Dr. Prithu Sundd

Pulmonary, Allergy and Critical Care Medicine

Dr. Sundd's Profile

Research Interests

1. Mechanisms of leukocyte rolling and arrest during inflammation.
   
   Inflammatory response following bacterial infection involves neutrophil adhesion to the inflamed endothelium of blood vessels with high wall shear stress ( t > 6 dyn/cm2). Neutrophil-endothelial adhesion starts with rolling along the vessel wall mediated by P-selectin on the endothelium binding to P-selectin glycoprotein ligand-1 (PSGL-1) on neutrophils, followed by firm arrest which is mediated by activated β2-integrins (LFA-1 and Mac-1) on the neutrophil binding to inter-cellular-adhesion-molecule-1 (ICAM-1) on endothelium. In October 2010 (Sundd, P. et al. Nature Methods, 2010), I introduced quantitative Dynamic Footprinting (qDF) microscopy which is an adaptation of TIRF microscopy and allows estimation of z -distances in the footprints (cell-substrate contact zone) of rolling neutrophils. This study revealed that neutrophils rolling at high shear stress (> 6 dyn/cm2) deform creating a four-fold larger footprint with the P-selectin substrate than that predicted by computational models and low resolution in vivo images, and that the rolling is further facilitated by three to four long membrane tethers which can extend up to 16 µm behind the rolling cell. In the most recent study ( Sundd P. et al,Nature, 2012), I have discovered 'sling', an autonomous adhesive structure made by rolling neutrophils. I have shown that long tethers made by neutrophils rolling at high shear stress (6-10 dyn/cm2) do not retract as postulated, but instead persist and appear as 'slings' at the front of rolling neutrophils (Movie 1). Slings are made by rolling neutrophils in vitro and in a model of acute inflammation in vivo . Selectin ligand PSGL-1 is presented as discrete sticky patches while integrin LFA-1 is expressed over the entire length on slings. As neutrophils roll forward, slings wrap around the rolling neutrophils and undergo a step-wise peeling from the P-selectin substrate which is enabled by the tandem failure of PSGL-1 patches under hydrodynamic forces (Movie 2). Currently, we are conducting experiments in an in vitro microfluidic flow chamber to elucidate the cytoskeletal organization responsible for the ability of slings to withstand hydrodynamic forces at high shear stresses.
   We are also conducting qDF experiments to study the nature of sling formation by different mouse circulating monocyte subsets (Gr-1+/Ly6Chi and Gr-1-/Ly6Clow) and their role in monocyte adhesion during inflammation.

2. Role of neutrophils in pulmonary vaso-occlusion during sickle cell disease Acute Chest Syndrome.
   
   Sickle cell disease (SCD) is an autosomal recessive point mutation in the beta globin gene, generating a mutant hemoglobin S (Hb-S) molecule that will polymerize when deoxygenated. Accumulation of intra-erythrocytic Hb-S polymer leads to cellular rigidity, altered rheological properties, expression of surface integrins and other adhesion molecules (ICAM-4, VLA-4, CD36, sulfated glycolipids and tetrasaccharides), and ultimate entrapment of the cells in the microcirculation. As a result, the microcirculation can become occluded in SCD patients, producing ischemia and reperfusion injury to almost any organ in the body. This occurs frequently in the lung, where red cell-leukocyte-endothelial adhesive interactions can cause a form of acute lung injury called the Acute Chest Syndrome (ACS). While the mechanisms of vaso-occlusion have been characterized in the systemic circulation, little is known about the molecular vaso-occlusive events in the lung microcirculation. Unlike systemic vasculature where leukocyte recruitment takes place primarily in the postcapillary venules, pulmonary capillaries are the primary site of leukocyte sequestration in lungs during bacterial infection. However, several factors are likely to contribute to pulmonary vaso-occlusion including the pro-adhesive properties and enhanced rigidity of sickle RBCs, the large marginated pool of neutrophils in the pulmonary capillaries, and the hyper-inflammatory state present in SCD. We are using multi-photon intra-vital microscopy to study how inflammation drives pulmonary vaso-occlusion in SCD mice.