HLA-DR is incorporated into the HIV virion envelope [52] and is transcriptionally activated by CIITA, therefore its contribution to Gag processing was assessed. Virions from HLA-DR-expressing cells exhibited less Gag processing than virions from CIITAexpressing cells ( Figure 5F , bottom panel). When virions from both cell lines were affinity-purified on HLA-DR binding columns, Gag processing was reduced in those produced in the HLA-DRexpressing cells as compared to virions from cells which express CIITA ( Figure 5F , upper panel and 5G), suggesting that HLA-DR alone does not contribute to increased Gag processing. Similar to our previous results, there are fewer processing intermediates present in virions from CIITA-expressing cells, suggesting that CIITA expression enhances virion infectivity by increasing maturation through more complete Gag processing.