Depletion of Functionally Active CD20+T Cells by Rituximab Treatment

Objective. Rituximab is a therapeutic anti-CD20 antibody used for in vivo depletion of B cells in proliferative and autoimmune diseases. However, the mechanisms of action are not fully understood, since not all of the therapy-mediated effects can be explained by the depletion of antibody-secreting cells. In addition to B cells, there is also a small population of T cells coexpressing CD20 in all individuals. This study was conducted to examine the phenotype and function of CD3+CD20+ T cells in patients with rheumatoid arthritis (RA) and healthy controls. Methods. The phenotype and apoptosis of peripheral blood mononuclear cells from healthy donors and RA patients were examined by 4-color fluorescence-activated cell sorting analyses. Cytokine production was determined by intracellular staining and measurement of cytokines in the supernatants. Proliferation of sorted T cell populations was analyzed using H-3-thymidine uptake assays. Results. In healthy individuals, 0.1-6.8% of peripheral blood T cells (mean 1.6%; n = 142) coexpressed CD20, which was not significantly different from that in the peripheral blood of RA patients, in whom 0.4-2.6% of T cells (mean 1.2%; n = 27) were CD20+. During rituximab therapy, the CD20+ T cells along with the 13 cells were eliminated from the RA peripheral blood. Among the CD20+ T cells, 45% coexpressed CD8 and 55% coexpressed CD4. Polyclonal CD3+CD20+ cells were functionally characterized by constitutive cytokine production (i.e., interieukin-1 beta and tumor necrosis factor a), a low proliferative capacity, a high activation state, and enhanced susceptibility to apoptosis. Conclusion. These findings suggest that CD20+ T cells represent a terminally differentiated cell type with immune-regulatory and proinflammatory capacities. Depletion of CD20+ T cells may be an additional mechanism by which anti-CD20 therapy functions in patients with RA.