Renal Section, Division of Medicine, Imperial College School of Medicine, Hammersmith Hospital, London, UK
Sir,
In glomerular disease complement components (especially C3) are usually found immunohistologically in the glomerulus in association with immunoglobulins and are thought to be part of an immune complex. However, in some cases, particularly of acute proliferative [1] and mesangiocapillary glomerulonephritis [2] glomerular C3 may be present without demonstrable accompanying immunoglobulins. This failure is usually explained as being due to steric hindrance of all the immunoglobulin antigenic sites by complement.
Other possible explanations might be that the complement is fixed directly by bacterial products, by the alternative pathway as has been demonstrated for a number of bacteria (including Streptococci) [3], or by the lectin pathway [4]. In neither case is it clear why the complement should localize in the glomeruli. In a case of acute glomerulonephritis we tested the possibility that the binding of complement to the glomeruli involved a lectin interaction.
Case.
A 60-year-old Indian male had a history suggestive of acute glomerulonephritis. He had been given ampicillin and blood and throat cultures were negative, but the ASO was more than 800 units. CH50 was normal, but levels of alternative pathway activity [5] were low. Biopsy showed 27 glomeruli with increased lobularity, cellular proliferation and polymorph infiltration: immunofluorescence showed extensive granular deposits of C3 in both the mesangium and capillary loops but no immunoglobulins except segmental deposits of IgM in two glomeruli.
Cryostat sections were covered with 20 µl of solutions of 0.5 M glucose, glucosamine, N-acetyl-glucosamine, N-acetyl-galactosamine, mannose, alpha-methyl-mannoside or L-fucose each prepared in 0.15 M phosphate buffer pH 7.2 with 0.01% sodium azide and incubated overnight at 20°C in a humid atmosphere. After washing in phosphate-buffered saline, sections were processed for immunofluorescence using a fluorescein-labelled antihuman C3 antibody (Dakopatts). Glucosamine eluted almost all the glomerular C3 deposits, but the control buffer and the other sugars including N-acetyl-glucosamine had no effect.
A similar effect was sought in other cases of glomerulonephritis where the C3 was associated with immunoglobulins (three cases each of mesangial IgA disease, membranous nephropathy and mesangiocapillary nephritis) but in these cases sugars failed to elute C3.
Discussion.
The involvement of glucosamine was quite unexpected. Non-acylated glucosamine is much rarer in nature than its acetylated derivatives, though it is known to be present as part of the membrane anchor by which proteins, for example decay accelerating factor are attached to eukaryotic cell surfaces [6]. Non-acylated glucosamine is also present in a variety of bacterial components including the cell walls of some bacteria, and in staphylococcal adhesins [7].
Lectin type binding with glucosamine specificity does not appear to have been demonstrated in man. This specificity is shown by several plant lectins e.g. [8], and by the blood meal induced lectin from Glossina morsitans (which agglutinates Trypanosoma brucei) [9]. There is also evidence that adhesion of Candida to skin involves a D-glucosamine specific interaction [10].
These findings suggest that in some cases of acute glomerulonephritis a glucosamine specific lectin interaction is involved in the binding of complement in the glomeruli.
Acknowledgments
I wish to thank Professor C. Pusey and Dr S. Bartolotti for helpful discussion.
References