Intracellular trafficking of antigen in B lymphocytes

Abstract

B cells are an essential part of the adaptive immune system by providing the humoral arm of the response, and specific and long-lasting protection against a pathogens. Successful antigen internalization, intracellular trafficking, processing and presentation are critical for full B cell activation and high-affinity antibody production. Despite the fundamental importance of this pathway for the immune response, the mechanisms underlying antigen trafficking for successful processing and MHCII presentation remain unclear. By using confocal and super-resolution microscopy, we followed the traffic of internalized antigen along the endosomal pathway in B cells. To identify different vesicle populations, we took advantage of different Rab GTPases as well-known markers for early-, late-and recycling-endosomes. After internalization, antigen traffics from early-to late-endosomes and typically reaches a dense endosomal compartment located in the perinuclear region. This compartment is rich in lysosomal markers and Cathepsin-S, as well as MHCII, suggesting antigen loading compartment. Interestingly however, when we investigated the early timepoints after activation in detail, we found the antigen in peripheral endosomes with atypical features combining both early (EEA1+, Rab5+) and late (LAMP1+) markers. These endosomes also showed colocalization with Rab11, a marker of recycling-endosomes, and MHCII, suggesting that already these early peripheral vesicles could degrade antigen and load peptides on MHCII for presentation. In support of their role in antigen processing, these vesicles also contained Cathepsin-S and showed hydrolysis of antigen labeled with DQ, a dye that only fluoresces upon hydrolysis. We postulate that these early vesicles could play a role in fast antigen processing, or could promote presentation of different peptide repertoire as compared to the central, perhaps more mature, perinuclear compartment. Further studies are needed to unravel how these pathways are regulated and how their balance affects T cell help and thereby B cell fate and production of high-affinity antibodies.