Class II MHC molecules and antigen enter the same vesicles during internalization by resting B lymphocytes

Area of Science:

• Immunology
• Cell Biology

Background:

• B cell antigen presentation to T cells involves antigen receptor (Ig) binding, internalization, and processing.
• The interaction of processed antigens with class II MHC molecules during antigen presentation is not fully understood.
• It is unclear if processed antigens bind to internalized class II MHC or nascent class II within the cell.

Purpose of the Study:

• To investigate whether cell surface class II MHC molecules are cointernalized with antigens (Ag) during B cell internalization.
• To determine if class II MHC is excluded from antigen-containing vesicles or cointernalized.

Main Methods:

• Utilized 5- and 16-nm gold particles labeled with anti-class II and anti-Ig antibodies, respectively.
• Observed particle internalization in B cells using electron microscopy after incubation at 37°C.
• Quantified the presence and colocalization of both particle types within vesicles over time.

Main Results:

• Significant internalization of both anti-class II and anti-Ig labeled particles was observed within 10 minutes.
• Between 40-64% of vesicles contained both types of particles, indicating cointernalization.
• Cell surface class II MHC molecules were not selectively excluded from antigen-containing vesicles during internalization.

Conclusions:

• Cell surface class II MHC molecules are cointernalized with antigens bound to the B cell receptor.
• Cointernalization of antigen and class II MHC provides a mechanism for processed antigen to encounter class II MHC within the cell.
• This finding contributes to understanding the molecular mechanisms of B cell antigen presentation.