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Homeostatic Mechanisms Affect Reconstitution Of CD4+ FoxP3+ Regulatory T Cells After Hematopoietic Stem Cell Transplantation

Year: 2009

Type: article

Abstract: CD4+FoxP3+ regulatory T cells (Treg) play a critical role in immune tolerance after allogeneic HSCT but the homeostatic mechanisms that affect Treg reconstitution post-HSCT have not been identified. To address this issue we simultaneously examined phenotypic and functional characteristics of Treg and CD4+FoxP3- conventional T cells (Tcon) in peripheral blood after allogeneic HSCT (n = 16, median 10 months post-HSCT). Post-HSCT Treg expressed higher levels of mitotic marker Ki-67 than Tcon (median 5.2% vs 1.5%; p<0.0001) and Treg from normal donors (median 5.2% vs 2.5%; p = 0.03). %Ki-67+ Treg was inversely correlated with peripheral CD4 count (r = -0.86, p<0.0001). To assess survival of Treg, we purified 4 different CD4+ T cell subsets by cell sorting (CD45RA+ Tcon, CD45RA- Tcon, CD45RA+ Treg and CD45RA- Treg) from 4 healthy donors, 8 patients post-autologous and 8 post-allogeneic HSCT. Each subset was cultured with or without agonistic FAS antibody and apoptosis was measured using Annexin-V staining. Anti-FAS rapidly induced apoptosis of CD45RA- Treg from patients (median 25.3%) while all other Treg and Tcon (median 4.5%, p<0.0001) subsets were relatively resistant to apoptosis. The degree of memory CD45RA- Treg apoptosis is equivalent after autologous and allogeneic HSCT, but inversely correlated with the peripheral lymphocyte count in the patient (r = -0.54, p = 0.03). The effect of lymphopenia was also evaluated by prospectively monitoring Treg and Tcon recovery 3–24 months post-HSCT in 46 patients who received peripheral stem cell grafts from HLA-identical donors after myeloablative conditioning. In 27 patients with >220/ul CD4 T cells at 6 months, both Tcon and Treg gradually increased over the subsequent 18 months. In contrast, in 19 patients with <220/ul CD4 T cells at 6 months, Tcon gradually increased but Treg decreased over the subsequent 12–24 months. These results indicate that Treg reconstitution is characterized by increased peripheral proliferation but this is counterbalanced by increased sensitivity to apoptosis. These characteristics are more evident in lymphopenic patients, suggesting that lymphopenia-driven proliferation and apoptosis skew Treg reconstitution. This homeostatic process does not affect the survival of other CD4 T cell subsets, resulting in a relative deficiency of Treg post-HSCT. Our findings provide important information for developing strategies to modulate Treg and promote immune tolerance after HSCT.

Source: Transplantation and Cellular Therapy

DOI: 10.1016/j.bbmt.2008.12.417