Role of NF kappa B in antigen presentation and development of regulatory Tcells elucidated by treatment of dendritic cells with the proteasome inhibitor PSI

Dendritic cells (DC) are the most potent antigen-presenting cells for naive T cells, due to their high expression of MHC and costimulatory molecules, but relatively little is known about the biochemical pathways that regulate this function. We used the proteasome inhibitor N-benzyloxycarbonyl-IIe-Gl u(O-tert-butyl)-Ala-leucinal (PSI) to demonstrate that DC antigen presentat ion is NF kappaB dependent. As PSI is not a specific inhibitor of NF kappaB , we reproduced this finding using a very specific approach, namely adenovi ral gene transfer of I kappaB alpha, the naturally occurring inhibitor of N F kappaB. The mechanism for this inhibition of DC antigen presentation invo lves at least three aspects of antigen presenting function: down-regulation of HLA class II, down-regulation of CD86, and inhibition of the immunostim ulatory cytokines IL-12 and TNF-alpha. In the light of the marked down-regu lation of antigen-presentation cell function, it was of interest to investi gate what effects exposure to PSI-treated DC might have on T cell function. It was found that immunological tolerance was induced, as challenge of T c ells previously exposed to PSI-treated DC, with normal DG from the same don or did not restore their response, despite the presence of viable T cells. There were also changes in T cell surface markers, with down-regulation of CD3 and CD25 expression, and inhibition of the production of Th1 cytokines like IL-2 and IFN-gamma. These results demonstrates that NF kappaB is an ef fective target for blocking DC antigen presentation and inhibiting T cell-d ependent immune responses, and this has implications for the development of therapeutic agents for use in multiple conditions, including transplantati on, allergy and autoimmune diseases.