Naive B cells express IgM and IgD receptors. Both receptors share the same unique immunoglobulin variable region but differ in immunoglobulin constant regions. IgM is secreted in response to antigenic signaling but not much is known regarding the role of IgD in immune response.
New study in Nature Immunology provided very interesting data about the function of IgD. It turned out that unlike IgM, IgD receptors only respond to polyvalent antigens.
The authors, led by Hassan Jumaa at the Institute of Immunology (Ulm, Germany), first showed that unlike IgM receptor, cell line expressing IgD receptors specific for hapten (NIP) or antigen (HEL) responded only to polyvalent antigenic forms.
Next, the authors find that these difference in response between IgM and IgD was related to the difference in hinge region (that connects variable and constant regions). IgD with no hinge region (IgDΔhinge) responded as if IgM and IgM with IgD hinge region responded as if IgD.
The authors observed that anergic B cells obtained from antigen (HEL)-specific B cell double transgenic mouse expressing soluble HEL (s-HEL) could still respond to polyvalent HEL antigen.
The authors showed that monovalent antigen could competitively inhibit IgD signaling in response to polyvalent antigens.
Next, the authors observed that functionally, absence of IgM receptor could compromise innate B-1 cells scavenging response (natural antibody response) to a soluble auto-antigen phosphatidylcholine (PtC).
Finally, the authors also showed that the absence of IgM signaling compromised IgG scavenging functions against multiple auto-antigens (oxidized LDL, etc) as well.
In summary, this study revealed that anergic B cells (IgMlowIgDhigh) can respond to antigen when stimulated with polyvalent antigens. Probably the role of IgD receptor is to prevent improper activation of naive B cells in response to soluble antigens. I wonder what is the (a) phenotype of IgD KO mice or (b) whether auto-antigens targeted by natural antibodies are mono or polyvalent in nature?