Transplanted individuals in operational tolerance (OT) maintain long-term stable graft function after completely stopping immunosuppression. Understanding the mechanisms involved in OT can provide valuable information about pathways to human transplantation tolerance. Here we report that operationally tolerant individuals display quantitative and functional preservation of the B-cell compartment in renal transplantation. OT exhibited normal numbers of circulating total B cells, naive, memory and regulatory B cells (Bregs) as well as preserved B-cell receptor repertoire, similar to healthy individuals. In addition, OT also displayed conserved capacity to activate the cluster of differentiation 40 (CD40)/signal transducer and activator of transcription 3 (STAT3) signaling pathway in Bregs, in contrast, with chronic rejection. Rather than expansion or higher activation, we show that the preservation of the B-cell compartment favors OT.

Our findings indicate that the recommended polymyxin B doses should not be reduced for patients on CVVHD.

The combination of a depleting anti-Cd43 monoclonal antibody (mAb) and a single donor-specific transfusion before transplantation has been shown to induce operational transplantation tolerance in the majority of cardiac allograft recipients in a mouse model. To examine a protocol which might be more clinically relevant, we have modified this tolerance-inducing protocol by substituting the depleting with a nondepleting anti-Cd4 mAb. We show that this form of pretreatment can also induce immunologic unresponsiveness in most recipients (C3H/He, H2(k)), provided a critical period of time, in this case 28 days, is allowed between pretreatment and transplantation of a fully mismatched heart graft (H2(b)). When only 1 or 2 weeks were allowed between pretreatment and transplantation, only slight graft prolongation was obtained when compared with recipients receiving anti-Cd4 mAb alone, at these time points. Maintenance of tolerance in this model was due, at least in part, to active mechanisms as immunologic unresponsiveness to donor antigens could be transferred to naive syngeneic mice by splenocytes from recipients bearing long-term functioning grafts. These findings suggest that a population of regulatory cells develop after pretreatment with nondepleting anti-Cd4 mAb and donor-specific transfusion, and that it takes at least 1 month for these cells to expand and effectively drive the recipient's immune system toward immunologic unresponsiveness.