Phenotypic Analysis and Single-Cell Gene Profiling of Human Regulatory T Cell Subsets in Human Graft-Versus-Host Disease

Abstract 1975

Acute graft versus host disease (aGVHD) is an important cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (alloHSCT). CD4+FOXP3+ regulatory T cells (Treg) are a population with potent immunosuppressive properties and may offer a new way to prevent or treat aGVHD. Treg therapy has demonstrated high efficacy in mouse models; however, translation to human has been hampered by the identification of different subsets of human Treg and doubt about in vivo conversion of Treg into effector Th17 cells. Based on the expression of CD45RA and HLA-DR, we identified 3 different subsets of human FOXP3+ Treg in healthy subjects' peripheral blood as well as in cord blood. All 3 subsets were suppressive in vitro. Gene expression profiling combined with global pathway analysis revealed clearly distinct immune signatures for each subset, which were validated by analysis at the single-cell level. Single-cell gene profiling also uncovered a striking heterogeneity of gene expression within these Treg subpopulations and revealed that cytokine-expressing Treg did not downregulate FOXP3 and other Treg markers.

We prospectively studied Treg subsets in 18 consecutive alloHSCT recipients' peripheral blood. Median age was 47 years (range: 26 to 65). Analysis was performed before steroid initiation in patients with aGVHD (n=7) and at hematopoietic recovery in the control group (n=11). First sample was analyzed a median of 20 days after alloHSCT (range: 11 to 36) with no difference between the 2 groups. Percentages of FOXP3+ cells in CD4+ cells were not significantly different in aGVHD patients and in the control group (10.4 and 12.6%, p=0.53). However, we observed in the aGVHD group a strong alteration of Treg subsets compared to the control group, with a pronounced bias towards an activated phenotype. RA-DR+ cells were significantly more represented among FOXP3+ T cells in aGVHD patients than in the control group (80.8 versus 53%, p=0.003). Conversely, RA-DR- and RA+DR- cells were more frequent in the control group than in patients with aGVHD (26.8 versus 10.6% and 13.5 versus 2.1%, p=0.014 and p=0.0012, respectively).

Our data suggest that frequencies of specific Treg subpopulations, rather than the frequency of the total pool of CD4+FOXP3+ Treg, is altered in aGVHD and may serve as a biomarker for this condition.

Current work addresses the molecular and functional characteristics of Treg subsets in aGVHD patients. Since Treg have been suggested to convert into effector T cells in pro-inflammatory and lymphopenic conditions, we will also assess the potential “plasticity” of the three Treg subsets.