Event

PhD Dissertation Defense - Ann Ramirez

Tuesday, December 3, 2024
12:00 p.m.
AJC 3104
Rachel Chang
301 405 8268
rachel53@umd.edu

Title: Characterizing the biophysical properties of mouse lymph nodes using multiple particle tracking

Committee members:

Dr. Katharina Maisel, Chair

Dr. Gregg Duncan

Dr. Giuliano Scarcelli

Dr. Kimberly Stroka

Dr. Martha Wang

Dr. Birthe Kjellerup, Dean's Representative

Abstract:

Lymph nodes (LNs) are scattered throughout the body and essential for mounting an adaptive immune response. They are highly spatially organized, which helps to ensure that these immune responses are efficiently mounted. LN spatial organization is supported by an extracellular matrix (ECM), which is thought to be more densely packed in the inner paracortex and less densely packed in the outer cortex. ECM provides tissue with viscoelasticity, which is a property of a material that exhibits both viscous and elastic responses. Elasticity is the ability of a material to be pulled or compressed without permanent deformation, whereas viscosity is the liquid-like response of the material. Recent studies have shown that LN viscoelasticity affects cell functions, including cell proliferation, migration speed, and morphology. However, the viscoelasticity of LN has not yet been characterized.

Traditional methods for studying material rheology are inadequate for studying the intricate architecture of LNs. In this study, live LN tissue slices were used to maintain LN architecture, and viscoelasticity was measured using an established method called multiple particle tracking (MPT). In MPT, fluorescent probes are placed on tissue slices, and the Brownian motion of the particles is recorded. The displacements of probe trajectories can be applied to the Stokes-Einstein relationship to calculate the values of moduli, pore size, and viscosity. 

In this thesis, we developed an inert probe for use with MPT that does not interact with other probes or the environment. We then characterized skin draining and mesenteric LN viscoelasticity at homeostasis. We used an acute mouse model in which lipopolysaccharide (LPS) was intradermally injected to inflame inguinal LNs, euthanized at specific time points, and viscoelastic properties were characterized using MPT. To observe any changes in ECM components during inflammation, we used immunohistochemistry and image analysis to quantify changes in ECM components such as collagen, elastin, and fibronectin. Additionally, we characterized viscoelasticity in a chronic inflammation mouse model (weekly LPS injections for 5 weeks) and euthanized the mice at peak inflammation and resolution. We found that LN viscoelasticity is site-specific, where skin-draining LNs exhibit stiffer properties than mesenteric lymph nodes. We also found that during acute inflammation, LNs exhibited tissue stiffening at peak inflammation and were permanently restructured after resolution. Lastly, we found that chronic inflammation did not significantly change LN viscoelasticity. Our findings show the characteristics required to use MPT in LN tissues to characterize the viscoelasticity of LNs while maintaining their structure. We also successfully measured the viscoelastic properties of LNs during acute and chronic inflammation and began probing how the ECM components change in relation to viscoelasticity.