Mucosal Surfaces in Human Tissues Localization of Lung Surfactant

Localization of Lung Surfactant Protein D on Mucosal Surfaces in Human Tissues Jens Madsen, Anette Kliem, Ida Tornøe, Karsten Skjødt, Claus Koch and Uffe Holmskov This information is current as of September 29, 2021. J Immunol 2000; 164:5866-5870; ; doi: 10.4049/jimmunol.164.11.5866 http://www.jimmunol.org/content/164/11/5866 Downloaded from

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The Journal of Immunology is published twice each month by The American Association of Immunologists, Inc., 1451 Rockville Pike, Suite 650, Rockville, MD 20852 Copyright © 2000 by The American Association of Immunologists All rights reserved. Print ISSN: 0022-1767 Online ISSN: 1550-6606. Localization of Lung Surfactant Protein D on Mucosal Surfaces in Human Tissues1

Jens Madsen,* Anette Kliem,* Ida Tornøe,* Karsten Skjødt,* Claus Koch,† and Uffe Holmskov2*

Lung surfactant protein-D (SP-D), a collectin mainly produced by alveolar type II cells, initiates the effector mechanisms of innate immunity on binding to microbial carbohydrates. A panel of mRNAs from human tissues was screened for SP-D mRNA by RT-PCR. The lung was the main site of synthesis, but transcripts were readily amplified from trachea, brain, testis, salivary gland, heart, prostate gland, kidney, and pancreas. Minor sites of synthesis were uterus, small intestine, placenta, mammary gland, and stomach. The sequence of SP-D derived from parotid gland mRNA was identical with that of pulmonary SP-D. mAbs were raised against SP-D, and one was used to locate SP-D in cells and tissues by immunohistochemistry. SP-D immunoreactivity was found in alveolar type II cells, Clara cells, on and within alveolar macrophages, in epithelial cells of large and small ducts of the parotid Downloaded from gland, sweat glands, and lachrymal glands, in epithelial cells of the gall bladder and intrahepatic bile ducts, and in exocrine pancreatic ducts. SP-D was also present in epithelial cells of the skin, esophagus, small intestine, and urinary tract, as well as in the collecting ducts of the kidney. SP-D is generally present on mucosal surfaces and not restricted to a subset of cells in the lung. The localization and functions of SP-D indicate that this collectin is the counterpart in the innate immune system of IgA in the adaptive immune system. The Journal of Immunology, 2000, 164: 5866–5870. http://www.jimmunol.org/ ung surfactant protein D (SP-D)3 is one of the collectins, lysosomal granules of alveolar macrophages (18, 19), suggesting a a family of oligomeric proteins whose individual chains receptor-mediated uptake by these cells; a putative receptor for L consist of a collagen region linked to a C-type lectin do- SP-D, gp-340, has been characterized (20). SP-D also seems to main via an ␣ helical neck region (1, 2). The structural subunits of have an immunomodulatory function, inhibiting T lymphocyte SP-D are homotrimers of these chains, and the subunits are them- proliferation and IL-2 production (21) as well as inhibiting specific selves oligomerized into cross-like tetramers and higher oligomers IgE binding to allergens and blocking allergen-induced histamine (3–8). release from human basophils (13, 22). SP-D binds to oligosaccharides on the surface of a variety of In addition to its role in antimicrobial defense, SP-D may be pathogenic microorganisms. This binding initiates several effector involved in pulmonary surfactant homeostasis. It interacts with by guest on September 29, 2021 mechanisms, including the recruitment of inflammatory cells to phospho- and glycolipids in vitro (23–25), and mice made SP-D destroy the pathogens. SP-D has been shown to bind to carbohy- deficient by gene targeting accumulate surfactant lipids and alve- drate residues on influenza A virus, thereby inhibiting its hemag- olar macrophages in the alveolar space (26, 27). In this situation, glutination activity and causing viral aggregation (9, 10). SP-D the macrophages may appear as multinucleated foam cells, also enhances the binding of influenza A virus to neutrophil gran- whereas the alveolar type II cells are hyperplastic and contain giant ulocytes and promotes the neutrophil respiratory burst in response lamellar bodies. to the virus (11). SP-D binds to and induces aggregation of other SP-D is generally recognized as a molecule expressed in alve- microorganisms, such as Gram-negative bacteria (12) and the olar type II cells and Clara cells, but it has also been demonstrated fungi Cryptococcus neoformans and Aspergillus fumigatus (13, in mucus cells of the rat gastric mucosa (28), and extrapulmonary 14). SP-D binds directly to alveolar macrophages in the absence of expression has been detected in murine tissues (29). In this report, microbial ligands, thereby mediating the generation of oxygen rad- the extrapulmonary expression of human SP-D was investigated icals (15), and also acts as a potent chemotactic agents for phago- by RT-PCR and immunohistochemistry. SP-D was found to be cytes (16, 17). SP-D has been localized in endocytic vesicles and expressed in relation to mucosal surfaces in numerous glands and organs.

*Department of Immunology and Microbiology, Institute of Medical Biology, Uni- versity of Southern Denmark, Odense University, Odense, Denmark; and †Statens Materials and Methods Serum Institut, Copenhagen, Denmark SDS-PAGE and Western blotting Received for publication July 15, 1999. Accepted for publication March 21, 2000. Electrophoresis was performed on 4–14% (w/v) polyacrylamide gradient The costs of publication of this article were defrayed in part by the payment of page gels with discontinuous buffers using the FAST system (Pharmacia, Pis- charges. This article must therefore be hereby marked advertisement in accordance cataway, NJ). Samples were reduced by heating to 100°C for 3 min with 40 with 18 U.S.C. Section 1734 solely to indicate this fact. mM DTT in 0.1 M Tris-HCl buffer, pH 8.0, containing 1.5% (w/v) SDS 1 This work was supported by the Danish Medical Research Council, Michaelsen and 5% (v/v) glycerol, and carboxamidated by the addition of iodoacet- Fonden, the Novo Nordisk Foundation, Fonden til Lægevidenskabens Fremme, Na- amide to 90 mM. Unreduced samples were heated in sample buffer with 90 tionalforeningens Fond, and the Benzon Foundation. mM iodoacetamide. After electrophoresis, proteins were electroblotted 2 Address correspondence and reprint requests to Dr. Uffe Holmskov, Immunology onto polyvinylidene diﬂuoride membranes (Immobilon P, Millipore, and Microbiology, Institute of Medical Biology, University of Southern Denmark, Odense Bedford, MA). University, DK-5000 Odense, Denmark. E-mail address: [email protected] The membrane was incubated with primary Ab (monoclonal SP-D Ab, 3 Abbreviations used in this paper: SP-D, lung surfactant protein D; DMBT1, deleted 50 ng/ml) and secondary alkaline phosphatase-coupled rabbit anti-mouse in malignant brain tumors 1. Ig in 10 mM Tris-HCl, pH 7.4, containing 0.5 M NaCl, 15 mM NaN3, and 0.05%

FIGURE 1. RT-PCR analysis of SP-D expression in human tissues. Total mRNA (2 ␮g) from 19 different tissues were ampliﬁed, separated in 1.2% (w/v) agarose gel, blotted onto nylon membrane, and hybridized with either a human SP-D or a ␤-actin oligonucleotide probe as described in Materials and Methods.

(v/v) Tween 20 (polyoxyethylene sorbitan monolaurate, Merck-Schuchardt, Ho- were fixed in 4% formalin in 10 mM sodium phosphate buffer, pH 7.4, henbrunn, Germany). The membranes were washed and developed with containing 140 mM NaCl (PBS) for 24 h and then conventionally dehy- nitroblue tetrazolium and potassium 5-bromo-4-chloro-3-indolylphosphate. drated and embedded in paraffin. A biotin-streptavidin immunoperoxidase technique was used on paraffin Reverse-transcriptase PCR sections. Briefly, the paraffin sections were pretreated in 10 mM sodium Downloaded from citrate buffer, pH 6.0, in a microwave oven for three 5-min periods at 650 Total RNA was obtained from whole organs from various human tissues W. The sections were left in citrate buffer for 15 min; washed in 10 mM (Clontech, Palo Alto, CA). Two micrograms of total RNA were used for Tris-HCl, pH 7.2, containing 140 mM NaCl and 7.5 mM NaN (TBS); first-strand synthesis using the antisense SP-D primer, SP-DR1, 5Ј-TCA- 3 preincubated with 2% (w/v) BSA in TBS for 10 min; incubated for 30 min GAACTCGCAGACCA-3Ј. This sequence corresponds to nucleotides with Hyb 245-1, 5 ␮g/ml in TBS containing 1% (w/v) BSA; washed with 1282–1298 of the cDNA sequence of SP-D (2). The RNA was incubated TBS; incubated for 30 min with biotin-labeled goat anti-mouse Ig (Dako, in a volume of 20 ␮l with 20 U Moloney murine leukemia virus reverse

Carpinteria, CA) diluted 1:200 in TBS; washed with TBS; incubated with http://www.jimmunol.org/ transcriptase (Stratagene, La Jolla, CA) at 37°C for 1 h. One-fourth of the HRP-coupled streptavidin (Dako) diluted 1:300 in TBS without NaN ; resulting cDNA was subjected to 40 cycles of PCR amplification in a 3 washed with TBS and water; incubated for 20 min with 0.04% 3-amino- volume of 30 ␮l with a denaturation temperature of 94°C for 30 s, anneal- 9-ethylcarbazole (Sigma, St. Louis, MO) and 0.015% H O in 50 mM ing at 62°C for 1 min, extension at 72°C for 2 min, and finally an extension 2 2 sodium acetate buffer, pH 5.0; washed; counterstained with Mayer’s he- at 72°C for 7 min, using Taq polymerase (Stratagene) with two SP-D matoxylin for 2 min; and mounted in Aquatex (Sigma). The specificity of primers spanning the neck and carbohydrate recognition domain region of immunostaining was verified by replacing the primary Ab with an unre- human SP-D: a sense primer, SP-DF1, 5Ј-ATGTTGCTTCTCTGAGG-3Ј, lated mAb of the same subclass as the SP-D Ab, as well as other conven- corresponding to nucleotides 839–855; and an antisense primer, SP-DR3, tional staining controls. 5Ј-TCAGAACTCGCAGACCACAAG-3Ј corresponding to nucleotides 1278–1298. As a control, RT-PCR was performed on ␤-actin under the same conditions as for SP-D, except that the annealing temperature was Results by guest on September 29, 2021 Ј Ј 58°C and the primers used were: 5 -actin (sense), 5 -GGCATGGCTT Expression of SP-D in human tissues analyzed by RT-PCR TATTTGTTTT-3Ј; and 3Ј-actin (antisense), 5Ј-GTAAGCCCTGGCTGCC TC-3Ј. The reaction mixtures were run on a 1.2% agarose gel, blotted onto A fragment of 461 bp corresponding to the neck and carbohydrate a nylon membrane (Biotrace HP, Gelman Science, Ann Arbor, MI) by recognition domain region of human SP-D was amplified by RT- alkaline Southern blotting, and hybridized with the respective SP-D and PCR applied to total RNA from 19 different human tissues (Fig. 1). ␤-actin probes labeled with [␣-32P]dATP. The SP-D probe was amplified by PCR using a human cDNA clone of Expression of SP-D mRNA was most pronounced in the lung but SP-D (kindly provided by Dr. Jinhua Lu, Singapore University, Singapore) was also found in the kidney, trachea, brain, testis, pancreas, sal- as template and the same conditions and primers as used for RT-PCR. The ivary gland, heart, prostate, small intestine, and placenta. Low ex- ␤ -actin probe was generated from a template of total RNA from the uterus, pression was found in the uterus, stomach, mammary gland, under the same conditions and with the same primers as those used for ␤-actin RT-PCR. Each reaction mixture was run on an 1.2% agarose gel; spleen, adrenal gland, and liver. No expression was observed in the bands were extracted from the gel by means of the QIAquick Gel skeletal muscle or thymus. All samples were normalized with re- Extraction Kit (Qiagen, Chatsworth, CA) and labeled with [␣-32P]dATP spect to ␤-actin. The PCR products obtained from lung and sali- using “large fragments of DNA polymerase I” (Life Technologies) and vary gland were isolated and sequenced. In both tissues the se- random hexamers as primers. quences obtained were identical with the corresponding sequence The PCR products obtained from lung and salivary gland were cut out from the agarose gel, subcloned with the TA cloning kit (Invitrogen) and published for human SP-D, confirming that the correct transcript sequenced. was being amplified in the PCR. Production of mAbs against SP-D Characterization of Abs SP-D was purified from amniotic fluid by the method described in Ref. 30. Mice were immunized with 10 ␮g purified human SP-D adsorbed onto The specificity of the Ab used for immunohistochemistry was ver- aluminum hydroxide gel and emulsified with 0.25 ml Freund’s incomplete ified by Western blots of partially purified SP-D that has been adjuvant. Fusions were performed with the cell line X63Ag8.6.5.3, and mixed with Triton X-100-solubilized alveolar macrophages and positive clones were identified by ELISA using purified SP-D coated onto separated on SDS-PAGE in the reduced state (Fig. 2). the wells. Cells from wells with Abs against SP-D were recloned three times by the limiting dilution method. The specificity of the mAbs for SP-D was checked by Western blotting of partially purified SP-D mixed with Immunohistochemical analysis of SP-D Triton X-100 solubilized alveolar macrophages separated on SDS-PAGE In the lung, strong immunoreactivity was observed in alveolar type in the reduced and unreduced state. A panel of nine Abs was obtained, and Hyb 245-1 was selected for immunohistochemistry. II cells (Fig. 3, a and b), in unciliated bronchial cells (Fig. 3, b and c) and in a subset of alveolar macrophages (Fig. 3, d–f). Some Immunohistochemistry alveolar macrophages were stained intracellularly (Fig. 3f), Normal human tissues were from the tissue bank at the Department of whereas others showed a distinct staining of the cell membrane Pathology, Odense University Hospital (Odense, Denmark). The tissues (Fig. 3, d and e). 5868 TISSUE LOCALIZATION OF SP-D

meruli were unstained. The epithelium of both ureter and bladder were stained (Fig. 5, h and i). Some of the prostate gland epithelium was clearly stained, whereas gland epithelium from other parts of the prostate showed only faint staining (not shown). Stain- ing and substitution controls were negative. Discussion The present report describes the tissue expression of SP-D in the lung and 18 extrapulmonary tissues as determined by RT-PCR and immunohistochemistry. SP-D was demonstrated in epithelial cells in a variety of tissues throughout the body. It has long been established that alveolar type II cells and unciliated bronchial epithelial cells (Clara cells) in the lung are the major sites of synthesis of SP-D (18, 31, 32). Northern blotting and RT-PCR have previously been used to determine SP-D expression by selected extrapulmonary tissues of different species. In humans, SP-D transcripts have been detected in heart, pancreas, stomach, small, and large intestine (29, 33), whereas they have been found in the stomach, kidney, and heart of the mouse (29). In the rat, Downloaded from FIGURE 2. Specificity of the monoclonal SP-D Ab analyzed by West- SP-D has been found in the gastric mucosa (28) and mesentery ern blotting. Lane 1, reduced partially purified SP-D mixed with Triton cells (34) by RT-PCR, immunohistochemistry, and Western blot- X-100 solubilized alveolar macrophages. The blots were incubated with 50 ting, whereas it could not be demonstrated in the small or large ng/ml Hyb 245-1. The bound Ab was visualized by means of alkaline intestine. Other cells expressing SP-D outside the lung have not phosphatase-labeled goat anti-mouse IgG and substrate. Lane 2, Gold been characterized in the rat. staining of reduced partially purified SP-D mixed with Triton X-100 sol- In the present study, SP-D was found to be strongly expressed in http://www.jimmunol.org/ ubilized alveolar macrophages. the human lung and trachea, but extrapulmonary organs such as the kidney, brain, testis, pancreas, salivary gland, heart, prostate, small Various exocrine glands were stained. In the parotid gland both intestine, and placenta also produced clear signals, whereas the uterus, large and small ducts were stained (Fig. 4, a and b), as were ducts stomach, mammary gland, spleen, adrenal gland, and liver showed of oral mucous glands (Fig. 4c). In the pancreas, the intercalated weak expression. Sequencing of the RT-PCR product from lung and ducts showed a strong granular staining (Fig. 4d). Small bile ducts parotid gland confirmed that correct transcript had been amplified. were stained in the liver, but no staining was observed in hepato- Of the panel of 9 mAbs raised against purified human SP-D, two cytes (Fig. 4e). Staining was seen in the ducts of sweat glands (Fig. could be used for immunohistochemical staining on paraffin-em- 4f), and a similar staining pattern was observed in mammary and bedded tissue, and the staining pattern of these two Abs was iden- by guest on September 29, 2021 lachrymal gland (not shown). tical in lung and salivary glands. Hyb 245-1 was selected for its In the epidermis, only the basal cells were stained (Fig. 5a), good immunohistochemical staining properties. In Western blot- whereas staining was seen throughout the unkeratinized stratified ting, this Ab reacted with a 43-kDa band from the reduced SP-D squamous epithelium of the esophagus (Fig. 5b). Little or no stain- preparation, corresponding to single chain of SP-D chains. In the ing was observed in the stomach and large intestine, whereas a unreduced preparation, the Ab reacted with a 180-kDa band cor- slight but significant staining was seen in the epithelial cells of the responding to SP-D chain oligomers (not shown). small intestine (Fig. 5c). The gallbladder epithelium was stained Immunohistochemistry with this Ab confirmed the presence of (Fig. 5d). SP-D immunoreactivity in alveolar type II cells, Clara cells, and In the kidney, the collecting ducts were stained (Fig. 5, e–g), and the mesothelial cells lining the pleural cavity; those lining the peri- faint staining was also seen in the loops of Henle, while the glo- toneal cavity were also stained for SP-D (not shown). The weak

FIGURE 3. Immunohistochemical localization of SP-D in normal human lung. SP-D was found in alveolar type II cells (a) and Clara cells (b and c). SP-D was also found on and within alveolar macrophages (d–f). The tissues were stained by an indirect immunoperoxidase technique and counterstained with Mayer’s hematoxylin as described in Materials and Methods. Original magniﬁcation, ϫ400. The Journal of Immunology 5869

FIGURE 4. Immunohistochemical localization of SP-D in exocrine ducts in normal human tissues. SP-D was found in duct epithelial cells within the salivary gland (a–-c), in the intercalated ducts of pancreas (d), in the small bile ductules of the liver (c), and in the sweat glands (f). The tissues were stained by an indirect immunoperoxidase technique and counterstained with Mayer’s hematoxylin as described in Ma- terials and Methods. Original magniﬁcations: a and f, ϫ200; b–e, ϫ400.

signal seen in the RT-PCR analysis of, e.g., the spleen could be be secreted at the lining of these ducts, pointing to a major potential Downloaded from derived from mesothelial cells. In most of the alveolar macro- role in the mucosal defense system against invading microorganisms. phages, the staining appeared to be present in the phagolysosome Surprisingly, SP-D was also located in the basal cells of epider- compartment, but a few macrophages showed a distinct granular mis and throughout the epithelial cell layer of the esophagus. SP-D staining of the cell membrane. Ultrastructural studies have previ- could be released from these cells in inﬂammatory states and play ously shown the presence of SP-D in the endocytic compartment a role in wound healing and combating cutaneous infection. There

of alveolar macrophages but not in the biosynthetic organelles is evidence of some species variation in SP-D expression in the http://www.jimmunol.org/ (18). This suggests that SP-D is not produced by these cells but is gastrointestinal tract. Whereas SP-D was clearly detected in the taken up by endocytosis. The localization of SP-D to the mem- gastric mucosa but not in the small intestine of the rat (28), both brane is the ﬁrst step in such an event, and a putative receptor for the body and pyloric mucosa of human gastric mucosa showed SP-D that may be responsible for this membrane binding has re- faint immunostaining, and only a weak RT-PCR signal was found cently been characterized (20, 35). This glycoprotein, named gp- when RNA from whole gastric mucosa was used as template. Both 340, exists both in a soluble form and in a form associated with the these parameters were stronger in the small intestine. membrane of the alveolar macrophages. It is a multidomain protein In the urinary tract, SP-D was found in the renal collecting ducts composed of 14 scavenger receptor cysteine-rich domains, four and loops of Henle, as well as in the epithelium of the ureter and

CUB domains, and a zona pellucida domain (35) and appears to be bladder. Hensin, the rabbit analogue of DMBT1, is found in the by guest on September 29, 2021 an alternative splicing product of the DMBT1 (deleted in malignant collecting ducts of the kidney and has been implicated in the ter- brain tumor 1) gene (36). minal differentiation of the intercalated cells (37, 38). Because SP-D was identiﬁed in epithelial cells lining the ducts of a number SP-D colocalizes with gp-340 in the collecting ducts, as well as of exocrine glands and the biliary and urinary tracts. SP-D may thus in epithelial cells in the lung, salivary glands, pancreas, and

FIGURE 5. Immunohistochemical localization of SP-D in epithelial cells in normal human tissues. SP-D was found in the skin (a), esophagus (b), small intestine (c), gallbladder epithelium (d), collecting ducts of the kidney (e–g), and the epithelia of the ureter (h) and urinary bladder (i). The tissues were stained by an indirect immunoperoxidase technique and counterstained with Mayer’s hematoxylin as described in Materials and Methods. Original magnifica- tions: e and h, ϫ100; b–d, ϫ200; a, f, and g, ϫ400. 5870 TISSUE LOCALIZATION OF SP-D small intestine, it is possible that the interaction between SP-D and 17. Cai, G. Z., G. L. Griffin, R. M. Senior, W. J. Longmore, and M. A. Moxley. 1999. gp-340, also a DMBT1 gene product, may be involved in processes Recombinant SP-D carbohydrate recognition domain is a chemoattractant for human neutrophils. Am. J. Physiol. 276:L131. other than immunoprotection. DMBT1 has been suggested as a 18. Voorhout, W. F., T. Veenendaal, Y. Kuroki, Y. Ogasawara, L. M. van Golde, and candidate tumor suppressor for brain, gastrointestinal, and lung H. J. Geuze. 1992. Immunocytochemical localization of surfactant protein D (SP-D) in type II cells, Clara cells, and alveolar macrophages of rat lung. J. His- cancers (36, 38–41), and it is well known that interruption of tochem. Cytochem. 40:1589. terminal differentiation pathways can lead to tumor formation. 19. Crouch, E., S. F. Kuan, and A. Persson. 1994. Surfactant protein D: possible role We conclude that the localization and synthesis of SP-D are not in pulmonary host defense. Prog. Respir. Res. 106:132. 20. Holmskov, U., P. Lawson, B. Teisner, I. Tornoe, A. C. Willis, C. Morgan, restricted to the respiratory system. SP-D was found to be widely C. Koch, and K. B. Reid. 1997. Isolation and characterization of a new member distributed in exocrine glands and epithelial cells throughout the of the scavenger receptor superfamily, glycoprotein-340 (gp-340), as a lung sur- body. In many tissues, SP-D colocalizes with gp-340, and it cannot factant protein-D binding molecule. J. Biol. Chem. 272:13743. 21. Borron, P. J., E. C. Crouch, J. F. Lewis, J. R. Wright, F. Possmayer, and be excluded that SP-D may have additional functions related to its L. J. Fraher. 1998. Recombinant rat surfactant-associated protein D inhibits hu- interaction with gp-340. SP-D also colocalizes with IgA of the man T lymphocyte proliferation and IL-2 production. J. Immunol. 161:4599. 22. Madan, T., U. Kishore, A. Shah, P. Eggleton, P. Strong, J. Y. Wang, S. S. Aggrawal, adaptive immune system. The main function of SP-D, like that of P. U. Sarma, and K. B. Reid. 1997. Lung surfactant proteins A and D can inhibit IgA, is to prevent microbial colonization of the epithelium and specific IgE binding to the allergens of Aspergillus fumigatus and block allergen- inhibit growth of pathogens once attached to the epithelium, at- induced histamine release from human basophils. Clin. Exp. Immunol. 110:241. 23. Kuroki, Y., S. Gasa, Y. Ogasawara, M. Shiratori, A. Makita, and T. Akino. 1992. tracting phagocytes and promoting phagocytosis by binding to spe- Binding specificity of lung surfactant protein SP-D for glucosylceramide. Bio- cific receptors as part of this function. In contrast to IgA, however, chem. Biophys. Res. Commun. 187:963. SP-D is constitutively produced from birth and may be up-regu- 24. Persson, A. V., B. J. Gibbons, J. D. Shoemaker, M. A. Moxley, and W. J. Longmore. 1992. The major glycolipid recognized by SP-D in surfactant is lated in the course of infection (42). phosphatidylinositol. Biochemistry 31:12183. Downloaded from Like mannan-binding lectin, SP-D may be considered as an “an- 25. Ogasawara, Y., F. X. McCormack, R. J. Mason, and D. R. Voelker. 1994. Chi- meras of surfactant proteins A and D identify the carbohydrate recognition do- te-Ab” with a broad binding profile, and the localization and func- mains as essential for phospholipid interaction. J. Biol. Chem. 269:29785. tion of SP-D indicates that this collectin is the counterpart in the 26. Botas, C., F. Poulain, J. Akiyama, C. Brown, L. Allen, J. Goerke, J. Clements, innate immune system to IgA in the adaptive immune system. E. Carlson, A. M. Gillespie, C. Epstein, et al. 1998. Altered surfactant homeostasis and alveolar type II cell morphology in mice lacking surfactant protein D. Acknowledgments Proc. Natl. Acad. Sci. USA 95:11869.

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