigen binding to the T cell receptor (TCR), and it plays a critical role in T cell signaling.

<span class="mw-page-title-main">Tyrosine-protein kinase SYK</span>

Tyrosine-protein kinase SYK, also known as spleen tyrosine kinase, is an enzyme which in humans is encoded by the SYK gene.

<span class="mw-page-title-main">CD22</span> Lectin molecule

CD22, or cluster of differentiation-22, is a molecule belonging to the SIGLEC family of lectins. It is found on the surface of mature B cells and to a lesser extent on some immature B cells. Generally speaking, CD22 is a regulatory molecule that prevents the overactivation of the immune system and the development of autoimmune diseases.

<span class="mw-page-title-main">CD5 (protein)</span> Protein-coding gene in the species Homo sapiens

CD5 is a cluster of differentiation expressed on the surface of T cells and in a subset of murine B cells known as B-1a. The expression of this receptor in human B cells has been a controversial topic and to date there is no consensus regarding the role of this receptor as a marker of human B cells. B-1 cells have limited diversity of their B-cell receptor due to their lack of the enzyme terminal deoxynucleotidyl transferase (TdT) and are potentially self-reactive. CD5 serves to mitigate activating signals from the BCR so that the B-1 cells can only be activated by very strong stimuli and not by normal tissue proteins. CD5 was used as a T-cell marker until monoclonal antibodies against CD3 were developed.

<span class="mw-page-title-main">LYN</span> Mammalian protein found in Homo sapiens

Tyrosine-protein kinase Lyn is a protein that in humans is encoded by the LYN gene.

<span class="mw-page-title-main">CRKL</span> Protein-coding gene in the species Homo sapiens

Crk-like protein is a protein that in humans is encoded by the CRKL gene.

<span class="mw-page-title-main">INPP5D</span> Protein-coding gene in the species Homo sapiens

Src homology 2 (SH2) domain containing inositol polyphosphate 5-phosphatase 1(SHIP1) is an enzyme with phosphatase activity. SHIP1 is structured by multiple domain and is encoded by the INPP5D gene in humans. SHIP1 is expressed predominantly by hematopoietic cells but also, for example, by osteoblasts and endothelial cells. This phosphatase is important for the regulation of cellular activation. Not only catalytic but also adaptor activities of this protein are involved in this process. Its movement from the cytosol to the cytoplasmic membrane, where predominantly performs its function, is mediated by tyrosine phosphorylation of the intracellular chains of cell surface receptors that SHIP1 binds. Insufficient regulation of SHIP1 leads to different pathologies.

<span class="mw-page-title-main">B-cell linker</span> Mammalian protein found in Homo sapiens

B-cell linker (BLNK) protein is expressed in B cells and macrophages and plays a large role in B cell receptor signaling. Like all adaptor proteins, BLNK has no known intrinsic enzymatic activity. Its function is to temporally and spatially coordinate and regulate downstream signaling effectors in B cell receptor (BCR) signaling, which is important in B cell development. Binding of these downstream effectors is dependent on BLNK phosphorylation. BLNK is encoded by the BLNK gene and is also known as SLP-65, BASH, and BCA.

<span class="mw-page-title-main">FCGR2B</span>

Fc fragment of IgG receptor IIb is a low affinity inhibitory receptor for the Fc region of immunoglobulin gamma (IgG). FCGR2B participates in the phagocytosis of immune complexes and in the regulation of antibody production by B lymphocytes.

<span class="mw-page-title-main">CD79A</span> Mammalian protein found in Homo sapiens

Cluster of differentiation CD79A also known as B-cell antigen receptor complex-associated protein alpha chain and MB-1 membrane glycoprotein, is a protein that in humans is encoded by the CD79A gene.

<span class="mw-page-title-main">CD79B</span> Mammalian protein found in Homo sapiens

CD79b molecule, immunoglobulin-associated beta, also known as CD79B, is a human gene.

B10 cells are a sub-class of regulatory B-cells that are involved in inhibiting immune responses in both humans and mice. B10 cells are named for their ability to produce inhibitory interleukin: Interleukin-10 (IL-10). One of their unique abilities is that they suppress the innate and adaptive immune signals, making them important for regulating the inflammatory response. Like the B cell, the B10 cell requires antigen specific binding to the surface of CD5 receptor to illicit a response from the T-cell. Once an antigen binds to the CD19 receptor, immediate downregulation in B-cell receptor (BCR) signal expression occurs and mediates the release of IL-10 cytokines. In mice and humans, B10 cells are distinguishable in their expression of measurable IL-10 due to the lack of unique cell surface markers expressed by regulatory B cells. However, IL-10 competence is not limited to any one subset of B cells. B10 cells do not possess unique phenotypic markers or transcription factors for further identification. B10 cells predominantly localize in the spleen, though they are also found in the blood, lymph nodes, Peyer's patches, intestinal tissues, central nervous system, and peritoneal cavity. B10 cells proliferate during inflammatory and disease responses.

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Further reading

This article incorporates text from the United States National Library of Medicine, which is in the public domain.