Page 69 - Veterinary Immunology, 10th Edition

VetBooks.ir Soluble Pattern-Recognition

Receptors

Although the TLRs, NLRs, and RLRs are expressed on cell surfaces,
soluble PRRs also bind PAMPs. Because these molecules function in
the extracellular fluid, they generally serve to promote destruction
(phagocytosis) of any organisms they encounter. In general they do

not induce the expression of inflammatory cytokines. That task is
accomplished by the cell-surface PRRs.
Since many bacterial PAMPs are glycoproteins and
polysaccharides, circulating carbohydrate-binding proteins called

lectins play important roles in innate immunity. Three extracellular
lectin families, the P-, S-, and C-type lectins, are involved in innate
immunity.
P-type lectins are also called pentraxins. Pentraxins are formed by

five protein subunits arranged in a ring. Two pentraxins, C-reactive
protein (CRP) and serum amyloid P (SAP), are important acute-
phase proteins (Chapter 7). (They are called acute-phase proteins
because their blood levels climb greatly during acute infections or

trauma.) Pentraxins have multiple biological functions, including
activation of complement and stimulation of leukocytes. They bind
to bacterial LPS in a calcium-dependent manner and activate the
classical complement pathway by interacting with C1q (Chapter 4).

They also interact with neutrophils, monocyte-macrophages, and
NK cells and augment their activities.
Galectins are extracellular S-type lectins. Their name derives from
their specificity for galactosides. They play a role in inflammation

by binding leukocytes to the extracellular matrix.
C-type lectins (CLRs) are a large family of carbohydrate-binding
proteins with many different roles. (At least 1000 have been
identified.) All require calcium to bind to carbohydrates. Each end

of a CLR has a distinct function; the C-terminal domain binds to
carbohydrates, whereas the N-terminal domain interacts with cells
or complement components, thereby exerting their biological effect.
There are both soluble and membrane-bound CLRs. The most

important of the soluble CLRs is mannose-binding lectin (MBL).

69