Welcome! You have reached the homepage for the laboratory of Dr. Bryan Heit. Our lab is part of the Department of Microbiology and Immunology at Western University, and we are members of the Center for Human Immunology, the lead centre for the CIHR Human Immunology Network.
Our interests surround the function of phagocytes – white blood cells which ingest (phagocytose) pathogens, particles, and dead cells. We focus on the cellular and molecular processes which control the function of these cells during the maintenance of homeostasis, infection and chronic inflammatory disease. Central to most of our studies is the study of efferoctyosis – the phagocytic removal of apoptotic (dying) cells, and how failures in this process lead to inflammation, autoimmunity and infection.
Phagocytes are a class of white blood cells which have the capacity to engulf large particles such as bacterial and fungal pathogens, and subsequently destroy the engulfed material. The term phagocyte literally translates to “cell that eats”, which is an apt description of the primary function of these cells in our bodies. While there are many types of phagocytes, the Heit lab focuses primarily on macrophages, which play key roles in both maintaining our bodies and in fighting infections.
In this short Youtube video we demonstrate a simple method of measuring cholesterol uptake and intracellular transport in macrophages. In this experiment, macrophages are cultured in a media containing oxLDL (bad cholesterol) and a cholesterol/lipid-reactive dye (Nile red). Live cell time-lapse microscopy is then used to monitor the accumulation of cholesterol by the cell. The movement of cholesterol within the cell can also be observed; accumulating first in the peri-nuclear region (within the Endoplasmic Reticulum) and later trafficking to discrete punctuate structures – lipid droplets. This is the first step in foam cell formation, with the stress of cholesterol accumulation eventually leading to the death of the cell.
We have published a brief Youtube video showing a phagosomal pH measurement experiment. This method can be used to assess phagosome, efferosome or endosome acidification using any target that can be conjugated to FITC, and using any cell type. The method automatically compensates for photobleaching and incorporates an in situ pH calibration to convert FITC ratios into pH units.
In this video the pH of phagosomes in primary human macrophages following uptake of IgG-coated 5µm beads (pathogen mimics) is quantified. pH is measured by imaging FITC conjugated to the beads using two excitation wavelengths – a 440 nm excitation, which is pH-independent and allows for photobleaching correction, and a 490 nm excitation which is pH-dependent, with emission decreasing with decreasing pH.
After the time-lapse video is completed a calibration curve is calculated by profusing the imaging chamber with nigericin-containing buffer at known pH’s (4.0, 5.0, 6.0 and 7.0). The nigericin acts as a proton ionophore, equalizing the pH in the phagosome/efferosome lumen to the pH of the extracellular media. FITC images at 440 nm and 490 nm excitation are captured for each pH.
Post-imaging, the background is subtracted from the Ex440 and Ex490 channels and the 490/440 ratio calculated. A pH can be assigned to each bead based on the calibration curve generated using he nigericin-containing media.
A detailed protocol can be found at: Steinberg, B.E., and S. Grinstein. 2007. Assessment of phagosome formation and maturation by fluorescence microscopy. Methods Mol. Biol. 412: 289–300.
The Heit lab is excited to announce the publication of our newest paper, a review on how neutrophil granules contribute to the potent antimicrobial capabilities of neutrophils.
— Bryan Heit (@BryanHeit) November 29, 2017
A free read-only full text version of the paper can be found here.
Subscribers to Cell and Tissue Research can get a downloadable copy here.
We are happy to have published two new papers with our collaborators over the past two weeks.
The first paper was published with our collaborators Dr. Warren Lee and Dr. Greg Fairn, both from the St. Michael’s Hospital in Toronto, where we identified the mechanisms mediating the transport of HDL (good cholesterol) into the brain. Unlike other vascular beds in the body, which allow HDL to slowly “leak” into tissues, the vasculature of the brain is very tight, and materials must be actively transported across. We discovered that the scavenger receptor SR-BI binds to HDL in the blood and then transports the HDL across the blood-brain barrier via a process termed “transcytosis”, in which the cells lining the blood vessels shuttle material from the blood-side to the brain-side inside of small intracellular vesicles. More details can be found in our paper, available free at this link.
The second paper was published with our collaborator Dr. Dawn Bowdish at McMaster University. This study investigated the recent (hominid) evolution of the phagocytic receptor MARCO which is required for the destruction of many pathogens via phagocytosis. This study demonstrated that MARCO has undergone recent evolution, with some of the newly occurring mutations enhancing the bacteria-clearing capabilities of MARCO. Additional details can be found in the paper, available at this link.