A role for HSC70 in regulating antigen trafficking and presentation during macronutrient deprivation

Abstract

Globally, protein malnutrition remains problematic, adversely affecting several systems including the immune system. Although poorly understood, protein restriction severely disrupts host immunity and responses to infection. Induction of high-affinity, long-lasting immunity depends upon interactions between B and T lymphocytes. B lymphocytes exploit several pathways including endocytosis, macroautophagy, and chaperone-mediated autophagy to capture and deliver antigens to the endosomal network. Within the endosomal network antigens are processed and loaded onto major histocompatibility complex (MHC) class II molecules for display and recognition by T lymphocytes. To examine the effect of macronutrient malnutrition on MHC class II antigen presentation, we grew B lymphocytes in media containing amino acids, sugars and vitamins but lacking serum, which contains several types of macronutrients. Our studies show macronutrient stress amplified macroautophagy, favoring MHC class II presentation of cytoplasmic antigens targeted to autophagosomes. By contrast, macronutrient stress diminished MHC class II presentation of membrane antigens including the B cell receptor (BCR) and cytoplasmic proteins that utilize the chaperone-mediated autophagy pathway. The BCR plays a critical role in MHC class II antigen presentation, as it captures exogenous antigens leading to internalization and degradation within the endosomal network. While intracellular protease activity increased with macronutrient stress, endocytic trafficking and proteolytic turnover of the BCR was impaired. Addition of high molecular mass macronutrients restored endocytosis and antigen presentation, evidence of tightly regulated membrane trafficking dependent on macronutrient status. Cytosolic chaperone HSC70 has been shown to play a role in endocytosis, macroautophagy, chaperone-mediated autophagy and proteolysis by the proteasome, potentially connecting distinct routes of antigen presentation. Here, altering the abundance of HSC70 was sufficient to overcome the inhibitory effects of nutritional stress on BCR trafficking and antigen presentation suggesting macronutrient deprivation alters the availability of HSC70. Together, these results reveal a key role for macronutrient sensing in regulating immune recognition and the importance of HSC70 in modulating distinct membrane trafficking pathways during cellular stress. These results offer a new explanation for impaired immune responses in protein malnourished individuals.