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.
The CD79a protein together with the related CD79b protein, forms a dimer associated with membrane-bound immunoglobulin in B-cells, thus forming the B-cell antigen receptor (BCR). This occurs in a similar manner to the association of CD3 with the T-cell receptor, and enables the cell to respond to the presence of antigens on its surface.
The mouse CD79A gene, then called mb-1, was cloned in the late 1980s, followed by the discovery of human CD79A in the early 1990s. It is a short gene, 4.3 kb in length, with 5 exons encoding for 2 splice variants resulting in 2 isoforms.
CD79A is conserved and abundant among ray-finned fish (actinopterygii) but not in the evolutionarily more ancient chondrichthyes such as shark. The occurrence of CD79A thus coincides with the evolution of B cell receptors with greater diversity generated by recombination of multiple V, D, and J elements in bony fish contrasting the single V, D and J elements found in shark.
CD79a is a membrane protein with an extracellular immunoglobulin domain, a single span transmembrane region and a short cytoplasmic domain. The cytoplasmic domain contains multiple phosphorylation sites including a conserved dual phosphotyrosine binding motif, termed immunotyrosine-based activation motif (ITAM). The larger CD79a isoform contains an insert in position 88-127 of human CD79a resulting in a complete immunoglobulin domain, whereas the smaller isoform has only a truncated Ig-like domain. CD79a has several cysteine residues, one of which forms covalent bonds with CD79b.
CD79a plays multiple and diverse roles in B cell development and function. The CD79a/b heterodimer associates non-covalently with the immunoglobulin heavy chain through its transmembrane region, thus forming the BCR along with the immunoglobulin light chain and the pre-BCR when associated with the surrogate light chain in developing B cells. Association of the CD79a/b heterodimer with the immunoglobulin heavy chain is required for surface expression of the BCR and BCR induced calcium flux and protein tyrosine phosphorylation. Genetic deletion of the transmembrane exon of CD79A results in loss of CD79a protein and a complete block of B cell development at the pro to pre B cell transition. Similarly, humans with homozygous splice variants in CD79A predicted to result in loss of the transmembrane region and a truncated or absent protein display agammaglobulinemia and no peripheral B cells.
The CD79a ITAM tyrosines (human CD79a Tyr188 and Tyr199, mouse CD79a Tyr182 and Tyr193) phosphorylated in response to BCR crosslinking, are critical for binding of Src-homology 2 domain-containing kinases such as spleen tyrosine kinase (Syk) and signal transduction by CD79a. In vivo, the CD79a ITAM tyrosines synergize with the CD79b ITAM tyrosines to mediate the transition from the pro to the pre B cell stage as suggested by the analysis of mice with targeted mutations of the CD79a and CD79b ITAM. Loss of only one of the two functional CD79a/b ITAMs resulted in impaired B cell development but B cell functions such as the T cell independent type II response and BCR mediated calcium flux in the available B cells were intact. However, the presence of both the CD79a and CD79b ITAM tyrosines were required for normal T cell dependent antibody responses. The CD79a cytoplasmic domain further contains a non-ITAM tyrosine distal of the CD79a ITAM (human CD79a Tyr210, mouse CD79a Tyr204) that can bind BLNK and Nck once phosphorylated, and is critical for BCR mediated B cell proliferation and B1 cell development. CD79a ITAM tyrosine phosphorylation and signaling is negatively regulated by serine and threonine residues in direct proximity of the ITAM (human CD79a Ser197, Ser203, Thr209; mouse CD79a Ser191, Ser197, Thr203), and play a role in limiting formation of bone marrow plasma cells secreting IgG2a and IgG2b.
The CD79a protein is present on the surface of B-cells throughout their life cycle, and is absent on all other healthy cells, making it a highly reliable marker for B-cells in immunohistochemistry. The protein remains present when B-cells transform into active plasma cells, and is also present in virtually all B-cell neoplasms, including B-cell lymphomas, plasmacytomas, and myelomas. It is also present in abnormal lymphocytes associated with some cases of Hodgkins disease. Because even on B-cell precursors, it can be used to stain a wider range of cells than can the alternative B-cell marker CD20, but the latter is more commonly retained on mature B-cell lymphomas, so that the two are often used together in immunohistochemistry panels.
^ abcdSeda V, Mraz M (Mar 2015). "B-cell receptor signalling and its crosstalk with other pathways in normal and malignant cells". European Journal of Haematology. 94 (3): 193–205. doi:10.1111/ejh.12427. PMID25080849.
^ abAnthony S-Y L, Cooper K, Leong FJ (2003). Manual of Diagnostic Cytology (2 ed.). Greenwich Medical Media, Ltd. pp. XX. ISBN1-84110-100-1.
^Pelanda R, Braun U, Hobeika E, Nussenzweig MC, Reth M (Jul 2002). "B cell progenitors are arrested in maturation but have intact VDJ recombination in the absence of Ig-alpha and Ig-beta". Journal of Immunology. 169 (2): 865–72. doi:10.4049/jimmunol.169.2.865. PMID12097390.
^Wang Y, Kanegane H, Sanal O, Tezcan I, Ersoy F, Futatani T, Miyawaki T (Apr 2002). "Novel Igalpha (CD79a) gene mutation in a Turkish patient with B cell-deficient agammaglobulinemia". American Journal of Medical Genetics. 108 (4): 333–6. doi:10.1002/ajmg.10296. PMID11920841.
^Castello A, Gaya M, Tucholski J, Oellerich T, Lu KH, Tafuri A, Pawson T, Wienands J, Engelke M, Batista FD (Sep 2013). "Nck-mediated recruitment of BCAP to the BCR regulates the PI(3)K-Akt pathway in B cells". Nature Immunology. 14 (9): 966–75. doi:10.1038/ni.2685. PMID23913047.
^Patterson HC, Kraus M, Kim YM, Ploegh H, Rajewsky K (Jul 2006). "The B cell receptor promotes B cell activation and proliferation through a non-ITAM tyrosine in the Igalpha cytoplasmic domain". Immunity. 25 (1): 55–65. doi:10.1016/j.immuni.2006.04.014. PMID16860757.
Müller B, Cooper L, Terhorst C (Jun 1992). "Cloning and sequencing of the cDNA encoding the human homologue of the murine immunoglobulin-associated protein B29". European Journal of Immunology. 22 (6): 1621–5. doi:10.1002/eji.1830220641. PMID1534761.
Hutchcroft JE, Harrison ML, Geahlen RL (Apr 1992). "Association of the 72-kDa protein-tyrosine kinase PTK72 with the B cell antigen receptor". The Journal of Biological Chemistry. 267 (12): 8613–9. PMID1569106.
Yu LM, Chang TW (Jan 1992). "Human mb-1 gene: complete cDNA sequence and its expression in B cells bearing membrane Ig of various isotypes". Journal of Immunology. 148 (2): 633–7. PMID1729378.
Venkitaraman AR, Williams GT, Dariavach P, Neuberger MS (Aug 1991). "The B-cell antigen receptor of the five immunoglobulin classes". Nature. 352 (6338): 777–81. doi:10.1038/352777a0. PMID1881434.
Lankester AC, van Schijndel GM, Cordell JL, van Noesel CJ, van Lier RA (Apr 1994). "CD5 is associated with the human B cell antigen receptor complex". European Journal of Immunology. 24 (4): 812–6. doi:10.1002/eji.1830240406. PMID7512031.
Vasile S, Coligan JE, Yoshida M, Seon BK (Apr 1994). "Isolation and chemical characterization of the human B29 and mb-1 proteins of the B cell antigen receptor complex". Molecular Immunology. 31 (6): 419–27. doi:10.1016/0161-5890(94)90061-2. PMID7514267.
Brown VK, Ogle EW, Burkhardt AL, Rowley RB, Bolen JB, Justement LB (Jun 1994). "Multiple components of the B cell antigen receptor complex associate with the protein tyrosine phosphatase, CD45". The Journal of Biological Chemistry. 269 (25): 17238–44. PMID7516335.