P1.061: Ongoing B-Cell Dynamics May Influence Engraftment Following IUHCT

Background: In-utero hematopoietic cell transplantation (IUHCT) utilizes the tolerogenic fetal immunologic development to facilitate engraftment of donor cells for the potential treatment of any disease correctable with postnatal bone marrow transplantation. Despite IUHCT’s therapeutic potential, clinical application has been limited by subtherapeutic levels of hematopoietic chimerism which impacts successful correction of pathologic phenotypes in targeted disorders. B-cells play an integral role in antibody-mediated graft rejection and this study sought to evaluate host and donor B-cells with changes in chimerism.

Methods: BALB/cJ fetuses were injected with 20x106 adult B6-GFP bone marrow mononuclear cells (n=28) or phosphate-buffered saline (sham, n=13) on embryonic day 14. For six months, peripheral blood samples were drawn monthly for flow cytometry. Peripheral blood chimerism was determined by determining the percentage of circulating leukocytes that were GFP+. B-cells were measured as percentages of total B-cell percentage (B220+/total lymphocytes), donor B-cell percentage (B220+/GFP+ Lymphocytes), and host B-cell percentage (B220+/GFP- lymphocytes). Mice were grouped as high (>50%) or low ( < 50%) chimerism groups at one month and compared to fostered littermate controls (n=20).

Results: 25% of chimeric mice were highly chimeric. The mean chimerism at one month was 81.0 ± 16.5% and 12.8 ± 10.7% for the high (n=7) and low (n=21) chimeric groups, respectively. Low chimeric mice exhibited further reduction in peripheral blood (PB) chimerism over time (4.7% ± 1.2 at 6 months). Two mice had no detectable GFP, and seven others had less than 2% peripheral blood chimerism at harvest (Figure 1a). By contrast, PB chimerism continued to increase in high chimeric mice time (93.9% ± 3.7 at 6 months). (Figure 1)

Conclusion: Total B cell populations increase over time in both high and low chimeras IUHCT. High chimeric mice have an increase in donor B cells while host B cells are suppressed; however, the opposite occurs in low chimeric mice. This may represent a donor response to suppress host B cell activation and proliferation, reducing the production of alloantibodies that could lead to graft failure. The suppression of host B cells may be a critical step to increasing engraftment in IUHCT to overcome one of the greatest barriers to successful IUHCT.