Lipoglycans Are Putative Ligands for the Human Pulmonary Surfactant Protein A Attachment to Mycobacteria: CRITICAL ROLE OF THE LIPIDS FOR LECTIN-CARBOHYDRATE RECOGNITION

Stéphane Sidobre,Jérome Nigou,Germain Puzo,Michel Rivière

doi:10.1074/jbc.275.4.2415

Abstract

The human pulmonary surfactant protein A (hSP-A) has been implicated in the early capture and phagocytosis of the pathogenic Mycobacterium tuberculosis by alveolar macrophages. In this report, we examined the interaction of alveolar proteinosis patient hSP-A with Mycobacterium bovis BCG, the vaccinating strain, as a model of pathogenic mycobacteria, and Mycobacterium smegmatis, a nonpathogenic strain. We found that hSP-A binds to the surface of M. bovis BCG, but also to a slightly lesser extent, to M. smegmatis, indicating that hSP-A does not discriminate between virulent and nonpathogenic strains. Among the various glycoconjugates isolated from the mycobacterial envelope, we found that the best ligands are the two major lipoglycans: the mannosylated lipoarabinomannan (ManLAM) and the lipomannan. In contrast, the mannose-capped arabinomannan, structurally close to the ManLAM, as well as the LAMs from the non pathogenic M. smegmatis are poorly recognized by hSP-A. These results clearly show that the presence of both the terminal mannose residues and the phophatidyl-myo-inositol anchor are necessary to achieve the highest binding affinity. Selective removal of either the terminal mannose or the acyl residues esterifying the glycerol moiety of the ManLAM abrogates the interaction with hSP-A, further supporting the notion that the hSP-A recognition of the carbohydrate epitopes of the lipoglycans is dependent of the presence of the fatty acids.

Highlights

The human pulmonary surfactant protein A has been implicated in the early capture and phagocytosis of the pathogenic Mycobacterium tuberculosis by alveolar macrophages
We found that human pulmonary surfactant protein A (hSP-A) binds to the surface of M. bovis BCG, and to a slightly lesser extent, to M. smegmatis, indicating that hSP-A does not discriminate between virulent and nonpathogenic strains
The maximum level of specific binding (Bmax) to M. bovis BCG is 8.4 ng/1 ϫ 107 cells, and the apparent equilibrium dissociation constant, calculated from the saturation curve fitted by nonlinear regression, is KDBCG ϳ4.5 Ϯ 0.7 ϫ 10Ϫ9 M consistent with the previous values observed for the binding of hSP-A to

Summary

Introduction

The human pulmonary surfactant protein A (hSP-A) has been implicated in the early capture and phagocytosis of the pathogenic Mycobacterium tuberculosis by alveolar macrophages. Two key components of pulmonary innate immunity, the human surfactant proteins A and D (hSP-A, hSP-D), were found to interact with M. tuberculosis Despite their great structural analogy, they have opposite effects toward the phagocytosis of invading mycobacteria. Weikert et al [9] reported that hSP-A may act as an opsonin, enhancing the phagocytosis of Mycobacterium bovis BCG by macrophages This result suggests that opsonization mechanisms leading to the phagocytosis of invading pathogenic mycobacteria may occur in the alveolar space, just following the initial inoculation, despite the low concentration of soluble serum complement opsonins. The phosphatidylinositol mannosides (PIMs), the glucan, and the mannose-capped lipoarabino-

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