A Comparative Analysis of Immune Reconstitution Following Reduced Intensity Conditioning with CAMPATH-1H and Total Lymphoid Irradiation/Anti-Thymocyte Globulin Prior to Allogeneic Stem Cell Transplantation.

Abstract 1148

Poster Board I-170

Graft versus host disease (GVHD) remains a significant cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (HCT). In vivo quantitative T-cell depletion using CAMPATH-1h (anti-CD52) has been explored in an effort to prevent acute GVHD. More recently, a regimen consisting of total lymphoid irradiation and anti-thymocyte globulin (ATG) has been shown to polarize T cells towards an inhibitory phenotype potentially reducing the associated risk for GVHD. However, these strategies may be associated with impaired post-transplant immune reconstitution, increased risk of tumor relapse and opportunistic infection. In this study we examined the pattern of cellular immune recovery following T cell depletion with CAMPATH-1h and compared results with an initial cohort of patients undergoing reduced intensity conditioning with TLI and ATG. Immunologic analyses were performed on twenty patients undergoing reduced intensity conditioning in conjunction with low dose CAMPATH -1h (50 mg) and an initial cohort of 5 patients treated with TLI/ATG. Conditioning with CAMPATH-1h resulted in the significant depletion of CD3, CD4, and CD8 T cells in the early post-transplant period and persistence of CD4 T cell depletion (< 200 cells /uL) for more than 6 months. Following TLI/ATG, persistent depletion of CD4+ T cells was also observed but no significant decrease in CD8 T cells was seen. A two-fold increase in circulating CD56+ NK cells, 21.8 to 41.6% (p=0.004), was seen following TLI-ATG, which was not noted following Campath conditioning. CAMPATH-1h conditioning was associated with a significant decrease in mean CD45RO+ memory T cells in the early post-transplant period (27.2 to 5.7% of the total population of nonadherent peripheral blood mononuclear cells, p=0.034). Relative percentages of naïve T cells (CD45RA+), central memory (CD45RO+CD62L+CCR7+) (CM), and effector memory (CD45RO+CD62L-CCR7-) (EM) T cells remained stable in the pre- and post-transplantation period. The CM:EM was 0.6 pre-transplant and at day 60, respectively. In contrast, T cell recovery in early post-transplant following the TLI/ATG regimen was associated with no reduction in CD45RO+ memory T cells. A significant rise in the relative percentages of naïve T cells from 39 to 61.3% (p=0.04), CM cells from 12 to 32.8% (p=0.05), a corresponding fall in EM cells from 57.9 to 32.5% (p=0.10), and a significant change in the CM:EM levels (0.2 pre-transplant, 1.0 day 60 post-transplant) was noted after TLI/ATG. The mean percentage of regulatory T cells as defined by the percentage of CD4+/CD25+ cells that express FoxP3 rose in the early post-transplant period following both regimens (8 to 20.7% at Day 30, p=0.003 in the CAMPATH group; 5.6 to 16.9% at Day 30, p=0.03 in the ATG/TLI group). Functional analyses demonstrated that the T cell proliferative response to the mitogen, Phytohemagglutinin (PHA), was profoundly depressed following CAMPATH-1h with mean SI decreasing from 34 pre-transplant to 1.4 at Day 30. In contrast, treatment with TLI/ATG resulted in no significant change in T cell proliferation in response to PHA with SI only decreasing from 45 pre-transplant to 36 at Day 30. Assessment of T cell polarization following stimulation with PHA or phorbol-ester (PMA)/ionomycin, recipient derived dendritic cells (DCs) or third party DCs demonstrated a rise of CD8+ T cells expressing, IL-4 and IL-10 consistent with a suppressor phenotype. Minimal T cell proliferation was observed following stimulation with patient derived DCs, which is consistent with suppression of the expansion of alloreactive T cells. In summary, both CAMPATH and TLI/ATG result in CD4+ T cell depletion but TLI/ATG resulted in relative preservation of CD8+ T cells, persistence of memory cells, relative preservation of central memory as compared to memory effector cells and intact response to mitogens. TLI/ATG therapy was also associated with the polarization of CD8+ T cells towards a Tc2 phenotype and lack of proliferation in response to recipient derived DCs. As such, TLI/ATG appears to be associated with more modest level of functional T cell depletion characterized by Tc2 polarization and suppression of host/donor alloreactivity.