Low molecular weight IgM in B cell lymphoproliferative disorders

IgM and its receptors: structural and functional aspects

This review combines the data obtained before the beginning of the 1990s with results published during the last two decades. The predominant form of the IgM molecule is a closed ring composed of five 7S subunits and a J chain. The new model of spatial structure of the pentamer postulates nonplanar mushroom-shaped form of the molecule with the plane formed by a radially-directed Fab regions and central protruding portion consisting of Cµ4 domains. Up to the year 2000 the only known Fc-receptor for IgM was pIgR. Interaction of IgM with pIgR results in secretory IgM formation, whose functions are poorly studied. The receptor designated as Fcα/µR is able to bind IgM and IgA. It is expressed on lymphocytes, follicular dendritic cells, and macrophages. A receptor binding IgM only named FcµR has also been described. It is expressed on T- and B-lymphocytes. The discovery of new Fc-receptors for IgM requires revision of notions that interactions between humoral reactions involving IgM and the cells of the immune system are mediated exclusively by complement receptors. In the whole organism, apart from IgM induced by immunization, natural antibodies (NA) are present and comprise in adults a considerable part of the circulating IgM. NA are polyreactive, germ-line-encoded, and emerge during embryogenesis without apparent antigenic stimuli. They demonstrate a broad spectrum of antibacterial activity and serve as first line of defense against microbial and viral infections. NA may be regarded as a transitional molecular form from invariable receptors of innate immunity to highly diverse receptors of adaptive immunity. By means of interaction with autoantigens, NA participate in maintenance of immunological tolerance and in clearance of dying cells. At the same time, NA may act as a pathogenic factor in atherosclerotic lesion formation and in development of tissue damage due to ischemia/reperfusion.

IgM paraproteinaemia: disease associations and laboratory features

AIMS

To review the disease associations and laboratory features occurring in IgM paraproteinaemia.

METHODS

Systematic review of all new serum IgM paraproteins detected over a 6-year period in an immunodiagnostic laboratory serving a population of 400,000 people. Clinical diagnoses were ascertained from a computerised laboratory database or clinical notes, whilst associated laboratory features were obtained from the same sources.

RESULTS

The 125 IgM paraproteins detected constitute 19.7% of all new paraproteins observed over the period of study. IgM paraproteinaemia occurred more commonly in males and its frequency increased with age. Approximately 30% were associated with B cell lymphoproliferative disorders (Waldenstrom's macroglobulinaemia, non-Hodgkin's lymphoma, chronic lymphocytic leukaemia, amyloid, etc.) with the remainder being labelled as monoclonal IgM gammopathies of uncertain significance (four having a peripheral neuropathy). At clinical presentation, patients with lymphoproliferative disorders tended to have higher levels of IgM, beta2-microglobulin, the presence of free urinary light chains and demonstrated molecular size heterogenicity of the paraprotein (presence of decamers, oligomers and monomers in addition to the pentamer) but there was considerable overlap. A good correlation was noted between paraprotein concentration and viscosity in most patients.

CONCLUSION

IgM paraproteinaemia was most frequently encountered in the context of a gammopathy of uncertain significance. Features which suggested lymphoproliferative disorders included higher levels of paraprotein (>15 g/l) elevated levels of beta2-microglobulin and the presence of urinary free high chain. However, as much overlap was seen, regular monitoring of paraprotein levels is considered mandatory in the management of these patients.

Acquired angioedema as the presenting feature of lymphoproliferative disorders of mature B-lymphocytes

BACKGROUND

Acquired complement component 1 (C1) inhibitor deficiency with consequent angioedema is a rare condition that may indicate an underlying neoplasm of B-lymphocytes or plasma cells.

METHODS

Three patients with C1 inhibitor deficiency and late-onset angioedema were studied to establish the presence and to characterize the nature of any underlying lymphoproliferative disorder.

RESULTS

Each of the patients was found to have a low-grade B-lineage lymphoma. In one patient, the underlying disorder, best characterized as splenic lymphoma with villous lymphocytes, was occult initially. The other two patients had splenic lymphoma with villous lymphocytes and small lymphocytic lymphoma, respectively. Two of the patients had an immunoglobulin M (IgM) serum paraprotein with cold agglutinin activity; the third patient had no paraprotein, but his cells expressed monoclonal membrane IgM. Two of the patients initially did not require specific anti-lymphoma therapy and responded to therapy with an attenuated androgen; in the third patient, splenectomy and cytotoxic chemotherapy led to resolution of the acquired angioedema. In all three patient, there was considerable delay in diagnosis consequent to lack of awareness of this syndrome.

CONCLUSIONS

The presence of a neoplasm of B-lymphocytes was established eventually in all three patients, although in one patient it was occult initially. In each case, the lymphocytes were mature B-lymphocytes that either showed cytologic evidence of plasma cell differentiation or secreted a paraprotein. Underlying lymphoma is common in patients who are seen initially with late-onset angioedema. Angioedema is an important, potentially life-threatening complication of lymphoma. Optimal patient management requires that both angioedema and the underlying lymphoma are recognized.

An enhanced chemiluminescence detection system combined with a modified immunoblot technique for the detection of low molecular weight IgM in sera from healthy adults and neonates

An enhanced chemiluminescence detection system combined with a modified immunoblot technique is described for the detection of low molecular weight IgM (LMW IgM) in human sera and cell culture supernatants at levels down to 2 pg/ml. This detection system is reliable, specific and more sensitive than the previously described chromogenic detection system. Importantly, this method has, for the first time, revealed LMW IgM in the sera from all 34 healthy adults and 26 neonatal cord bloods tested. A significant linear correlation was observed between the LMW IgM and the total circulating IgM in both the healthy adult sera (r = 0.88, p less than 0.001) and cord blood sera (r = 0.98, p less than 0.001).

Molecular size heterogeneity of immunoglobulins in health and disease

The molecular size of serum IgG, IgA and IgM in patients with a variety of monoclonal and polyclonal immune disorders has been determined by a sensitive immunoblotting technique. IgM, IgA and IgG3 paraproteins from patients with B cell lymphoproliferative disorders frequently polymerize with IgA paraproteins demonstrating two polymeric series, the basic unit of the more dominant series being monomeric IgA, and the second consists of a basic unit having a molecular mass of approximately 70 kD heavier than monomeric IgA. The molecular nature of this heavier IgA moiety was shown to be IgA covalently bound with a single molecule of serum albumin or alpha 1 antitrypsin and this moiety was also observed in small quantities in sera from healthy subjects. IgM paraproteins, particularly from patients with malignant lymphoproliferative disorders, consisted of varying proportions of decamers, pentamers and monomers together with other low molecular weight IgM oligomers. Paraproteins from patients with benign conditions showed less tendency to exist in multiple molecular weight forms. Serum immunoglobulins from patients with polyclonal immune disorder also showed molecular sized heterogeneity. Significantly increased levels of dimeric IgA were observed in patients with alcoholic cirrhosis and Felty's syndrome, while low molecular weight IgM was commonly seen in patients with autoimmune disorders. In these latter disorders monomeric IgM correlated significantly with the serum IgM level suggesting a disorder of assembly of the IgM subunits during an ongoing IgM immune response. We have demonstrated considerable molecular size heterogeneity of serum immunoglobulins both in health and disease and have indicated some possible clinical associations.