Graft Failure After Hematopoietic Cell Transplantation

Introduction and Historical Perspective

The first hematopoietic cell transplantation (HCT) was performed in 1956 and demonstrated neutrophil recovery and transient donor cell engraftment, as documented by red cell engraftment. However, in these original transplants only two of the six reported patients had transient evidence of donor cell engraftment. From 1958 to 1968, a total of 203 patients underwent allogeneic bone marrow (BM) transplant, and the results of their outcomes were compiled by Dr. Mortimer Bortin. Of these patients, 125 failed to have any evidence of donor cell engraftment and only 11 patients were documented to be stable allogeneic chimeras. Thus, the first barrier for successful HCT faced by the pioneers of this field was to successfully engraft donor cells in the host.

Graft failure was one of the most prominent causes of treatment failure during the initial experience with HCT in patients with aplastic anemia. Thirteen of 49 patients reported by Storb et al. failed to engraft, of whom only one was successfully retransplanted. With improved conditioning and supportive care, along with human leukocyte antigen (HLA) typing, the incidence of primary graft failure (PGF) has decreased significantly. However, PGF and secondary graft failure (SGF) are still important causes of treatment failure. In this chapter, we will review current concepts and definitions of graft function as well as a modern approach to the diagnosis and treatment of graft failure after HCT.

Concepts and Definitions

• Hematopoietic cell recovery (HCR): This concept applies for both autologous and allogeneic HCT and is defined by the evidence of an absolute neutrophil count (ANC) of ≥ 500/mm ³ (or ≥ 0.5 × 10 ⁹ /L) in peripheral blood (PB) for 3 consecutive days. The date of ANC recovery is the date of the first of three consecutive laboratory values where the ANC is ≥ 500/mm ³ . HCR commonly occurs within 28 days after PB or BM infusion and after 42 days of cord blood (CB) infusion.

• Engraftment: This term should be preferentially used for allogeneic HCT and refers to the ability of donor hematopoietic stem cells to adhere to the recipient BM and restore adequate blood cell production. Engraftment is defined by the evidence of donor-derived cells in PB or BM and is confirmed by chimerism studies. In the autologous HCT setting, engraftment can be used to define the presence or absence of genetically modified cells, such as those used for the treatment of thalassemia or sickle cell disease.

• Delayed hematopoietic recovery: Failure to show HCR in the known predictable time, depending on the cell type infused (PB, BM, or CB), is considered delayed engraftment. Several factors can contribute to the prolonged time in reinitiating the hematopoietic process. Underlying disease, damage to the BM microenvironment from previous chemotherapy, donor-recipient HLA compatibility, origin of the donor cells, CD34 ⁺ cell dose infused, type of conditioning regimen, and opportunistic infections are among the factors that contribute to a significantly longer time to ANC recovery.

• Graft failure: Failure of the transplanted stem cells to achieve or sustain HCR after HCT defines graft failure in the absence of disease recurrence. This phenomenon is most commonly observed after allogeneic HCT. Failure to achieve HCR after HCT is considered PGF. After initial engraftment, patients may experience loss of their graft, as evidenced by a sustained decline of ANC < 0.5 × 10 ⁹ /L for 3 consecutive days. This is classified as SGF. Depending on the cause, SGF may be transient and reversible.

• Autologous reconstitution: In the setting of allogeneic HCT, HCR in which a majority of the cells are host-derived is referred to as autologous reconstitution . This phenomenon can occur initially after transplant or later and is another form of graft failure when it occurs in the absence of recurrence of the original malignancy. Autologous reconstitution does not necessarily require intervention.

• Graft Rejection: Graft rejection is an immune-mediated reaction between the donor allograft and the residual host immunity. Graft rejection is usually observed during the first months after HCT.

• Poor graft function: Severe cytopenia of at least two cell lines with or without a transfusion requirement in the presence of hypoplastic or aplastic BM with full donor chimerism can occur. It is possible for this to occur in the absence of severe graft-versus-host disease (GVHD) or relapse. It is a serious complication postallogeneic HCT that can occur any time posttransplant.

Evaluation of Engraftment

Engraftment is confirmed by evidence of donor-derived cells in the BM. The proportion of donor:host cells in the BM is referred to as chimerism . Full chimerism is defined as when the hematopoietic and lymphoid cells are all donor-derived. In 2001, the American Society of Blood and Marrow Transplant and the Center for International Blood and Marrow Transplant Research (CIBMTR) convened a workshop on chimerism postallogeneic HCT, and in that workshop full chimerism was defined as 100% of the lymphoid and hematopoietic cells being donor-derived. However, at that time most assays were only semiquantitative, and small numbers of host cells could go undetected. Furthermore, mixed chimerism was generally defined as the presence of at least 5% to 10% donor cells. For data management purposes, the European Society for Blood and Marrow Transplantation and CIBMTR define predominantly or complete donor chimerism if there is evidence of 80% or more donor cells that are T-cells, B cells, or myeloid cells. The presence of 5% to 79% donor cells defines mix-chimerism.

Chimerism testing after HCR is used for different purposes besides evaluation of engraftment. In general, we and others perform evaluation of engraftment at 30 days, 100 days, 6 months, and a year posttransplant, in concordance with the CIBMTR reporting schedule.

Testing methods to evaluate engraftment have evolved over the years. The semi-quantitative polymerase chain reaction (PCR)-based method to evaluate chimerism seems to be the most widely used. This method uses whole blood to identify T-cells and myeloid cells. Other methods that are commonly used are quantitative PCR and next-generation sequencing (NGS). Further subset analyses requires that PB or BM cells undergo separation or sorting into purified T-cells, B cells, or lymphoid versus myeloid populations to perform subset chimerism analysis.

Table 32.1 summarizes the different assays used to document engraftment. Red blood cell phenotyping, cytogenetics, or fluorescence in situ hybridization techniques are only informative in sex-mismatched transplants and are only rarely used. The current gold standard recommended by the Eurochimerism consortium are PCR assays looking for short-tandem repeats or variable number tandem repeats.