Bacterial Killing

Aims and Objectives

• To understand the terms complement, opsonin, opsonisation, phagocytosis.
• To understand the principles of complement activation.
• How complement is involved in the elimination of bacteria.
• To understand the types of cell involved in phagocytosis.
• To understand the process of phagocytosis.
• To understand the biochemical mechanisms of killing and degradation of bacteria by phagocytes.
• To understand the clinical features and management of patient's with antibody, complement or neutrophil deficiences.

Complement, Opsonisation and Phagocytosis

C3 is activated by limited proteolysis by enzymes called C3 convertases. There are two C3 convertases, one produced by activation of the classical pathway which is antibody dependent (IgM or IgG), while the alternative pathway can be activated in the absence of antibody. The classical pathway is inactivated when the C1q subcomponent of first component of complement binds to two adjacent IgG molecules or a single IgM molecule. The C1s subcomponent of C1 becomes an active protease and activates C4 and C2 to form the classical pathway C3 convertase which is a complex of activated C4 (C4b) and C2 (C2a) it is designated C4b2a. The alternative pathway is activated when micro-organisms come into contact with body fluids. The pathway is always being activated slowly to generate small amounts of activated C3 (C3b). When C3b binds to a micro-organism activation of the alternative pathway is amplified so that a large amount of the alternative pathway C3 convertase is formed on the surface, followed by deposition of activated C3 (C3b).

Complement can also kill micro-organisms directly. Once C3b has been formed, same binds to the C3 convertase to change it to a C5 convertase which activates C5 to form C5b. Subsequently C6, C7, C8 and C9 bind together with C5b to form a hollow cylinder which is inserted into cell membranes to produce lysis.

1. Opsonisation followed by phagocytosis and intracellular killing (indirect).
2. Assembly of the cytolytic C5b-9 membrane attack complex (direct).

Neutrophils and monocytes/macrophages are the two important phagocytic cells. Neutrophils and monocytes circulate in the blood and migrate into the tissues at the post-capillary venule. Initially they adhere to endothelium and then migrate through intercellular junctions. In the tissues they migrate towards bacteria by means of a process called chemotaxis, which is defined as direct movement along a concentration gradients of chemotactic agents (e.g. C5a leukotriene B4, IL-8, bacterial peptides). Phagocytes recognise their targets by specific sugar residues (e.g. mannose or LPS) but binding is greatly enhanced if the organism is opsonised with IgG and/or C3b. Phagocytes possess Fc and C3b receptors and there is co-operation between these receptors. Thus an organism opsonised with IgG and C3b is more effectively phagocytosed.

Ingestion (phagocytosis) is a localised endocytosis process requiring energy. The plasma membrane envelopes the particle and buds off to form an intracellular vesicle, the phagosome. Following fusion of the phagosome with lysosomal granules the phagolysosome is formed and the bacteria are killed by oxygen-dependent and oxygen-independent process.

Intracellular killing mechanisms:

Oxygen dependent:

• Hydrogen peroxide
• Sight oxygen
• Hydroxyl radical
• Hypohalite
• Nitric oxid

Oxygen independent:

• Lysozyme
• Lysosomal contents
• acid hydrolases
• cationic proteins
• Lactoferrin
• Neutral proteases

© AJC 2007.