About 80% of patients with multiple myeloma develop a severe osteolytic bone disease causing pain and fractures. The myeloma cells secrete immunoglobulins and the presence of monoclonal immunoglobulins in serum is a hallmark of the disease. Immunoglobulins play a role in bone loss, but have not been linked with the bone disease in multiple myeloma.
In this work, we isolated immunoglobulins from serum of myeloma patients using protein G coupled magnetic beads and columns. We found that immunoglobulins from patients with bone disease (n=16) promoted osteoclast differentiation when added to human monocyte-derived pre-osteoclasts (p=0.006). We next fractionated the immunoglobulin samples by size-exclusion chromatography and found that the "osteoclast promoting activity" was in the high-molecular weight fractions, suggesting that they are in complexes. Since extent of complex formation may be determined by glycosylation, we examined whether there is a difference in immunoglobulin glycosylation between healthy controls and patients, and whether it changes during disease progression. To this end we analysed IgG glycosylation in serum samples from patients (n=72) and age and sex matched controls (n=51). These analyses showed that patient IgG was less galactosylated (p=0.02) and less sialylated (p=0.04) compared with control IgG. Moreover, patients with bone disease (n=43) had significantly less galactose on IgG compared with patients without bone disease (p=0.02, n=33). Supporting this data, we found that galactosidase treatment of immunoglobulins from patients without bone disease induced osteoclastogenesis (p=0.03), whereas addition of galactose to immunoglobulins of patients with bone disease removed their pro-osteoclastogenic effect (p=0.01). Further, the glycosyltransferases ST6GAL1 and B4GALT11, which add sialic acid and galactose to the sugar chain, respectively, are less expressed in plasma cells obtained from patients with bone disease (n=137) compared with those without (n=36, p<0.002, p<0.001, GSE755). Importantly, we observed a significant reduction of IgG glycosylation (p=0.02, n=8) in serum samples obtained from individual patients before and after the onset of bone disease.
Taken together, our data support that immunoglobulins promote bone loss in multiple myeloma.
No relevant conflicts of interest to declare.
Author notes
Asterisk with author names denotes non-ASH members.
