Complement components

Optimal phagocytosis requires the presence of both antibodies and complement components on the particle. Particles opsonized with IgG or IgM antibodies can activate complement through the classical pathway, resulting in binding of fragments of complement components to the particle. These particles will be ingested to a higher extent than particles opsonized with antibodies alone because of the presence of receptors for complement components on the surface of the phagocytes. Fey receptors and complement receptors have been shown to act synergisti-cally in mediating the phagocytosis of particles opsonized with IgG and complement components. Many microoorganisms do not require the presence of specific antibodies for activation of the complement system. Gram-negative bacteria can activate the alternative pathway of complement due to the presence of lipopolysaccharide in their outer membrane. Other microorganisms bear surface structures that activate complement in the absence of antibodies. These microorganisms can be opsonized with complement components in nonimmune serum or serum deficient in immunoglobulins.

The complement components known to be involved in opsonization are G lq, C4b, C3b, iC3b, C3dg and C3d. Bound C3-derived fragments are the most important opsonins generated upon complement activation by either the classical or the alternative pathway. Activation of either pathway-results in the formation of a C3 convertase that cleaves C3. The major cleavage product of C3, C3b, binds covalently to the particle surface. Subsequently C3b can be degraded by a number of inactivators of complement to iC3b, C3dg and C3d. Because G3b is rapidly degraded to iC3b (especially on particles that activate the classical pathway of complement), and C3dg and C3d are poor opsonins, iC3b is probably the most important opsonin generated upon cleavage of C3.

Phagocytes bear three types of receptors that may-mediate the binding and uptake of C3-coated particles: CR1, CR3 and CR4 (pi50/95). The role of CR1 and CR3 in enhancing phagocytosis of lgG-coated particles by neutrophils, monocytes and macrophages is well established. However stimulation of either CRI or CR3 alone by, respectively, C3b- or iC3b-coated particles does not result in their ingestion, although the particles are bound to the receptors. Ingestion requires the presence of very small amounts of IgG on the particle or an additional stimulus that activates CR1 or CR3. Stimuli that can induce phagocytosis in vitro through CR1 or CR3 alone are extracellular matrix proteins (fibronectin, serum amyloid P and laminin) or soluble components such as C5a and the tumor-promoting phorbol ester phorbol myristate acetate (PMA).

The site of binding of the C3 fragments to the activator is essential for its capacity to function as an opsonin. Studies with encapsulated pneumococci have demonstrated that the capsule forms a physical barrier between the C3b bound to the cell wall of the bacteria and C3 receptors on the phagocyte.

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