Mixed chimerism is frequent after allogeneic peripheral blood stem cell transplantation with positive CD34 selection, and is not reverted by low doses of donor T-cells add-back

Favorable outcomes in patients with high donor-derived T cell count after in vivo T cell-depleted reduced-intensity allogeneic stem cell transplantation

Patients with hematologic malignancies were conditioned using a rabbit antithymocyte globulin-based reduced-intensity conditioning regimen for allogeneic stem cell transplantation. Donor-derived CD3(+) cell count (ddCD3), a product of CD3(+) cell chimerism and absolute CD3(+) cell count, when <110/μL at 8 weeks post-stem cell transplantation predicted a high risk of sustained mixed chimerism and relapse. Alternatively, patients with a higher ddCD3 developed graft-versus-host disease more frequently, and when partially chimeric, had higher rates of conversion to full donor chimerism after withdrawal of immunosuppression. Early data from our small cohort of patients indicate that ddCD3 at 8 weeks may be used to guide decisions regarding withdrawal of immunosuppression and administration of donor lymphocyte infusion in partially T cell-depleted reduced-intensity regimens.

Genetically modified donor leukocyte transfusion and graft-versus-leukemia effect after allogeneic stem cell transplantation

Seven patients with acute myeloid leukemia (AML) and two patients with chronic myelogenous leukemia (CML) were transplanted from HLA-identical sibling donors with CD34(+) cell-enriched stem cells (HSCTs) without further immunosuppression. The myeloablative standard transplantation protocol was adapted to include transfusion of gene-modified donor T cells after HSCT. Donor T cells were transduced with the replication-deficient retrovirus SFCMM-3, which expresses herpes simplex thymidine kinase (HSV-Tk) and a truncated version of low-affinity nerve growth factor receptor (ΔLNGFR) for selection and characterization of transduced cells. Transduced T cells were detectable in all patients during follow-up for up to 5 years after transfusion. Proteomic screening for development of acute graft-versus-host disease (aGvHD) was applied to five of the seven patients with AML. No positivity for the aGvHD grade II-specific proteomic pattern was observed. Only one patient developed aGvHD grade I. To date, three of the patients with AML relapsed; one responded to three escalating transfusions of lymphocytes from the original donor and is in complete remission. Two were retransplanted with non-T cell-depleted peripheral blood stem cells from their original donors and died after retransplantation of septic complications or relapse, respectively. In one patient with CML, loss of bcr-abl gene expression was observed after an expansion of transduced cells. Seven of nine patients are alive and in complete remission.

Mixed chimerism in SCT: conflict or peaceful coexistence?

Stem cell transplants that follow both myeloablative and non-myeloablative conditioning regimens can result in states of mixed chimerism, which can be stable over time. With widespread availability of Y chromosome FISH in sex-mismatched transplantation and DNA-based methodologies for analysis of chimerism in other donor-recipient pairs, further insights have been gained regarding the implications of the mixed chimeric state. In transplants performed for inherited and acquired marrow failure disorders, disease status can be improved with only 10-20% donor cells, and it appears that stable mixed chimerism at that level is an acceptable outcome often leading to a state of tolerance, but an increasing level of recipient cells often precedes graft rejection. In transplants performed for malignant conditions, increasing levels of mixed chimerism may indicate disease relapse, but some cases with stable levels of mixed chimerism have been compatible with prolonged remission states. Understanding when mixed chimerism is an indication of secondary graft failure or impending graft rejection vs a state of tolerance and ongoing propensity for the establishment of a graft-vs-tumor effect is often difficult with currently available technologies and immunologic assays. The ability to understand the implication of mixed chimerism of multiple cell lineages and of varied lymphocyte subsets will remain important areas for future research to best harness the immunologic and other therapeutic benefits of allogeneic transplantation.

Chimeric protein for selective cell attachment onto cellulosic substrates

We have developed a fusion protein (CBD-LG) incorporating a cellulose-binding domain and an antibody binding domain, protein LG, to provide an adaptor molecule for cell separation with regenerated cellulose hollow fiber arrays. A single hollow fiber cell adhesion assay utilizing a CD34+ cell line, KG1a, was used to investigate whether ligand affinity interactions were strong enough for cell attachment and separation. CBD-LG efficiently captured CD34+ cells labeled with the mouse IgG2a monoclonal antibody MHCD3400. However, it was not possible to bind CD34+ cells labeled with an IgG1 antibody (HPCA-2). The low affinity of HPCA-2 for LG was overcome by secondary antibodies: KG1a cells that were dual labeled with HPCA-2 followed by rat anti-mouse IgG1 adhered inside hollow fibers coated with CBD-LG. Alternatively, immobilized rabbit polyclonal anti-mouse IgG1 captured cells labeled with HPCA-2. The development of an adaptor molecule to display recombinant domains at the surface of hollow fibers will be an effective tool to investigate cellular ligand-receptor interactions, a necessary step in the development of hollow fiber bioreactors for manufacture of human cellular products.