Group A Streptococcus (GAS) is an important human pathogen causing diseases ranging from simple pharyngitis ("strep throat") to invasive necrotizing fasciitis ("flesh-eating disease") to the immune-mediated syndrome of rheumatic fever. Our lab aims to discover and characterize GAS virulence factors by coupling genetic approaches (e.g. targeted mutagenesis) with tissue culture and murine infection models. Of particular interest are genetic mechanisms of GAS innate immune resistance and the shift from mucosal colonization to systemic disease. Among the GAS virulence factors we study are the pore-forming toxins streptolysins S and O (SLS and SLO), the antiphagocytic and proinflammatory surface M protein, DNAse Sda1, cysteine protease SpeB, hyaluronic acid capsule, serum opacity factor, IL-8 peptidase, and the cell wall group A carbohydrate. Together, these studies aim to provide new targets for drug therapy and vaccine prophylaxis. Our collaborators include M. Walker (U. Queensland), P. Ghosh (UCSD), J. Dixon (UCSD), M. Kotb (Cincinnati) and E. Hanski (Jerusalem).

Staphylococcus aureus causes nosocomial and community-acquired diseases including skin and soft tissue infections, osteomyelitis, bacteremia, abscesses, endocarditis and septicemia. Antibiotic resistance has reached epidemic proportions in many regions, with methicillin-resistant S. aureus (MRSA) now exceeding HIV/AIDS as a cause of death in the U.S. Our S. aureus research includes study of how the pathogen resists killing by human phagocytes, including  the antioxidant properties of its golden carotenoid pigment, staphyloxanthin. Pigment inhibition, achieved through repurposing of human cholesterol-lowering agents, may hold promise as an adjunct to antibiotic therapy of MRSA.  Other  immune evasion factors under investigation include S. aureus nitric oxide synthase, phenol soluble modulins, Ig-binding protein A and the pore-forming α-hemolysin. We are also investigating S. aureus colonization and skin infection, to better understand how the pathogen both activates and resists cutaneous innate defenses. Our collaborators include George Liu (Cedars-Sinai), Eric Oldfield (U. Illinois), Pieter Dorrestein (UCSD), Suzan Roiijakkers (Utrecht) and AuricX Pharmaceuticals (Houston).

Bacillus anthracis  is a Gram-positive spore-forming bacterium and the causative agent of anthrax.  Primarily a disease of livestock, anthrax can infect humans through cutaneous, respiratory or gastrointestinal routes of infection.  Inhalational anthrax occurs when endospores are introduced to the lung and taken up by resident phagocytes -- the high lethality of this disease makes anthrax a foremost biodefense concern, as evidenced by the 2001 postal attacks.  We are studying novel functions of the anthrax toxins edema factor (EF) and lethal factor (LF) to inhibit endocytic recycling by the Rab11/Sec15 exocyst, leading to disruption of tight junctions and cell barriers, a finding we have generalized to other cAMP-inducing toxins. We are also studying the survival response of macrophages to LF inhibition of p38 MAPK, a pathway that involves activation of NOD2-dependent inflammasomes via ATP release and adenosine receptor signaling.  Finally, we are looking at novel antimicrobial peptide resistance factors such as the ClpX protease which promote anthrax innate immune resistance and virulence.  Collaborators include Ethan Bier (UCSD),  Michael Karin (UCSD) and Shauna McGillivray (TCU).

Cathelicidin antimicrobial peptides (AMPs)  In a complex environment, higher organisms face the constant threat of microbial infection.  Effective first lines of defense against infectious pathogens comprise the innate immune system. A key component of innate immunity is the production of small, cationic AMPs, a protection strategy conserved from insects through man. In a longstanding collaboration with the laboratory of Richard Gallo (UCSD), we have adopted a combined mammalian and bacterial genetic approach to decipher the contributions of the cathelicidin family of AMPs to host immunity.  Our studies with cathelicidin KO mice and GAS infection provided the first in vivo demonstration that endogenous expression of a mammalian antimicrobial peptide protects against  invasive bacterial infection. Ongoing collaborative projects now explore additional immunostimulatory functions of the cathelicidin molecule, its transcriptional regulation by HIF and VitD in response to infectious challenge, the molecular  and phenotypic basis of bacterial sensitivity or resistance to AMP action, and the impact of bacterial AMP resistance on  virulence and infectious disease epidemiology.

Glycobiology of host-pathogen interactions.  The surface of all bacterial and human cells are covered with glycan molecules that play a primary role in arbitrating the outcome of the host-pathogen encounter.  We are members of the UCSD Program in Excellence in Glycosciences (PEG) and Glycobiology Research and Training Center (GRTC) spearheading collaborative projects that examine the role of bacterial glycans and glycosidases in modulating myeloid cell innate immune and inflammatory responses.  One major focus is the role of sialic acid binding lectins known as Siglecs in regulation of leukocyte function, and their subversion through molecular mimicry by GBS expressing its sialylated polysaccharide capsule. We are also studying the role of GAS hyaluronic acid in interactions with CD44, proinflammatory effects of bacterial sialidases and hyaluronidases, the role of host glycosaminoglycans in neutrophil and endothelial barrier function during infection, and the potential of reprogramming natural antibodies against the nonhuman αGal epitope to clear drug-resistant pathogens via engineered RNA aptamers. Collaborators include A. Varki (UCSD), J. Esko (UCSD), R. Gallo (UCSD) and Altermune Technologies (London).

Neutrophil extracellular traps (ETs) consist of nuclear (or mitochondrial) DNA as a backbone with embedded antimicrobial peptides, histones, and cell-specific proteases providing a matrix to entrap and kill microbes . NETs are formed after stimulation with mitogens, cytokines, or pathogens themselves, in a specialized cell death process involving a ROS-mediated signaling cascade and particular chromatin modifications. We are exploring  roles of HIF-1α and cathelicidins in the generation of NETs at peripheral foci of infection, and have uncovered a novel contribution of the cholesterol biosynthetic pathway in the regulation of NET formation. We are also investigating other immune cells including mast cells and macrophages can themselves produce extracellular traps to control pathogens. Companion projects examine how specific bacterial factors (e.g. GAS M protein) stimulate NET production, whereas other virulence factors promote bacterial resistance to NET killing, e.g. by degradation of the NET architecture (DNAses of GAS and S. aureus) or resistance to the embedded cathelicidins.  Collaborators include Maren von Köckritz-Blickwede (Hannover).