Phosphorylation of CIITA directs its oligomerization, accumulation and increased activity on MHCII promoters.

The class II transactivator (CIITA) is the master regulator of major histocompatibility complex class II (MHCII) transcription. Its activity is regulated at the post-transcriptional level by phosphorylation and oligomerization. This aggregation mapped to and depended on the phosphorylation of residues between positions 253 and 321 in CIITA, which resulted in a dramatic accumulation of the protein and increased expression of MHCII genes in human promonocytic U937 cells, which represent immature antigen-presenting cells. Thus, the post-transcriptional modification of CIITA plays an important role in the immune response.