CD20

Synonyms: Cluster of differentiation 20, MS4A1

by Assia Bassarova

Background

CD20 (MS4A1) is the first member of the MS4A (membrane-spanning 4-domain family, subfamily A) family. It comprises proteins that are predicted to span the cellular membrane four times and share similarities in their polypeptide sequence and predicted overall topological structure. MS4A genes are clustered on chromosome 11q12 in humans. The CD20 antigen is a 33-37 kDa membrane-embedded, non-glycosylated phosphoprotein. MS4A1 is associated with a variety of membrane proteins including major histocompatibility complex (MHC) class I, MHC class II, tetraspanins (CD53, CD81 and CD82), CD40 and the B-cell receptor (BCR) and is involved in the regulation of B-cell activation, proliferation and differentiation. It is commonly thought that MS4A1 is involved in Ca2+ conductance, possibly by serving as an ion channel, but recent studies have shown that MS4A1 can homo-oligomerize into tetramers and that it physically associates with the antigen-binding component of the BCR to positively regulate BCR-induced cytoplasmic Ca2+ mobilization. MS4A1 is selectively expressed on mature B cells and most malignant B cells, making it a clinically useful target for monoclonal antibody therapies (for example, rituximab and ofatumumab) in treating B-cell non-Hodgkin’s lymphoma, mantle cell lymphoma and certain autoimmune diseases. CD20 appears on the surface of the pre-B lymphocyte between the time of light chain rearrangement and expression of intact surface immunoglobulin and is lost just before terminal B-cell differentiation into plasma cells. Surface expression of CD20 on activated B cells is approximately 4-fold greater than that found on resting B cells. CD20 is virtually specific for normal B-cells. A weak expression has been demonstrated in a subpopulation of T-cells, but not in any other cell type.

Staining in normal cells/tissues

It is expressed on pre-, naïve, and mature B cells but not on plasma cells or early pro-B cells.

Staining in tumors

CD20 is expressed in the large majority of cases of B-cell leukaemia/lymphoma. Early stage precursor B lymphoblastic leukaemia/lymphoma may be negative, and chronic lymphocytic leukaemia/small cell lymphoma may show a weak staining. Plasma cell neoplasms are as a rule CD20 negative. However, a special type of CD20 positive, Cyclin D1 positive myelomas with t(11;14) translocation account for 10-20% of the cases. T-cell lymphomas are almost always CD20 negative, but CD20 has been demonstrated in few cases of various types of T-cell lymphoma. The nodular lymphocyte-predominant subtype of Hodgkin’s lymphoma shows CD20 staining of L&H cells in most cases, while Reed-Sternberg cells in the other subtypes reveal CD20 positivity in about 40%, albeit in a minority of neoplastic cells.
Acute myeloid leukaemia is CD20 positive in few cases, while blastic transformation in chronic myeloid leukaemia is accompanied by CD20 positivity in about 30%. Thymoma may reveal CD20 positivity in a spindle cell component.
In patients treated with rituximab (a humanized anti-CD20 antibody) for malignant B-cell lymphoma, the CD20 epitopes disappear (both in normal and neoplastic B-cells) as a result of down-modulation of CD20 m-RNA in the cells in some cases. In other cases CD20 negativity may be attributed to deletion, mutations in within CD20 coding region or to epigenetic mechanisms.  Negative CD20 staining may potentially lead to erroneous diagnosis in these cases. During treatment, the B-cells can be identified with other antibodies, anti-CD79a or PAX-5.

Staining pattern

Distinct membranous.

Control tissue

Tonsil or appendix are appropriate controls. The B-cell areas with primary or secondary lymphoid follicles are populated with small B-cells in the marginal and mantle zones and centrocytes and centroblasts in germinal center – all they show very strong membranous staining reaction.

Application

  • Together with CD79a and PAX5, CD20 is one of the most important markers for the identification of B-cell neoplasms and lymphocyte-predominant Hodgkin lymphoma.

Selected references

  1. Ferry JA. There Are No Magic Bullets in Hematopathology: Even Immunostains for CD20 and CD3 Can Get You Into Trouble. Adv Anat Pathol. 2018 Jan;25(1):14-23.  doi: 10.1097/PAP.0000000000000174. 
  2. Kuek LE, Leffler M, Mackay GA, Hulett MD. The MS4A family: counting past 1, 2 and 3. Immunol Cell Biol. 2016 Jan;94(1):11-23. doi: 10.1038/icb.2015.48. Epub 2015 Apr 3. PMID: 25835430.
  3. Nelson BH. CD20+ B cells: the other tumor-infiltrating lymphocytes. J Immunol. 2010 Nov 1;185(9):4977-82. doi: 10.4049/jimmunol.1001323. PMID: 20962266. 
  4. O’Malley DP, Fedoriw Y, Weiss LM. Distinguishing Classical Hodgkin Lymphoma, Gray Zone Lymphoma, and Large B-cell Lymphoma: A Proposed Scoring System. Appl Immunohistochem Mol Morphol. 2016 Sep;24(8):535-40. doi: 10.1097/PAI.0000000000000236. PMID: 26447896.
  5. Riley JK, Sliwkowski MX. CD20: a gene in search of a function. Semin Oncol. 2000 Dec;27(6 Suppl 12):17-24. PMID: 11225995. 
  6. Tedder TF, Klejman G, Schlossman SF, et al. Structure of the gene encoding the human B lymphocyte differentiation antigen CD20 (B1). J Immunol. 1989 Apr 1;142(7):2560-8. PMID: 2466899.