Although CD20 is classically a B cell marker, in the last three decades, dim expression has been noted on a subset of T cells as well that has been independently verified by a number of groups. Our understanding of these cells and their function is not well established.
A thorough review of original articles on CD20+ T cells was undertaken of Pubmed by using combination of phrases including “CD20+”, “CD20-positive” and “T cells”. Articles in English were considered, and there was no time restriction.
CD20+ T cells express the standard T cell markers and, in comparison to CD20¯ T cells, appear to express greater inflammatory cytokines and markers of effector function. Although the ontogeny of these cells is still being established, the current theory is that CD20 may be acquired by trogocytosis from B cells. CD20+ T cells may be found in healthy controls and in a wide range of pathologies including autoimmune diseases, haematological and non-haematological malignancies and human immunodeficiency virus (HIV) infections. One of the best studied diseases where these cells are found is multiple sclerosis (MS) where a number of therapeutic interventions, including anti-CD20 depletion, have been shown to effectively deplete these cells.
This review summarises the latest understanding of CD20+ T cells, their presence in various diseases, their putative function and how they may be an ongoing target of CD20-depleting agents. Unfortunately, our understanding of these cells is still at its infancy and ongoing study in a wider range of pathologies is required.
Cluster of differentiation 20 (CD20) is a lymphocyte surface molecule that has become a ubiquitous marker for B cells in flow cytometry, being expressed on pre-B cells to mature B cells, and lost in plasmablasts and plasma cells. It is a molecule that ranges from 33 to 37 kDa and is encoded by the MS4A1 gene located on chromosome 11. Functionally, it is thought to serve as a calcium channel and is important in regulating B cell function by associating with the B cell receptor.
Since the 1990 s, observations were made, by flow cytometry, that a small percentage of human CD3+ T cells in peripheral blood dimly express CD20 on their surface. This is in contrast to the traditional bright expression of CD20 found on B cells which are approximately 15-fold brighter by flow cytometry. CD20+ T cells may range from 1 to 6% of T cells in peripheral blood (PB) of healthy controls (HCs), and do not express any other markers of B cells such as CD19. The frequency of these cells bear no relationship to age of patient, sex or CD4:8 ratio. They may also be found in other human tissues such as the thymus, tonsils, bone marrow and cerebral spinal fluid (CSF).
However, the appearance of CD20 on T cells has generated some controversy. Some maintain that part of the CD20 expression on T cells may be a product of T-B cell doublets. As evidence for this, ethylenediaminetetraacetic acid treatment of PB reduces the frequency of CD20+ T cells, as does the exclusion of doublets via forward and side scatter gating. However, dim expression of CD20 create a distinct subset from the bright CD20 found on B cells and furthermore, these cells lack CD19, effectively ruling them out as T-B cell doublets. Multiple anti-CD20 clones have reproduced similar results and blocking studies have revealed that CD20+ T cells are not a product of non-specific binding of these anti-CD20 clones. Nevertheless, they are, to this day, recognised as a bona fide subset that has been independently verified by a large number of groups and using other techniques such as single-cell imaging flow cytometry and immunohistochemistry.
The origin of CD20+ T cells is not entirely clear. Certainly, these cells express MS4A1 mRNA suggesting an intrinsic ability to generate CD20 protein. In monkey experiments, isolated CD20+ T cells up-regulate MS4A1 mRNA and surface CD20 expression after concanavalin A and interleukin (IL)-2 stimulation in vitro indicating that expression may also be regulated by the cells’ microenvironment.
CD20 protein expression is absent on T cells of human cord blood, suggesting acquisition of this molecule later in development. In mice, CD20+ T cells only appear in the spleen after week 2 of age supporting this notion. By co-culturing T and B cell lines, de Bruyn et al. were able to show an increase in CD20 expression on T cells after just 1 h. They speculated that the mechanism of this was via trogocytosis when T cells make contact with their cognate B cells. This theory was proven more recently when co-culturing experiments of mice lymphocytes revealed that direct contact with B cells was necessary for the transference of CD20 to T cells.
