T cell evolution in the blood and organs during acute graft-versus-host disease

Abstract

Hematopoietic Stem Cell Transplantation (HSCT) can be curative for a variety of diseases, including hematologic malignancies, immunodeficiencies, and bone marrow failure syndromes. Graft-Versus-Host Disease (GVHD) remains one of the deadliest complications of HSCT. Even with the use of current GVHD prophylaxis regimens, which are broadly acting, non-targeted agents, GVHD still occurs in 50-80% of allogeneic transplant recipients. There is a need for more selective therapies to prevent GVHD while still preserving the protective effects of the immune system. T cells are the main instigators of acute GVHD (aGVHD), given that their removal ameliorates this disease. However, their removal does not decrease transplant mortality, due to diminished engraftment, increased relapse, and higher risks of infection. Considering the essential protective effects of T cells as well as their role in GVHD, better understanding and targeting of alloreactive T cells while preserving non-pathogenic T cells will be important for the future of HSCT. Chapter two describes our investigations into T cell programming changes that occur in the liver and lungs during aGVHD in non-human primates (NHP) and identified organ-specific gene expression programs specific to the lung and liver. We demonstrated how these programs developed through clonal expansion of the T cells within the organ-microenvironment in an antigen-independent manner. These discoveries laid a foundation for better targeted and possible organ-specific treatment development for aGVHD in the future. Chapter three describes our investigation of candidate GVHD-modifying gene edits in a xenogeneic model of aGVHD, using CRISPR/Cas9-edited human T cells transplanted into immunodeficient (NSG) mice. We edited target genes that were implicated in aGVHD T cell pathogenesis and evaluated how their deletion affected aGVHD development in the model. We focused on RGS1 for its role in aGVHD development, with results suggesting that knocking out RGS1 could decrease aGVHD. Chapter four describes our evaluation of the differences in T cell reconstitution after clinical HSCT in two cohorts of patients treated with distinct GVHD prophylaxis regimens: tacrolimus and methotrexate (Tac/MTX) compared to post-transplant cyclophosphamide, tacrolimus and MMF (PT-Cy). Additionally, we evaluated the differences in T cell gene expression programs when patients developed aGVHD in the two cohorts. We found evidence for wholesale T cell depletion with PT-Cy, resulting in long term deficits in TCR diversity, relative enriched contributions of host-derived cells, and prolonged deficits in naïve T cells compared to patients receiving Tac/MTX. T cell subsets in patients who developed aGVHD in either cohort demonstrated distinct immune activation patterns. The data presented in this thesis evaluate T cell reconstitution in the blood and organs of two model systems, mice and NHP, as well as human clinical trial samples, to further understand how T cells evolve throughout HSCT and the development of aGVHD. These discoveries have implications for the efficacy and toxicity of current aGVHD prophylaxis regimens, as well as for developing aGVHD prevention and treatment using targeted agents in the future.