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Mannose-Binding Lectin (MBL) Facilitates Opsonophagocytosis of Yeasts but Not of Bacteria despite MBL Binding¹

Nannette Brouwer^2,*, Koert M. Dolman^*,, Michel van Houdt^*, Marleen Sta^*, Dirk Roos^* and Taco W. Kuijpers^*,

^* Sanquin Research and Landsteiner Laboratory and Emma Children’s Hospital, Academic Medical Centre, University of Amsterdam, Amsterdam, The Netherlands

Mannose-binding lectin (MBL) is a serum protein of the innate immune system. After binding to a microorganism, MBL in complex with MBL-associated serine proteases activates the complement system, resulting in cleavage of complement factor C3. Cleaved C3 on the surface of the microorganism mediates opsonization for clearance, but the impact of MBL on subsequent phagocytosis has not been widely studied. We investigated the role of MBL in complement activation and phagocytosis of various bacteria and yeast species by flow cytometry. We measured both the C3 deposition during serum opsonization of fluorescent-labeled microorganisms as well as subsequent uptake of these microorganisms by human neutrophils. In MBL-deficient sera, a consistently decreased C3 deposition on both zymosan and Candida albicans was found and a reduced phagocytosis by neutrophils that was restored by exogenous MBL. This indicates that the lectin pathway of complement activation is important for the opsonophagocytosis of yeasts. In contrast, the C1q-dependent classical pathway dominated in the opsonization and phagocytosis of Staphylococcus aureus, Streptococcus pneumoniae, and Escherichia coli, whereas no effect of MBL was found. Both the lectin and the classical pathway of complement activation were highly amplified by the alternative route for opsonophagocytosis by neutrophils of yeast as well as microbial species. In summary, our data demonstrate that yeast species are preferentially opsonized and subsequently phagocytosed via activation of the lectin pathway of complement, whereas the uptake of bacterial strains was found to be largely MBL independent.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ This work was supported by Landsteiner Stichting voor Bloedtransfusie Research (LSBR 0207).

² Address correspondence and reprint requests to Dr. Nannette Brouwer, Sanquin Research, Phagocyte Laboratory, U209b, Plesmanlaan 125, 1066 CX Amsterdam, The Netherlands. E-mail address: n.brouwer{at}sanquin.nl

³ Abbreviations used in this paper: MBL, mannose-binding lectin; MASP, MBL-associated serine protease; SNP, single nucleotide polymorphism; PMN, polymorphonuclear leukocyte.

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