Imaging and Defining Emergent Behaviors of the Immune Response

Search for Pathogens

In studying a myosin1g loss-of-function T cells,,(Gerard et al. 2014)  we identified this motoro as a ‘steering’ motor in cells that forces them to make periodic turns. Without the motor, T cells crawl more quickly but in straighter paths.  This allowed us to show that random walks allows T cells efficiently scan tissues.

We had first hypothesized that control over cell motility and surface-to-surface contact underlies immune function. T cells cannot respond to incoming pathogens until they physically find them or find antigen-presenting cells. Additionally, we proposed that motile forces and their control underlie the control of immunological synpases (IS) and similar synapses between immune cells—sites where information exchange is enhanced.

We entered this field by focusing on candidate myosin motors that are expressed in T cells. Our lab was the first to identify and characterize the major Myosin II isoform in T cells.

In Jacobelli et al. Nature Immunology, 2004, we showed MyosinIIA (“MyoIIA”. from the gene MyH9) was the only isoform expressed in T cells and was necessary for effective motility in vitro in a non-confined environment. In this setting, myosinII contractility was associated with stable motile propagation as well as generation of cortical tension associated with the amoeboid shape of motile T cells in such non-confined environments.  We also showed TCR and Ca-triggered phosphorylation of the MyoIIA protein, consistent with a transient inactivation of this motor during the generation of the immunological synapse (IS).  We now hypothesize that this ‘de-activation’ is a critical part of the ability of T cells to quickly ‘spread’ onto antigen presenting cells and set up an effective IS.

In Jacobelli et al. J. Immunology, 2009, we demonstrated that this motor plays a critical role in determining how cells approach a substrate, especially under confinement.  This study used TIRF imaging to demonstrate two unique ‘modes’ of amoeboid movement.  In the first, which we term ‘walking’, T cells utilize MyosinIIA to generate multiple ‘footprints’ on substrates. Typically as MyosinIIA contracts inward and extinguishes one footprint, another adhesion is licensed nearer the leading edge. This mode is quite distinct from what we term ‘sliding’, which more closely resembles what is observed for mesenchymal motility.  Under sliding, which does not require MyosinIIA, cells maintain a continuous and much larger adhesion with substrate. This mode is slower than ‘walking’ but also likely permits more effective surface scanning and we thus hypothesize that inactivation of MyoIIA at the start of IS formation (via phosphorylation as described in Jacobelli, 2004) is likely critical to increasing sensitivity to pMHC complexes.

In Jacobelli et al. Nature Immunology, 2010, we sought to analyze the full import of MyosinIIA expression in T cells in complex 3-dimensional environments. By generation of mice in which MyoIIA was specifically deleted in naïve T cells or activated T cells derived from them, we demonstrated that these cells showed profound overadhesion during motility, resulting in their slowed movement as they scan through lymph nodes. To analyze the membrane dynamics underlyin this, in greater detail, we generated artificial 3-dimensional environments via microfabrication techniques. Beyond recapitulating the overadhesion/slowing in lymph nodes, we found that T cell migration rate is regulated by the degree to which the cells are confined rather than merely by the stickiness of the surfaces along which they crawl as had previously been thought (the ‘haptokinetic model’). For amoeboid cells, we found a confinement-optimized ‘channel size’ for optimal motility rate that is just a bit wider than the cells average width. We hypothesize that when cells are further squeezed, they slow down due to increased friction with the substrate walls. In contrast when they are confined less, they appear not to be able to efficiently generate the multiple footprints that defined the ‘walking’ mode of our previous studies and that characterized the optimum movement. This ability to optimize speed in turn was reliant on the presence of MyosinIIA to regulate the adhesions made during ‘walking’.

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