Immune reconstitution after allogeneic bone marrow transplantation involves peripheral T cell apoptosis (2004)

Post-transplant T cell deficiency (especially in adults) is related to impaired thymus-dependent lymphopoiesis. However, others and we have previously found that peripheral T cell apoptosis in recipients of allogeneic bone marrow transplantation (allo BMT) is increased. We used murine allo BMT models to analyze peripheral T cell apoptosis in three different T cell populations after allo BMT: T cells derived from (a) donor bone marrow (BM) precursors (de novo generated), (b) infused mature donor T cells in the allograft (alloreactive and non-alloreactive), and (c) residual host T cells. In the first experiments, we found an increased percentage of apoptotic (Annexin-V+) T cells in both donor derived de novo generated CD4+ and CD8+ T cells at day 28 after transplant in recipients of a T cell depleted (TCD) BMT without GVHD. This was associated with decreased levels of intracellular Bcl-2 in de novo generated T cells from both young and old recipients of an allo TCD BMT. The increased percentages of apoptotic peripheral T cells and down regulation of Bcl-2 levels were more prominent in the older recipients. In recipients with GVHD, we found a severe loss of thymic cellularity, low numbers of BM derived de novo generated T cells with a high fraction of apoptotic cells and very low Bcl-2 expression and increased caspase 8 and 9 activation, Conversely, alloreactive T cells (derived from the infused mature donor T cells in the allograft) showed upregulation of Fas expression, higher Bcl-2 levels, less caspase activity and finally, lower numbers of apoptotic T cells compared to BM derived T cells. To determine the effects of Bcl-2 and Fas on T cell reconstitution in allo BMT recipients, we transplanted donor allografts consisting of BM from Bcl-2 overexpressing transgenic mice (Bcl-2 Tg) or BM from Fas-deficient lpr mice with or without wild type congenic T cells (B6.Ly5.1) into lethally irradiated recipients. Bcl-2 Tg as well as lpr BM rescued post-transplant thymic cellularity and distribution and increased CD4+ and CD8+ T cell numbers in the periphery, however no change in the fraction of apoptotic cells was observed. In contrast, post transplant administration of Interleukin-7 and Interleukin-15 reduced the number of apoptotic T cells in the periphery.We conclude that peripheral T cell apoptosis is an important factor in the delay of post transplant T cell reconstitution especially in older recipients of allo BMT and in recipients with GVHD. Post-transplant T cell deficiency (especially in adults) is related to impaired thymus-dependent lymphopoiesis. However, others and we have previously found that peripheral T cell apoptosis in recipients of allogeneic bone marrow transplantation (allo BMT) is increased. We used murine allo BMT models to analyze peripheral T cell apoptosis in three different T cell populations after allo BMT: T cells derived from (a) donor bone marrow (BM) precursors (de novo generated), (b) infused mature donor T cells in the allograft (alloreactive and non-alloreactive), and (c) residual host T cells. In the first experiments, we found an increased percentage of apoptotic (Annexin-V+) T cells in both donor derived de novo generated CD4+ and CD8+ T cells at day 28 after transplant in recipients of a T cell depleted (TCD) BMT without GVHD. This was associated with decreased levels of intracellular Bcl-2 in de novo generated T cells from both young and old recipients of an allo TCD BMT. The increased percentages of apoptotic peripheral T cells and down regulation of Bcl-2 levels were more prominent in the older recipients. In recipients with GVHD, we found a severe loss of thymic cellularity, low numbers of BM derived de novo generated T cells with a high fraction of apoptotic cells and very low Bcl-2 expression and increased caspase 8 and 9 activation, Conversely, alloreactive T cells (derived from the infused mature donor T cells in the allograft) showed upregulation of Fas expression, higher Bcl-2 levels, less caspase activity and finally, lower numbers of apoptotic T cells compared to BM derived T cells. To determine the effects of Bcl-2 and Fas on T cell reconstitution in allo BMT recipients, we transplanted donor allografts consisting of BM from Bcl-2 overexpressing transgenic mice (Bcl-2 Tg) or BM from Fas-deficient lpr mice with or without wild type congenic T cells (B6.Ly5.1) into lethally irradiated recipients. Bcl-2 Tg as well as lpr BM rescued post-transplant thymic cellularity and distribution and increased CD4+ and CD8+ T cell numbers in the periphery, however no change in the fraction of apoptotic cells was observed. In contrast, post transplant administration of Interleukin-7 and Interleukin-15 reduced the number of apoptotic T cells in the periphery. We conclude that peripheral T cell apoptosis is an important factor in the delay of post transplant T cell reconstitution especially in older recipients of allo BMT and in recipients with GVHD.