IJAZ MUHAMMAD M.PHIL 2ND SEMESTER

Madam SUMERA SHAMS

ADVANCE PROTO- ZOOLOGY

PRESNTATIONBRUCELLA TYPE IV SECRETION SYSTEM

TYPE IV SECRETION SYSTEM OF BRUCELLA SPP. AND ITS EFFECTORS

Brucella spp. Are intracellular bacterial pathogens that cause infection in domestic c and wild animals. They are often used as model organisms to study intracellular bacterial infections. Brucella VirB T4SS is a key virulence factor that plays important roles in mediating intracellular survival and manipulating host immune response to infection. In this review,we discuss the roles of Brucella VirB T4SS and 15 effectors that are proposed to be crucial for Brucella pathogenesis. VirB T4SS regulates the inflammation response and manipulates vesicle trafficking inside host cells. VirB T4SS also plays crucial roles in the inhibition of the host immune response and intracellular survival during infection. Here, we list the key molecular events in the intracellular life cycle of Brucella that are potentially targeted by the VirB T4SS effectors. Elucidating the functions of these effectors will help clarify the molecular role of T4SS during infection. Further more, studying the effectors secreted by Brucella spp. Might provide insights into the mechanisms used by the bacteria to hijack the host signaling pathways and aid in the development of better vaccines and therapies against brucellosis. Yuehua Ke1, Yufei Wang2, Wengfeng Li 3* and Zeliang Chen1* 1 Institute of Disease Control and Prevention, AMMS, Beijing, China, 2 Department of Laboratory Medicine, General Hospital of Chinese People’s Armed Police Forces, Beijing, China, 3 Department of Orthopedics, the first Affiliated Hospital of General Hospital of People’s Liberation Army, Beijing, China

Frontiers in Cellular and Infection Microbiology REVIEW published: 13 October2015 doi: 10.3389/fcimb.2015.00072

ENTRY OF BRUCELLA TO HOST CELLS

• On entering the host cells, the bacteria interact with the early and late endosomes and resulting in the formation of a “Brucella-containing vacuole” (BCV).• The BCVs then fuse rapidly with the

lysosome.• The contents of the BCVs are

subjected to phagolysosomal degradation.

90% of internalized Brucella are killed by the action of hydrolyzing enzymes.

Remaining 10% evade the host killing mechanisms probably involves the acidification of the BCVs, triggering of the virB operon[which encodes (T4SS)],and release of a large variety of effectors into the host cells’ cytosol.

• The bacteria then traffic and arrive at the endoplasmic reticulum (ER).• Within the ER, the bacteria

survive and establish their replicative niche, and multiply to large numbers. • The formation of the aBCV is

essential for the completion of the intracellular life cycle of Brucella and for cell-to-cell spreading

VIRULENCE FACTORS

In general, Brucella species do not express toxins or virulence factors that cause direct damage to the host.

Upon initial infection with Brucella, innate immune responses are evaded. For example, surface-exposed lipopolysaccharide (LPS) from B. abortus is poorly endotoxic to hosts as it does not bind complement and is poorly recognized by the innate immune sensor Toll-like receptor (TLR).

Brucella smooth LPS, which contains full length O-antigens, is required for entry of the bacteria into host cells.

VIRULENCE FACTORS THAT ARE ESSENTIAL FOR BRUCELLA INFECTION, INCLUDING……

• MucR • SagA • BtaE • BacA and • BetB and……….• T4SS.

• Lipopolysaccharide (LPS) • B- cyclic glucan • BvrS/BvrR, • Outer membrane proteins (Omps) • BmaC

IMPORTANT VIRULENCE FACTORS FOR INTRACELLULAR SURVIVAL OF BRUCELLA

Cyclic β‑1,2-glucan

Is a molecule secreted into the periplasm of Brucella and is required for intracellular Brucella to avoid fusion of the phagosome with lysosomes.

Type IV secretion system (T4SS).

T4SS translocates Brucella proteins, also called effectors, into host cells and is critical for both Brucella survival and replication in infected host cells.

TYPE IV SECRETION SYSTEM

Type four secretion systems (T4SS) are multiprotein complexes present in many Gram-negative bacteria such as Agrobacterium tumefaciens, Helicobacter pylori, Bordetella pertussis, Legionella pneumophila and Brucella.

The VirB T4SS of Brucella is a translocation complex in the bacterial membrane capable of injecting Brucella effector proteins into the host cytosol.

• T4SS is a key virulence factor that plays important roles in mediating intracellular survival and manipulating host immune response to infection.

• T4SS functions in the translocation of effector proteins across bacterial and host membranes into the host cytosol.

• To understand the functions of T4SS, it is important to identify these effectors.

• 15 effectors are secreted by Brucella in a T4SS-dependent manner.

In Brucella, T4SS is encoded by the virB operon.

virB operon consists of 12 genes (virB1–12) located on chromosome II.

ORGANIZATION AND STRUCTURE OF TYPE IV SECRETION SYSTEM

THE COMPLEX CAN BE DIVIDED INTO FIVE PARTS:

The Stretching Needle Complex

Composed of VirB2

The Core/Outer Membrane Complex

Composed of VirB7 VirB9 and VirB10

The Linking Stalk

Probably composed of fragments from VirB5 or VirB10

The Inner Membrane Complex

Composed of VirB3 VirB4 VirB6 VirB8, and the N-terminus of VirB10

The ATPases/Energy Center

Consisting of VirB4 andVirB11

Exception

Except for VirB1, VirB7, and VirB12, all the other subunits play crucial roles in the virulence of Brucella

FUNCTION OF BRUCELLA T4SS DURING INFECTION

• in vitro in macrophages Brucella virB mutants are not able to replicate and are killed.• whereas T4SS mutants are slowly

cleared.

• in vivo virB mutants survive at similar numbers compared with wild-type Brucella in spleens of mice until 3 days post infection.• It is possible that Brucella is extracellular during

the first few days after infection or is present in a different cell population in which Brucella does not require a T4SS to survive.• After 8 weeks post infection, the Brucella wild-

type bacteria are able to persist and replicate.

At later stages of infection, it has been shown that Brucella wild-type bacteria elicit innate immune responses in mice.

Brucella T4SS is required for maturation of the Brucella phagosome into an ER-derived compartment.

Approximately 12–24 h after Brucella infection of a host cell Brucella start to multiply to high numbers inside host cells.

However, T4SS mutants do not.

while Brucella virB mutants never reach the ER-derived vacuole and are killed in phagolysosomes.