Why Do We Need More than One Pulmonary Collectin?
There are several biochemical differences between SP-A and SP-D that likely confer distinctive functional properties in vivo. The most important of these include (1) differences in solubility at physiologic ionic strength, which may influence compartmentalization and availability in the airspace lining material; (2) differences in the length of the collagen domain, which determine their capacities to participate in long-range bridging interactions; (3) differences in CRD specificity or valency; and (4) differences in the localization of Asn-linked sugars. Differences in ligand specificity may allow these proteins to complement one another in vivo (Figure 8) (150). For example, colonization and initial invasion by gram-negative bacteria is probably favored by a capsule-deficient phenotype that is expected to facilitate SP-D binding. By contrast, evasion of leukocyte host defenses may depend on an encapsulation, which could favor interactions with SP-A. What Determines Specificity for Fo
