Monocyte-Induced Regulatory T Cell Differentiation

Immediately after birth, thousands of foreign antigens challenge the newborn immune system. Many of the invaders are harmless, such as food, pollen, and beneficial bacteria. Newborns have a tolerant immune system that keeps them from developing inflammation or allergies to these new antigens. In utero, this immunoregulatory tendency is important for establishing tolerance to self and maternal antigens. Multiple processes contribute to fetal tolerance, including clonal deletion, anergy, changes in antigen presenting cells (APCs), and the generation of regulatory T cells (Tregs). However, the mechanism(s) of fetal Treg differentiation and the specific APCs required are unknown. Our lab has previously shown that many CD4+ and CD8+ T cells from umbilical cord blood (UCB) differentiate into Forkhead box P3 (Foxp3)+ Tregs after T cell receptor (TCR) stimulation ex vivo. Depleting CD14+ monocytes from UCB abrogates Treg generation, while purified CD14+CD36hi monocytes are sufficient to induce Treg differentiation from naïve T cells. The function of monocytes in protecting against bacterial infection, maintaining blood vessel integrity and promoting tissue repair are well known. However, their immunoregulatory properties have largely gone unrecognized. The goal of this dissertation is to identify the mechanisms monocytes use to induce Treg generation and describe how this process is impaired in adult blood or during disease states. In this work, I demonstrate that monocytes induce Treg differentiation by providing three critical signals to naïve T cells: membrane-bound transforming growth factor beta (TGF-β), retinoic acid and Notch ligands. CD14+CD36hi monocytes are the only UCB cells capable of presenting all three molecules to T cells, highlighting their importance for immune homeostasis. Ligand binding to the CD36 receptor can impair Treg generation and skew T cells to produce effector cytokines, such as IL-4. Pathogenically elevated levels of CD36 ligands, such as oxidized low-density lipoprotein (ox-LDL) during atherosclerosis or beta-amyloid during Alzheimer’s, may aggravate inflammation by impairing Treg generation. IL-4 potently blocks Treg generation from UCB, more than other inflammatory or effector cytokines. IL-4 has several reported mechanisms of inhibiting Foxp3 expression in T cells. We found that IL-4 also differentially regulates the mediators of TGF-β signaling, Smad2 and Smad3. Downregulation of Smad3 by IL-4 correlates with impaired Treg generation and knockdown of Smad3 alone is sufficient to decrease Foxp3 expressing cells. Together, these results demonstrate that CD14+CD36hi monocytes are an important immunoregulatory cell, capable of simultaneously producing multiple signals required for Treg differentiation. The impairment of monocyte-induced Treg generation by CD36 ligands may be an unrecognized cause of inflammation during diseases such as atherosclerosis. Furthermore, enhanced IL-4 production by T cells in the presence of ox-LDL may be one mechanism of impairing Treg differentiation