The primary object of the conditioning regimen for allogeneic reduced-intensity stem cell transplantation (RIST) is immunosuppression to achieve stable engraftment of donor cells, rather than bone marrow ablation. Therefore, immune reconstitution after RIST might be different from that after conventional stem cell transplantation (CST). In this study, 22 patients underwent RIST and 28 underwent CST. The RIST regimen consisted of cladribine (2-CdA; 0.11 mg/kg/day for 6 days), BU (4 mg/kg/day for 2 days), and rabbit anti-thymocyte globulin (ATG; 2.5 mg/kg/day for 2-4 days). The CST group received either the BU (4 mg/kg/day × 4 days)/CY (60 mg/kg/day × 2 days) (n = 13) or CY (60 mg/kg/day × 2 days)/TBI (4 Gy/day × 3 days) regimen (n = 15). All patients underwent transplantation with G-CSF-mobilize blood stem cells. Engraftment speed after RIST was fast and seven of 22 patients did not require platelet transfusion. We noted that the numbers of CD4+, CD4+ CD45RA+, and CD4+ CD45RO+ T cells after transplant in the RIST group were significantly lower than those in the CST group (P = 0.0001 for both the comparisons). However, the reconstitution of CD20+ B cells was faster in the RIST group (P = 0.0001). The response of T cells to PHA stimulation was lower in the RIST group (P = 0.0001 on day 30 and P = 0.02 on day 90). Nevertheless, there were no significant differences in the incidence of bacterial, fungal, or viral infections between the two groups. We concluded that our RIST regimen might delay laboratory-evaluated T-cell immune reconstitution compared to CST; however, the observed setbacks did not directly translate into clinically significant increases in infectious episodes.
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