Effects of IL-7 On Dynamics of T-Cell Recovery After Transplantation with Purified HSC and Common Lymphoid Progenitors (CLP).

Abstract 2442

Poster Board II-419

Strategies to accelerate immune reconstitution include the administration of lymphopoietic cytokines, e.g., IL-7, and the co-transplantation of lymphoid progenitors. CLP are rapidly cycling, non-self-renewing, IL-7R+ marrow cells that give rise to lymphoid progeny. We assessed the effects of IL-7 on the in vivo dynamics of T cell generation from purified HSC and CLP after murine transplantation. Groups of C57Bl/6 mice, 9-11 weeks old, were irradiated at a dose of 950 cGy (two 450 cGy fractions), followed by congenic transplantation with either 500 HSC or 500 HSC + 3000 CLP. Control and IL-7 treated groups received 3E5 murine mesenchymal stem cells (MSC) retrovirally transduced with either the EGFP or IL-7 genes, respectively. Thus, the four experimental groups were HSC with EGFP-MSC, HSC with IL-7-MSC, HSC+CLP with EGFP-MSC, and HSC+CLP with IL-7-MSC. Negative controls for MSC administration were mice that received HSC only, but no MSC (no difference from HSC with EGFP-MSC recipients). Compared to recipients of HSC with EGFP-MSC or HSC with IL-7-MSC, mice transplanted with HSC+CLP with EGFP-MSC had increased numbers of donor-derived triple negative (TN)(>3×), double positive (DP)(>5×), and CD4 or CD8 single positive (SP)(>3×) thymocytes at Day 21. The effects of HSC+CLP co-transplantation on donor-derived thymopoiesis were greatly increased by IL-7-MSC (TN: HSC + CLP with IL-7-MSC >3× more than HSC with EGFP-MSC; DP: >20×; SP CD4 or CD8: >50×). The IL-7 induced increases in thymopoiesis were due to expansion of CLP-derived cells. In contrast, by D28, almost all donor thymopoiesis was derived from HSC in both the HSC and HSC+CLP recipients; IL-7 treatment decreased the numbers of CLP-derived progenitors and had minimal effects on the numbers of HSC-derived thymocytes. At D21 and D28, HSC+CLP with EGFP-MSC recipients had 3× more peripheral (splenic) CD4 and CD8 mature T lymphocytes than HSC recipients. IL-7-MSC significantly increased the numbers of donor-derived mature T lymphocytes in HSC+CLP recipients to approximately 8× greater than that of recipients of HSC, again mainly by expansion of CLP-derived cells. The data indicate that 1) most thymopoiesis early after transplant is derived from non-self-renewing lymphoid progenitors while HSC contribute later; 2) IL-7 treatment accelerates thymopoiesis and peripheral T-cell reconstitution, mainly by expansion of CLP and their progeny, 3) IL-7 treatment may accelerate the exhaustion of the non-self-renewing CLP population. Despite this progenitor depletion, the net effect of IL-7 treatment is to increase the generation of CLP-derived T-cells. Commitment to lymphoid differentiation by HSC is a bottleneck in T lymphoid reconstitution which may be overcome by combined CLP transplantation and IL-7 treatment to bridge the delay in production of HSC-derived lymphoid progenitors.