Murine bone marrow chimeras developing autoimmunity after CTLA-4-blockade show an expansion of T regulatory cells with an activated cytokine profile

Autoimmune adverse events are a concern in patients treated with blocking anti-CTLA-4-mAb for solid and hematological tumors. Patient and mouse data on the contribution of a quantitative or qualitative defect of regulatory T cells (Treg) in this autoimmune phenomenon are conflicting.We have previously shown that a treatment course with blocking anti-CTLA-4-mAb in murine allogeneic bone marrow chimeras induces an antileukemic response in close association with systemic autoimmunity. Here, we used this model to investigate the effect of CTLA-4-blocking therapy on the kinetics of Treg frequency and function. As previously published, CTLA-4-blocking treatment, initiated on day 20 after bone marrow transplantation, led to overt autoimmunity by day 35. CD4+Foxp3+ Treg frequency was determined (flowcytometry) on day 21, 23, 25 and 35: treated chimeras showed an expansion of CD4+Foxp3+ Treg frequencies on day 25 and 35, without a prior frequency decrease. The Treg expansion occurred selectively in the recipient-derived CD4+ T-cell compartment. In vitro, purified CD4+CD25+FR4high Treg from ‘day 35’ autoimmune and control chimeras showed equal suppressive effects towards self-antigen-specific autoimmune T cells. Purified CD4+CD25highFR4high Treg from ‘day 35’ treated chimeras showed increased IL-10 and IFN-γ mRNA-expression (RT-PCR) relative to control chimeras.In this model of CTLA-4-blockade-induced autoimmunity after allogeneic bone marrow transplantation, anti-CTLA-4-mAb gives rise to a progressive expansion – without a prior transient reduction – of Treg cells. Treg of autoimmune animals do not show a defect in in vitro suppressive function but show an in vivo activated cytokine profile, suggesting that the expansion occurs as a compensatory phenomenon to control autoimmunity.