SummarySurfactant protein D (SP-D) is a pattern-recognition molecule of the innate immune system that recognizes various microbial surface-specific carbohydrate and lipid patterns. In vitro data has suggested that this binding may lead to increased microbial association with macrophages and dendritic cells. The aim of the present in vivo study was to study the expression of porcine SP-D (pSP-D) in the lung during different pulmonary bacterial infections, and the effect of the routes of infection on this expression was elucidated. Furthermore, the aim was to study the in vivo spatial relationship among pSP-D, pathogens, phagocytic cells and dendritic cells. Lung tissue was collected from experimental and natural bronchopneumonias caused by Actinobacillus pleuropneumoniae or Staphylococcus aureus, and from embolic and diffuse interstitial pneumonia, caused by Staph. aureus or Arcanobacterium pyogenes and Streptococcus suis serotype 2, respectively. By comparing normal and diseased lung tissue from the same lungs, increased diffuse pSP-D immunoreactivity was seen in the surfactant in both acute and chronic bronchopneumonias, while such increased expression of pSP-D was generally not present in the interstitial pneumonias. Co-localization of pSP-D, alveolar macrophages and bacteria was demonstrated, and pSP-D showed a patchy distribution on the membranes of alveolar macrophages. SP-D immunoreactivity was intracellular in dendritic cells. The dendritic cells were identified by their morphology, the absence of macrophage marker immunoreactivity and the presence of dendritic cell marker immunoreactivity. Increased expression of pSP-D in the surfactant coincided with presence of pSP-D-positive dendritic cells in bronchus-associated lymphoid tissue (BALT), indicating a possible transport of pSP-D through the specialized M cells overlying (BALT). In conclusion, we have shown that pSP-D expression in the lung surfactant is induced by bacterial infection by an aerogenous route rather than by a haematogenous route, and that the protein interacts specifically with alveolar macrophages and with dendritic cells in microbial-induced BALT. The function of the interaction between pSP-D and dendritic cells in BALT remain unclear, but pSP-D could represent a link between the innate and adaptive immune system, facilitating the bacterial antigen presentation by dendritic cells in BALT.

Summary Surfactant protein D (SP‐D) is a collectin believed to play an important role in innate immunity. SP‐D is characterized by having a collagen‐like domain and a carbohydrate recognition domain (CRD), which has a specific Ca2+‐dependent specificity for saccharides and thus the ability to bind complex glycoconjugates on micro‐organisms. This paper describes the tissue immunolocalization of porcine SP‐D (pSP‐D) in normal slaughter pigs using a monoclonal antibody raised against purified pSP‐D. Porcine SP‐D was purified from porcine bronchoalveolar lavage (BAL) by maltose‐agarose and immunoglobulin M affinity chromatography. The purified protein appeared on sodium dodecyl sulphate–polyacrylamide gel electrophoresis as a band of ∼53 000 MW in the reduced state and ∼138 000 MW in the unreduced state. Porcine SP‐D was sensitive to collagenase digestion and N‐deglycosylation, which reduced the molecular mass to ∼24 000 MW and ∼48 000 MW respectively, in the reduced state. N‐deglycosylation of the collagen‐resistant fragment, reduced the molecular mass to ∼21 000 MW showing the presence of an N‐glycosylation site located in the CRD. Porcine SP‐D bound to solid‐phase mannan in a dose and Ca2+‐dependent manner with a saccharide specificity similar to rat and human SP‐D. The purified protein was used for the production of a monoclonal anti‐pSP‐D antibody. The antibody reacted specifically with pSP‐D in the reduced and unreduced state when analysed by Western blotting. Immunohistochemical evaluation of normal porcine tissues showed pSP‐D immunoreactivity predominantly in Clara cells and serous cells of the bronchial submucosal glands, and to a lesser extent in alveolar type II cells, epithelial cells of the intestinal glands (crypts of Lieberkühn) in the duodenum, jejunum and ileum and serous cells of the dorsolateral lacrimal gland.