Umbilical Cord Blood Xenografts in Immunodeficient Mice Reveal That T Cells Enhance Hematopoietic Engraftment Beyond Overcoming Immune Barriers by Stimulating Stem Cell Differentiation

Experimental design.

Infused UCB MNCs. CB MNCs were thawed, and for TCD recipients, depleted of CD2⁺ cells immunomagnetically prior to transplantation. Percentages of CD34⁺ and CD3⁺ cells in the infused product were evaluated by flow cytometry, and a representative flow cytogram is shown. TCD efficiency was >97% and CD34⁺ cells were consistently enriched in the TCD infused product compared to unmanipulated CB MNC.

Despite absent host barriers and more CD34⁺ cells, the level of engraftment of TCD UCB is inferior to UCB MNC. Percent human leukocyte engraftment was evaluated in BM from recipient mice between days +22 and +27 or +39 and +45 following transplantation. A, Overall mean engraftment as assessed by human CD45 in UCB MNC recipients: 4.69% ± 0.85% (n = 32, range: 0-27.3%) and TCD UCB: 1.90% ± 0.87% (range: 0-10.6%, n = 31, P = .02). Diamonds represent mean (center) and 95% confidence intervals (top and bottom) for each distribution. B, Scatter plot of percent human CD45 by dose of cells transplanted. Linear regression lines are shown for UCB MNC (solid line; linear prediction human CD45 = 2.47 + 2.45 × cell dose; R²: 0.54) and TCD MNC (interrupted line, linear prediction human CD45 = 0.56 + 1.25 × cell dose; R²: 0.56). Limiting dilution analysis to determine functional stem cell numbers was performed using a single unit of UCB at 5 dose levels between 5 × 10⁴ and 5 × 10⁶ cells (2 to 8 mice/group; total mice transplanted: 58, 54 survived to analysis) Animals with >0.5% human CD45⁺ cells, and >0.1% human lymphoid (CD 19⁺) and myeloid (CD33⁺) cells in the FSC/SSC gate were considered to be engrafted. The difference in SRC frequency was not significant (P = .20).

Coinfusion of CD3/CD28 activated UCB T cells with TCD UCB restores engraftment. A, Immunophenotype of CD3/CD28 costimulated UCB T cells on day 7 of culture. Although transplanted CD3/CD28 costimulated T cells were derived from CD4⁺- and CD8⁺-enriched cells, T cells could also be expanded from unpurified MNCs (fresh or thawed) with a comparable purity. Left and middle panels demonstrate >93% purity of T cells in terms of CD3 expression (62.9% CD4⁺, 28.8% CD8⁺, not shown). There was bright expression of CD25 and CD28 in the entire population; CD11b was more highly expressed on T cells derived from MNC (gray line) compared to T cells expanded from CD2⁺ selected cells (black line); immunophenotype was otherwise similar. The T cell product that was adoptively transferred into mice was expanded from CD4⁺ and CD8⁺ isolated cells. B, Engraftment of CD45⁺ cells minus adoptively transferred CD45⁺CD3⁺ cells. Lines represent means and standard error. C, Representative histopathologic examination (liver, 20×) of recipients of CD3/CD28 costimulated UCB T cells showed no evidence of inflammation or significant pathology of sampled organs (4 of 7 recipient tissues examined, 0 of 4 affected).

Facilitation of early multilineage engraftment by CD3/CD28 costimulated UCB T cells is maintained in NOG mice. A, Marked erythroid engraftment in recipients of TCD UCB with costimulated CD3/CD28 UCB T cells accounted, in large part, for the overall engraftment advantage in this cohort of mice (n = 6 per group; 16 of 18 mice survived to analysis). Contour plots (A) of bone marrow cells from representative mice gated on live cells: x-axes: human CD45; y-axes: Glycophorin A (A, top panels) and CD3 (A, lower panels). The majority of human CD45⁺ cells in recipients of UCB MNC were expanded adoptively transferred CD3⁺ cells. B, Engraftment of the composite sum of CD45⁺CD3⁻ cells together with the GlyA⁺CD45⁻ cells. C, Photomicrographs of spleen (H&E: left, 10×, middle, 40×) of a recipient of TCD UCB with costimulated CD3/CD28 UCB T cells demonstrating extramedullary hematopoiesis. Right panel: liver (H&E; 10×) showing mild periportal and intrahepatic inflammatory infiltrates.