Generation of an immunodominant CTL epitope is affected by proteasome subunit composition and stability of the antigenic protein

Generation of the HLA-A0201 (A2) influenza Matrix 58-66 epitope contained w ithin the full-length Matrix protein is impaired in cells lacking the prote asome subunits low molecular protein 2 (LMP2) and LMP7, This Ag presentatio n block can be relieved by transfecting the wild-type LMP7 cDNA into LMP7-d eficient cells. A mutated form of LMP7, lacking the two threonines at the c atalytic active site, was equally capable of relieving the block in present ation of the influenza Matrix A2 epitope, These observations were extended by analyzing whether modification of the influenza Matrix protein could ove rcome the block in presentation of the A2 Matrix epitope, Expression of eit her a rapidly degraded form of the full-length Matrix protein or shorter Ma trix fragments led to an efficient presentation of the A2 influenza Matrix epitope by LMP7-negative cells. These findings demonstrate two main points: 1) LMP7 incorporation into the proteasome is of greater importance for the generation of the influenza A2 Matrix epitope than the presence of the LMP 7's catalytic site; and 2) the interplay between cytosolic proteases and st ability of target proteins is of importance in optimization of Ag presentat ion. These observations may have relevance to the immunodominance of tumor and viral epitopes and raise the possibility that generation of shorter pro tein fragments could be a mechanism to ensure optimal Ag presentation by ce lls expressing low levels of LMP7.