Improved Reconstitution of CD4 T Cells and B Cells But Worsened Reconstitution of Serum IgG Levels After Allogeneic Transplantation of Blood Stem Cells Instead of Marrow

To the Editor:

In a recent issue of Blood, Ottinger et al1 described that the numbers of circulating CD4 T cells and B cells are higher after peripheral blood stem cell transplantation (PBSCT) compared with bone marrow transplantation (BMT). We wish to present data that support the findings of Ottinger et al and to add new findings on B-cell immunity, including the surprising finding of decreased serum IgG levels after PBSCT compared with BMT.

Blood from 57 HLA-identical sibling PBSC (n = 26) or BM (n = 31) transplant recipients was studied at approximately 2 months after grafting. All patients had a hematologic malignancy and were conditioned with high-dose chemotherapy and/or radiation (usually cyclophosphamide [120 mg/kg] with busulfan [16 mg/kg] or fractionated total body radiation [13.2 Gy]). PBSCs were obtained from donors after mobilization with filgrastim (15 to 30 μg/kg/d subcutaneously for 4 days). The PBSC and BM grafts were not cryopreserved or depleted of any subset of nucleated cells. Graft-versus-host disease prophylaxis usually consisted of methotrexate and cyclosporine.2 All patients underwent a thorough work-up to determine disease status on approximately day 80. Relapse was diagnosed in 7 of the 57 patients; these patients were excluded from analysis. One other patient was excluded because only 25% of his day 80 marrow nucleated cells were of donor origin by variable nucleotide tandem repeats (VNTR).3 The remaining 49 patients (20 PBSC and 29 BM recipients) had greater than 90% marrow or blood nucleated cells of donor origin by VNTR or Y-chromosome in situ hybridization.4 Demographic and clinical characteristics of the 49 patients analyzed are given in Table 1.

Because at least some lymphocytes after grafting are derived from the mature lymphocytes transferred with the graft,5 we also quantified lymphocyte subpopulations in 18 PBSC and 15 BM grafts. For each lymphocyte subset the ratio of median number of cells in a PBSC graft to median number of cells in a BM graft was calculated to show how much more cells of each subset are typically transferred with a PBSC graft compared with a BM graft.

The enumeration of lymphocyte subsets in blood was performed by three-color flow cytometry as described.6,7 Lymphocyte subsets in the grafts were detected analogically, except for initial (forward × side scatter) gating on all nucleated cells rather than mononuclear cells; the absolute count of each lymphocyte subset was calculated as the total nucleated cell count multiplied by the proportion of these cells. Serum IgG levels were determined by standard nephelometry.

T cells.As displayed in Table 2, PBSC recipients received greater than 11 times higher numbers of naive and memory CD4 T cells and greater than 7 times higher numbers of CD8 T cells than BM recipients. At 2 months after grafting, circulating naive CD4 T cells were approximately 6 times higher, whereas memory CD4 T cells were only slightly (1.6 times) higher after PBSCT than after BMT, and CD8 T cells were similar in both patient groups. The discrepancy in the PBSC/BM ratio of memory CD4 T cells and CD8 T cells in the grafts versus in the blood after transplantation remains to be explained. It could be due to increased propensity of PBSC graft-derived memory CD4 T cells and CD8 T cells to die,8 due to greater proliferation of the BM graft-derived memory CD4 T cells and CD8 T cells, or due to another mechanism.

B cells.The number of both naive as well as memory B cells was substantially (>3 times) higher both in the PBSC grafts (compared with BM grafts) and in the blood of PBSC recipients (compared with BM recipients) at 2 months after grafting. However, total serum IgG on day 80 was lower in PBSCT versus BMT recipients (Table 2). It is unknown whether this could be due to the transfer of a larger number of plasma cells with BM than with PBSCs (Table 2), due to the increased number of B-cell precursors in the BM graft (Table 2) that might differentiate into plasma cells, or due to another mechanism.

It remains to be determined whether the increased numbers of naive CD4 T cells and naive as well as memory B cells after PBSCT translate into improved antibody responses to vaccination (particularly with neoantigens), improved serum levels of specific antibodies to common recall antigens such as Haemophilus influenzae, and decreased rates of infections.