Towards Immunotherapy of Midgut Carcinoid Tumors

Abstract: Classical midgut carcinoids belong to neuroendocrine tumors of the gastroenteropancreatic tract (GEP-NETs) and are associated with serotonin overproduction. The term midgut is derived from the tumors’ embryological site of origin: enterochromaffin cells in the lower jejunum, ileum, caecum and the ascending colon. Despite their rather benign nature, these tumors can metastasize to mesentery and liver, putting patients at risk for the so-called carcinoid syndrome. This syndrome is characterized by flushes, diarrhoea and valvular heart disease due to the excessive serotonin secretion by tumor cells. Treatment of metastatic disease is currently ineffective and T cell immunotherapy has been suggested as a novel approach.We propose a number of midgut carcinoid-associated proteins as potential antigens for immunotherapy. Chromogranin A (CGA), tryptophan hydroxylase 1 (TPH-1), vesicular monoamine transporter 1 (VMAT-1), caudal type homeobox transcription factor 2 (CDX-2), islet autoantigen 2 (IA-2) and survivin represent interesting candidates based on their fairly restricted neuroendocrine tissue expression.In pursuit of potential antigens we identified a novel splicing variant of VMAT-1, lacking the second last exon. The variant, denoted VMAT1?15, encodes a differently translated C-terminal compared to the native form, is localized in the endoplasmic reticulum (ER) instead of large dense core vesicles and is unable to accumulate serotonin.We identify several immunogenic HLA-A'0201-binding peptide epitopes derived from our proposed antigens by analyzing CD8+ T cell responses in blood from midgut carcinoid patients. We demonstrate immune recognition of midgut carcinoid tumors in patients and in vitro generation of activated CD8+ T cells recognizing these peptide epitopes in blood from healthy controls. Patients also exhibit increased frequencies of circulating regulatory T cells (Tregs) with suppressive quality and patient lymphocytes display a decreased proliferative capacity compared to healthy controls. Midgut carcinoid tumors are frequently infiltrated by T cells, however always in the presence of Foxp3-expressing Tregs. Midgut carcinoid-associated antigens recognized by CD8+ T cells are of great interest for cellular therapies such as modified DC vaccines or adoptive T cell transfer. However, the systemic and local suppression of Th1 immunity must be considered and likely corrected in order to obtain clinically effective immunotherapies.