CD4⁺ T-cell recognition of human 5T4 oncofoetal antigen: Implications for initial depletion of CD25⁺ T cells

Background: The human 5T4 (h5T4) oncofoetal antigen is expressed by a wide variety of human carcinomas including colorectal, ovarian, gastric and renal, but rarely on normal tissues. Its restricted expression on tumour tissues as well as its association with tumour progression and bad prognosis has driven the development of a MVA-based vaccine (TroVax) which has been tested in several early phase clinical trials and these studies have led to the start of a phase III trial in renal cell carcinoma patients. We have recently shown that CD8 ⁺ T cells recognizing h5T4 can be generated in the absence of CD4⁺ T cells from peripheral blood lymphocytes of human healthy individuals. Results: We report the existence and expansion of human CD4 ⁺ T cells against h5T4 by stimulation with autologous monocyte-derived dendritic cells infected with a replication defective adenovirus encoding the h5T4 cDNA (Ad-h5T4). The h5T4-specific T-cell responses in normal individuals are enhanced by initial depletion of CD25⁺ cells (putative T regulatory cells) prior to the in vitro stimulation. We have identified a novel h5T4-derived 15-mer peptide recognized by CD4⁺ T cells in HLA-DR4 positive healthy individuals. Interestingly, CD4⁺ T cells spontaneously recognizing a different 5T4 epitope restricted by HLA-DR were identified in tumour-infiltrating lymphocytes isolated from a regressing renal cell carcinoma lung metastasis. Conclusion: Our data show that CD4 ⁺ T cells recognizing h5T4 can be expanded and detected in healthy individuals and a renal cell carcinoma patient. Such h5T4-specific CD4 ⁺ T cells boosted or induced by vaccination could act to modulate both cell or antibody mediated anti-tumour responses.