Allogeneic blood and marrow transplantation is performed with growing success worldwide as highlighted by several centers reporting comparable outcome after unrelated donor bone marrow transplantation (BMT) and cord blood transplantation that match the outcome of genotypically HLA-matched sib recipients [3
]. Although for many, chronic GVHD remains a major barrier to achieving a sufficiently high quality of life, those patients who are successfully weaned off systemic IS not only demonstrate freedom of underlying malignancy, marrow failure, or primary immunodeficiency but also achieve a state of transplantation tolerance [6
]. Full-donor chimerism as a way to “protect from relapse” is an oft-stated goal of transplanters caring for leukemia patients. Nevertheless, not all patients require 100% of their hematopoietic and immune cells to be of donor origin, in particular, those with nonmalignant disorders. Long-term stable coexistence of host and donor cells without clinical evidence of immune-mediated pathology is often referred as persistent mixed chimerism
. It existed long before the advances of reduced-intensity conditioning. Despite undergoing myeloablative conditioning, a sizeable proportion of patients with β-thalassemia [7
] or sickle cell disease [8
] reconstitute with mixed host and donor hematopoietic cells, each represented in excess of 10% for many years. Similarly, infants with severe combined immunodeficiency receiving HLA-disparate marrow grafts have demonstrated a sustained mixture of donor and host cells in the absence of GVHD or graft rejection. The concept of split chimerism is best illustrated by ex-severe combined immunodeficiency patients whose fractionated blood chimerism may display predominantly host B lymphocytes and myeloid cells in contrast with overwhelmingly donor-derived T cells, because of selective advantage of donor cells over genetically defective host cells. The underlying immune mechanisms responsible for the absence of alloreactive immunopathology in both full-donor and in persistent mixed chimerism is increasingly being characterized. The presence of circulating T cells with immune reactivity against host tissues even in the tolerant state points to incomplete clonal deletion of newly emerging thymocytes and/or indicating long-term survival of adoptively transferred host-reactive T cells. Nevertheless, peripheral regulatory mechanisms are operational, as highlighted by the identification of IL-10–secreting T regulatory cells (Tregs) and other regulatory T- and B-lymphocytes [6
]. Recently, CD4+
Tregs have entered the clinical arena with an encouraging safety profile; however, their efficacy in restoring immune tolerance, that is, successful treatment of GVHD, has not yet been established [9
A novel approach for achieving tolerance and immune competence is illustrated by the recent unique case of a 17-year-old patient with combined immunodeficiency disease who required bilateral orthotopic lung transplantation to treat pulmonary failure caused by recurrent bacterial (Stenotrophomonas, Escherichia coli) and mycobacterial pneumonia. Chronic hypoxia and recurrent infectious gastroenteritis led to severe growth failure necessitating total parental nutrition for years. She presented in 2009 with the clinical necessity for lung and hematopoietic transplantations to correct pulmonary insufficiency and the underlying combined immunodeficiency disease. At this age, her lymphocytes had declined to a stage of extreme lymphopenia of T and B cells (30–70 CD3+ T cells/mm3) but normal natural-killer cell numbers.
A single-patient protocol proposing the use of a lung and BMT from the same donor to reduce the probability of pulmonary graft rejection or GVHD of the lungs was approved by Duke University institutional review board and FDA (IDE #14206). A 4 of 8-HLA-matched unrelated cadaveric donor was identified by the United Network for Organ Sharing, who underwent iliac crest marrow harvest yielding 5.4 × 108 cells/kg. Lungs were procured and transplanted following the marrow harvest. The marrow was cryopreserved following CD3 and CD19 depletion. Bilateral orthotopic lung transplantation was performed in December 2009 following pulse steroids and basiliximab. Within days, the patient became independent of supplemental oxygen. Three months later, while receiving FK506 and low-dose prednisone, she underwent conditioning with rituximab, alemtuzumab, antithymocyte globulin, hydroxyurea, a dose of Thiotepa, and a single fraction of total-body irridiation (lung shielding) before infusion of the T cell-depleted thawed bone marrow. She was discharged on day +20 on FK506 monotherapy with outstanding performance status and full-donor chimerism in whole blood, CD3+ T cell, and CD15+ myeloid cell fractions. She has remained without detectable host leukocytes—last tested at 15 months post-BMT. Serial repeat lung biopsies have shown absence of immune rejection. Severe gut GVHD developed in September 2010 following a bout of norovirus gastroenteritis and an associated drop in FK506 levels. GVHD resolved after a single dose of infliximab and low-dose steroid therapy; however, her course was complicated by enterococcus sepsis and DIC. FK506 wean was started ~1 year post-BMT and completed by ~15 months after BMT. The pretransplantation lymphopenia has resolved, and since age 9 months post-BMT, her CD4+ cells are >250 cells/mm3. CD8+ cells have been >500 cells/mm3 since ~3 months post-BMT with associated significant anticytomegalovirus proliferative responses (stimulation index >10). Functional B cell recovery is evident with normal serum IgA levels. Thymic output has been slow and became detectable only at last testing (15 months post-BMT) when the percentage of CD4+ T cells dually expressing CD45RA and CD62L rose to 4%. Biological studies were performed at 12 months posttransplantation to test for alloreactivity. Highly purified peripheral blood (donor) T cells showed hyporeactivity (proliferation, IL-2 and tumor necrosis factor secretion) against host-derived Epstein-Barr virus-transformed B cell lines compared with Epstein- Barr virus-lymphoblastic cell lines generated from haplotype mismatched maternal (<7%) or fully mismatched unrelated donor (<5%). This “tolerant” state could not be broken by in vitro Treg depletion and neither HLA class I or II blockade led to further attenuation of self-reactivity. There is no detectable IL-10 in coculture supernatants. Taken together, in this T cell-depleted BMT setting, the ancillary immune data support a central clonal deletion mechanism to explain the lack of antirecipient reactivity. In sum, this unique case shows for the first time, the feasibility and immune consequences of tandem cadaveric lung and T cell-depleted marrow transplantation from the same HLA-mismatched unrelated donor to create solid organ and recipient-specific tolerance and fully functional donor-derived hematopoietic, immune, and pulmonary organ function in the absence of systemic immunosuppression. However, with all leukocytes of donor origin, it cannot be determined if there is a threshold prerequisite for donor cell chimerism to achieve tolerance.