The Role of Innate Immunity in the Pathogenesis and Treatment of Experimental Pulmonary Hypertension

Abstract

In this thesis, the monocrotaline (MCT)-induced rat model of pulmonary arterial hypertension (PAH) was used to investigate the role of innate immunity in the pathogenesis of PAH and the mode of action of experimental therapies. The first section of this thesis is an investigation of the therapeutic mechanism of human, early and late-outgrowth endothelial progenitor cells (EPCs) in the MCT-induced, nude rat model of PAH. While late-outgrowth EPCs provided no therapeutic benefit in this model, early EPCs (E-EPCs) prevented the elevation of right ventricular systolic pressure (RVSP, P<0.001) and right ventricular (RV) hypertrophy (P<0.01). Ablation of natural killer (NK) and natural killer T cells with anti-asialo GM-1 antiserum (ASGM-1) enhanced human cell retention in the lung but abrogated the therapeutic capacity of E-EPCs. In vitro studies demonstrated that E-EPCs are similar to monocyte-derived regulatory dendritic cells (DCs) and possess the capacity to stimulate both autologous and rat NK cells in co-culture. Imatinib mesylate has been reported to reverse established PAH both clinically and in the MCT model. Imatinib can also induce NK activation through inhibition of c-kit signaling in DCs, suggesting that imatinib and the DC-like E-EPCs may prevent PAH through a similar, NK-mediated mechanism. In the second section of this thesis, imatinib prevented MCT-induced increases in RVSP (P<0.001) and RV hypertrophy (P<0.01) in immunocompetent Fisher 344 rats, but not in nude rats or Fisher rats following ablation of NK cells and T lymphocytes with ASGM-1. These data suggest that the stimulation of NK activity by imatinib is insufficient to prevent disease in the absence of T lymphocytes. Hyaluronan (HA) fragments are a potent inflammatory stimulus, capable of inducing macrophage activation and DC maturation. In the third section of this thesis, HA synthesis and degradation were investigated in the MCT model of PAH. While the early stages of disease were characterized by enhanced hyaluronidase-1 activity and a loss of high molecular weight (HMW) HA, severe disease was associated with HMW HA synthesis and HA accumulation in the lungs. The early degradation of HMW HA may drive inflammation and stimulate pathological vascular remodeling in PAH.