Introduction: Higher frequency of transitional and naive B cells in patients with operational tolerance (stable allograft function despite absence of immunosuppression ≥1 year) suggests abnormal differentiation of B cells in those patients. The association of transitional B cells with stable 1-year graft function was confirmed also in patients on standard immunosuppression, however, the long-term data remains unclear.
Methods: Stable graft function was defined as >15 years with creatinine <150 µmol/L (n=24). As a control group patients with biopsy-proven chronic antibody mediated rejection (CAMR, cg >1) >2 years after transplantation were selected (n=18). While almost all CAMR patients were on standard triple immunosuppression based on calcineurin inhibitors, mycophenolate mofetile (MMF) and prednisone) (n=17), patients with stable graft function were on double immunosuppression regimes (calcineurin inhibitors and MMF or prednisone (n=11); azathioprine and prednisone (n=4); sirolimus and MMF or prednisone (n=3); MMF and steroids (n=1) or on sirolimus or TAC monotherapy (n=5). Subpopulations of B-lymphocytes (transitional, naive, non-switched, switched memory B cells and plasmablasts) were compared by flow-cytometry using DuraClone IM B cell panel (Beckman Coulter) on 10 colour Navios flow cytometer and data were analyzed using KALUZA software (Beckman Coulter). Absolute numbers of B-lymphocytes subsets were calculated from the absolute numbers of leukocytes analyzed using the Sysmex hematology analyzer and relative numbers of particular subsets are expressed as a ratio of CD45 positive cells.
Results: Both groups didn´t differ in main demographic characteristics but patients with stable graft function were younger (median 35 vs 53.5 years, p=0.001). Creatinine level at 3 months and at sampling was lower in stable graft function compared to CAMR (at 3 months: 113 vs 139.6 µmol/l, p=0.008 and at sampling: 112.4 vs 269.5 µmol/l, p<0.001, respectively). There were no differences in either absolute or relative numbers of naive, non-switched and switched memory B cells between both groups, however absolute and relative numbers of transitional B cells and plasmablasts were higher in patients with stable graft function (Fig. 1).

Fig. 1 Absolute numbers of plasmablasts and transitional B cells in patients with stable kidney graft function (SF) compared to chronic rejection (CAMR).
Conclusion: Similarly to operational tolerance patients, the observation of higher numbers of peripheral transitional B cells and short-living plasmablasts in immunosuppressed patients with very long-term normal graft function supports their protective role in kidney transplantation.