LAMP-2-deficient human B cells exhibit altered MHC class II presentation of exogenous antigens

Major histocompatibility complex (MHC) class II molecules present antigenic peptides derived from engulfed exogenous proteins to CD4⁺ T cells. Exogenous antigens are processed in mature endosomes and lysosomes where acidic proteases reside and peptide‐binding to class II alleles is favoured. Hence, maintenance of the microenvironment within these organelles is probably central to efficient MHC class II‐mediated antigen presentation. Lysosome‐associated membrane proteins such as LAMP‐2 reside in mature endosomes and lysosomes, yet their role in exogenous antigen presentation pathways remains untested. In this study, human B cells lacking LAMP‐2 were examined for changes in MHC class II‐restricted antigen presentation. MHC class II presentation of exogenous antigen and peptides to CD4⁺ T cells was impaired in the LAMP‐2‐deficient B cells. Peptide‐binding to MHC class II on LAMP‐2‐deficient B cells was reduced at physiological pH compared with wild‐type cells. However, peptide‐binding and class II‐restricted antigen presentation were restored by incubation of LAMP‐2‐negative B cells at acidic pH, suggesting that efficient loading of exogenous epitopes by MHC class II molecules is dependent upon LAMP‐2 expression in B cells. Interestingly, class II presentation of an epitope derived from an endogenous transmembrane protein was detected using LAMP‐2‐deficient B cells. Consequently, LAMP‐2 may control the repertoire of peptides displayed by MHC class II molecules on B cells and influence the balance between endogenous and exogenous antigen presentation.