Mapping of T Cell Subsets in the Tumor Microenvironment of Multiple Myeloma and its Precursor Conditions

Abstract

The bone marrow tumor microenvironment (BMME) in multiple myeloma (MM) is essential for maintaining anti-tumor activity. Dysregulation of normal immune responses is believed to be an important factor in tumor evasion and a contributing factor in the development of active MM from its precursor conditions, monoclonal gammopathy of undetermined significance (MGUS) and smoldering multiple myeloma (SMM). T cells are key immune regulators and are essential for maintain effective anti-tumor activity with their cytotoxicity and suppressor abilities. However, with those cells being altered and dysfunctional in MM it has been documented that they suppress the immune system more than the disease itself, allowing clonal plasma cells to keep growing and surviving. The aim of this study was to map T cell subsets in the BMME in a screened population of all stages of MM development. Participants were recruited from the Iceland Screens, Treats, or Prevents MM (iStopMM) study, a population-based screening study (N = 80,759) for MM precursors and randomized trial-follow strategies. BM aspirate samples from individuals with MM and its precursors conditions were analyzed using multiparameter flow cytometry (MFC) and automatic clustering used to quantify T cell subsets, notably, CD4+, CD8+, regulatory (Tregs), and TCRγδ T cells, along with the respective maturation stages of CD4+ and CD8+ T cells: naïve, effector memory (EM), central memory (CM), terminally differentiated effector memory (Temra). The distribution of T cell subsets was compared between disease stages and according to the presence of the independent risk factor for progression of MGUS and SMM to active MM, 95% or more of clonal plasma cells within the plasma cell compartment in BM. A total of 138 individuals were included in the study cohort (40 had MGUS, 78 had SMM and 20 had MM). The percentage of total T cell was found to be significantly higher in individuals with SMM and MM when compared to MGUS (p = 0.003 for MGUS vs SMM and p = 0.013 for MGUS vs MM). The highest median percentage of CD4+ T cells was observed for SMM compared to MGUS but higher in MM compared to MGUS (p = 0.036 for MGUS vs SMM). CD8+ T cells were found to be significantly higher in MM compared to MGUS (p = 0.042 for MGUS vs SMM and p = 0.031 for MGUS vs MM). No difference was observed for regulatory T cells and TCRγδ between disease stages. Of the characterized maturation stages of CD4+ and CD8+ T cells, CD4+naïve were found to be significantly higher in MM compared to MGUS and SMM (p = 0.037 for SMM vs MM), however no difference was observed of others maturation stages between disease stages. The count of total T cells and respective T cell subsets (CD4+, CD8+, T regulatory cells) were found to be higher in those who had 95% or more of abPC ratio compared to those who had less than 95% abPC ratio (p<0.05 for every T cell subset). No difference was observed for TCRγδ between groups (abPC). Of the characterized maturation stages of CD4+ and CD8+ T cells, CD4+Temra were found to be significantly higher in those who had 95% or more of abPC ratio compared to those who had less than 95% abPC ratio (p = 0.042), however no differences were observed of other maturation stages between groups. In this thesis, based on a large unique nationwide population-based study on MM and its precursor conditions, distribution of T cell subsets was evaluated in MM and its precursor conditions and by the established progression risk factor of an abnormal plasma cell ratio for precursor conditions of MM. We found differences in the distribution of distinct T cell subsets between MM and its precursor conditions and between individuals with precursor conditions according to an independent risk factor for MM progression. Our findings suggest that T cell expansion and changes in the T cell compartment in the BMME occur during MM development and may be a contributing factor in progressive disease. These findings, based on an unbiased precursor condition and MM cohort, will clarify some of our understating of the role of BMME in the development of MM, which may lead to improved diagnostics and potentially to targeted treatment for patients with early MM disease, ultimately leading to improved outcomes in MM.