Comprehensive Immune Reconstitution without Development of Graft Versus Host Disease (GvHD) Using Limited T-Cell Add Back at the Time of Unrelated Multi-Antigen Mismatched Pan-T-Cell Depleted Transplant.

Application of unrelated multi-antigen mismatched transplant is limited by a high risk of severe GvHD. Pan-T-Cell depletion successfully limits GvHD in this setting, but graft failure, early disease relapse and prolonged immunodeficiency have prevented broad use of this approach. In an attempt to limit GvHD, obtain engraftment and early disease control, we evaluated the effect of adding back 1 x 10⁵ bone marrow derived CD3⁺ cells/kg patient body weight, following full conditioning, to a pan-T-cell depleted (Isolex, Baxter, Chicago IL) marrow graft (1 x 10⁶ CD34⁺ cells/kg). In this pilot trial, three consecutive male patients with high risk malignancies without matched donors (disease/age: ALL-CR2/22, refractory AML/31 and CML blast crisis/55 failed imatinib) consented to receive multi-antigen mismatched unrelated marrow grafts (ALL at B/C, AML at B/DR/DQ and CML at A/DQ). Two patients (AML/CML) received transplant conditioning with busulfan/cyclophosphamide and one received TBI/cyclophosphamide (ALL). In addition, all patients received rabbit derived ATG (3 mg/kg days -3 and -2). GvHD prophylaxis included standard tacrolimus and methotrexate with prednisone (0.5 mg/kg starting day +7 tapered off by day + 90). All patients achieved neutrophils 1000/ul (days +15, 21 and 26). No patient developed detectable acute or chronic GvHD. One patient (ALL) died on day +110 in remission, from drug resistant CMV pneumonia. The other two are long term clinical disease-free survivors at 25 (CML) and 40 months (AML) with complete blood counts and differentials within the normal range. Neither survivor has experienced any serious late infection and both have undetectable EBV DNA. Immunoglobulin levels (IgG, A and M) are within the normal range for both patients except for a low IgA level (29 mg/dl) in the CML patient. At 40 months post-transplant, the AML patient is 100% female by FISH, and has normal T and B cell counts (CD3, CD8, helper/suppressor ratio, CD19) with a borderline low CD4 count(507; nl range >666cells/cu.mm). At 25 months, the CML patient has normal CD3 and CD8 counts, increasing CD4 counts (260) and improving chimerism (CD33⁺ 100% donor; CD3⁺ 33% donor). TCR Vb CDR3-size spectratype analysis of CD4 and CD8 subsets of these two patients were compared to 6 healthy donors to assess the complexity of their reconstituting T cell repertoire. Of 24 Vb families, 21–23 were resolvable revealing an overall complexity in the CD4 compartment of 91% (AML) and 82% (CML), and in the CD8 compartment 86% (AML) and 73% (CML). Limited T-cell add back at the time of unrelated multi-antigen transplant did not cause GvHD, was associated with prompt and stable engraftment and appears to facilitate comprehensive immune reconstitution. Further study is warranted.