Research

To efficiently fight pathogens, the innate immune system disposes of an arsenal of receptor meant to recognize unique molecule on the surface of microorganisms. Through these proteins, macrophages and dendritic cells (i) recognize foreign particles, (ii) internalize them via phagocytosis and (iii) trigger signaling pathways leading to the activation of the acquired immune system.

During my post-doctoral training in Dr. Sergio Grinstein laboratory, my research interest was to elucidate the dynamic of plasmalemmal components during the formation of phagosome and to follow the remodeling steps taking place early during phagosome maturation.
Using fast live-cell imaging techniques, we were able to visualize the rapid turnover of plasma membrane components initially present on the phagosome membrane (see movie on the left).

The focus of the laboratory is to understand the mechanisms underlying the activation of Dectin-1 when engaged by its specific ligands. Biochemical and biophysical approaches are undertaken to answer questions about receptor organization in the plasma membrane of innate immune cells (macrophages and dendritic cells), and about the establishment of interaction with other membrane proteins and signaling molecules.

PAMPs and PRRs. Cells of the innate immune system are equipped with an arsenal of pattern recognition receptors (PRRs) to recognize either unique pathogen associated molecular patterns (PAMPs), or opsonins covered pathogens.

Dectin-1 (figure above) is unique because of its capacity to trigger immune responses to fungal and bacterial pathogens exhibiting ß-glucans. Cellular responses include phagocytosis, induction of reactive oxygen species, activation of antigen presentation, cytokines secretion for further activation of the acquired immune system.

Dectin-1 recognizes various forms of ß-glucans. ß-glucans consists of a chain of glucose units attached through ß(1-3) linkages with or without ß(1-6) glucose branching.
ß-glucans are a major constituent (more than 50%) of the yeast cell wall extract zymosan, these particles are recognized by dectin-1 on the surface of macrophages, which are then able to internalize them efficiently. The figure on the left is a DIC picture of a RAW macrophages performing phagocytosis of FITC labeled zymosan.

In the past three decades, fungal infections have become much more prevalent and have emerged as a major public health problem. A growing number of fungi have been recognized as important human pathogens, and mycosis is ranked the seventh most common cause of infection-related death in North America. (from McNeil MM, Nash SL, Hajjeh RA, Phelan MA, Conn LA, Plikaytis BD, and Warnock DW. (2001) Clin Infect Dis 33, 641-7.) This disturbing trend is attributed to the increasing number of immunocompromised medical situations, either pathological (such as in HIV infected patients) or induced pharmacologically (during recovery from organ transplantation or chemotherapy).
Common fungal pathogens include Candida albicans, Coccidioides posadasii, Pneumocystis carinii, and aspergillus strains.

Using highly sensitive camera, it is now possible to detect light from single fluorescent molecules.
Using fluorescent antibodies, the organization and motion of each receptor molecule can be tracked and analyzed on living cells challenged or not with pathogenic molecules.

On the left, single molecules of the membrane protein CD36 are visualized using specific fluorescent antibodies.
Each dot or feature on the image sequence corresponds to one or multiple (brighter spots) molecules.

The image sequence is then processed through several algorithms allowing the determination of the position and intensity of each features (programs developed by Dr. Khuloud Jaqaman in Dr. Gaudenz Danuser’s laboratory at the Scripps Institute, San Diego).

Our investigations are aimed to identify the molecular mechanisms by which an immune receptor, dectin-1, is activated by its ligands and how this activation is translated into cellular responses. New approaches, such as single molecule tracking, will provide a very high level of molecular details on the mechanisms involved in host-pathogen interactions.