Human Leukocyte Antigen (Hla) Class I Assembly and Trafficking in Transporter Associated with Antigen Processing (TAP)-Deficient Cells

Abstract

Human leukocyte antigen (HLA) class I molecules play a crucial role in immunity against infectious pathogens and cancers by presenting antigens to cytotoxic T lymphocytes (CTLs) and regulating natural killer (NK) cell activities. HLA class I assembly largely depends on peptide availability via the transporter associated with antigen processing (TAP). Viral infections and cancers often inhibit TAP to evade immune responses. While TAP deficiency typically leads to HLA class I degradation, some TAP-deficient cells maintain low but detectable surface HLA class I expression through poorly understood mechanisms. This thesis investigated the assembly and trafficking of HLA class I molecules under TAP-deficient conditions. TAP deficiency induces increased high mannose, Endoglycosidase H (Endo-H)-sensitive glycans on HLA class I molecules, but without activating canonical ER stress mediators. Factors implicated in unconventional glycoprotein trafficking do not selectively affect surface HLA class I expression i n TAP-deficient cells. Instead, HLA class I surface expression in these cells is brefeldin A sensitive, suggesting the involvement of the conventional Golgi-dependent secretory route. TAP deficiency does not significantly alter ER and Golgi alpha-1,2 mannosidase levels, and other conditions that compromise HLA class I assembly, such as tapasin deficiency, also induce increased high mannose HLA class I surface glycans. Notably, TAP-deficient cells share over 80% of HLA class I-eluted peptides with wild-type cells, and the surface HLA class I expression in both cell types is sensitive to pH gradient-dependent assembly in endocytic compartments. Based on these findings, we propose a model where saturated recognition mechanisms for misfolded HLA class I molecules result in their partial Golgi-dependent secretory trafficking, accounting for impaired glycan maturation. This Golgi-mediated export of HLA class I with high mannose glycans may represent a homeostatic response to misfolded HLA class I accumulation in the ER. These results also highlight the relevance of supplemental HLA class I assembly in endocytic compartments. Together, these findings provide new insights into HLA class I assembly and trafficking in TAP-deficient cells and could inform the development of novel immunotherapies against infectious pathogens and cancers that inhibit TAP.