UBC Theses and Dissertations
Determining the role of hyaluronan as an environmental cue for macrophages and dendritic cells Dong, Yifei (Jeff)
CD44 is a cell surface glycoprotein that binds to hyaluronan (HA), an extracellular matrix glycosaminoglycan. Immune cells widely express CD44, but only a few types of cells such as alveolar macrophages (AMФ) in the lung alveoli bind fluorescein-conjugated HA constitutively. During inflammation and immune responses, other immune cells such as monocytes are activated and can gain the ability to bind HA. However, the functional significance of CD44 and HA interactions remains unclear. Therefore, the aim of this study was to investigate the function of CD44 and HA binding and how they regulate immune cells such as AMФ in the tissue environment. HA has been described as a regulator of tissue inflammation, with HA fragments reported to stimulate immune cells. To test if HA fragments can induce inflammation or are consequences of inflammation, I stimulated macrophages and dendritic cells with various sizes of HA from different sources. Pharmaceutical grade HA and endotoxin-free HA fragments failed to stimulate an inflammatory response in vitro and in vivo, demonstrating they were not pro-inflammatory. Since AMФ constitutively bind HA, I then compared these cells from CD44+/+ and CD44-/- mice to study the role of CD44 and HA binding as regulatory environmental cues. Using adoptive transfer experiments and a mouse model of inflammation, I found CD44 expression and HA binding were required for the survival of mature AMФ, but not for the recruitment and differentiation of monocytes into AMФ. CD44 expression by AMФ was required for a cell surface HA coat, which maintained AMФ survival and numbers in the lung. Since AMФ are essential for regulating pulmonary surfactant lipid homeostasis, CD44 deficiency and the partial loss of AMФ in CD44-/- mice disrupted lipid homeostasis in their lungs. CD44-/- mice had elevated lung surfactant phosphatidylcholine levels and CD44-/- AMФ exhibited an abnormal phenotype and accumulated cellular lipid droplets. They also suffered greater inflammation caused by oxidized phosphatidylcholine. Thus, CD44 deficiency led to a reduction of AMФ numbers and caused intrinsic defects in AMФ surfactant lipid homeostasis. Together, these results demonstrate a role of CD44 and HA binding in maintaining AMФ and lung homeostasis.
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