A mycoplasma peptide elicits heteroclitic CD4+ T cell responses against tumor antigen MAGE-A6.

Although T-helper (Th) epitopes have been previously reported for many tumor antigens, including MAGE-A6, the relevant HLA-DR alleles that present these peptides are expressed by only a minority of patients. The identification of tumor antigenic epitopes presented promiscuously by many HLA-DR alleles would extend the clinical utility of these peptides in vaccines and for the immunomonitoring of cancer patients. A neural network algorithm and in vitro sensitization assays were employed to screen candidate peptides for their immunogenicity. The MAGE-A6(140-170), MAGE-A6(172-187), and MAGE-A6(280-302) epitopes were recognized by CD4+ T cells isolated from the majority of normal donors and melanoma patients evaluated. Peptide-specific CD4+ T cells also recognized autologous antigen-presenting cell pulsed with recombinant MAGE-A6 (rMAGE) protein, supporting the natural processing and MHC presentation of these epitopes. Given the strong primary in vitro sensitization of normal donor CD4+ T cells by the MAGEA6(172-187) epitope, suggestive of potential cross-reactivity against an environmental stimulus, we identified a highly homologous peptide within the Mycoplasma penetrans HF-2 permease (MPHF2) protein. MPHF2 peptide-primed CD4+ T cells cross-reacted against autologous APC pulsed with the MAGE-A6(172-187) peptide or rMAGE protein and recognized HLA-matched MAGE-A6+ melanoma cell lines. These responses seemed heteroclitic in nature because the functional avidity of MPHF2 peptide-primed CD4+ T cells for the MAGE-A6(172-187) peptide was approximately 1,000 times greater than that of CD4+ T cells primed with the corresponding MAGE-A6 peptide. We believe that these novel "promiscuous" MAGE-A6/MPHF2 Th epitopes may prove clinically useful in the treatment and/or monitoring of a high proportion of cancer patients.