2013 Theses Doctoral

Spatial Dynamics and the Mechanoresponse in CD4+ T Cell Activation

Bashour, Keenan T.

The activation of naïve CD4+ T cells by antigen presenting cells is a critical step in the response of the immune system to foreign pathogens and in its acclimation to host tissues. Activation of naïve T cells proceeds through TCR engagement and is further augmented by CD28 costimulation: ensuring T cell survival and conferring numerous functional capabilities. The work in this dissertation highlights the spatial and temporal dynamics that regulate the initial coupling of CD28 with TCR signaling and also dissects the mechanical properties conferred by downstream effectors that are required to relay CD28 costimulation. A reaction-diffusion model that describes the spatial regulation of costimulation in activating human T cells is developed. The Src kinase Lck, though predominantly cytosolic, is an ideal candidate for the coupling of the TCR and CD28 pathways. Membrane associations bring Lck in contact with these receptors, where mediation of its active state by kinase activity and regulation of its spatial dynamics dictate its capacity to integrate early TCR and CD28 signaling.

This developed reaction-diffusion model focusing on Lck is then extrapolated to mouse cells that do not share similar sensitivity to segregation of TCR and CD28 triggering: indicating that while Lck is essential for costimulation, it does not confer spatial sensitivity in activating mouse T cells. A comparison of human and mouse cells demonstrate underlying differences in the diffusivity of Lck across the membrane and the enrichment of the cytoskeleton at the interface. The role of the cytoskeleton in generating TCR-driven contractile forces is then investigated through use of micropillar arrays. This approach also enables the quantification of forces generated by T cells during cellular activation.

The impact of CD28 costimulation on TCR-driven force generation is assessed and noted to increase cellular forces by 80% beyond what is induced through TCR triggering. By manipulating the presentation of CD28 activation, CD28 is determined to be a mechanoresponsive receptor that is not directly responsible for mechanosensitivty. Rather, CD28 mediates a change in cellular forces through PI3 kinase, whose inhibition normalizes force generation in T cells activated by TCR and those costimulated with TCR and CD28. Downstream of PI3 kinase, PDK1 is identified as being essential in both TCR and CD28 costimulatory force generation; inhibition of PDK1 fully abrogates cellular forces.

Lastly, we qualitatively characterize T cell activation on micropillar arrays, where their complex topology reveals a multiphasic behavior during activation. Whereas T cells activated on planar surfaces are relatively stationary, T cells activated on micropillars slowly migrate towards the base of the array. Forces exerted during this migration are substantially greater than those previously measured, and the slow migration leads to the characterization of multiple phases and the relocalization of key cellular proteins.