# Two MHC class II-restricted epitopes are processed in distinct endocytic compartments as a result of the acid-induced structural changes in a viral glycoprotein

#### Abstract

Exogenous antigens are processed and loaded onto Major Histocompatibility Complex (MHC) class II molecules in the endocytic pathway, although the factors that influence the intracellular location(s) of class II-restricted epitope loading remain poorly understood. I was interested in determining whether (1) two epitopes derived from the same viral protein could be processed in distinct endocytic compartments and (2) the structural context of an epitope dictates where it is processed in the endocytic pathway. To investigate this, I utilized the hemagglutinin molecule (HA) of the A/PR/8/34 (PR8) influenza virus as a model antigen. The studies reported here focus upon two well-defined I-E$\rm\sp{d}$ restricted class II epitopes within HA termed site 1 (S1) and site 3 (S3). Based on previous processing and presentation data and the positioning of S1 and S3 within the HA structure, I predicted that S1 requires the harsh proteolytic conditions of the late endosomal/lysosomal compartments for its excision. In contrast, I predicted that S3 is processed in an early endocytic compartment, relying on the structural changes in HA following acidification. Using an epitope-specific monoclonal antibody (MAb), I show that S1 becomes detectable in late endosomal/lysosomal vesicles. Using a mutant cell line, I show that its presentation depends on expression of H2-DM, a protein that accumulates in the late endocytic pathway and is involved in class II loading. Furthermore, I determined that S1 presentation is enhanced when delivered directly to the lysosome by insertion of a lysosomal targeting signal in the cytoplasmic domain of HA. In contrast to S1, S3 presentation is H2-DM-independent. S3 is available for processing in early endosomes as a result of acid-induced structural changes in HA. Unlike S1, S3 presentation is not enhanced when targeted directly to the lysosome. Presentation of both epitopes can be made H2-DM-independent by denaturing HA, and H2-DM-dependent by preventing the acid-induced changes from occurring. These findings indicate that two distinct epitopes derived from the same viral protein can be processed in distinct endocytic organelles and that the structural context of an epitope can determine where it is processed in the endocytic pathway.

#### Subject Area

Immunology|Cellular biology|Microbiology

#### Recommended Citation

Chianese-Bullock, Kimberly A, "Two MHC class II-restricted epitopes are processed in distinct endocytic compartments as a result of the acid-induced structural changes in a viral glycoprotein" (1998). ETD Collection for Thomas Jefferson University. AAI9829085.
https://jdc.jefferson.edu/dissertations/AAI9829085

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