Brucella Lipopolysaccharide and pathogenicity: The core of the matter

Brucellosis is one of the most prevalent zoonotic infections worldwide. The half million reported new cases per year likely represents a gross underestimate related to challenges in diagnosis and incomplete reporting. In humans, brucellosis initially manifests as undulating fever accompanied by flu-like myalgias and arthralgias. However, chronic infection may lead to peripheral arthritis, sacroiliitis, orchitis, endocarditis and neurobrucellosis. Treatment involves prolonged courses of multiple antibiotics and relapses occur in up to 10% of patients. The causative agents, Brucella species, are Gram-negative facultative intracellular bacteria which infect herd animals such as goats, cattle and swine. In animals, Brucella species cause abortion, resulting in tremendous economic losses. Although animal vaccine strains such as Rev1, RB51 and S19 have been effective at controlling disease in the herds of nations with sufficient infrastructure and regulation, Brucella infections remain intractable in many parts of the world: Brucellosis is endemic in Central and South America, Sub-Saharan Africa, the Middle East and Asia. Brucella are primarily transmitted to human hosts by ingestion of contaminated unpasteurized dairy products, but Brucella also poses a risk to herders and abattoir workers. Because of the low number needed for infection (10-100 organisms), and ready aerosolization, Brucella is considered a bioterror threat. Current vaccine strains cause disease in humans. Thus at present, no safe and effective human vaccine exists. With the goal of improving therapeutic and vaccine strategies, research has focused on understanding the pathogenic determinants that allow Brucella to establish successful chronic infections and evade immune eradication. Ultimately, immune control of Brucella involves the development of effective Th1 cellular immunity. However, mobilization of Brucella-responsive T cells first requires recognition by “sentinel” dendritic cells of the innate immune system, and activation of these cells to become effective antigen presenting cells. Innate immune cells detect pathogens via repeated cellular patterns (PAMPs) such as the Lipopolysaccharide (LPS) coat on Gram-negative bacteria. Intriguingly, one of the major Brucella virulence factors identified to date is its non-canonical LPS. LPS plays a vital role in the integrity of Gram-negative bacterial outer membranes. It comprises three regions: a Lipid A moiety composed of a disaccharide backbone linked to up to seven hydrophobic acyl chains that are embedded in the outer membrane, a species-conserved core polysaccharide linker, and the external most Opolysaccharide chain. The inner core contains unusual sugars (e.g. 3-deoxy-D-manno-octulosonic acid (Kdo)), but the outer core has more common sugars such as hexoses and hexosamines. The O-polysaccharide, a long chain of repeating glycosyl subunits, exhibits the most variability, providing the means for strain differentiation in labs and antigenic stimulation of humoral immune responses. Innate immune cells recognize LPS via a heteromeric receptor composed of Toll-like receptor 4 (TLR4) and its binding partner myeloid differentiation-2 (MD2). MD2 contains a large hydrophobic pocket that accompanies 5 of the Lipid A acyl chains, whereas the LPS polysaccharide forms polar interactions with the rim of MD2 and TLR4. Agonist activity (endotoxicity) is generally thought to reflect the acyl chain number, length, and chemical modifications. Brucella LPS counters innate immune defenses on multiple levels: the Lipid A moiety contains overly long C16-18 fatty acids (including up to C28), rather than the optimal 12–14 carbons, correlating with poor MD2