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Infect. Immun., Sep 1997, 3896-3905, Vol 65, No. 9
HC Hoppe, BJ de Wet, C Cywes, M Daffe and MR Ehlers
The molecular basis for the binding of Mycobacterium tuberculosis to
nonphagocytic cells, which are readily infected in vitro, and the in vivo
significance of this interaction are incompletely understood. Of six cell
types tested, we found that only two, Chinese hamster ovary (CHO)
fibroblasts and primary porcine aortic endothelial cells, were able to bind
M. tuberculosis H37Rv efficiently in vitro. Binding to both CHO and
endothelial cells was markedly (three- to fivefold) enhanced by 10 to 20%
human or bovine serum, suggesting that the bacteria were coated by a serum
opsonin. Preincubation with individual candidate opsonins revealed that
recombinant human mannose-binding protein (rMBP), fibronectin, and
transferrin were each able to enhance binding threefold. Preincubation of
bacteria in serum depleted of mannan-binding lectins or in genetic
MBP-deficient serum resulted in enhancements that were only approximately
60 and 58%, respectively, of that produced by preincubation in control
serum. In contrast, serum depleted of fibronectin or transferrin retained
its opsonizing capacity, suggesting that the latter two are not significant
opsonins in whole serum. Binding of M. tuberculosis and Mycobacterium
smegmatis to both CHO and endothelial cells in the presence or absence of
serum was blocked (60 to 70%) by a monoclonal antibody, MAb 1D1, selected
for recognition of intact bacilli. The 1D1 antigen was purified from
mycobacterial cell walls and chemically identified as a polar
phosphatidylinositol mannoside (PIM). Latex beads coated with purified 1D1
antigen bound to CHO cells, which was enhanced threefold by serum and
abolished by periodate treatment, suggesting a requirement for the PIM
mannoses in opsonic adhesion. This was likely mediated, at least in part,
by serum MBP, as rMBP bound strongly to 1D1 antigen in both thin- layer
chromatography overlay and plate binding assays, the latter in a
mannan-inhibitable manner. This is the first demonstration that
mycobacterial PIMs can function as adhesins for binding to nonphagocytic
cells, both directly and after opsonization with serum proteins, including
MBP.
Copyright © 1997, American Society for Microbiology
Identification of phosphatidylinositol mannoside as a mycobacterial adhesin mediating both direct and opsonic binding to nonphagocytic mammalian cells
Department of Medical Biochemistry, University of Cape Town Medical School, South Africa.
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