Christopher V. Carman
Our laboratory investigates the cell biological basis of inflammation and wound healing with special emphasis on leukocyte-endothelial interactions. The vascular endothelium is the monolayer of cells that lines the cardiovascular system and serves as the critical barrier between the tissues and the blood. Thus, the endothelium is strategically positioned to serve as a unique sentinel for communicating interstitial information to the circulating immune cells (i.e., leukocytes). In this way, the endothelium plays critical roles in directing the trafficking patterns, as well as activation states of leukocytes. The endothelium can also become damaged by inappropriate immune/inflammatory responses, creating a dysfunctional barrier that gives rise to many of the pathogenic features of inflammatory diseases such as sepsis, anaphylaxis, multiple sclerosis and arthritis. Our work makes extensive use of advanced fluorescence imaging, electron microscopy and biomechanical approaches to understand the fundamental basis and consequences of leukocyte-endothelial interactions and how they may become perturbed during inflammatory and immune-related diseases. Topics we are currently investigating include i.) the basis for and regulation of leukocyte trafficking (i.e. trans-endothelial migration), ii.) the role of the endothelium as an antigen presenting cell, iii.) the cytoskeletal and biomechanical regulation of vascular integrity, and iv.) the cell biologic basis for sepsis pathogenesis.
Our investigations of the cell biological basis of immune cell trafficking have generated a range of insights. Dynamic, high-resolution imaging revealed that endothelium form actin-based structures termed ‘transmigratory cups' that embrace' the immune cells and help guide their egress into the tissue. We also, showed for the first time in vitro that leukocytes can cross the endothelium not only by squeezing between endothelial cells but by penetrating individual endothelial cells directly (i.e., ‘trans-cellularly'). We further uncovered new actin-rich protrusion ‘Invadosome-like protrusions' (ILPs) that facilitate leukocyte pathfinding and barrier breaching. Our current studies endeavor to understand biomechanical sensing by leukocytes that facilitates these critical capabilities. We have also developed analogous studies to investigation how mesechymal stem cells (an emerging anti-inflammatory therapeutic) trafficking across the endothelium through both in vitro and intravital imaging approaches.
Antigen Presentation by Endothelium
Our group is interested in characterizing potentially critical, but poorly understood, contribution of endothelium to antigen (Ag)-specific immunity and pathologies such as allograft rejection, myocarditis, vasculities, diabetes, lupus and multiple sclerosis. Besides the canonical ‘professional' Ag Presenting Cells (APCs; e.g., dendritic cells), endothelial cells are one of the few cell types that express both Major Histocompatibility Complex (MHC)-I and –II along with co-stimulatory molecules, which are necessary to present peptide Ag to CD8+ and CD4+ T cells. We and others have shown that endothelium can activate previously primed CD8+ and CD4+ T cells (but not naïve T cells) in an Ag-specific manner, through a novel ILP enriched immunological synapse. Thus the endothelium may play unique roles as ancillary or ‘semi-professional' APCs that function during the effector phase of immune responses. Our ongoing studies seek to determine specific contributions of the endothelium to Ag-specific immunity mediated by CD4+ and CD8+ T cells in vivo through new endothelial-specific knockout strains of Ag presentation molecules that we have developed. We are also exploring how MHC-related proteins expressed on endothelial surfaces, may analogously instruct Natural Killer (NK) and NKT cells.
Vascular Integrity and Wound Healing
Our group is also investigating the mechanisms by which the endothelium maintains its integrity and how it may be driven to break down during inflammatory disease. In this regard we recently uncovered a novel actin-remodeling activity (‘ventral lamellipodia') that is able to heal micro-wounds formed a consequence of leukocyte trafficking. We showed that this activity is regulated through mechano-sensing of barrier discontinuities, which activates the actin regulator Rac-1 and, as a result, production of reactive-oxygen species (ROS) by NADPH oxidases. Our ongoing work in this area seeks to 1.) understand how oxidative stress may compromise normal homeostatic integrity maintenance and 2.) characterize novel therapeutic strategies that target this processes. We are also investigating how the newly uncovered tension/Rac1/ROS activities participate in collective cell migration during wound healing and angiogenesis.
Pathogenesis of Sepsis: Bedside-to-Bench
Finally, our laboratory is interested in creating a better understanding of sepsis, a devastating and life-threatening systemic inflammatory response to infection for which no specific therapies currently exist. Aberrant leukocyte activation and endothelial breakdown are thought to be central to the pathogenesis of sepsis, but through ill-defined mechanisms. To address this, we have established a unique ‘bedside-to-bench' approach for studying clinical samples of human septic blood. These studies are exploring alterations in leukocyte activation, migration, degranulation and formation of Neutrophil Extracellular Traps (NETs), all of which might contribute to endothelial damage. In this way we hope to identify new therapeutic targets and strategies to treat sepsis.
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