Proteolysis of lymphocytic surface immunoglobulin

Area of Science:

• Immunology
• Cell Biology
• Biochemistry

Background:

• Surface immunoglobulins (Ig) on lymphocytes play crucial roles in immune responses.
• Understanding the structural integrity and membrane interactions of these surface Ig is vital.
• Limited proteolysis offers a method to probe Ig structure and membrane attachment.

Purpose of the Study:

• To investigate the limited proteolysis of lymphocytic surface immunoglobulins (IgM, IgD) in guinea pigs, rabbits, and humans.
• To determine the fate of immunoglobulin fragments after enzymatic cleavage.
• To elucidate the mechanism of Ig attachment to the lymphocyte membrane.

Main Methods:

• Immunofluorescence microscopy was employed using antisera specific for immunoglobulin fragments.
• Papain digestion was used for limited proteolysis of cell surface immunoglobulins.
• Analysis of fragment attachment to lymphocyte membranes was performed.

Main Results:

• Papain cleaved cell surface IgM on guinea pig and rabbit lymphocytes at the hinge region, leaving the Fcmicron fragment attached to the membrane.
• Human chronic lymphocytic leukemia cells showed complete clearance of surface IgD and IgM from the cell surface after papain treatment.
• The differential clearance suggests either proteolytic degradation of membrane-bound Fc or cleavage at a different site than the hinge region.

Conclusions:

• The Fcmicron fragment of IgM can remain membrane-associated after hinge region cleavage.
• Human IgD and IgM exhibit distinct susceptibility to papain-mediated cleavage and release from the cell surface.
• These findings highlight variations in the membrane anchoring or susceptibility to proteolysis among different immunoglobulin isotypes and species.