Bootcongres

Background: The co-stimulatory inhibitor of the CD28-CD80/86-pathway, belatacept, is a promising alternative for calcineurin inhibitors in kidney-transplantation. However, a somewhat higher rejection rate is observed in belatacept-treated patients than in cyclosporine-treated patients. Differences in sensitivity for belatacept and alternative activations of CD28-positive (CD28^pos) T-cells could explain this. Therefore, we studied immunological behavior of CD28^pos T-cells under belatacept. Methods: The IC₅₀ of belatacept was determined in a 7-day MLR. By flow cell-sorting we obtained purified populations of peripheral CD28^pos and CD28^null lymphocytes of 16 kidney-transplant candidates. Using flowcytometry, we measured their proliferation, their production of the effector-cytokine IFNγ, and CD28-expression after donor-antigen stimulation, in the presence or absence of belatacept or IgG-control. Results are reported as medians with the interquartile range (IQR).

Results: The IC₅₀of belatacept was 138 ng/mL (115-165 ng/mL); a concentration comparable to serum levels observed in clinical studies. Belatacept in a concentration of 100 ng/mL inhibited proliferation of CD28^pos T-cells by 28.6% [IQR 18.5-34.4%;p<0.001), while leaving IFNγ-production unaffected. Irrespective of the presence of belatacept, more CD28^pos T-cells converted into CD28^null T-cells in the proliferated population than in the non-proliferated population [21.1%, IQR 6.4-32.9% vs. 3.7%, IQR 1.9-11.1%; p<0.05]. Upon stimulation with donor-antigen, a higher percentage of these induced CD28^null T-cells produced IFNγ [7.0%, IQR 1.9-19.2%] than T-cells that remained CD28^pos[4.8%, IQR 0.7-6.4%](p<0.01). Neither CD28^null-induction nor the IFNγ-production by these induced CD28^null T-cells was inhibited by belatacept. As expected, belatacept only minimally inhibited the proliferation of sorted CD28^null T-cells [1.3%, IQR -0.6-3.0%]. Interestingly, in the presence of belatacept, sorted CD28^null T-cells converted into IFNγ-producing CD28^pos T-cells [18.6% CD28^pos, IQR 7.9-34.2%: 10.1% of which produced IFNγ, IQR 5.5-19.9%].

Conclusions: Using the less-intensive dosing regimen of belatacept, not only CD28^null T-cells, but also a subset of CD28^pos T-cells can escape co-stimulatory blockade. This could explain the rejection rate observed in belatacept-treated kidney-transplant patients.