HLA-B44 Polymorphisms at Position 116 of the Heavy Chain Influence TAP Complex Binding via an Effect on Peptide Occupancy

A single residue polymorphism distinguishes HLA-B*4402(D116) from HLA-B*4405(Y116), which was suggested to allow HLA-B*4405 to acquire peptides without binding to tapasin-TAP complexes. We show that HLA-B*4405 is not inherently unable to associate with tapasin-TAP complexes. Under conditions of peptide deficiency, both allotypes bound efficiently to TAP and tapasin, and furthermore, random nonamer peptides conferred higher thermostability to HLA-B*4405 than to HLA-B*4402. Correspondingly, under conditions of peptide sufficiency, more rapid peptide-loading, dissociation from TAP complexes, and endoplasmic reticulum exit were observed for HLA-B*4405, whereas HLA-B*4402 showed greater endoplasmic reticulum retention and enhanced tapasin-TAP binding. Together, these studies suggest that position 116 HLA polymorphisms influence peptide occupancy, which in turn determines binding to tapasin and TAP. Relative to HLA-B*4405, inefficient peptide loading of HLA-B*4402 is likely to underlie its stronger tapasin dependence for cell surface expression and thermostability, and its enhanced susceptibility to pathogen interference strategies.