Abatacept (CTLA-4Ig) blocks CD28-mediated T cell activation by binding to the costimulatory B7 ligands CD80/CD86 on antigen presenting cells. Costimulatory molecules, however, can also be expressed on T cells upon activation. Therefore, the aim of our study was to investigate direct effects of CTLA-4Ig on distinct T cell subsets in RA patients.
Phenotypic and functional analyses of CD4(+) T cells, including CD4(+) FoxP3(+) CD25(+) regulatory T cells (Treg), from RA patients were performed before and during CTLA-4Ig therapy. In addition T cells from healthy volunteers were analysed on in vitro culture with CTLA-4Ig or anti-CD80 and anti-CD86 antibodies. Apoptotic DNA fragmentation in CD4(+) and CD4(+) FoxP3(+) T cells was measured by TUNEL staining.
We observed an increase in T cells, including Treg cells, after initiation of CTLA-4Ig therapy, which was linked to a downregulation of activation-associated marker molecules and CD95 on CD4(+) T cells and Treg cells. CTLA-4Ig decreased CD95-mediated cell death in vitro in a dose-dependent manner. Functional analysis of isolated Treg cells from RA patients further revealed a diminished suppression of responder T cell proliferation. This was found to be due to CTLA-4Ig-mediated blocking of CD80 and CD86 on responder T cells that led to a diminished susceptibility for Treg cell suppression.
CTLA-4Ig therapy in RA patients exerts effects beyond the suppression of T cell activation, which has to be taken into account as an additional mechanism of CTLA-4Ig treatment.