Major histocompatibility complex class II compartments in human and mouse B lymphoblasts represent conventional...

Area of Science:

• Immunology
• Cell Biology
• Endocrinology

Background:

• Intracellular major histocompatibility complex (MHC) class II molecules are crucial for antigen presentation.
• MHC class II molecules typically reside in late endocytic compartments (MIICs) in antigen-presenting cells.
• A discrepancy exists regarding MHC class II localization in mouse A20 B cells, with early endocytic vesicles (CIIVs) implicated.

Purpose of the Study:

• To investigate and clarify the endocytic pathways of major histocompatibility complex (MHC) class II molecules in human and mouse B cell lines.
• To resolve the conflicting reports on the location of MHC class II molecules within B cell endocytic compartments.

Main Methods:

• Quantitative immunoelectron microscopy was employed on cryosections of human (6H5.DM) and mouse (A20.Ab) B cell lines.
• Cells were pulse-chased using endocytic tracers like transferrin-HRP and BSA-gold.
• Antibodies against invariant chain and endogenous MHC class II-peptide complexes were used for identification.

Main Results:

• Up to six endocytic subcompartments were identified, including an early MIIC enriched in invariant chain.
• Early MIICs served as the earliest endocytic compartment containing MHC class II-peptide complexes.
• The identified early MIIC shared characteristics with CIIVs and was located downstream of early endosomes.

Conclusions:

• B cells utilize conventional endocytic compartments for major histocompatibility complex (MHC) class II presentation.
• No unique class II-containing endocytic structures were found in B cells beyond those in non-antigen-presenting cells.
• Early MIICs appear to be the primary entry point for newly synthesized MHC class II/invariant chain complexes into the endocytic pathway.