Surfactant protein D binding to alveolar macrophages

Abstract

Surfactant protein D (SP-D) is a lung-specific protein, synthesized and secreted by lung epithelial cells. It belongs to group III of the family of C-type lectins; each member of this group has an unusual overall structure consisting of multiple globular 'head' regions (which contain the C-type lectin domains) linked by triple-helical, collagen-like, strands. This group includes the surfactant protein A (SP-A) and the serum proteins mannan-binding protein, conglutinin and collectin-43, all of which have been shown to bind to the C1q receptor found on a wide variety of cells, including macrophages. Both SP-D and SP-A have been shown to enhance oxygen radical production by alveolar macrophages. Although this strongly suggests a direct interaction between SP-D and a specific receptor on alveolar macrophages, it is still unclear whether SP-D binds to the same receptor used by SP-A and/or C1q. Human SP-D was isolated from amniotic fluid and was radiolabelled using 125I. Alveolar macrophages were isolated from human bronchioalveolar lavage fluid, and also from bovine lung washings, by differential adhesion to 24-well tissue-culture plates. The study was carried out using EDTA-containing buffers, to eliminate Ca(2+)-dependent C-type lectin binding, and was also carried out at 4 degrees C to eliminate possible internalization by the cells. 125I-SP-D showed specific binding to alveolar macrophages in both a time- and concentration-saturable manner. The binding was inhibited, by approx. 90%, on addition of a 200-fold excess of unlabelled SP-D. The apparent dissociation constant (Kd) was (3.6 +/- 1.3) x 10(-11) M, based on the assumption that native SP-D is assembled as a dodecamer of 12 identical polypeptides of 43 kDa to yield a protein of 516 kDa. C1q was also shown to bind alveolar macrophages (Kd 3 x 10(-6) M), but addition of C1q did not show inhibition of the binding of 125I-SP-D to the macrophages. We conclude that SP-D binds specifically to alveolar macrophages and the receptor involved is different from that utilized by C1q.

Pulmonary infections in swine induce altered porcine surfactant protein D expression and localization to dendritic cells in bronchial‐associated lymphoid tissue

“…37 The direct binding of SP-D to alveolar macrophages, without the presence of microbial ligands, has been shown previously 37,38 and it is likely that pSP-D binds to, and 'arms', alveolar macrophages that express the appropriate receptor. Another putative function was indicated in a recent in vitro study, where mouse SP-D bound apoptotic neutrophils in a localized, patchy pattern, promoting the phagocytosis of such cells.…”

Collectins — soluble proteins containing collagenous regions and lectin domains — and their roles in innate immunity

“…Comparison of the N-terminal regions of the chains of C l q , MBP, conglutinin, CL-43, and SP-A has led to the identification of a possible receptor bind-ing site, composed of 5 Gly-Xaa-Yaa triplets with many charged residues in the X and Y positions, within the N-terminal portions of the collagenous regions of all these molecules (Malhotra et al, 1993b). However, this site is also present in SP-D, which does not appear to bind to the C l q receptor but to bind to a different, but probably related, receptor on alveolar macrophages (Miyamura et al, 1994).…”

“…The interaction with the complement system and host cell receptors via the collagenous/ N-terminal regions of the collectins might allow for the classification of microorganisms according to their ability to be recognized by different collectins. The collectin-microorganism interaction could direct the innate immune response toward either complement activation, opsonization, phagocytosis via the C l q receptor, or utilization of a recently postulated distinct receptor for SP-D (Miyamura et al, 1994). A combination of these reactions may take place, depending on the collectin in question, and the state of inflammation already in progress, especially, since MBP, for example, behaves as an actue phase reactant.…”

Molecular Structures and Interactions of Pulmonary Surfactant Components

“…Both SP-A and SP-D bind to the lipopolysaccharide of certain bacteria and specifically bind to and activate alveolar macrophages (van Iwaarden et al, 1990(van Iwaarden et al, , 1992aLim et al, 1994;Miyamura et al, 1994). The binding of LPS to SP-D has been shown to be mediated by the CRD and to require a trimeric structure (Lim et al, 1994;Kishore et al, 1996).…”