Endogenous presentation of a nuclear antigen on MHC class II by autophagy in the absence of CRM1-mediated nuclear export

Accumulating evidence suggests that intracellular antigens are endogenously presented on MHC class II, but it is still unknown whether antigens within different subcellular compartments are presented with similar efficiency, and via the same or different pathways. We have previously shown that endogenous MHC class II presentation of the cytosolic bacterial antigen neomycin phosphotransferase II (NeoR) is mediated by autophagy. Here, we addressed whether secluding NeoR from this cytoplasmic pathway by directing the protein into the cell nucleus (NucNeoR) would affect antigen presentation. Unexpectedly, NucNeoR was presented at least as efficiently as the cytosolic version of the antigen. Furthermore, presentation of NucNeoR was also dependent on autophagocytosis and lysosomal processing, indicating that both antigens were presented via the same pathway. Inhibition of CRM1-mediated nuclear export did not impede antigen presentation, indicating that NucNeoR gained access to this autophagy-dependent MHC class II presentation pathway by a CRM1-independent route. Thus, this endogenous presentation pathway broadens the spectrum of intracellular antigens surveyed by CD4⁺ T cells by efficiently sampling cytoplasmic as well as nuclear antigens.