Following an in vivo T cell depleting non-myeloablative conditioning regimen, 5 patients, aged 22–49, received combined kidney and bone marrow transplantation from a haploidentical related donor. Rituximab was included in the conditioning for patients 4 and 5. All patients developed initial mixed chimerism but lost it by day 21; no patient developed GVHD. Four patients discontinued immunosuppression from 240 to 422 days after BMT and have remained off immunosuppression for 9 to 52 months with no evidence of allograft rejection. Flow cytometry was used to assess lymphocyte subsets recovering after transplant. CD3 counts recovered slowly, exceeding 500 cells/μl at days +271, +365, +640 and +450. While memory CD45RO+ cells were most prevalent among CD4+ cells, naïve-type CD4+CD45RA+ cells, presumably arising from the recipient thymus, ranged from 8% to 56% at the time when total CD4 counts recovered to >100 cells/μl (days +165, +21, +352, +240). Notably, a very high proportion of initially recovering T cells were CD3+CD4+ expressing CD25 in all patients as early as day 7 and persisted over 1 year in 2 patients. At approximately day +120 and +365, we further characterized these cells for CD127, FOXP3, CD45RO, CD45RA, HLA-DR and CD62L expression. At Day +120, all 4 patients showed increased frequencies (10.7±4.6%) of CD25+CD127-FOXP3+ regulatory T cells (Treg) within the CD4 population compared to healthy subjects (3.8±0.4%). Expression of CD45RO, CD45RA, CD62L and HLA-DR was variable. By 1 year post-transplant, frequencies of Treg had decreased to levels similar to those in normal subjects. In vitro assays for CD8 and CD4 T cell-mediated alloreactivity (CML/MLR) showed development of long-lasting donor-specific unresponsiveness by 3 months after transplant in Patients 2, 4 and 5, and by 9 months in Patient 1. Responses to 3rd party recovered in all patients after a period of unresponsiveness. In Patient 1, in whom anti-donor CML reactivity declined gradually to become unresponsive by 9 months, depletion of CD4+CD25+ cells revealed a residual anti-donor CML and MLR response at 1year but not at 18 months. In 2 other patients, depletion of CD4+CD25+ cells did not reveal an anti-donor response at time points analyzed from day +122 to 2 years. In patients in whom renal tubular epithelial cells (RTEC) were cultured from the donor kidney, loss of killing activity against donor RTEC was observed post-transplant. The high percentage of Treg recovering early after transplant suggests that they may play a role in initial tolerance induction. This regulatory mechanism may be followed by later deletion of donor-reactive T cells. The variable ability to detect regulation of anti-donor reactivity may reflect the strength of the initial response, as patients with weak pre-transplant anti-donor responses and rapid post-transplant development of donor unresponsiveness did not reveal anti-donor response when Treg were depleted. In addition, infiltration of Treg at the graft site, not revealed by the assays described, might be responsible for tolerance in these patients.

Author notes

Disclosure: No relevant conflicts of interest to declare.