Abstract A027: HLA-DP restricted responses to citrullinated proteins are pre-existing in the memory pool and can be rapidly reactivated to provide good tumor therapy

Abstract

Abstract Citrullination is the post-translational modification of arginine to citrulline. Citrullinated proteins are known to play a significant role in the pathogenesis of autoimmune diseases and are presented in complex with MHC class II. Citrullination is mediated by Peptidylarginine deiminases (PADs), which are a family of calcium dependent enzymes found in a variety of tissues. A recent elegant report by Ireland et al. demonstrates that the presentation of citrullinated T cell epitopes on APCs is dependent upon autophagy and PAD activity. Autophagy is also induced by stress in other cells to enable processing of endogenous antigens for presentation on MHC class II molecules. Stressful conditions in tumor microenvironment induce autophagy in cancer cells as a mechanism to promote their survival and we have shown that citrullinated epitopes presented on HLA-DR4 are excellent targets for anti-tumor immunity (Brentville et al). CD4 T cell responses to citrullinated proteins have also been characterized in association with the HLA-DRB1 shared epitope alleles in rheumatoid arthritis. However, if citrullination is a generic response to cellular stress it should not be HLA-restricted. In this report we show that there is a response to citrullinated vimentin and MOG peptides in C57Bl mice and that these peptides induce anti-tumor immune responses against established B16F1 or B16F1-MOG tumors respectively. We have shown that there is also a repertoire of T cells capable of responding to citrullinated peptides in a wide range of normal individuals that were HLA-DR4 negative. This suggested that the peptides were being presented on other HLA alleles. The only common HLA-allele amongst the responding patients was HLA-DP4 which is expressed by 70% of the Caucasian population. Indeed, motif analysis of the peptides revealed an HLA-DP super motif (Sidney et al.,) suggesting that these peptides could be presented on a range of HLA-DP molecules. To confirm this observation, we vaccinated HLA-DP4 mice with citrullinated vimentin and enolase peptides and showed a rapid CD4 T cell response suggesting that these mice had a pre-existing response. These responses could also target tumors and showed efficient tumor therapy in both an ovarian (ID8-DP4, 80% survival p>0.001) and an aggressive melanoma tumor model (B16-DP4, 70% survival p = 0.0058). These results demonstrate a vaccine incorporating citrullinated peptides may have a universal application in cancer therapy.